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!

Just a link to a post by Gianandrea Giacoma on the site of the sci.bzaar.net workshop, an event about which I wrote here, here and here.

In the post, Gian uses very kind words to introduce a video on my thoughts on the need of horizontality in scientific blogs. I already posted a link to my video yesterday (beware, it is in Italian - I will try to find the time for an English version though, or at least provide a transcript in English), but the one on the sci.bzaar.net site does not need to be downloaded before playing - a huge bonus since you might get bored halfway through (oh well, damned if you do. It’s just 7 minutes).

Today we had our meeting of the CMS analysis group in Padova, a monthly recurrence where we get adjourned of the various efforts going on. It was my turn to chair the meeting (I am co-convener of the meeting with Ezio Torassa and we alternate), and I had put together a tightly packed agenda, which included updates on the global cosmic runs (weeks of data taking when muons from cosmic rays are collected and used to understand the detector response), the tracker checkout (issues with the final commissioning of the silicon tracker), the trigger studies for SLHC (or how to measure muon momenta accurately enough to prevent being overwhelmed by the huge rate of fake muons of low transverse momentum, when we will take data with CMS at a luminosity of ), plus analyses of the decay, ttH production, and dimuon mass spectra.

Ignazio Lazzizzera, from the associated group of Trento, presented some kinematical distributions of muon tracks extracted from minimum bias Monte Carlo that will be used for SLHC studies. Minimum bias is a jargon that particle physicists use to describe events that withstood no selection whatsoever: events which suffered the minimum possible bias by the fact of having been collected by the detector. Such a collection of events is useful to understand what our “priors” are: at the full LHC luminosity (just a factor 10 below SLHC ones), every 25 nanoseconds we will have 20 proton-proton collisions to deal with, and only very rarely these interactions originate a high-momentum muon, which tags a potentially very interesting event. We have to rely on these minimum bias simulations to understand how easy it is for a light hadron -a pion or a kaon- to fool our detection system and be identified as a muon by our trigger, if we want to understand our chances of tuning trigger cuts and select good muons with high efficiency without being drowned in impossibly high rates from fake muons.

As Ignazio showed the plot below, which is the distribution of rapidity of simulated muon tracks in minimum bias data, I jumped on my chair. What was going on ? The two-humped distribution resembled a camel’s back!

To let you understand why such a distribution is unphysical, I need to take a step back. When you collide protons with other protons at high energy, what you are actually doing is creating hard interactions about proton constituents: quarks and gluons. Each of these constituents of a high-energy proton carries a fraction of the proton momentum: the two streams of “partons” (i.e. quarks or gluons) travel together in the positive and negative direction along the z axis - the beam direction- inside each proton; but some carry a larger, and many a smaller fraction of the total protons momentum.

Because of the variable amount of momentum carried by each parton, the collision center-of-momentum reference frame is not at rest in the detector reference frame: if a 90mph truck hits a 50mph compact car head on the debris will fly away following the truck direction!

What governs the probability that quarks and gluons carry a certain momentum fraction of the proton containing them are some functions called “Parton Distribution Functions“. They are shown below for the different constituents of protons.

As you see, it is increasingly probable (in a measured described by the PDF xf(x)) to find a parton carrying a smaller and smaller momentum fraction x (forget the u-distribution, which has a local maximum due to valence quarks: we are discussing the low-x tail of these shapes, since we are discussing not-so-high-energy interactions which constitute the bulk of collisions). Is this enough to figure out what will be the distribution of the debris, and in particular, the motion of the most energetic particles produced in the collision in the detector frame ?

Well, basically yes. If we label the momentum fractions of the colliding partons (which can be assumed massless for all practical purposes at LHC), the center-of-mass energy will be their geometric average times the 14 TeV globally possessed by the colliding protons. The motion of the center-of-momentum frame in the detector frame will instead be described by rapidity - the quantity , which reduces to .

Rapidity is, for the muons, the quantity plotted in the two-humped histogram above. Can there be a hole at zero in this distribution ? Not really! It does not take complicated math to realize that if you pick at random two values from a monotonous function, their values are most likely to be close to each other, and so their ratio will be close to one more often than not. The logarithm of one is zero, and at zero there cannot be a minimum! The distribution has to have a single maximum at zero rapidity instead!

You might find the above reasoning rather complicated. It is. However, had you worked at a hadron collider for 16 years, you would not need the math at all: the rapidity distribution of any physics process is (with very few exceptions) a broad distribution with a maximum at zero, unless the data have been biased by selection cuts.

I could thus explain what was going on in the distribution Ignazio was showing: the data he was plotting had been stripped of events which could fire the CMS trigger -that is, events with high-Pt, central muons in our case. Take a dromedar, substract stuff in the middle (the muons which are central), and you are left with a camel!

It remains to be seen why the minimum bias Monte Carlo had been selected this way. I suppose one such sample is rather useless for trigger studies!

On Saturday, May 15th, a conference called “sci.bzaar.net” will take place in Milano. It will bring together a restricted group of researchers, psychologists, bloggers, designers, physicists, writers, philosophers, computer scientists and web experts, who will discuss scientific divulgation, production of knowledge, and open culture in the academic world.

I will not be there in person, but a video I produced for the event will be shown - and I will connect with skype or some other means to take questions. You can see the agenda of the workshop here.

In addition, I produced for the web site of the event another short video where I discuss the importance of horizontality in a blog aimed at scientific divulgation. Unfortunately, I only have a version in Italian so far (the event is aimed at an italian public). I will paste below a writeup as I have the time, but if you are interested you can see me in the 7-minutes video here (beware though, it is kind of heavy - 500 Mbytes!).

Yesterday I had a dinner I would not have missed for anything in the world. After a lot of work with internet searches and email exchanges, we were able to organize a rendez-vous with my colleagues of high school, some of which I had never ever seen again after the day of the last written test for the final exam, which in Italy is called “Maturità”, maturity exam.

We were able to get together only two-thirds of the class. 13 people willing to meet after so long time was already a result to be proud of and to look forward to. Italy is not a place where people tend to move away for travel: we tend to remain anchored to the places where we have our parents, and where we lived our childhood. Indeed, of the 13 people who met, only two were coming from outside the area (Antonino, from Mantova, and Gianluca, from Milano).

It was an immense pleasure to see some of them after 25 full years, and being able to take on the puns and the jokes where we had left them, as if time had not passed. Funnily, a bunch of 42- and 43-year-old men and women looked to me like a bunch of teenagers. It is still difficult for me to shake that impression, as I look at the pictures of the evening.

Here is my class 25 years ago, in front of our school:

Can you locate me ? Not too easy, but you should manage.

And here is the bunch who met yesterday, in Piazza Ferretto (Mestre):

Time passes for everybody, but we had a great time. We already agreed we will meet again in September, on a hopefully sunny Sunday on the beach of Lido di Venezia, for a lunch and a game of soccer, hoping that a few of the nine who missed the dinner yesterday will show up…

Here is an excerpt of the latest LHC schedule for the following few months, as agreed in a meeting at CERN chaired by the Director-General, with the experiments and LHC machine heads.

Based on the good progress for the cool down of the LHC sectors, and on the powering tests from two sectors, the following planning was arrived at:

1. End of June: The LHC is expected to be cooled down. [...]
2. Mid of July: The experimental caverns will be closed [...]
3. End of July: First particles may be injected, and the commissioning with beams and collisions will start.
4. It is expected that it will take about 2 months to have first collisions at 10 TeV.
5. Energy of the 2008 run: Agreed to be 10 TeV. The machine considers this to be a safe setting to optimize up-time of the machine util the winter shut-down (starting likely around end of November).[...]
6. The winter shut-down will then be used to commissioning and train the magnets up to full current, such that the 2009 run will start at the full 14 TeV design energy.

The above means that the machine will deliver collisions from the end of September on, for at most nine weeks in 2008. More safely, one can assume 6 full weeks of data-taking. What luminosity do we expect to collect ?

A state-of-the-art estimate was made by a colleague, who used his past experience with LEP as well as the information on the current limitations of the RF system -which will make the proton bunches shorter than planned (RMS of 5.4 cm), and with a transverse size of 46 microns. At the lower energy the low-beta squeeze will also be loosened from 2 to 3 meters. These figures reduce the instantaneous luminosity, and the estimate for 6 weeks of collisions are of about 40 inverse picobarns of data in 2008.

If ATLAS and CMS will be fully on during the weeks of collisions, these 40 inverse picobarns will fruit, in my opinion:

• A top pair production cross section with 10-15% accuracy
• A sizable sample of vector boson decays to leptons, very useful for calibrations and checks of lepton efficiency studies
• The first estimates of b-tagging and tau-tagging capabilities of current algorithms
• no information on the Higgs
• no SUSY discovery (of course!)

All the above will have a chance of being ready for the 2009 winter conferences, if all goes well…

I stumbled today into two old booklets advertising a concert. One in Conegliano, on Friday, March 13th 1981; the other in Udine on Wednesday, May 28th, 1980. These were times when I toured north-eastern Italy with the orchestra of the Venice Conservatory, directed by m. Fabio Pirona. I was a teenager, but I could already play the recorder (straight flute) rather well.

I remember that already back then I did not really think that a career in music would suit my taste nor my talents -my interest was not focused on Physics yet but I had a pretty good idea I liked science already- but I nevertheless enjoyed playing the part of the musician. Probably this has been some sort of constant in my life: I have been an amateur musician, an amateur astronomer, an amateur chessplayer, an amateur reporter and photographer, but then I decided to become a professional physicist. In other words I seem to have applied to arts, sports, and intellectual activities what is commonplace to do with sentimental relationships: women and men flirt with the most attractive counterparts, but end up marrying the one which promises more stability.

So what were we playing back then, in Conegliano and Udine (but also in Venice, Mirano, and other places I can’t even recall) ? The offer was a trio of concerts by Johann Sebastian Bach: the Brandemburg Concerts number V, IV, and III. I was the second flutist in the fourth concert, as you can see in the scans I paste below.

Above, the front page of the booklet of Concert season in Conegliano, 1981

…and the page with the three concerts, and a few signatures from my colleagues.

The one above is instead the leaflet advertising the concert in Udine…

…and the back, with the program of the afternoon.

I have warm memories of those concerts. In the one in Conegliano, we performed excellently the fourth concert (I remember I was really pleased of the outcome and by my own performance) until -at the very end of the third movement- my instrument had become soaked with condensed breath, and it literally dripped. The condensed moisture flowed down the hole at the end and, what’s worse, down the hole on the back, which is closed by the left thumb to play bass tones and only closed halfways -by using the fingernail- to play high pitches. And one of those high pitches was needed towards the end of the Presto, when in the culmination of a forte I had to play a high mi. The thumb was unable to close the hole the way it should have, and my instrument let out a broken note which was probably heard even by the ticket seller outside the hall. That evening was spent on a pleasant restaurant on the hills of Conegliano, with the whole orchestra having fun of me -but it was cheerful and I did not resent it.

In the concert in Udine another incident happened. I was rather tense (I think it was the first time we performed the concert outside the walls of our Conservatory) and when the fifth concert was over, the solists came backstage, and I went on stage with my buddy Francesco and the first violin Andrea. As we were about to sit down, I realized I had left my scoresheet backstage! A better player would have acted nonchalantly and played by heart, but I was too nervous -so I rushed back and grabbed it, re-entering on stage with the eyes of the public on me but, what’s worse, those of my director following me like a missile approaches a plane to be taken down.

Ah, memories… I wish I had a recording of those concerts! I remember the one in Conegliano was indeed recorded, and I was promised a copy of the tape which never came.

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!

Although ten short meaningless posts won’t outvalue a longer thoughtful one, for today I stick with the former. So let me just paste here a link to a post about me at sci.bzaar.net, the site of a workshop I will attend virtually next week.

In a few days I plan to provide the site owner, Gianandrea Giacoma, with a couple of short videos where I discuss some limits of blogs in the context of scientific outreach. If I am not too lazy I will produce an English version of those (the event is for an italian audience).