A Quantum Diaries Survivor

Jeff pointed out to me today the remarkable world wide telescope, a site where you can download a software created by Microsoft to browse the heavens as if you were commanding a powerful telescope. The constellations are not maps, but actual pictures, into which you can zoom as much as the images of the digital sky surveys (SDSS and others) allow.

My jaw dropped as I started using the software, which you can download and install on your computer, and which works pretty much like google Earth - downloading the region you are visualizing from the internet. A nice feature is the appearance of a frame of thumbnail pictures around the zoomed area, highlighting the most interesting celestial objects present there. If you click once on each pic the relevant object is highlighted on the map; clicking twice will allow you to download full-resolution image of the object directly from the online databases, including Hubble images.

What I find amazing, however, is the fact that browsing the night sky becomes a thrilling experience at your fingertips in front of the computer. The realism is perfect - these are pictures, in pure google earth style. However, while we never have the need to find a feature on the Earth surface by hovering over it in our real life, that is exactly what we do when we observe the night sky: so the learning experience provided by the program for a user who wants to get better at locating celestial objects is invaluable.

Above you can see a screenshot of part of the WWT window, which I centered on the Deer Lick group of galaxies - NGC7331, a milky way-like galaxy which is the largest member of the group, is on top. Below you can see Stephan’s quintet - a group of five small galaxies of 13th-14th magnitude which is among my favorite targets in deep-sky observing sessions. By zooming in (below), you get to see stars fainter than 18th magnitude, at a resolution comparable to that of  a meter-class instrument. Amazing!

I highly recommend downloading the software. Learning to locate objects will become a wonderful pastime!

Here is a selected list of interesting links from blogs I read:

• Bee at Backreaction has the most complete list of reasons why you should not be bothered by the LHC destroying the Earth. Instructive, entertaining, to the point. With useful furthering of the matter in the comments thread.
• Peter at Not Even Wrong has two interesting posts out. In one he reports about Witten’s take on dark energy. In the other the question on what string theorists would do if their pet theory was proven wrong is discussed. Don’t miss the comments thread.
• Carl at Mass explains in detail why the current cosmology does not explain the angular correlations in the fluctuations of cosmic microwave background for large angles, while a changing speed of light would fit the data better. Controversial!
• Lubos at the Reference Frame discusses whether a theory that makes no predictions is to be preferred or disfavored, in relation to one that is more predictive. He also has a poll. Let’s all ask him to add a bullet, “A and B are equally unlikely because they are both favored by Lubos”,
• Jester at Resonaances has a short but poignant post on how to be a good crackpot. Recommended.
• Kea at Arcadian Functor has reached lesson 182 in category theory. Her explanations make you believe you know those things, and there are a bunch of graphs you cannot miss. Esthetically pleasing.
• Chad at Uncertain Principles has one of his imperdible dog dialogues out. Highly recommended.

Long overdue, here is the final part of a long post on the searches for new particles that may be the solution of a long-standing problem in astrophysics today: the missing mass in our Universe.

The large majority of cosmologists have become convinced, through the analysis of masses of data collected in the last two decades, that four-fifths of the matter in the Universe is non-baryonic. If we neglect particles which can only be created in high-energy collisions and decay in ridiculously small amounts of time, Baryons exists in just two forms: protons and neutrons. These make up the nuclei of atoms, and provide the fuel for stars to shine as they fuse into helium nuclei.

Non-baryonic matter does exist, and we know it well: we have electrons and neutrinos; but these are irrelevant. Electrons weigh less than a thousandth of a proton -and there are just as many electrons as protons around, to a very good approximation. As for neutrinos, despite our ignorance on their mass, they cannot make up the deficit of mass observed in the rotation speed of galaxies (exhibit one in support to Dark Matter: the speed of rotation does not decrease as much as it should if their mass was concentrated in stars) or in clusters of galaxies (exhibit two: gravitational effects we may detect visually do not match the observed distribution of galaxies in these agglomerates).

One intriguing solution to the problem lies in hypothesizing that a massive particle called neutralino wanders around in huge amounts, slow and unbothered by its close encounters with ordinary matter. Neutralinos would be electrically neutral, they would not interact strongly with matter, and they would be perfectly stable, lest they violate a very convenient quantum-mechanical conservation law. For more details on these hypotheses, see part II of this post.

So how can collider experiments detect this evanescent particle ? By producing pairs of higher-mass supersymmetric particles, which would chain-decay into non-supersymmetric ones plus a pair of those lightest supersymmetric particles, LSP. On the right you can see a decay chain whereby a gluino - a SUSY particle produced in large amounts in hadron collisions, due to its strongly interacting nature - emits a squark, the squark in turn emits another quark and decays into an excited neutralino, this emits a slepton, and the slepton ends up producing the lightest neutralino. All in all, from each of these chains (one per decay of each of the produced gluinos) one should observe two jets of hadrons from the quark hadronization, two leptons, and some missing energy. The missing transverse energy stolen by each neutralino would add as two vectors add in a plane: only rarely they would cancel each other out. In the graph below, for instance, two neutralinos leaving in different directions (the two dashed lines pointing towards the upper and lower left, in the transverse cut-away view of the ATLAS detector) would create a missing transverse energy vector pointing roughly mid-way between their exit directions.

The Tevatron experiments have searched for these events in their Run II data. The search in CDF considered the signature of two, three, or four hadronic jets plus a significant amount of missing energy from the neutralinos. This signature can be mimicked very effectively by the frequent, generic production of many jets by quantum chromodynamics interactions between quarks and gluons; the missing energy is thus required to be large and significant to suppress these processes.

The CDF experiment applied three different sets of selection cuts on their data to seek sensitivity to different regions of the parameter space of Supersymmetry. Indeed, as the mass of gluinos, squarks, and sleptons varies, so does the visible final state. For instance, if squarks and gluinos have a similar mass one is unlikely to detect a hadronic jet from the quark that is emitted in the transformation of the former into the latter. The signature pf pair-produced gluinos then more closely resembles one with only two jets and missing energy.

The figure on the right shows the final selection of the data in one of the three search regions. It is clear that known Standard Model processes provide a good modeling of the observed distribution of missing transverse energy in the data (black points with error bars), whereas a supersymmetric signal (the empty histogram in green, overlaid to SM contributions) would have instead stood out and created a disagreement.

From the distributions an upper limit can be extracted on the amount of signal contained in the data, and from the latter a limit is obtained in the cross section of gluino pair production: this translates into a mass exclusion range for squarks and gluinos. The final summarizing plot is shown below.

The plane is spanned by the mass of the two hypothetical particles. Colored areas have been excluded by different experiments; the CDF search extends the excluded region by the size of the red-painted area. We thus learn that gluinos cannot be lighter than 300 GeV, whatever the squark mass, otherwise CDF would have seen a bunch of anomalous events with large missing energy and jets.

The Tevatron protons and antiprotons do not have enough energy to investigate supersymmetric particles of mass much larger than the limit discussed above: so if Supersymmetry is the right theory of Nature, it may turn out to be the job of the Large Hadron Collider to discover it. With its 7-fold increase in energy and hundred-fold increase in interaction rates, the LHC is expected to provide a clear-cut answer: discover supersymmetry, or rule it out for good. As you can see in the plot below (where the plane is spanned by two convenient parameters among the multitude of choices: and ), the discovery reach of the CMS experiment extends to mass values in excess of a TeV - where supersymmetric particles would be close to useless, because they would not have a chance to solve the problems of electroweak symmetry breaking for which they were once invented.

The graph is complicated and it requires some more explanation: the blue areas are excluded by theoretical constraints and experimental searches, and the green area is also excluded. The colored wavy lines show instead the limits that CMS will be able to set in the plane -intending it will exclude anything to the left of the curves - with different searches, labeled by their respective “smoking guns”. The red curve is labeled for missing transverse energy, and it is one of the most performant in excluding the parameter space.

So, indeed, CMS and ATLAS will have an easy way to find signals of supersymmetry across the table -the wide space of parameters: they just need to study their distribution of missing transverse energy, just as we saw CDF do in the analysis mentioned above. The fanthom signal of a neutralino, which cannot interact with the detector and leaves unseen, turns out to be more striking at the end of the day than the multitude of jets and charged leptons the pyroclastic Supersymmetric production events would give rise to. Seeing events with a large amount of missing transverse energy would not allow us to determine which form of supersymmetry we are dealing with - whether a minimal supersymmetric extension of the Standard Model with two higgs boson doublets, or more complicated schemes. However, it would still allow us to claim that we have evidence for THE candidate particle which constitutes 80% of the stuff the Universe is made of.

I need to warn the reader here: of course, ATLAS and CMS have already studied dozens of methods, some of which are quite complicated, to extract very detailed information on Supersymmetry and very clean signatures of its presence from LHC data. These analyses focus on kinematical properties of the supersymmetric decays which are very model-dependent, and very complicated to explain. Although I reported about these methods in my seminar, I take the liberty here of jumping ahead a little…

So what instead if SUSY is not, after all, the right idea ?

Despite the general enthusiasm of theorists, phenomenologists, and other assorted believers, in fact, we have to keep a cool mind. Let’s review the cost of the purchase we have to make if we are to marry Supersymmetry:

• twenty brand-new particles, never before seen
• at least 104 new parameters, whose value is unknown and to be determined by improbable experiments
• a strict conservation of R-parity, the number you get by adding together spin, baryon, and lepton number in a suitable combination - the combination allows the proton and the lightest neutralino to remain stable
• We also have to agree that despite the fact that in principle the Tevatron and LEP colliders could have well stumbled into Supersymmetry, they haven’t - new physics chose to hide in the far away corner, just like the small coin that you dropped from your pocket.

Some of us think the above is too much to buy, for a theory which “solves” the mystery of a unnaturally small mass of the Higgs boson (provided the Higgs exists and is light as every evidence still suggests) and which collapses two crossings between running coupling constants into one single point. Ockham’s razor comes a-slashing: “entia non sunt multiplicanda praeter necessitatem“, one must not multiply entities. The most economical explanation is the best one… The razor cuts unnecessary entities.

One should mention, at the end of this long post which focused on the searches for just one candidate for dark matter - the one which hadron colliders may have a chance to find, the neutralino - that there is a long list of alternatives, of many flavors: kaluza-klein gravitons, sneutrinos, gravitinos, little higgses, axions, primordial black holes, charged massive particles, heavy neutrinos, sterile neutrinos, you name them.

It is for this very reason that in the end, LHC searches will require to follow the very important two-step procedure outlined by M.Mangano in a recent paper: first establish that an anomaly exists in the data, and only after it has been demonstrated to be utterly unexplainable by known phenomena, proceed with an exotic explanation.

To conclude, dark matter candidates have been searched at past and present collider experiments with no success. LHC appears to have the right energy and the potential to finally discover the source of this astounding enigma. In any case, we will know in a few years whether Supersymmetry is real or just a crazy concoction. If SUSY exists, new accelerators will be needed to investigate it in detail, but if it doesn’t, particle physics may be at a dead end. Despite this threatening possibility, we have extremely exciting years ahead of us!

In part I of this long post I gave a writeup of part of the seminar I gave last Tuesday. There, I discussed some of the tools necessary for the searches that have been carried out at the Tevatron collider experiments, and will be performed at the LHC experiments, for dark matter candidates. In particular, I focused the attention on missing transverse energy (MEt), which is a measure of the amount of imbalance in the momentum flow out of the proton-proton collision, in the plane transverse to the beam. A dark matter (DM) candidate produced in a high-energy collision would create that imbalance by carrying away unseen a sizable amount of momentum: we assume such a DM candidate is weakly interacting, and so it leaves undetected just like a neutrino. In this post, I will continue the discussion, and I will give one first example of a direct search for DM performed at the Tevatron.

Cosmologists assure us that we need new particles beyond the Standard Model to accommodate a dark matter candidate. One possibility which is dear to many is the lightest neutralino, a particle belonging to the rich spectrum of new states predicted by supersymmetric (SUSY) theories. The neutralino is the lightest supersymmetric particle (LSP) and it is a quantum superposition of as many as four electrically neutral superpartners of the neutral bosons predicted by the model. The exact recipe depends on a few of the many parameters defining the particular kind of supersymmetry that Nature (the bitch, not the magazine) might have chosen for the Universe we live in. Those parameters are, of course, still unknown to us, and so are the phenomenological details of SUSY.

Indeed, supersymmetry is not even a model, but just a framework which dictates a new symmetry between ordinary and supersymmetric matter and fields. SUSY predicts the existence of one superpartner for each ordinary particle, as shown in the table on the left (SUSY particles have wiggles on their names). The introduction of these new entities solves one grievious problem in the Standard Model: the fact that a light Higgs boson -necessary for the experimental consistency of electroweak observations- is at odds with the expected huge corrections on its mass necessary to renormalize some divergent loops involving the boson coupled to ordinary matter. It is as if the mass of the Higgs boson ended up being of order one after having withstood subtraction and addition of a dozen different contributions of the order of a billions of billions each. The introduction of supersymmetric particles cancels the divergent loops, solving the problem at its root.

Supersymmetry has a second charming feature: it allows the running coupling constants which determine the strength of the three basic interactions -strong, electromagnetic, and weak- to become one and the same at a very high energy scale. These couplings do depend on the value of the energy at which they are measured: and it is indeed expected that they “become one single interaction” above a energy scale where they unify. In the standard model, one sees the three couplings meet at different values of energy, whilst supersymmetry allows them to have the same value at a common energy scale.

And supersymmetry allows a neutral weakly interacting particle, massive just enough to make a perfect candidate for the dark matter we infer exists in the Universe. Since dark matter has survived to our time from the big bang, this neutralino has to be perfectly stable: it simply cannot, CANNOT decay to anything else. Supersymmetric theories which include R-parity - a conserved integer quantum number which is a sum of particles spin, baryon and lepton numbers- have this feature built in.

R-parity was not invented to make the neutralino stable: rather, it was introduced to solve a couple of other outstanding problems of the theory, namely to maintain the stability of the proton and the universality of weak couplings despite the addition of new states. However, it is just what we need if we are to assume that neutralinos make up 20% of our universe today, rather than have decayed to ordinary matter and radiation. R-parity also has an important phenomenological consequence at colliders: it dictates that supersymmetric particles can only be produced in pairs in the collision of ordinary matter.

The CDF experiment carried out a search for neutralinos in its Run II dataset by considering the pair-production of chargino and neutralino as in the diagrams shown on the right. The neutralino emits a charged lepton, converting into the lightest state which leaves the detector without a trace; the chargino (a supersymmetric analog of the W boson) is expected to decay with the emission of one or two charged leptons and another light supersymmetric particle, LSP in short, as we already mentioned. The final state may thus include two or three charged leptons and a large amount of missing transverse energy from the combination of the two LSP.

The CDF detector, which collects proton-antiproton collisions at Fermilab 2-TeV Tevatron collider, is good at finding such a signature. Charged leptons are only produced in rare weak interaction processes at a proton-antiproton collider: the production of a W or Z boson, or the decay of a heavy quark. Electrons and muons of large transverse momentum are identified very effectively by a online trigger system, so the collection efficiency of events with two or three leptons is very high. In order to search for chargino-neutralino production, two different “signal regions” are defined by a set of selection cuts on the observed characteristics of the events before looking at the data. Similar “control regions“, which are expected to contain a negligible fraction of the searched process, are also defined.

Monte Carlo simulations of all known weak processes capable of yielding leptons in the final state are then compared to the data contained in the control region in a number of kinematical distributions. The comparison allows to gain confidence that the simulation is capable of predicting both the number and the kinematics of the experimental data. Only after these checks are successful, the signal region is opened, and data contained within are compared numerically to the expected sum of standard model processes contributing to the mixture.

CDF thus finds 6 events in a signal region defined to contain events with large missing Et, two well-identified leptons, and a third lepton candidate. Here, simulations predict events, mainly from diboson production and top pair production. In the other signal region, defined to have a third good lepton candidate, only one event is found, with an expectation of from standard model processes. The distribution of missing transverse energy observed in this latter case and the expected contributions from standard model processes and from supersymmetric contributions is shown in the plot above. There, you see the one candidate (the point with error bars with missing Et above 20 GeV, the cut defining the signal region in events with three charged leptons) compared to SM backgrounds: mostly diboson production. The white histogram is the SUSY contribution.

Simulations in fact can predict the amount of chargino-neutralino events the two signal regions would contain, as a function of the value of supersymmetric parameter space. One thus gets to know that, for instance, 6.9 events would be expected in the first signal region, and 4.5 in the second. The data clearly do not allow that interpretation.

Since no signal is found, the experiment can set a limit on the production rate of the sought process. The reasoning is quite down-to-earth:

“I observed one event; on average, standard model reactions should produce 0.88 events in that dataset, give or take a small error. Now, that one event could well be the result of SUSY, and the standard model fluctuated to yield zero events; similarly, SUSY could have contributed with an average of two, or even three events, to the selected dataset, and a unlucky fluctuation could have brought our observation to one single event.“

There is a limit to our credibility, of course. In particle physics, we use to set credible chances for these searches at one-in-twenty odds: a complicated but conceptually simple computation allows one to compute the “95% confidence level” (C.L.) upper limit on the average number of events that the cuts defining our signal region should include. It is the number N such that 95% of the times would yield, together with the 0.88 expected standard model yield, more (at least two, that is) than the one event we observed.

Once N is computed, converting it into a 95% C.L. on the chargino-neutralino cross section only requires accounting for the total luminosity of the collected data and the expected efficiency with which our signal region would capture those events: .

In the plot below, you can see the result of the exercise. The cross section limit is shown by the black line with blue and yellow bands signalling the one- and two-standard deviations boundaries expected for the particular search. The limit is plotted as a function of the chargino mass -one of the many free parameters of the considered model; the limit varies as a function of it because so does signal efficiency. Since the theoretical model would foresee a cross section (the red line) larger than the limit for all chargino masses lower than 140 GeV, there follows an exclusion of chargino masses below that value. You can see that CDF sizably extends the LEP limit on this particle, set at 103 GeV (the hatched band on the left).

(To be continued…)

Yesterday I gave a seminar on searches for dark matter at the Tevatron and LHC in Padova, to a wide audience. This was a one-afternoon-workshop intended to educate students and publicize the LHC experiments, but it gathered more audience than undergraduates: quite a few of the Department staff came to listen.

My talk was the last one in a tightly packed agenda, and it indeed started with some 40 minutes of delay, as I had predicted. However, despite the late time -5.40 in the afternoon is about time to catch a train on normal workdays, even for me- the audience stayed to listen.

I already posted my slides here, but since they are in italian, I feel the need to give a summary of my seminar in English here, now that I have some more time to do so. I will do this in at least two parts, because I am swamped with other obligations these days!

I started my seminar by comparing the Tevatron and the LHC (in the aerial view of Fermilab above, the Tevatron ring is compared to the size of LHC, overimposed as a red circle courtesy M.Schmitt): the former collides protons against antiprotons, the latter collides protons with other protons. The crucial differences are however not the projectiles, but two parameters: energy and luminosity at the Tevatron, and and at LHC. While E sets the limit of investigation in new physics phenomena - particles more massive than a few hundred GeV cannot be produced at the Tevatron - L is a parameter which dictates the rate of rare processes. The dumb product of the increases in E and L offered by LHC is a factor 1000, which can be thought as a rule of thumb for the increase in discovery reach of the ATLAS and CMS detectors with respect to their smaller, older brothers CDF and D0. Sure, discovery reach scales only with the square root of the collected data (proportional to L), but cross sections of rare phenomena scale with more than the square of the energy increase: for instance, top production at LHC is 100 times more frequent, at equal L.

I had to mention the huge legacy that the Tevatron offers to LHC: twenty years of investigations, discoveries, and measurements. The top quark mass is known with a 0.8% accuracy thanks to CDF and D0’s recent measurements. This grants CMS and ATLAS a standard candle with which to calibrate their calorimeter response to hadronic jets: it will be extremely important in the initial phase of running, when top quark pairs will be available for a check of the jet energy scale. But the Tevatron’s high precision studies of electroweak physics will do much more for the LHC: the tuning of parton distribution functions performed by CDF and D0 with detailed QCD studies will be crucial to tune the simulation and understand the cross section of rare phenomena.

I then spent five minutes discussing why the important quantity at a hadron collider is the momentum flow in a plane orthogonal to the direction of the beams. While in electron-positron colliders the center-of-mass of the collision is at rest (unless beams are asymmetric in energy on purpose, such as at BaBar or Belle), and particle momenta are equally important regardless of their outgoing direction, a hadron collision of high energy is in fact a collision between quarks and gluons. These constituents of hadrons (drawn as colored lines in the cartoon above, where protons are the black circles) carry a variable fraction of their container’s momentum, and as a result the collision center-of-mass may move in either direction along the beam. What characterizes a hard interaction is instead the momentum flowing orthogonally from this direction (the two red and blue lines exiting at large angle from the protons direction in the cartoon): transverse momentum is therefore a measure of the acceleration that the proton constituents participating in the collision underwent during the mindboggingly brief moment of their interaction.

As a quark or gluon escapes the collision point, it extends a gluon string. The QCD potential grows linearly with distance decelerating the outgoing parton, until it finds it energetically favorable to break in two, materializing a quark-antiquark pair at its midpoint. The process continues until a stream of colorless hadrons are created. These then decay with strong and weak interactions, producing a final stream of particles which collectively carries memory of the originating parton’s momentum. It is what we call a hadronic jet.

Jets are measured in the detector elements called calorimeters (see a description in two parts here and here) by destroying the particles they contain, both charged and neutral ones, in electromagnetic and nuclear interactions with heavy elements - typically tiles of lead or iron. What is measured in these devices is the total track length - the sum of paths of all secondary particles produced in the shower originated by the chain of interactions in the absorber. That quantity is proportional to the energy of the incident bodies. Ultimately, the originating quark or gluon energy and its direction are reconstructed with an accuracy sufficient to understand the characteristics of the process which caused its emission.

In general, a hadronic collision produces jets of particles. Sometimes, though, rarer and fancier objects -ones that are not present in the projectiles- are produced: leptons and photons of high energy. These do not feel the strong interactions, and are due to electroweak interactions, which involves the exchange of W and Z bosons, or heavy quarks which decay weakly. In general, electrons and muons are objects that the detectors are trained to detect with high efficiency. But for dark matter, the signal which is by far the most important of all is an indirect one: missing transverse energy.

Missing transverse energy -the energy carried away by a body which leaves the detector unseen- is reconstructed thanks to the law of conservation of momentum: the incoming projectiles carry no momentum in the direction orthogonal to the beam, and so the final products of a collision must balance their momenta in the transverse plane. When this does not happen, it may be due to an imperfect reconstruction of momenta -a likely cause only if missing Et is not large and not significantly different from zero-, or to the escape of a high-energy neutrino. A dark matter candidate would similarly cause the same imbalance.

The graph above shows an event with two electrons (giving pink energetic deposits) and large energy imbalance -indicated by the downward arrow. Most probably, this rare event collected by CDF is the decay of a pair of Z bosons: , where the two neutrinos escape giving collectively a trace of their creation by the energy imbalance they leave behind.

Missing transverse energy is defined as the opposite of the vector sum of all detected energetic deposits in the calorimeters, in the transverse plane. It is measured with a resolution with depends on the total transverse energy detected: in fact, its resolution scales with the square root of total transverse energy. The reason is the way energy is extracted from the number of track segments caused by hadronic showers: integer numbers follow Poisson statistics, and their uncertainty scales with the square root of the number -and so does energy, and so does missing transverse energy.

Why is a dark matter candidate going to cause missing energy in the detector ? Because dark matter particles cannot be electrically charged -or they would have been found quite easily in the Universe-, they cannot feel strong or electromagnetic interactions -or they would create exotic atoms we do not see-, and they are massive -they need to, if they are to solve the matter-energy balance equation of the Universe, which foresees that dark matter makes up for 20% of the total budget as compared to baryonic matter’s 4%.

One of the most appealing candidates for dark matter is the Supersymmetric particle called Neutralino. Supersymmetry is a model extending the Standard Model of particle physics. It predicts the existence of a new partner for each known quark, lepton, or boson we know - only, with different values of spin. This multiplication of known bodies is the price to pay for a theory that solves one big issue in the standard model: the inconsistency of the mass of the Higgs boson, which must be light if the Standard Model is to be consistent with the many measurements colliders performed at the electroweak scale, but should be far heavier to avoid having to invoke a delicate and unnatural cancelation of huge contributions from virtual divergent diagrams that are present in the theory. WIth Supersymmetry, the Higgs mass is “stabilized at the electroweak scale“: supersymmetric particles cancel automatically the unwanted loop effects of SM particles. SUSY also predicts a unification of forces at a common, very high-energy scale, in a way that is pleasing to the eye but admittedly not called for by any intrinsic requirement.

(To be continued in Part II)

In twenty minutes the mini-workshop on Dark Matter at LHC that we organized for Physics students will start in Aula B at our Physics Department “Galileo Galilei”, here in Padova. I will be closing the workshop with a talk named as this post: which is both a good and a bad thing. It is good to have the last word, but it is not good to see other unmoderated talks straggling past their allotted time and the audience leaving to catch the last train before you had a chance of hypnotizing them.

In any case, I have prepared a reasonably light-weight presentation. The slides are unfortunately in Italian, but I will give a transcript here as an update, later this evening or tomorrow. They are tightly packed - a feature which I call a annoying defect in other people’s presentations, but I find always excusable in my own. No, really - the reason for filling the slides up with text is to make the slides usable without the speaker: a commendable, unselfish reason, you will agree.

So, please find the slides here. I will remove the link once I manage to put together a transcript, since I am running short of space in the public area where I store my stuff. Incidentally, I will have to find a solution for that. Does anybody have an advice on free sites offering permanent access to a Gig of disk space ?

Due and happy thanks to a friend for pointing me to the following sentence, appeared minutes ago at the Cosmic Variance site in a guest post by Juan Collar:

“Thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).”

No, I think Sean will not edit it - by now it is on record. In any case, I have two comments. The first is that I am happy that a comment I recently made in this blog about the presentation of the new DAMA result sounds polite and positive if compared with the above. The second is that I think we should all back off and realize that no matter whether an experiment will one day win the Nobel prize or be proven laughably wrong, every scientist who works in our field deserves our respect until proven an imbecile. Doing otherwise harms the whole field, and ourselves.

Oh, and - I still thank Sean for linking to my own commentary of the DAMA-LIBRA signal.

I listened carefully to Antonio’s talk this morning at the final session of the 2008 conference “Neutrino Oscillations in Venice”, for two reasons. One is that the director of INFN-Padova is well-known for giving exceptionally clear and insightful talks, and the other is that he will speak of the theoretical side of dark matter searches at the LHC just before I deal with experimental details of the same topic, next week in Padova. So I am able to offer you a rather detailed report of his speech, with the note that I am solely responsible for omissions and mistakes. Another caveat is that these notes are not for outsiders: sorry, but making the text below understandable for non-physicists is just above my possibilities. I will expiate soon with a more accessible discussion of the same topics.

The purpose of Antonio’s talk was to discuss what we mean by complementarity of LHC with the effort going on in astroparticle physics to look for new physics at the electroweak scale.

We can foresee two different scenarios. In the first one, LHC is turned on and very soon discovers new physics: this is what most of us (not me) believe and all of us (including me) hope. In this case, it could be difficult, if not impossible, to reconstruct the fundamental theory lying behind those signals of new physics: that is, even if signals were evident, it would be hard to trace back to which kind of SUSY theory gives rise to these particles. All these questions will be very difficult to address in a hadronic machine. We thus need additional, independent information.

The second scenario occurs if LHC has difficulty finding new particles apart from the Higgs boson.
Indeed, we insist on the presence of new physics at the electroweak scale for the sake of stabilizing the Higgs potential. Such a stabilization could however jolly well imply the presence of new particles at the TeV scale but not below. In this case LHC could have trouble finding new physics. Having a complementary tool to address these signatures of “low-energy, but above the TeV scale” new physics becomes important.

Thus, very important besides LHC are the complementary roads, one linked to Astroparticle Physics, in particular the issue of dark matter and the issue of baryogenesis, which are keys for this complementarity to LHC; the second is the high-intensity road, in particular the study of flavor-changing neutral current phenomena and CP violating phenomena and their relation to lepton flavor violation in neutrino physics.

Today, what are the observational reasons pushing us beyond the SM ? The traditional road of high energy physics as much as that of flavor physics did not lead to any conclusive evidence for physics beyond the standard model. We have some possible hints, such as the 3.something discrepancy from theoretical estimates in the gyromagnetic factor of the muon, and some three-sigma discrepancies in transitions. Concerning the latter, a month ago there was a study showing some discrepancy from SM for the phase in mixing. We have some possible indications, but however how much we can like this approach, we have to admit that after thirty years or so of searches for discrepancies we did not get any firm evidence for deviations from SM yet.

To go beyond the SM the observational evidence may come from astroparticle neutrino physics and cosmoparticle physics. It can be interesting to put one close to the other: the standard models we have for particle physics and cosmology. If we put the two models together, their marriage is a quite happy one, with incredibly successful fruits, such as the clear picture of the big bang we have with studies of nucleosynthesis. However, fortunately enough, there are some points of friction: some pepper in the marriage. This points of friction are a clue for us that there is new physics either beyond the standard model in particle physics, or in cosmology. We should keeop in mind that both the options are present together.

Besides the issue of neutrino mass, the three main points of friction are dark matter, matter-antimatter asymmetry, and inflation. This kind of Michelin rating in stars (dark matter gest three stars, matter asymmetry gets two, and inflation gets one) for these three topics reflect Masiero’s point of view about their relevance to individuating new physics. Dark matter is the major issue in this context.

Before going to dark matter, Masiero discusses the matter-antimatter asymmetry in the universe. If you want a dynamical mechanism to produce the asymmetry starting from a symmetric early universe, you need some new physics (NP) beyond the particle physics SM. Indeed, after many years of a lot of work trying to produce baryons, we concluded that we need new physics. Which kind of new physics can produce the matter-antimatter asymmetry ? It is very tempting to adopt the view of it coming from lepton-antilepton asymmetry through the decay of right-handed massive neutrinos. It is appealing trying to build up upon the existing models created for other purposes: this is the case for heavy right-handed neutrinos, which were introduced from neutrino mixing observations. Introducing a new source of CP violation in the leptonic sector may have important links with CP violation in neutrino mixing.

The seesaw mechanism brings in a new element: if you insist on right-handed neutrinos and if you like to have Supersymmetry (SUSY) at least at the large scale, then you are led to a SUSY seesaw mechanism. In that case, it is given as a bonus because of the running from the large scale that you generate large lepton flavor violation in the scalar sector of the leptons. The important message here is that if you consider lepton flavor violation, you can probe this region of parameter space of SUSY seesaw models. The muon decay experiment looking for the reaction , MEG, can give bounds on the mu-egamma branching ratio at or about . These foreseen further two orders of magnitude of improvement in the branching ratio for MEG are not just important for making it to the PDG booklet, but also because it enters the heart of the region for SUSY seesaw models for lepton flavor violation (LFV). Something similar exists for the decay, which will reach a bound in branching ratio. We have to keep an eye to these LFV phenomena because they might give very important surprises even before LHC starts being operative.

Concerning inflation, again the main message to bring here is that if one wants to have an inflationary epoch in the early universe it is not possible to use a simple scalar potential of the standard model. One needs to do some heavy gymnastics to produce inflation. It means that if one wants an inflationary epoch in the Universe evolution, one needs an extension of the SM in the scalar sector, and the polarization of cosmic microwave radiation can evidence the new scale in inflationary physics, likely to be much larger than the mass of W or Z bosons.

No matter how important these four roads are, neutrino masses plus the other three just mentioned, they do not indicate which new scale could be present to produce new physics necessary to give neutrino mass, dark matter, baryogenesis and an inflationary epoch. There is no indication of where this scale could be. The only indication we have that the first NP is accessible through LHC comes from theoretical arguments: we need this stabilization of the electroweak scale, and it is thus better -more comfortable- to have the NP at the same scale. This is the major argument we have. There is a second point: the strong indication that we have from SM that there is some kind of unification of the fundamental coupling constant may point in the direction that the SM meets some kind of low energy completion in order to achieve the correct unification of these couplings. Either the SM tells that the three couplings show some trend to acquire a common value at a high scale, some new physics could be introduced to achieve a good unification at a larger scale. This is a 1-star support, because there is a lot of theoretical prejudice which enters this kind of argument, but it is still appealing.

What is important from a pheonomenological point of view, is that we have in our hands a powerful tool to assess this high energy scale, one handle to access this scale is related to proton decay. This is important thinking in a talk about astroparticle physics: proton decay must remain a complementary access to this high scale in addition to baryogenesis and dark matter tools.

Masiero focused on two numbers, enormously important for us: , the baryon number density, and . Their clash is very significant indication that we have NP beyond the SM; the non-baryonic dark matter is the most striking evidence we have of NP. Neutrino mass is just an addition to the SM, but in the case of dark matter it is even qualitatively new physics. You really need something new to account for this result. Alternatives related to deviations from standard gravitation, like MOND, have failed when studies of the bullet cluster were produced last year. In fact, we can use this cosomological argument to have in sight the neutrino masses. Instead of using neutrinos, we can use the cosmological argument to infer information about neutrino masses and properties.

Concerning dark matter, the strongest candidate is a particle in the tens of GeV range, which interacts weakly. Saying this does not mean that it is really the only candidate or in any case the most favorite one. It is the strongest because as it was pointed out, there is this amazing coincidence that putting together parameters that do not know each other, you end up with a density of WIMPS (weak interacting massive particles) which ends up in the ballpark to account for dark matter. The argument is that the density of WIMP particles which could be interesting to explain dark matter is of the order of , which is exactly what one finds if one hypothesizes GeV or so.

Of course in the history of physics we had other instances of remarkable coincidences that did not lead to the discovery of something new about Nature. Nature could be more original and creative than we are, so one should not emphasize too much this result, but this coincidence deserves further study. There is a convergence of two independent sectors, particle physics and cosmology conspire to give this kind of number.

Interestingly, if you ask about candidates for WIMPS, all the examples of new physics at the electroweak scale produced in recent years provide candidates: stable particles which have some kind of weak interaction in the range of 100 GeV. They can be bosons, scalars, etc. The message is important: a WIMP candidate could at the same time account, enter in a extension of the SM to provide a stabilization of the electroweak scale, and at the same time provide a candidate to solve our problem with the accounting of matter in the universe.

If you want to stick to the SUSY possibility of this WIMP, Masiero feels the need to insist on a point that sometimes people forget, which is that SUSY in itself does not predict a neutralino! If you write a SUSY lagrangian, in general you do not find a stable scalar. Since you want to eliminate terms that are dangerous for the violation of baryon and lepton number, so because of the proton decay problem, we add to the SUSY theory a new symmetry, this famous R-parity, and this makes the theory predicting a stable particle. So this connection between baryon violation and predicting a new stable particle is to be taken with care.

The lagrangian of any SUSY contains more than a hundred parameters. So when one talks about a “minimal” SUSY model, one has to buy at least 124 parameters. One then gets “reasonable” by having five independent parameters, which one can do by making reasonable assumptions. These are , , , , . Then, when you say “this is the prediction of SUSY” you give the impression that you give a prediction in a model. But SUSY is not a model but a framework. You can find very different kinds of light supersymmetrical particles. Gravitinos, for instance, could just as well be the light particle. You could make a gravitino with 1 keV or 10 keV which could play the role of the light particle.

The SUSY parameter space of this so-called “constrained minimal supersymmetric extension of the standard model”, CMSSM, is 5-dimensional, and there is a small region where you have
suitable production of dark matter. There are narrow bands in the SUSY parameter space in the plane when you consider the CMSSM. These results rely on a belief that so far has not any strong scientific support, that is that we know how things went in the early universe before nucleosyhnthesis. We are making an extrapolation of standard cosmology before nucleosynthesis. Nucleosynthesis is the first moment from where we have testability of the evolution of the Universe. We cannot observe the Universe before then.

This brings to the important issue of dark matter and radiation. We have become accustomed to sweep under the rug the problem of dark energy (DE). There could be a relation between the problem of DM and DE. For instance, if you decide to make some modification of gravity to account for DE, this can completely spoil the picture of your cosmology before big-bang nucleosynthesis took place. This creates a major departure from standard cosmology.

The presence of a WIMP today as the DM candidate implies some constraints on its annihilation cross-section: it exists, so it is rather stable. You can correlate annihilation to production, and create a link with how many of these particles can be produced at a collider. One would like to find this DM directly. One of the highlights of this meeting was the result of DAMA-LIBRA. A consistent result for 11 years, plus the statistical relevance we have at the moment for the modulation effect.This badly calls from our community to have some independent confirmation. This result is so important that it is mandatory that we make a major effort to confirm it independently.

How far are we in DM searches from some possible threshold to find this WIMP particle directly? This

brings to some evaluation of the cross sections in SUSY . We are three orders of magnitude away to probe multi-TEV SUSY, which could be difficult to access with LHC. The testable region by LHC is smaller than that accessible by dark matter searches. It must be mentioned in this respet that we are thus at an important stage now. In a month we have the launch of GLAST, which is very relevant to cover the region below 100 GeV. It could detect gamma rays from the annihilation of dark matter candidates, and it will extend our sensitivity in a range not well covered so far. In general, LHC searches for dark matter, and those that may follow at ILC, are complementary to the other searches because they cover different parts of the parameter space.

A recent roadmap of APPEC (the Astroparticle coordination in Europe) shows seven different experiments that may play a key role in giving us complementary information about dark matter. Funding of these experiments is off by a factor of two, but we can try to push all of them. We must thus make an effort from the scientific point of view to insist on the validity of this astroparticle tool to have this access to new physics at the electroweak scale. Together with them, the effort on flavor physics (a super B machine could be relevant to explore the 1% REGION of the flavor parameter space, to find a discrepancy with SM) and the LHC are crucial. All this together should make us able in the next decade to finally clear
up what is the center of this new physics at the electroweak scale.

The slides of Antonio’s talk can be found here.

The new result of the DAMA-LIBRA collaboration, which finds a yearly-modulated signal of interactions in their NaI crystals which is compatible with dark matter in our galactic halo, has caused some ripples in the web. I was surprised to find that a comment I had posted yesterday on the matter was linked by Symmetry magazine as well as by Peter Woit (who was however the true originator of my remark).

I must say I do not particularly enjoy to always sound skeptical. A scientist should keep an open mind, and if the DAMA-LIBRA signal has a cross-section which is apparently already excluded by the CDMS result, as well as orders of magnitude above the estimates for mainstream dark matter candidate models, one should wait before taking a step back, and rather consider questioning the exclusions rather than the signal. Indeed, establishing a signal comes before challenging it with specific models, and the comparison of DAMA’s result with CDMS exclusion contours belongs firmly to the second category. CDMS excludes specific models, while DAMA establishes a yearly frequency in its signal yield which could be due to particles we have still not even conceived.

So, why not focusing on the establishment of the signal ? I tried to build an opinion on the solidity of the DAMA result this morning by talking to a few people who attend the conference, and in particular with a young researcher who now works for CUORE and has been in the DAMA collaboration in the past. She in fact worked at the analysis of data, trying to interpret the modulation in photomultiplier counts with different density models of dark matter in the milky way halo as well as components from our close satellites, the magellanic clouds.

It looks like the phase of the cosine oscillation in signal yield, which is something I had doubts about, is indeed compatible with being zero for a time of the year compatible with June 2nd, which is the date when the Earth travels in the direction of the Sun’s motion in the galaxy (if one forgets about the inclination of the Earth orbit with respect to the Sun’s line of motion). Not perfectly matching June 2nd, but compatible with it.

Another issue is of course the one which was raised at yesterday’s talk: signal efficiency is steeply rising in the region between 2 and 6 keV where the modulated signal is observed. It turns out that the efficiency is not full but it is quite stable - stability is checked weekly - and the reason why it is not 100% is due to specific cuts that are made to exclude a background contamination. This, of course, might make the signal yield dependent on subtleties of the shape of the PMT response (the cuts are made on signal shape form factors), but I see no reason to doubt that the stability is well under control, although extraordinary claims require extraordinary evidence, and the latter implies going after the subtlest of possible non-exotic explanations.

One thing I would have done if I had designed the experiment myself would have been to prepare a mock-up of the active region, wrap it with black paper, and instrument with similar photomultipliers and the accessory set-up, the overpressure, the temperature controls. This would have allowed to have a real-time comparison between PMT counts from the real NaI detector and PMT counts in a fake one. It would dismiss any claim that PMTs are varying their response seasonally without appeal, but unfortunately we do not have such luxury available in the DAMA-LIBRA setup.

So the question remains. If the signal is strong and significant, and if it is not due to instrumental nuisances -I am sure about the former, less so about the latter- what is its source ? Can we get a model of dark matter particles which produce a similar flux ? It should not be too difficult. Maybe the ball is in the theorists’ court in fact. As for me, I keep cool. I still think there is no new particles to be found with these cunning but a bit overoptimistic endeavours.

Here follows a summary of the talk by Gary Steigman at the Neutrino Oscillations 2008 conference I am attending in Venice. Due to a chronic shortage of time, I will not attempt at reorganizing my notes in a coherent way -apologies if the text is obscure: the lack of corresponding figures and graphs, which Gary used in his talk, cannot unfortunately be substituted by argute explanations on my part. I therefore advise readers with no background in basic cosmology to jolly well skip this post and read something else. On the other hand, insiders will find here no really new information… So this post is essentially just for my own record! I will add pictures if I find the slides on the web, if not… Too bad.

Gary started by noting that in our attempts at understanding the evolution of the Universe, evidence of large scale structures allows us to study times from a few minutes after the big bang to about ten billion years after it. The evolution can be divided by three important moments.

When the Universe is a tenth of a second old, neutrinos decouple from matter. This transition, as the others, is not sharp: neutrinos continue to interact at this time but at a time scale which is becoming long with respect to the age of the Universe. A few minutes later, elements begin to form. Nuclear reactions continue to happen but there is primordial nucleosynthesis only when the Universe is a few minutes old. Finally, about 400 thousand years later, electrons combine with protons to form neutral atoms, and then is when relic photons are free, and they can propagate all the way to us.

So we have Big Bang Nucleosynthesis (BBN, 20 minutes after the big bang), the Cosmic Microwave Background (400 kiloyears after BB), and the Large Scale Structure of the Universe (10 gigayears after BB). They are all complementary probes of the early evolution of the Universe.

The question to ask oneself according to Steigman is whether the predictions and observations of baryon density and expansion rate agree at these different epochs.

The early hot and dense universe during part of its evolution is a cosmic nuclear reactor. As the universe expands, BBN begins when the temperature is of about 70 keV, when the ratio between neutron and proton abundances is . The ratio is crucial for helium abundance. Nucleosynthesis begins but very quickly ends, because the temperature drops and there are coulomb barriers between charged nuclei, neutrons get used up also because of beta decay, and at T =30 keV, 24 minutes after the start, nucleosynthesis ends.

When we talk about the baryon density we mean the nucleon density. But as the universe expands, the density changes. A parameter which remains invariant is the ratio , between nucleon and photon number densities. is of course very small, so is defined as the same number, 10 billion times larger. In terms of and h, the hubble parameter, we can write $\latex eta_{10} = 274 \Omega_B h^2$.

One of the key elements produced in the early universe is Deuterium. The abundance is maximum at about 300 seconds. There is none before 100 seconds. Then it burns to Tritium to end up into Helium four, and it decreases. When the universe is about 1000 seconds old the relative abundance D/H stops changing. This ratio depends on . More nucleon density means less Deuterium produced. So Deuterium is a baryometer: it measures the density of baryons. As the Universe evolves, D is destroyed. Anywhere, the relative abundance of Deuterium is smaller than its plateau value: its evolution is monotonic since the big bang. It can only decrease.

The predicted value of the D/H ratio is sensitive to the baryon density: a 10% determination of D abundance brings to a 6% determination of the baryion to photon abundance.

The way we observe deuterium is in absorption in light sources, high-redshifted quasi-stellar objects. H-I and D-I lines are seen in absorption, but their spectra are identical. An isotope shift is completely equivalent to a velocity shift, so we have to be very careful in interpreting it. An unresolved velocity structure in the measured object causes errors in N(H-I). We need to measure the heavy elements to determine the velocity structure.

Data on D/H can be plotted against metallicity -the ratio of heavy to light elements, such as the relative abundance of Silicon and Hydrogen, Si/H: a measure of heavy element abundance. We only have six data points from background sources at various (small) values of metallicity, and we have to understand well their velocity structure. The points show a lot of dispersion in D/H, and it is hard to see a clear plateau at low metallicity. However, we can fit , taking the dispersion around the mean as the uncertainty. We thus find a 10% error in deuterium abundance. We can use that to measure as , or 6% uncertainty.

The evolution of the Helium 4 mass fraction is represented by astronomers as . It evolves starting from 200 seconds. It increases up to 0.25 at 300 seconds and then it plateaus. Helium 4 starts after deuterium starts burning. Then all neutrons are quickly used up, and we get a plateau. With a neutron to proton ration of 1/7 when nucleosynthesis begins, is 0.25 with very little spread. So helium abundance is insensitive on the value of , but it depends crucially on the competition between the weak interaction rates, charged-current weak interactions, and the expansion rate of the Universe: so Helium abundance can provide constraints of the expansion rate of the early Universe.

The expansion rate is usually defined in terms of the Hubble parameter H, which provides a probe of non-standard Physics. There are many models where H deviates from SM values. The ratio of the square of H to the SM value provides an estimate of the energy density of relativistic particles to the Standard Model expectation, with three families of light neutrinos. Anything that changes that picture causes a deviation. An expansion rate parameter S measures the deviation . There can be many reasons why S departs from 1. parameterizes deviations as . Higher dimensions like those in the Randall-Sundrum model cause a difference in S. S also measures the difference of the gravitational constant from todays value.

We can determine from Helium abundance and we find $Y_P = 0.24 \pm 0.006$.
As a function of the oxygen to hydrogen abundance one can determine Y. Systems with about give a linear extrapolation to zero oxygen abundance, and one finds the value . Alternatively, instead than using 90 data points with uncertainty dominated by systematics, there are other analyses more careful with systematics, where the trend with metallicity is seen better. Any helium one sees is greater than the primordial value, so an upper bound can be extracted from the data, and is found at .

From standard big bang nucleosynthesis there is the prediction is . There is consistency. The deuterium and helium observations plotted together, and S can be seen as a function of and . The helium abundance depends on the expansion factor, while the deuterium abundance also depends slightly on expansion rate factor S. Putting these together we find that there is a consistent, not unexpected, possibility of explaining everything with . 2-sigma away from the standard value of three neutrinos.

In the vs $\eta_{10}$ plane, one has a nice contour plot from V-Simha and G.S. We are consistent with 3 neutrinos. In particular, the BBN constraint from He-4 shows very clearly that in the early universe at least one was present in the early Universe. At more than 2-sigma, there was at least one of them. Also, 4 flavors of neutrino from BBN are excluded.

About lithium, it is produced in the BB in low abundance, in the form of . There is a gap at mass 5, very hard to jump in nucleosynthesis, but there are some reactions that take you up to mass seven. But it stops there: only light elements are formed in the early Universe. From standard BB nucleosynthesis, the prediction of LI abundance is off. As a function of the ratio between iron and hydrogen abundance , one finds values a factor of three lower. Question, should we see a plateau, a speed plateau at low metallicity ? If there is a plateau, we can arbitrarily determine it by drawing a line through data points, and find Li abundance at There is too little lithium according to measurements.

On the CMB radiation, there is a complementary probe. The temperature fluctuation spectrum provides a constraint on the baryon density. Different curves can be drawn on the temperature fluctuation measured for the cosmic microwave background, corresponding to different values of the abundance ratio at 4.5, 6.1, 7.5. This allows to illustrate that it is possible for the data to discriminate the baryon density: so one has an early Universe baryometer which is better than deuterium. One finds , which has an uncertainty a factor two better than what we can get with deuterium abundance. In fact if we superpose them, we find excellent agreement.

If one puts the CMB results together with the BBN results, they overlap well, and taking the CMB values of baryon density and , and use BBN to predict abundances, one finds good matches in , , while no good agreement on lithium abundance.

What are the consequences of the good agreement of physics at 20 minutes and 400,000 year times ? Entropy conservation: the number of photons described by CMB and BBN we find . The ratio in the number of photons is one unless there is entropy creation in between. One can place upper limits on entropy production then.

A modified radiation density for a late decay of a massive particle also give different abundances at the two time scales, and one finds constraints on it too. For variations in the gravitational constant, one can interpret the expansion parameter in terms of G, and comparing the BBN value with the present value one finds values consistent with one.