EXOTIC MATTER RESEARCH IN SPACE Rom2F/2001/06

The direct detection of annihilation products in cosmic rays offers an alternative way to search for dark matter particles candidates. Here we will see in particular that the study of the spectrum of antiproton and positrons offers good possibilities to perform this search and we will review our experimental effort in this direction. 1 Probing for dark matter with cosmic rays space experiments Galactic dark matter was suggested to solve the discrepancy between observed (luminous) matter in the Universe and that inferred from dynamical considerations. A matter density of about 30-40% of the critical density of the Universe can be composed of dark matter. One possible form of dark matter could be weakly interacting massive particles (WIMPs) and a good candidate for WIMP's is the Lightest Supersymmetric Particle (LSP) in R-parity conserving SUSY models. In most of the Supersymmetric theories the LSP is the neutralino χ that is a combination of the partners of the γ, Z and the neutral Higgs particles (see 1 and references threin). Neutralinos are Majorana fermions and will annihilate with each other in the halo producing leptons, quarks, gluons, gauge bosons and Higgs bosons. The quarks, gauge bosons and Higgs bosons will decay and/or form jets that will give rise to antiprotons (and antineutrons which decay shortly to antiprotons). The idea of exploiting cosmic antiprotons measurements to probe unconven-tional particle physics and astrophysics scenarios has a long history 2 and was stimulated by early reports 3 of unexpectedly large values for the antiproton to proton ratio in cosmic rays. Quantitative estimation of the antiproton flux due to dark-matter annihilation requires assumptions about the WIMP mass and dark matter density. The solar modulation introduce incertanties in the antiproton spectrum at low energies (below 1 GeV), so the best place to look from a deviation of the spectra is at high energies where there can be a neutralino signal rather sharply peaked at an energy higher than the maximum in the background if its spectrum decreases rapidly at low energies. This effect can be produced by high mass neutralinos with negligible branching ratio into b ¯ b or t ¯ t, which is the case e.g. for a very pure heavy Higgsino-like neutralino. In figure 1 (on the right) there are the experimental data for the antiproton flux 5 together with the distortion on the antiproton flux (dashed line) due to one possible contribution from neutralino annihilation …