Cosmic Rays from Dark Matter Annihilation and Big-Bang Nucleosynthesis

📝 Abstract

Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios for the case of hadronic annihilation. Constraints on leptonically annihilating dark matter models are less severer.

💡 Analysis

Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios for the case of hadronic annihilation. Constraints on leptonically annihilating dark matter models are less severer.

📄 Content

arXiv:0901.3582v3 [hep-ph] 29 Jun 2009 ICRR-Report-536 IPMU 09-0008 TU-837 June, 2009 Cosmic Rays from Dark Matter Annihilation and Big-Bang Nucleosynthesis Junji Hisano(a,b), Masahiro Kawasaki(a,b), Kazunori Kohri(c), Takeo Moroi(b,d) and Kazunori Nakayama(a) aInstitute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582, Japan bInstitute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568, Japan cPhysics Department, Lancaster University, Lancaster LA1 4YB, UK bDepartment of Physics, Tohoku University, Sendai 980-8578, Japan Abstract Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios for the case of hadronic annihilation. Constraints on leptonically annihilating dark matter models are less severer. 1 Introduction Cosmological observations have revealed that about 20 percent of the total energy density of the Universe is dominated by the dark matter (DM) [1], whose detailed properties are still unknown, and many physicists believe that the DM is a kind of stable particle appearing in the physics beyond the standard model. Thus, to determine the origin and nature of the dark matter in the Universe is one of the most important topics in the particle physics, and some methods were proposed for detecting the signals of the DM directly or indirectly [2, 3]. One of such methods is that to search for high-energy cosmic- rays, including gamma-rays, positrons, anti-protons and neutrinos, which come from the DM annihilation in our Galaxy. Recent results of the cosmic-ray positron and electron fluxes by the PAMELA satellite experiment [4] and the ATIC balloon experiment [5] are now drawing a lot of attention, since the steep excess observed by these experiments can be interpreted as an extra contribution from the DM annihilation. (For earlier papers, see [6].) However, in order to explain these signals the annihilation cross section should be fairly large, as ⟨σv⟩∼ (10−24−10−23) cm3s−1, which may be achieved by the Sommerfeld enhancement effect [7]. This is orders of magnitude larger than the standard value ⟨σv⟩∼3×10−26 cm3s−1, which reproduces the observed DM abundance under the thermal freezeout scenario [2]. Then, a huge boost factor (BF ∼100), which is due to the enhancement of the DM annihilation rate due to the clumpy structure of the DM halo, should be introduced in the scenario if ⟨σv⟩is not varied in time. However, the DM may be produced nonthermally in other ways, such as late-decay of long lived particles [8, 9]. Once we give up the thermal freezeout scenario, a large annihilation cross section is still allowed. The DM with large annihilation rate leads to other observable signatures: gamma- rays, anti-protons, and neutrinos.1 In particular, even if the DM only annihilates into leptons, the internal bremsstrahlung processes always emit significant amount of gamma- rays, and it also predicts a comparable amount of neutrinos. Those may put stringent constraints on the DM models [11, 12]. Besides these cosmic-ray signals, in this paper we show that the DM annihilation also affects the prediction of the big-bang nucleosynthesis (BBN), since it injects high- energy particles in the nucleosynthesis epoch, which modify the light elements abundances. Such an effect of DM annihilation was pointed out by Jedamzik in Ref. [13], where the modification on the 6Li abundance due to the hadronic process was emphasized. (See also Refs. [14, 15] for early studies of DM annihilation effects on BBN.) This subject was recently studied by four of the present authors in connection with the observed positron excess [16]. In this paper we have performed more systematic studies on the effect of DM annihilation on BBN, including the case where the DM annihilates into only leptons, motivated by recent results of PAMELA/ATIC. In such a case, the photo-dissociations of light elements give constraint. In particular, we found that the 3He to D ratio gives the most stringent constraint on the annihilation cross section, which can be consistent 1Indirect detection signatures of non-thermally produced DM were investigated in Ref. [10] before PAMELA and ATIC. 1 with the PAMELA/ATIC results. We also consider the case that the DM annihilates into hadrons. Then, the BBN constraint becomes more stringent, and a boost factor larger than unity must be introduced in order to account for the PAMELA/ATIC anomalies. Therefore, the DM annihilation models as an explanation of the PAMELA/ATIC results should be treated carefully so as not to contradict with the BBN constraint presented in this paper. This paper is organized as follows. In Sec. 2 effects of DM annihilation on BBN are explained

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