CoGeNT Interpretations

Preprint typeset in JHEP style - PAPER VERSION MCTP-10-16 CoGeNT Interpretations Spencer Chang(a), Jia Liu(b), Aaron Pierce(c), Neal Weiner(b), and Itay Yavin(b) (a) Physics Department, University of California Davis, Davis, California 95616 (b) Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003 (c) Michigan Center for Theoretical Physics (MCTP) Department of Physics, University of Michigan, Ann Arbor, MI 48109 Abstract: Recently, the CoGeNT experiment has reported events in excess of expected background. We analyze dark matter scenarios which can potentially explain this signal. Under the standard case of spin independent scattering with equal couplings to protons and neutrons, we find significant tensions with existing constraints. Consistency with these limits is possible if a large fraction of the putative signal events is coming from an additional source of experimental background. In this case, dark matter recoils cannot be said to explain the excess, but are consistent with it. We also investigate modifications to dark matter scattering that can evade the null experiments. In particular, we explore generalized spin independent couplings to protons and neutrons, spin dependent couplings, momentum dependent scattering, and inelastic interactions. We find that some of these generalizations can explain most of the CoGeNT events without violation of other constraints. Generalized couplings with some momentum dependence, allows further consistency with the DAMA modulation signal, realizing a scenario where both CoGeNT and DAMA signals are coming from dark matter. A model with dark matter interacting and annihilating into a new light boson can realize most of the scenarios considered. arXiv:1004.0697v2 [hep-ph] 10 Aug 2010 1. Introduction Numerous experiments have been designed to search for the dark matter (DM), which constitutes the vast majority of all matter. Most searches have only placed limits. How do these null results influence the interpretation of a putative signal? This depends upon the properties of Weakly Interacting Particles (WIMPs) themselves. Variations in the couplings of WIMPs to protons, neutrons and spin [1], as well as possible inelastic [2, 3] or momentum dependent [4, 5] scatterings can all change expectations for signal rates. They also impact comparisons between different experiments. In addition, differences in target compositions and energy thresholds cause astrophysical unknowns, such as the local velocity distribution of WIMPs to impact experiments in different ways. When confronting a new signal, a broad exploration of these ideas is important to discern whether it can be of DM origin, or simply an unexpected background. Recently, the CoGeNT collaboration reported on a low energy ionization spectrum not immediately identifiable with background [6]. One possible explanation for these low energy events is a genuine signal from DM nuclear recoils against the germanium target. However, since CoGeNT does not discriminate between nuclear and electron recoils, one must be wary of unexpected backgrounds. It is thus important to determine the viability of a DM origin of the signal for various scenarios, given other experimental limits. Initial work in this direction has already appeared in [7] and the updated appendix of [8]. In finding a DM fit, [7] allows the possibility of a substantial contribution to the CoGeNT data from both DM and background, while [8] allows only a DM contribution to CoGeNT. They also differ in their treatment of possible systematic errors associated with the XENON experiment. Reference [7] entertains the possibility that systematic errors at the XENON experiment might degrade their published limits. In this work, we further explore the viability of DM recoils as an explanation to the CoGeNT data. We place an emphasis on the effect of the inclusion of background. In the absence of an exponential background which makes a significant contribution to the data, an interpretation of the CoGeNT data as spin-independent scattering is strongly constrained. With equal couplings to protons and neutrons, the CoGeNT 99% confidence region is entirely excluded by recent results from CDMS silicon (CDMS-Si) run [9]. We demonstrate that parameter space can open up for other relations between these cou- plings. We also examine other scenarios, such as light inelastic DM (iDM), spin-dependent couplings, and momentum dependent interactions. Aside from elastic spin-dependent in- teractions, we find that these generalizations are viable interpretations for CoGeNT not excluded by other experiments. These analyses can be sensitive to the inclusion of backgrounds. Therefore we consider the effect of an additional exponential background component at low energies. Depending on the amount of background allowed, one lessens the tension with CDMS-Si. However, completely avoiding the Si constraints requires a large amount of background, with at least

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