Dark matter search experiments
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ark Matter Search Experiments1 Wolfgang Rau Department of Physics, Queen’s University Kingston, Ontario, K7L 3N6, Canada Abstract—Astronomical and cosmological observations of the past 80 years build solid evidence that atomic matter makes up only a small fraction of the matter in the universe. The dominant fraction does not interact with electromagnetic radiation, does not absorb or emit light and hence is called Dark Matter. So far dark matter has revealed its existence only through gravitational effects. The strongest experimental effort to find other evidence and learn more about the nature of the dark matter particles concentrates around Weakly Interacting Massive Particles which are among the best motivated dark matter candidates. The two main groups of experiments in this field aim for indirect detection through annihilation products and direct detec tion via interactions with atomic matter respectively. The experimental sensitivity is starting to reach the parameter range which is preferred by theoretical considerations and we can expect to confirm or dismiss some of the most interesting theoretical models in the next few years. DOI: 10.1134/S1063779611040125
1.1 INTRODUCTION EVIDENCE FOR DARK MATTER The first observation of a significant discrepancy between the amount of matter deduced from optical observations, based on a good knowledge of a typical masstolight ratio of galaxies, and the gravitation in the respective system (in this case based on the velocity distribution and the virial theorem) came from Fritz Zwicky’s study of the Coma Cluster, published in 1933 [1]. He concluded that there must be more than two orders of magnitude more matter than could be accounted for by the observed luminous matter. To account for this difference he introduced the idea that there is a vast amount of dark matter present in galaxy clusters. Not much progress was made towards the under standing of dark matter until Vera Cooper Rubin and coworkers published their observation of rotation curves of spiral galaxies [2]: they documented that the rotational velocities of object outside the visible disk of these galaxies did not follow Kepler’s law as expected, but rather stayed constant out to very large radii, implying that galaxies are surrounded by a significant amount of invisible or dark matter. It is worth to note that Jan Hendrik Oort found already in 1932 discrep ancies between the observed rotation curve of our own galaxy and the expectation from the luminous matter [3]; however, from his observation he was not able to exclude (and actually seemed to favor the interpreta tion) that this discrepancy may have been caused by an underestimate of luminous matter due to the presence of absorbing matter. Since then numerous observations have been made which confirm that most of the matter in the universe 1 The article is published in the original.
is dark and nonbaryonic (i.e. not consisting of atoms). Only two more shall be mentioned here: the bullet cluster and the cosmic microwave background. The bullet cluster i
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