cdms-ii - CDMS Experiment - University of California, Berkeley

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14 CHAPTER 1. INTRODUCTION Other Dark Matter Candidates There are a number of other well motivated dark matter candidates in addition to the neutralino, not all of which require supersymmetry. It is worthwhile to briefly mention a few of these other candidates together with their motivations. • neutrinos: Recent measurements of the oscillations of the three neutrinos of the Standard Model indicate that they are not massless [23, 24, 25]. However, the neutrinos would not be massive enough to form dark matter that is cold. One possible dark matter candidate would be an extremely heavy neutrino that is sterile and only couples to other particles via neutrino mixing. Such a particle is very well motivated if one desires to explain the measurements of LSND [26] which measure a third mass splitting for neutrino oscillations. • axions: Axions are particles motivated by the need to solve the strong-CP problem. Axions couple extremely weakly to ordinary matter, and thus rapidly fall out of thermal equilibrium. Though constrained to be light by accelerators and astrophysical arguments, the axions are still attractive as cold dark matter because their mass is intimately tied to the QCD phase transition and the mechanism through which the axion addresses the strong-CP problem. In short, when the universe cools below the QCD phase transition, the axions produced are cold. • Other SUSY candidates: Supersymmetric extensions of the Standard Model have other dark matter candidates. The superpartner of the neutrino, the sneutrino, was favored for some time. However, the coupling of the sneutrino to nuclei is well constrained. The lack of detection of the sneutrino by elastic scattering off of nuclei currently disfavors the sneutrino as dark matter [27]. Other supersymmetric dark matter candidates are the axino [28] and gravitino both of which are difficult to detect due to extremely weak coupling to ordinary matter.
1.3. DIRECT DETECTION OF DARK MATTER 15 1.2.5 Accelerator and Cosomological Constraints on MSSMs There are a number of experimental constraints on the MSSM that come from measurements at accelerators. • Direct sparticle searches: Accelerators can search for supersymmetric particles directly. No evidence for sparticles has been found setting severe lower bounds on the mass of the LSP [1]. • Flavor changing neutral currents: Particles with large masses can have significant impact on other low energy measurements through radiative corrections. The effects of these quantum corrections can be most effectively detected through precise measurements of rare decay processes such as flavor changing neutral currents. The most severe constraint comes from measurements of b → sγ [29, 30]. • Anomalous Muon Magnetic Moment: The recent precision measurement of (g− 2) µ gives a result that differs from the Standard Model prediction by either 1.6σ or 3σ [31]. This measurement can be used to further constrain MSSMs. In particular, it constrains the Higgsino-mass parameter, µ, to be positive. Further constraints arise if the neutralino is to comprise the majority of the dark matter in the universe. If this is the case, then the neutralino must be the LSP and the model must yield a neutralino relic abundance that is consistent with the measured relic abundance of cold dark matter. Fig. 1.8 shows the reduction in the MSSM parameter space when all of the above constraints are considered. 1.3 Direct Detection of Dark Matter Goodman and Witten realized that it may be possible to detect the galactic dark matter through elastic scattering of the dark matter off of nuclei [33]. In this section, I will briefly discuss the expected signal for elastic scattering of neutralinos and describe some of the current direct detection experimental efforts. For more information, the reader can consult [34, 35].
Page 1 and 2: THE CRYOGENIC DARK MATTER SEARCH (C
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