Dark Matter-Photon Interactions: Are They Allowed?
We summarize the effects of Dark Matter-photon interactions on structure formation and discuss the shape of the CMB and matter power spectra in presence of such interactions. We find that cross sections of the order of \( {{10}^{{ - 33}}}\left( {\frac{{{{
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Introduction Since almost twenty years now, cosmologists are more or less convinced that if there exists a particle physics solution to the Dark Matter problem, it should be under the form of "collisionless" particles (i. e. with negligible interactions). The most popular solution originates from Supersymmetry which predicts the existence of a neutral, massive and stable particle: the so-called neutralino. However, the unsuccessful searches of this particle tend to indicate that it is probably relatively heavy (with a mass at least greater than 45 GeV if it has a coupling to the Z) making it, in a way, an even better solution. The Dark Matter particles are indeed expected to annihilate with a cross section a ann of the order of 10- 36 cm 2 (for their relic abundance to correspond to the observations [1,2]) 1 which is precisely of the order of magnitude of the annihilation cross section of a particle having a mass close to the W mass (mdm '"" mw). In fact, larger cross sections are possible but they do evoke special processes (like the co-annihilation mechanism for instance [3,4]) which will not be considered here. Anyway, even such a value of the cross section, although originating from weak interactions, is not negligible and one can wonder what is its effect on structure formation. In addition, the constraint on aann can also be alleviated if there exists a primordial asymmetry between the number densities of Dark Matter and anti-Dark Matter particles so that even particles with stronger interactions could in principle be a solution to the Dark Matter puzzle (c.f. baryons). It then appears reasonable to search for new constraints (i. e. not arising from relic density calculations) on the Dark Matter interactions. For instance, it is very well-known that Dark Matter made of Weakly Interacting Massive Particles (WIMPs), as proposed by [5,6]' is compatible with the existence of 1
assuming a symmetry between the number of particles and anti-particles
H. V. Klapdor-Kleingrothaus et al. (eds.), Dark Matter in Astro- and Particle Physics © Springer-Verlag Berlin Heidelberg 2002
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galaxies, clusters of galaxies and even smaller objects provided the Dark Matter particles have a mass greater than a few keV. However, no specific values of the maximum allowed cross section have been given up to now for these particles and one can wonder if values larger than 10- 36 cm2 are possible or not. Moreover, the present status of numerical simulations [7] tends to indicate that WIMPs candidates could be unable to explain some of the intrinsic properties of galaxies like, for instance, the dark halo profile (which could be more cuspy in numerical simulations than what is observed) etc .. Although some of the present problems can finally be alleviated by invoking astrophysical processes, the existence of many discrepancies between simulations and observations may justify to investigate more closely the Dark Matter properties and more specifically the role of their interactions (whether weak or not). A general study has
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