Antonio Masiero: Astroparticles in the LHC Era April 18, 2008Posted by dorigo in astronomy, cosmology, news, physics, science.
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.