Gamma-ray Constraints on Hadronic and Leptonic Activities of Decaying Dark Matter

arXiv:0910.2639v3 [hep-ph] 28 Dec 2009 IPMU 09-0126 Gamma-ray Constraints on Hadronic and Leptonic Activities of Decaying Dark Matter Chuan-Ren Chen1, Sourav K. Mandal1,2, Fuminobu Takahashi1 1 Institute for the Physics and Mathematics of the Universe, University of Tokyo, Chiba 277-8568, Japan 2 Department of Physics, University of California, Berkeley, CA 94720, USA Abstract While the excess in cosmic-ray electrons and positrons reported by PAMELA and Fermi may be explained by dark matter decaying primarily into charged leptons, this does not necessarily mean that dark matter should not have any hadronic decay modes. In order to quantify the allowed hadronic activities, we derive constraints on the decay rates of dark matter into WW, ZZ, hh, q¯q and gg using the Fermi and HESS gamma-ray data. We also derive gamma-ray constraints on the leptonic e+e−, µ+µ−and τ +τ −final states. We find that dark matter must decay primarily into µ+µ−or τ +τ −in order to simultaneously explain the reported excess and meet all gamma-ray constraints. 1 Introduction The existence of dark matter (DM) has been firmly established by numerous observations, though the nature of DM mostly remains unknown. In particular, it is not known whether DM is absolutely stable or not. If DM is unstable, it will eventually decay into lighter particles which may be observed as an excess in the cosmic-ray spectrum. If DM is related to new physics which appears at the weak scale, it is natural to expect that the DM mass is in the range O(100) GeV–O(10) TeV. However, the longevity of DM whose mass is of the weak scale is a puzzle and calls for some explanation. The (quasi)stability may be the result of a discrete symmetry or extremely weak interactions. For instance, in a supersymmetric (SUSY) theory, the lightest SUSY particle (LSP) is stable and therefore a candidate for DM if R-parity is an exact symmetry. However, R- parity violation may be a common phenomenon in the string landscape [1], in which case the LSP DM is unstable and eventually decays into Standard Model (SM) particles. On the other hand, if DM is in a hidden sector which has extremely suppressed interactions with the SM sector, the only way to probe DM may be to look in the cosmic rays for signatures of its decay products. Recently, much attention has been given to the electron/positron excess reported by PAMELA [2], ATIC [3], PPB-BETS [4] and Fermi [5]. The excess clearly suggests that we need to modify our current understanding of the production/acceleration/propagation of cosmic-ray electrons and positrons. Of the many explanations proposed so far for this excess, DM decay or annihilation remains an exciting possibility. In order to ac- count for the excess by DM decay/annihilation, DM should mainly produce leptons with suppressed hadronic branching ratios, otherwise the anti-protons produced would likely exceed the observed flux [6]. Further model-independent analysis also revealed that, if one requires that the PAMELA/Fermi excess be explained by DM annihilation, the gamma- rays accompanying the lepton production exceeds the observed flux unless a significantly less steep DM profile is assumed [7, 8]. Thus, the leptophilic decaying DM scenario has recently gained momentum [9, 10, 11, 12, 13, 14, 15]. Leptophilic decaying DM models can be broadly divided into two categories. One is such that DM first decays into additional light particles, which subsequently decay into 2 muons or electrons, while the decays into hadrons are forbidden by kinematics [16]. The other is such that the DM particle couples mainly to leptons due to symmetry [10] or geometric setup [17]. While the hadronic activities are absent in the former case, it is model-dependent to what extent the DM is leptophilic in the latter case. One example is the hidden gauge boson decaying into the SM particles through a mixing with a U(1)B−L gauge boson [9]; the DM is certainly leptophilic in the sense that it mainly decays into leptons, but a certain amount of quarks are also produced. The purpose of this paper is to study the current constraints on the hadronic and leptonic decay of DM. The anti-proton flux is known to provide a tight constraint on the hadronic activities, but there are large uncertainties in the propagation [18]. The other constraint comes from gamma-ray observation. In contrast to charged cosmic-ray particles, gamma-rays travel undeflected and there is no uncertainty in the propagation; the main uncertainty is the dark matter profile. Furthermore, the Fermi satellite has been measuring gamma-rays with both unprecedented precision and statistics, and we can expect a significant improvement over EGRET data [19, 20]. In this paper we will derive constraints on the partial decay rates of DM into WW, ZZ, hh, q¯q and gg as well as e+e−, µ+µ−and τ +τ −using the Fermi [21, 22] and HESS [23] data. The bounds obtained in this paper are not only generic but also can be used to know what branching ratios are allowed in d

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