Higgs-Portal Spin-1 Dark Matter with Parity-Violating Interaction for a Galactic Halo Gamma Ray Excess

TL;DR

This paper explores a dark photon dark matter model with a parity-violating Higgs-portal interaction, explaining a gamma-ray excess via Sommerfeld-enhanced annihilation without conflicting with existing constraints.

Contribution

It introduces a scalar mediator to enhance present-day annihilation rates in a parity-violating dark photon model, aligning with gamma-ray excess observations.

Findings

A 420 GeV dark matter mass explains the gamma-ray excess.

Sommerfeld enhancement increases annihilation rate in the Galactic halo.

The model remains consistent with dwarf galaxy and cosmological constraints.

Abstract

We study a dark photon dark matter scenario associated with a gauged $U(1)_X$ symmetry, stabilized by a dark parity that forbids kinetic mixing with the Standard Model. The leading interactions between the dark photon dark matter and the Standard Model arise from dimension-six Higgs-portal operators. In previous work, we found that for the parity-violating operator, the dark matter annihilation process is $p$-wave suppressed, naturally evading stringent direct-detection constraints while reproducing the observed relic abundance through thermal freeze-out. For a cutoff scale of 1 TeV, a dark matter mass of around 400 GeV is favored to realize the observed relic abundance. This scenario predicts cosmic-ray signals, in particular in the $W^{+}W^{-}$ channel, which can be targeted by indirect-detection experiments. Recently, a halo-like gamma-ray excess has been reported by Totani. Assuming a Navarro--Frenk--White $-\rho^2$ morphology for the dark matter distribution, a dark matter mass of 420 GeV is favored for the $W^{+}W^{-}$ final state. Motivated by this excess, we consider the present dark photon dark matter framework augmented by a CP-even scalar mediator with a mass of around 400 MeV, which couples to the dark photon mass operator in order to enhance the present-day annihilation rate in the Galactic halo without affecting the freeze-out dynamics. The exchange of this scalar induces a long-range attractive force, leading to Sommerfeld enhancement of the annihilation rate. The enhancement is saturating in dwarf spheroidal galaxies and the annihilation rate is dropping as $p$-wave remaining consistent with constraints from dwarf galaxies and cosmology.