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Short summary from an intense day at PPC08 – part 1 May 21, 2008

Posted by dorigo in cosmology, news, physics, science.
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Besides my talk, which opened the morning session, there were a number of interesting talks today at PPC 2008, the conference on the interconnection between particle physics and cosmology which is being held at the New Mexico University in Albuquerque. I will give some quick highlights below from the morning session, reserving the right of providing more information on some of them later on, as I will have time to reorganize my notes and go back to the talks slides for a second look; a summary of selected afternoon talks will have to wait tomorrow. In the meantime, you can find the slides of the talks at this link.

I. Sarcevic talked about “Ultra-high energy cosmic neutrinos“. Neutrinos are stable neutral particles, so cosmic neutrinos point back to their astrophysical point source and bring to us information from processes that are otherwise obscured by material in the line of sight. Extragalactic neutrinos have large energies, and they can probe particle physics and astrophysics, since they can escape from extreme environments and they point back to the sources.

Among the sources, active galactic nuclei are the most powerful sources of energy. The AGN flux is the largest below 10^10 GeV. There is a correlation important to discover between photons and neutrinos: if photons come from the hadronic interaction p \gamma \to p \pi^0 \to \gamma \gamma, they can be observed together with neutrinos yielded by p \gamma \to n pi^+ \to n \gamma \to p \pi^+ when the charged pions decay to neutrino of electron and muon kinds. A fraction of these may then also give tau neutrinos from oscillations. The sources of pion decays have a ratio of 1:2:0 between electron, muon, and tau neutrinos, but the neutrino oscillation changes this double ratio.

Experiments are looking for muon tracks (ICECUBE, RICE), electromagnetic and hadronic showers (ICECUBE and others). To determine the energy flux that reaches the detector we need to consider propagation through earth and ice. Tau neutrinos will give different contributions from muon ones because of the short tau lifetime and the regeneration effect.

Nicolao Fornengo spoke on “Dark Matter direct and indirect detection”. We know that we have non-baryonic cold dark matter (DM), which points to extensions of the standard model (SM), new elementary particles. The evidence is multi-fold: dynamics of galaxy clusters, rotational curves, weak lensing, structure formation, energy budget from cosmological determinations.

We are concerned with dark matter inside galaxies. This can be made of a new class of particles, WIMPS for instance – weakly interacting massive particles. We need to know how these new particles are distributed. The halo in the galaxy has uncertainties, a thermal component, some round spherical distribution. From the velocity distribution it can be thermal, or non-thermal (for instance related to the merging history of the galaxy).

We need to exploit specific signals to disentangle dark matter signals from backgrounds. It is important ot quantify the uncertainties for the signal if we want to do that. What we do is a multi-channel search for DM: we have in fact different possibilities. The first is the direct detection of DM related to the fact that we are in the halo, so the particle can interact with the nuclei of a detector, and a signal in this case is nuclear recoil due to scattering. There are specific signatures, annual modulation or directionality of the recoil in order to correlate with the direction of the detector in its motion in the galaxy.

The other class of searches are typically called indirect searches; they rely on the self-annihilation of these particles with themselves, which produce anything allowed by the process, neutrinos, photons, or antimatter.

In annihilation to neutrinos, you can have a signal which you can correlate with the density of the galaxy, and spectral features in order to disentangle signal from backgrounds. For photons you can produce a line, since they decay at rest, and the line is very much suppressed – for neutralino it occurs at one loop level, but it would be a very good signature.

You can also annihilate into matter and antimatter, and produce cosmic rays. Also, you can have neutrino fluxes from the center of the Sun or Earth. Spectral features can also be used ot discriminate these signals from atmospheric neutrino backgrounds.

Let us start with direct detection. Upper limits are compared to some theoretical models. The latest CDMS upper bound is compared to results from a isothermal sphere of DM. The exclusion depends on the uncertainty on the shape of the local density and the velocity distribution functions. A spherical halo with a non isotropic velocity distribution function can give a different limit.

For neutralino masses higher than 50 GeV you have a realization of supersymmetry (SUSY). On the lower mass side the model is a minimal SUSY model with implemented a different parameter to loosen the LEP bounds on neutralino mass. If instead we compare with the DAMA result, we have models for neutralino and models for sneutrino which have allwoed solution in the region where there is a signal.

The question which arises is the following: Is there a candidate of something that agrees with both DAMA and CDMS ? You can have the DM candidate, the lightest sneutrino interacts with a nucleus producing a heavier state. The \chi_1 couples with the Z only off-diagonal. You have a kinematical bound related to the threshold of the experiment such that scattering is possible if the difference in mass is smller than a function of the masses and the velocity of the particle. For sneutrinos the suppression factor for a germanium nucleus divided by the suppression factor for a iodine nucleus, you can have situation where they are very different (germanium is the constituent of CDMS, iodine the target in DAMA). The same point could satisfy the DAMA region and the CDMS bound.

Neutrinos from earth and sun: idea is that you can accumulate your particles by gravitational capture, these produce annihilation, and neutrinos emitted are found in neutrino telescopes. Typically these calculations were done with fluxes of \nu_\mu; but they can be obtained by oscillation as well. The correction due to oscillation for annihilations in the earth or in the sun is large. For the sun at very high energy there is absorption.

How much is the signal of upgoing muons affected depending on mass ? If you produce Z or W in the earth you are not much affected by propagation, while in the sun for large mass the flux is reduced by a large amount.

One can try to exploit some energy spectrum of atmospheric neutrinos versus DM annihilation.
Antimatter signals are due to the fact that particles annihilate in the halo, produce proton-antiprotons.

Backgrounds are in the disk. One can model diffusion and propagation in the galaxy, and solve the diffusion equation, a bunch of parameters to fix using cosmic ray data. Then you can make a prediction for signal and background. The predictions for the spectrum in energy show a difference, although there are large uncertainties in signal fluxes. Better data on cosmic rays will fix the parameters.

For antiprotons, there is not much room for an excess in the lowest energy bins over the background, and not a big handle on the energy spectrum to distinguish signal from backgrounds. In this case one can only set constraints. The uncertainty in the theoretical estimates for SUSY is large in cross section. Firm exclusion of points is not possible unless you make strict assumptions on the propagation parameters.

One interesting signal is an antideuteron signal: in the process of annihilations you can produce antineutrons with antiprotons, and in turn produce antideuterons. It is nice because you have a good handle to detect it with respect to backgrounds. The uncertainty on the background (you produce anti-D from standard processes) is on the nuclear processes, for the signal instead the situation is the other way around: propagation gives a much wider uncertainty. Nevertheless, for antideuteron the signal and backgrounds have very different spectra. At low energy there is good discrimination (background is suppressed in that kinematical region). No detection in space so far for antideuterons, but there are proposals. An experiment called GAP can work on a balloon flight, and the expected sensitivity could access the signal. By taking a gaugino non-universal model with MSSM, the coverage in the parameter space for one-event sensitivity cuts into the space of models. Models not excluded by antiproton searches can predict up to 100 events for a long balloon flight.

For cosmic-ray positrons, you have production through annihilation and backgrounds. There are uncertainties at low energy because of propagation, below 10 GeV. How much can you boost your signal because of clumps of DM in the halo ? You do not expect for positrons a very big enhancement.

In summary, as far as direct detection we have annual modulation and in the rate they have a modulation. This can be indeed due to a DM particle. If interpreted that way, is this compatible with the SUSY candidate ? Yes, compatible with neutralino, harder for minimal SUGRA. It is also compatible with sneutrino. In models where you give mass to the neutrino with induced majorana-like mass terms. On the other hand you have CDMS, XENON etc, which try to reduce the background and have upper bounds. If we compare the models, they exclude some configurations. In one specific example the two sets could be working at the same time.

For indirect detection we have many possibilities. Antideuterons would be the best chance. When GAPS will fly, it could exploit a strong feature at low energy, and a good chance of finally to have a signal detected. Antiprotons at the moment do not show a very good spectral feature, but we can have potentially strong bounds. Positrons may possess spectral features but typically they require some overdensity to match the data. Gamma rays may have good spectral features, like lines. GLAST will be able to test this. Neutrinos from earth and sun could be found through their energy and angle features. One possibility could be to find the tau neutrino component, a virtually background-free signal.

Maurizio Giannotti talked on “New physics and stellar evolution“. Stellar cooling can provide bounds that are much better than experiments in near future.

The golden age for astrophyscs and stellar evolution was the late 50’s, since the role of weak interactions was recognized. The reference paper for neutrino pysics is by Feynman and Gell-mann of 1957, V-A theory of weak interactions. Indeed, stars can provide a test of weak interactions theory. Already in 1963 Bernstein and colleggues showed that the stellar bound on neutrino magnetic moment was better than the experimental one at that time.

Can we use stars to test physics above the ew scale ? Yes. If there is new physics (NP) at the electroweak (EW) scale this will appear in stars. Electroweak physics enters at 4th power of Gf, while NP will bring about a different scaling law. So we have to use other scales, energy scales like the temperature of the stars or masses. All these are much smaller than the EW scale.

Orthopositronium experiments: orthopositronium is an electron-positron bound state of spin 1. It thus cannot decay to 2 photons. Main decay is to 3 photons. Lifetime is 150 ns, longer than spin 0 state. It is a clean bound state of pure leptons, non strong interaction, only electromagnetic. In fact there is a little bit of weak interactions, but their contribution is small. So one can hypothesize that one gamma makes itself invisible through its disappearance into extra dimensions. The goal is to measure the invisible width of orthopositronium to a part in 10^-8. The partial width of orthopositronium to two neutrinos is less than 10^-17 of the three photons mode.

For stars, the energy loss through photon decay into the extra dimensions would delay the ignition of helium in the core of a red giant. The new energy loss rate must not exceed the standard loss through plasmon decay by more than a factor 2-3. The decay provides extra cooling in stars.
The delay of the Helium flash tells us that for one extra dimension, the scale of the extra dimension is k>10^{21} TeV, for n=3 extra dimensions the bound is softer, k>10^2 TeV.

To keep the scales in the model close to the EW scale one either needs a large number of extra dimensions, or very high scales.

A terrestrial experiment sensitive to invisible decay modes should have the following sensitivities to provide analogous bounds on k: B< 2x10^{-24+1.75n} for red giant stars.

In conclusion, stars offfer a variety of interesting environments to test physics beyond the SM. Bounds from astrophysics can be much better than the experimental ones. The models which try to confine the photon on the brane through gravity only are severely constrained by stellar evolution considertions. In this case, the sensitivity required in the orthopositronium experiment to provide the same bound is beyond any possibility in the near future. The result is that the number of extra compact dimensions must be 4 or greater, in
order to keep the scale of extradimensions close to the electroweak scale.