A Quantum Diaries Survivor

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.

I spent the afternoon today at a conference on neutrino physics, . The conference is held this week in my beloved home town, and precisely in Palazzo Franchetti, which houses the Istituto Veneto di Scienze, Lettere e Arti, a pleasant venue close to the Grand Canal.

The program of the day was centered on astrophysics, and I decided to visit the conference to find inspiration for two forthcoming talks I will be giving, one in Padova next week and one in Albuquerque next month. The nice weather made for a pleasant coffee break: below you can see a pic of mself in palazzo Franchetti’s garden.

Later today - if I have enough stamina - I will post my summary of a couple of interesting talks:

• G. Steigman, Neutrinos and Big Bang Nucleosynthesis

• G.F. Giudice, Colliders and Cosmology

For now, I can only say that I did learn from the above talks some interesting details on the information about relic neutrinos one can extract from big-bang nucleosynthesis, and on the complementarity of searches for dark matter in direct astrophysics experiments and at the Large Hadron Collider…

UPDATE: I found out yesterday evening that Alexey Petrov is also at this conference, and in fact he discusses in his blog the talk given yesterday by R.Bernabei about the tentative dark matter signal observed by DAMA-LIBRA.

UPDATE 2: since this post is getting linked by high-traffic sites only because of a rather careless and potentially harmful remark I made in the comments section, I would rather direct you to a more accurate post which I wrote on the DAMA-LIBRA result today, which better represents my thoughts and is more politically correct on the matter.

I have been invited by David Orban, a friend and fellow blogger, to speak on the divulgation of Science next May 17th at the Scuola Politecnica di Design in Milano, at a meeting called SCI(bzaar)NET. The event, organized by Gianandrea Giacoma, is described in its web site as (my translation)

“Subjects active in the net meet in a new way to ponder on the challenges that Internet poses to scientific divulgation, production of knowledge, and Open Culture in the academic world.”

The meeting will have three main threads:

1. The hunger of scientific outreach: scientific research and the fast technological evolution are increasingly becoming, as is evident to all, among the main factors of change in the world and in our daily life. For these reasons a growing number of people, fascinated and awed, feel the need to understand and make their own opinion on the matter.
2. Production of knowledge: if internet is historically connected to the academic world, on the other hand one cannot claim that the majority of researchers as indivudials and the italian University institutions have adopted these new instruments for a more advanced presence online and a more effective handling of knowledge, students, researchers, and professors.
3. Open Culture: the growing impact of legal, economical, organizational and cultural scenarios of a diffusion of Open Culture in Universities under the pressure of internet.

I will contribute with a video, because I unfortunately cannot be there in person… On the following morning I am leaving to New Mexico for PPC 2008;. I am planning to post the video here, with a transcription (the language of the meeting is Italian…). The subject of my talk will be “Fare divulgazione scientifica con un blog: opportunita’ e limiti” (doing scientific outreach with a blog: opportunities and limits).

UPDATE - the name of this post has been modified according to the request of G.Giacoma on 4/23, reflecting the final name of the event.

In a country where the word “communist” has been increasingly used as an insult since 1993 - we have to give unshared credit of this to Silvio Berlusconi, who ever since his descent in politics used it as a synonym of “illiberal” or even worse - it might not come as a surprise that the new parliament after yesterday’s elections does not contain one single person who even loosely defines himself as such.

Despite the derogatory nature that the epiteth had taken in the eyes of many in recent years, however, the disappearance of a radical left in Italy’s political arena has generally not been greeted with enthusiasm. Not even members of National Alliance, the party born on the ashes of the filo-fascist MSI, seemed to rejoice yesterday evening on television post-mortem analyses: a rather confusing stand, and a demonstration that italian politics is not easy to understand by outside observers.

A country with no representation of a radical left in the parliament is drifting towards a policy of consensus that cuts corners and steam-rolls over dissent. Italy is not ready for that. It is not by chance that a veteran like Francesco Cossiga -who was prime minister during the most violent period in the history of the italian republic- warns today in an interview to the newspaper Il Corriere della Sera that political terrorism in Italy has its roots in the total lack of a dialogue of the government with the fringes of society, and that the conditions for a rebirth of violence are ripe again.

But what are the reasons of the incredible defaillance of the left, which presented a coalition of forces which had gathered no less than 11% of votes only two years ago, and is now at 3.1%, well below the 4% threshold which allows a party to be represented in the Camera dei Deputati, Italy’s lower chamber ? Analysts will have their hands full in the forthcoming months to understand fluxes and tendencies, but it is clear that this surprising result comes from at least two effects.

The first is the abstaining of many of the supporters of the radical left, disillusioned by the left parties who did not have anything to show for two full years of participation in Prodi’s 2006 government. One can see a signal of this in the increase of abstention by almost 3% in 2008.

The second is the sheer effect of bipolarism: the choice of a premier was recognized from the start to be only between Berlusconi and Veltroni, and many supporters of the radical left, moved by the wish to avoid a victory of Berlusconi, voted for Veltroni’s Democratic Party.

Veltroni cannot be too happy of this: he did well in convincing voters of center-left area, but he lost his elections because he did not convince any of the traditional voters of the center-right coalition. But one cannot really blame him, since his mission was impossible to achieve: Italy wanted a change from Prodi’s government, who tried hard in the past two years to mend the most grievious problem of Italy’s economy -its trillion-dollar debt- but forgot to protect the lower middle-class from price increases and ridiculous salaries.

I have many worries now. One is that INFN, my employer, will be seen as a conquer ground by the new government, who will cut funding and probably restructure the institute, for a better political control. Another is that Italy may be tempted to show an arrogant face again in the international arena, with military intervention in hot spots of this planet. A third is the stop of the attempts at saving the frail economy in the interest of tax cuts. A fourth is the boost to private schooling system, in a country where public schools work very well despite the ridiculous salaries of teachers. I could go on, but I have better think about research today.

The last votes have been cast minutes ago, and the first exit poll has just arrived. It appears that the fork between PDL (Berlusconi’s coalition) and PD (Veltroni’s party) is thinner than expected: at the lower chamber 42% to 40%, at the Senate 42.5% vs 39.5%.

Under such circumstances, one can foresee four different scenarios.

1) Exit polls are wrong, Berlusconi has a solid majority in both chambers, and Italy is condemned to 5 gloomy years of government by the right.

2) Despite the smaller-than-expected difference, Berlusconi has a majority of seats in both chambers. The numbers in the Senate (which is elected with a baroque system which never grants a solid majority) make his government very difficult to hold. Berlusconi gets blackmailed by Lega Nord from the start, and his government lasts at most two years. After which, the center-left led by Veltroni becomes a more credible alternative and wins.

3) No clear majority in the Senate for PDL forces a coalition of forces to change the electoral law and administer the country for a few months, and new elections happen in six-eight months time, with a unpredictable result.

4) The undecided response of the urns leads to a dismemberment in the big coalitions, and a coalition of forces, led by Pierferdinando Casini’s UDC, attempts to ride the tiger, with pitiful results.

Not a pretty picture in any case. More to come soon.

UPDATE:

At 9PM, about three fourths of votes have been scrutinized, and the result is not equivocal anymore. Indeed, it is a clear win for the right.

At the Camera dei Deputati the partial counts give PDL 46.2% vs PD 38.1%, while at the Senato della Repubblica the difference is even larger, PDL 47.1% vs PD 38.2%.

It remains to be seen how many seats will PDL win in the Senate. Due to a very strange electoral system, at the Senate the prize for majority is assigned on a regional basis - there are 20 regions in Italy. Because of that, the margin will be narrow, but probably still confortable, for Berlusconi.

We will have to wait tomorrow for a clear analysis, but it looks like Berlusconi is condemned to govern our country for five more years. And we are condemned to be led by him.

UPDATE:

It is now clear that Berlusconi has a full mandate to govern Italy. Even in the critical Senate, he collects 171 seats, which guarantee a solid majority. He said today that his first actions as a prime minister will be to abolish ICI, a very annoying tax - the one on the possession of the house one lives in; and to take care of the critical situation of Alitalia. We will judge him by facts this time.

I received this morning the poster of a workshop on Dark Matter searches at the LHC, which will be held in my University on April 22nd. This is a single afternoon of talks addressed to students of Physics, to educate them on the connection between particle physics and cosmology in view of the start of the collider this fall. Upon glancing at it, I immediately sensed the subliminal message it sends to whomever has been reached by the headlines on the recent lawsuit concerning the risk of black hole creation in the high-energy proton-proton collisions… Here is the poster:

The galaxy has of course nothing to do with an expanding black hole, but it still sends a sinister message. Let me say it here again: black holes will NOT be created at LHC. Scientists cannot even assess the chance of that happening, because the probability that 1) Large extra-dimensions exist in nature, 2) the scale of quantum gravity being both fine-tuned to allow black holes to be produced by LHC and not by past colliders, and orders of magnitude smaller than what it is most reasonable to conceive, is too small to be investigated meaningfully.

In any case, even if microscopic black holes were created at LHC, they would evaporate instantly, due to a phenomenon, Hawking radiation, which only rests on general relativity and quantum gravity, and is thus on much more solid ground than the very production of black holes. And in any case, even if black holes were created and they did not evaporate, they would escape the Earth without more than a few nuclear interactions. And in any case, even if scientists were wrong on all the previous counts, collisions like the ones LHC will produce are generated everywhere by cosmic rays, so the black holes generated inside the LHC would be nothing new under the sun.

For a more meaningful discussion of these issues, please visit this instructive post at backreaction.

In the first part of this long post I discussed some generalities of calorimeters, which are one of the most important components of modern detectors for high-energy particle physics experiments. To analyze in some detail the way calorimeters work it is now useful to distinguish electromagnetic from hadronic ones. In the former what is accurately measured is the energy of photons and electrons, while the latter targets particles interacting strongly with nuclear matter: mostly protons, neutrons, pions, and kaons.

Electromagnetic calorimeters

Electromagnetic calorimeters aim at measuring electrons and photons with energy above a few hundred MeV, when these particles lose energy mostly by pair production and bremsstrahlung, respectively.

Pair production is the process whereby an energetic photon “materializes” into a particle-antiparticle pair. It naturally occurs if the photon passes close to a nucleus (the heavier the better) which can “absorb” the excess transverse momentum that a conversion forcefully generates. Of course pair production can only happen if the photon energy exceeds the mass of the produced particle pair; moreover, although the production of any fermion-antifermion pair energetically allowed has a non-zero chance to occur, the only experimentally important process is electron-positron production.

Bremsstrahlung (braking radiation) is instead the “inverse” process: an energetic electron emits a photon, also yielding some fraction of its momentum to a spectator nucleus. The effect is a “braking” of the electron, and the emission of a energetic photon.

The cross section of these processes – or, if you prefer, the probability that they happen – depends on the square root of the atomic number Z (the number of protons) of the traversed material. The quantity which characterizes these phenomena is called radiation length and is universally labeled . Radiation length is defined as the thickness (in centimeters, or more usefully in grams per squared centimeter -you can switch from one unit to the other by multiplying by the material density in grams per cubic centimeter) of material crossing which an electron has a probability (or roughly 63%) of radiating a photon. For photons, can be considered as an attenuation length, because they “disappear”: if are initial intensity and intensity beyond a thickness , the formula dictates that , which means that a thickness converts 1-1/e of the photons, and only 1/e=37% remain in the initial beam.

One radiation length corresponds to about 300 meters in air, 9 cm in aluminum, and only 5.6mm in lead (a very useful formula for a quick approximation to keep in mind is , again in grams per squared centimeter; it is good to better than 20% accuracy for Z>13). Lead and other heavy materials allow the construction of compact calorimeters; for instance, a common design is that of thin lead sheets alternated with sensitive material like sheets of plastic scintillator. Another possibility is to use blocks of lead glass, where light is obtained by the Cherenkov effect.

Another small parenthesis: Cherenkov radiation, discovered in the 1930s, occurs when a charged particle travels in a medium at a speed larger than that of light in the material, creating a “shock wave” in the form of photons radiated at an angle depending on the particle speed [were you familiar with the fact that light travels at speed slower than in transparent media ? Its speed is indeed , where n is the refraction index of the material (). This phenomenon is at the basis of light refraction]. The radiation has a spectrum peaking in the near ultraviolet. In the picture on the right you see a particle path as a horizontal line; in the time it takes it to travel along the segment of length (with the ratio between particle speed and speed of light in vacuum), light only travel a distance , creating a coherent emission front at an angle such that .

Because does not depend on the energy of the incoming photon or electron, it is possible to estimate as a function of this quantity the total thickness of material which is needed to completely absorb an electromagnetic shower: the number of produced particles doubles per each additional thickness of traversed material, until particle energy reaches a critical value beyond which the process cannot be continued.

Below criticality, for electrons dominate energy losses with atomic electrons (what is called Moller scattering), while for photons at about the same energy starts dominating the Compton effect – the process whereby photons yield a fraction of their momentum to a nucleus and become softer. With some simple math one then finds than, given an initial energy of about 100 GeV, 20 radiation lengths are sufficient to absorb about 98% of the energy: in practical terms that means not more than 40-50 cm of thickness for sampling devices such as the frequently used lead-scintillator wafer.

Transverse containment can also be parametrized through the quantity . In this case, the widening of showers (principally due to the emission of bremsstrahlung photons not collinear with the incoming electron, since the angle is proportional to the fractionary momentum loss, and to Coulomb deflections only in the later stages of showering) scales with a quantity called Moliére radius, (where A is atomic weight and Z is atomic number of the material, and resulting units are grams per squared centimeter). The Moliére radius characterizes the typical deflection of electrons traversing one radiation length of material.

Energy resolution of electromagnetic calorimeters depends mostly on the stochastic nature of the processes of energy yield: since on average the total number of particles produced in a shower grows linearly with the energy of the original body, from Poisson statistics we know that energy resolution must scale with the square root of energy, , if one neglects systematic effects due to the loss of a part of the energy (longitudinally or transversely), to the non-linearity of the response of the active medium, and a multitude of other small nuisances.

Poisson statistics determines the distribution of counts for random processes which have integer outcomes, such as the number of tracks in a shower, as a function of the expected number, the average. The distribution has a width which is proportional to the square root of the average, so that the typical error that can be assigned to a number of counts N is .

For the constant values around 10% to 20% are common in sampling calorimeters. In CDF the central electromagnetic calorimeter is the classic lead-scintillator sandwich, and the resolution is . For a decay this results in a resolution of about .

Hadronic Calorimeters

In hadronic calorimeters what is measured is instead the energy of hadronic showers produced by nuclear interactions of mesons and baryons with the nuclei of the absorber. The processes causing energy loss are in this case much more complex and harder to measure accurately, for at least three reasons:

1. the presence of nuclear excitation phenomena, which reduce in a non trivial way the fraction of measurable energy because of the emission of fast neutrons and protons, or other non-radiative processes;
2. the decay in flight of pions and kaons into muons and neutrinos, since the latter do not release a significant amount of energy in the detector;
3. and finally, a sizable component of secondary hadrons is constituted by neutral pions (a third of the total of produced pions) and other particles which immediately decay to photons, with consequent losses of linearity in the energy response: photons give a larger response than charged pions (see below).

The resolution which can be obtained is much worse than that of electromagnetic shower detectors: the value of K ranges from 50% to 150% and above, depending on the quality of the active material. Moreover, one has to mention the fact that the quantity corresponding to the radiation length is, for hadronic showers, the interaction length , which is much longer, due to the smaller cross section of nuclear interactions. This forces much larger longitudinal dimensions in order to contain the hadronic showers. In iron, a material widely used as absorber in high-energy physics experiments, ; in uranium .

(Above, iron wedges of the CMS forward calorimeter).

The difference in response to electrons (or photons) and pions in hadronic calorimeters amounts typically to 30-40% and is mostly due to nuclear excitation phenomena by pions. The response can be equalized in the so-called compensated calorimeters: these usually have U-238 as absorbing material. Uranium yields back the energy loss “with interest”, in the form of nuclear fission. The detection of even a small part of the released energy may allow, through an accurate calibration and an optimization of the layers of uranium and scintillation material, to halve the K factor. One thus obtains values of K around 30-40%.

Some additional design considerations

One of the unavoidable constraints of calorimeters is the need to fully contain the development of particle showers in their volume. A leakage of penetrating tracks on the back of a calorimeter limits its resolution and worsens the measurement of the most energetic incoming particles. This suggests the use of very heavy materials, as and have been already shown to be inversely proportional to atomic weight.

A parameter of fundamental importance in the design of calorimeters is transversal segmentation. A finer segmentation with “towers” pointing back to the interaction vertex allows to obtain a precise map of the energy deposition as a function of the polar coordinates of particles generated in the interaction point, which proves very important for the identification of hadronic jets.

As a matter of fact, the segmentation is usually designed to be uniform in pseudo-rapidity, the quantity . Pseudorapidity is a monotonous function of the polar angle between the direction of a detector element and the beam, as seen from the interaction vertex. It transforms linearly for Lorentz boosts along the beam axis, and this makes jets show up as circular energy deposits in the calorimeter if mapped in the variables .

A Lorentz boost is a transformation of coordinates satisfying special relativity, and must be used to study interactions yielding a center-of-momentum which is moving in the detector frame of reference. In proton-proton collisions such as those produced by LHC, or proton-antiproton collisions at the Tevatron, this is exactly what happens, because the originators of the hard collision are partons within the projectiles, each of which carries a unknown fraction of the (anti-)proton energy.

The typical radius of hadronic jets is of 0.7 units in space, but jets have a transversal extension that becomes smaller as energy increases. This is because the momentum of hadrons originated from parton fragmentation is on average equal to 300 MeV in the direction transverse to the jet axis and only weakly dependent on the originating parton energy, while the longitudinal component scales linearly with the parton energy. Because of this fact, the reconstruction radius of clustering algorithms that recognize hadronic jets from the calorimeter deposits has slowly shrunk with years, following the increase of the energy of typical jets which the experiments strive to measure with accuracy.

Values of R around 0.4 are now commonly used by the Tevatron experiments in the reconstruction of heavy particle decays to hadronic jets, such as that of top quark pairs. With a cone of some of the jet energy is lost in what is called “out-of-cone”, slightly deteriorating the energy resolution because an average correction becomes then necessary; this is acceptable, though, because the detection of all the energy deposited by the jet is not as critical a factor as is the correct identification of jets traveling close together, which is a common feature of high jet-multiplicity final states produced by top-antitop decays. (In the event display on the left, energy depositions in the plane are represented as bars of red and blue color to describe electromagnetic and hadronic energy measurements. This is a candidate top pair decay to a tau, an electron, plus hadronic jets. Despite the leptonic decay of both W bosons, the event is still best reconstructed by using a small radius for jet clustering.)

The future

If new detectors will ever be built to explore a yet higher energy regime than the one about to be probed by LHC, calorimeters will be as necessary as they are today. The following characteristics will be desirable in a design of new generation:

• self-triggering (the ability of independent portions of the system to identify and measure a signal, interpreting it and sending an accept signal to the data aquisition system)
• stand-alone tracking (the ability of the calorimeter system to independently determine the direction of crossing particles)
• an integrated time-of-flight measurement (the capability to separate different particle signals based on the delay between their arrival time and the interaction time)
• high resolution and granularity (attainable with silicon technology)

The needs of these fancy features, however, rests on the specific hunt that we will decide to embark on. Which, in turn, critically depends on the discoveries that the Large Hadron Collider will produce!

Just a note to post here the permanent link to the proceedings of a conference I attended in Corfu (Greece) three years ago. This is a long (32 pages) report on “High- Physics: from the Tevatron to the LHC“, now published in the Journal of Physics: Conference Series [Tommaso Dorigo 2006 J. Phys.: Conf. Ser. 53 163-194]. I think I did post a draft of the paper on this blog a couple of years ago, but then I forgot to post the final version as well.

The paper is a bit dated in some parts, where the most recent (back then) results from the Tevatron are discussed; however, some parts -especially a discussion of the usefulness of Tevatron data for LHC physics- are still readable IMHO. Also worth noting is the fact that the acknowledgments section mentions the late Riqie Arneberg, a friend who passed away last fall, who had accepted the offer I had made to all readers of this blog to proofread the manuscript, and contributed in several places to the clarity of the text.

The publisher has now made available online all its 100 open access volumes through the JPCS home page. Of course I salute this contribution to the free diffusion of science with enthusiasm.