Abstract
Using neutron stars (NS) as a dark matter (DM) probe has gained broad attention recently, either from heating due to DM annihilation or its stability under the presence of DM. In this work, we investigate spin-$1/2$ fermionic DM $\chi$ charged under the $U(1)_{X}$ in the dark sector. The massive gauge boson $V$ of $U(1)_{X}$ gauge group can be produced in NS via DM annihilation. The produced gauge boson can decay into Standard Model (SM) particles before it exits the NS, despite its tiny couplings to SM particles. Thus, we perform a systematic study on $\chi\bar{\chi}\to2V\to4{\rm SM}$ as a new heating mechanism for NS in addition to $\chi\bar{\chi}\to2{\rm SM}$ and kinetic heating from DM-baryon scattering. The self-trapping due to $\chi V$ scattering is also considered. We assume the general framework that both kinetic and mass mixing terms between $V$ and SM gauge bosons are present. This allows both vector and axial-vector couplings between $V$ and SM fermions even for $m_V\ll m_Z$. Notably, the contribution from axial-vector coupling is not negligible when particles scatter relativistically. We point out that the above approaches to DM-induced NS heating are not yet adopted in recent analyses. Detectabilities of the aforementioned effects to the NS surface temperature by the future telescopes are discussed as well.
Highlights
It has been widely accepted that one-fifth of the total energy of the Universe consists of dark matter (DM)
In this work we have investigated the new dynamics arising from the kinetic mixing and V − Z mass mixing between the dark gauge boson V of the broken Uð1ÞX symmetry and neutral gauge bosons in Standard Model (SM)
V − Z mass mixing induces a resonance at mV ≈ mZ, which can be seen from the pole of εZ at mV 1⁄4 mZ
Summary
It has been widely accepted that one-fifth of the total energy of the Universe consists of dark matter (DM). [61] provides an example by introducing a two Higgs doublet Φ1, Φ2, with Φ1 coupled to SM fermions, and one scalar Higgs singlet φ Both Φ2 and φ carry the dark charge gd and the mixing among Φ1, Φ2, and φ are neglected for simplicity. Since V couples to neutral scalar bosons to acquire its own mass, the annihilation process χχ → VÃ → V þ s (with s one of the neutral scalar bosons) is possible for mV þ ms < 2mχ Such a process yields a comparable heating effect to that given by χχ → 2V but involves an additional mass parameter ms.
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