Papers

Sommerfeld enhancement from unstable final-state particles in dark matter annihilation

Tomohiro Abe, Ryosuke Sato, Takumu Yamanaka,  arXiv: 2603.02647[hep-ph]

We study the Sommerfeld enhancement of the annihilation cross section of dark matter into heavier unstable particles. In this process, the annihilation products become non-relativistic near the kinematical threshold. If they experience long-range interactions with each other, their wave function is distorted from a plane wave, and the annihilation cross section can be significantly enhanced. When evaluating the Sommerfeld enhancement from the long-range interactions between the annihilation products, the decay of the products needs to be taken into account. We treat this issue by including the decay width in the Schrödinger equations of the two-body wave function of the annihilation products. We find that bound states of the annihilation products with a narrow decay width enhance the annihilation cross section through a resonant effect. At the same time, this formulation automatically includes the annihilation process with off-shell final state particles, which is relevant for a wide decay width. We show that the resonant effect significantly affects the prediction of the dark matter relic abundance.

Excluding MeV-scale QCD axions by KL to pi0 pi0 a at KTeV

Takaya Iwai, Ryosuke Sato, Kohsaku Tobioka, Takumu Yamanaka, 

arXiv: 2602.15117 [hep-ph]

An interesting proposal suggests that a QCD axion $(a)$ coupling to the up quark, down quark, and electron remains viable for an axion mass near 10~MeV. In this paper, this possibility is reexamined by deriving new bounds from kaon decays. In particular, we perform a detailed analysis of the $K_L \to \pi^0 \pi^0 e^+ e^-$ measurement reported by the KTeV experiment, and reinterpret $K^+$ decay measurements at the E949 and NA62 experiments to constrain both the diphoton decay and effectively invisible decay modes of the axion. We find that, combining with the previously known bounds, the viable window for the MeV-scale QCD axion is excluded, primarily due to the KTeV bound. Uncertainties associated with the chiral Lagrangian are further examined, and the scenario remains excluded even after accounting for these uncertainties, except for a tiny region of parameter space where higher-order corrections must finely cancel the leading-order contribution, suppressing the branching ratio of $K_L\to \pi^0\pi^0 a$ by three orders of magnitude.

Composite Dark Matter with Forbidden Annihilation

Tomohiro Abe, Ryosuke Sato, Takumu Yamanaka,  JHEP 09(2024) 064,  arXiv:2404.03963 [hep-ph]

A dark matter model based on QCD-like SU(N_c) gauge theory with electroweakly interacting dark quarks is discussed. Assuming the dark quark mass m is smaller than the dynamical scale Λ_d ~ 4π f_d, the main component of the dark matter is the lightest G-parity odd dark pion associated with chiral symmetry breaking in the dark sector. We show that non-zero dark quark mass induces the universal mass contribution to both G-parity odd and even pions, and their mass tend to be degenerate. As a result, dark pion annihilation into heavier G-parity even dark pion also affects the dark matter relic abundance. Thus, our setup naturally accommodates forbidden dark matter scenario and realizes heavy dark matter whose mass is O(1-100) TeV, which is different from conventional electroweakly interacting dark matter such as minimal dark matter. We also discuss CP-violation from θ-term in the dark gauge sector and find that the predicted size of electron electric dipole moment can be as large as ~10^{-32} e cm.