Searching for Neutralino Dark Matter

Dark Matter in the Universe

The rotation curve for the spiral galaxy M33. The points represent the measured rotation velocities and the dashed curve is the contribution due to the observed matter . The existence of dark matter is inferred by the discrepancy between the observed rotation curve and the one due to the luminous disk .

What we can directly observe, by detecting photons emitted by sources in the Universe, it is not sufficient to explain some experimental evidences. The evolution of the radial velocity of spiral galaxies (see fig. 1), dynamics observation with respect to the luminous content of Coma Galaxy Cluster, and the measurements of the abundance of light elements in the Universe, suggest the presence in it of a significant amount of dark matter, totally different from baryonic matter. The CMB (Cosmic Microvawe Background) anisotropies also suggest non baryonic dark matter.

Dark Matter Candidates and MSSM

There are three groups of suitable candidates for dark matter, like Hot Dark Matter (HDM), Cold Dark Matter (CDM), and some combinations of them. The research work within IceCube Collaboration is concentrate on CDM and in particular on WIMPs (Weakly Interacting Massive Particles). These are non-relativistic particles produced in the Big Bang, being massive enough in order to undergo gravitational effects and interacting weakly enough to be still present in sufficient quantities. WIMPs are the most attractive and, hence, the most studied CDM candidates and in particular a class of them, the neutralinos.
These particles are introduced by supersymmetric extensions of the Standard Model (SM) of particle physics. This extension, called Supersymmetry (SUSY), is a development of grand unified theories which suggests that there is a symmetry between bosons and fermions, so that every SM particle has a "Supersymmetric" partner with the opposite statistics (i.e. a fermionic partner for each boson and vice versa), but the same mass and coupling. This mechanism modifies the running of the coupling constants and leads to their unification at high energies.
If the SUSY exist, it cannot be exact (it is said to be broken by some mechanism) as we would have already observed the supersimmetric partners of the SM particles.
We will work in a subset of SUSY, the Minimal Supersymmetric Standard Model (MSSM).
If the R-parity, that is a multiplicative quantum number in MSSM, is conserved, we deduce that the Lightest Supersymmetric Particle (LSP) is stable and that, if there were plenty of supersymmetric particles in the early Universe, they will have decayed to a large number of LSPs. The latter particles would be stable and remain until the present day; they therefore make a strong WIMP candidate.
In the context of Super-Gravity (SUGRA) theories the particle designated as LSP is the lightest combination of four fermions, partners of the neutral scalar bosons in the Higgs doublets (Higgsinos H⁰₁ and H⁰₂) and of the two neutral gauge bosons from electroweak symmetry (Bino B and Wino W³). These fermions are the "neutralinos", mentioned before, and the LSP is the lightest neutralino.

Neutralinos Detection

There are two different ways to search for neutralinos, via direct or indirect manner.

• The direct search studies the collisions of WIMPs on nuclei measuring their recoil energy. The most stringent direct detection limits to date come from the CDMS experiment at Soudan, that already started to exclude some MSSM models.
  

• The indirect search is based, instead, on the detection of products derived from neutralino annihilation, since they are Majorana particles (i.e. they are their own antiparticles). The most promising technique for indirect neutralino detection is to look for a high-energy neutrino signal from the center of celestian bodies. In fact, neutralinos passing through heavy objects can interact with nuclei and be elastically scattered; if the resulting neutralino's velocity is below the escape velocity, the particle becomes gravitationally trapped. Consequently, there will be an increase of neutralino concentration in the object's core and they will annihilate into different channels (leptons, bosons, b, c and t quarks and so on). Many theoretical calculations, especially of the possible neutrino flux from two extreme neutrino annihilation channels, b b-bar and W⁺W^- (further referred to as the "soft" and the "hard" channel), coming from the Sun and the Earth, have been performed. Energetic neutrinos from neutralino annihilations could be registered by large neutrino telescopes, through the neutrino induced muon flux. The muons are produced via charged-current interactions between neutrinos and nuclei of the medium surrounding the detector. Such muons are detected by the Cherenkov radiation caused by their passage through, for example, ice or water.
  

Analysis on WIMPs

The research work at IIHE consists in the analysis of the AMANDA, or possibly IceCube, data with the aim of searching for neutrinos originating from neutralinos accumulated in the center of the Sun or of the Earth. Using a good muon track reconstruction we can separate down-going atmospheric muons from neutrino-induced events and reduce the background. However, due to the limited accuracy of the reconstruction, additional quality criteria have to be applied to select reconstructed parameters.
After the optimisation of the WIMP simulation software, we will use it to optimise the event selection to obtain the best signal to noise ratio. If no neutralino signal is found, we will put an upper limit on the muon flux originating from neutralino annihilation in the Sun.