Using neutron stars to probe dark matter charged under a $L_μ-L_τ$ symmetry

TL;DR

This work investigates a dark matter candidate charged under the U(1)_{L_mu-L_tau} portal, interacting predominantly with muons, taus, and their neutrinos. By performing a fully relativistic treatment of dark matter capture in neutron stars, the authors quantify kinetic heating of old, cold neutron stars as a probe of the portal couplings and DM mass in the range 100 MeV–100 GeV. They compare astrophysical sensitivity to existing constraints from relic density, Planck CMB limits, and direct-detection experiments, showing that neutron-star heating can access substantial unexplored regions, particularly for DM charge Q_chi ~ 1, where direct detection is weak. The results demonstrate the complementarity of astrophysical observations in constraining leptophilic DM scenarios and motivate targeted searches for heated neutron stars in DM-rich environments.

Abstract

Kinetic heating of old cold neutron stars, via the scattering of dark matter with matter in the star, provides a promising way to probe the nature of dark matter interactions. We consider a dark matter candidate that is a Standard Model singlet Dirac fermion, charged under a $U(1)_{L_μ-L_τ}$ symmetry. Such dark matter interacts with quarks and electrons only via loop-induced couplings, and hence is weakly constrained by direct-detection experiments and cosmic-microwave background observations. However, tree-level interactions with muons enable the dark matter to interact efficiently with the relativistic muon component of a neutron star, heating the star substantially. Using a fully relativistic approach for dark matter capture in the star, we show that observations of old cold neutron stars can probe a substantial, yet unexplored, region of parameter space for dark matter masses in the range 100 MeV - 100 GeV.

Figures (3)

• Figure 1: Status of existing constraints on the $L_\mu-L_\tau$ model, in the $g_{\mu\tau}-m_{Z^\prime}$. The CCFR PhysRevLett.66.3117Altmannshofer:2014pba, BaBar BaBar:2016sci and CMS CMS:2018yxg constraints do not depend on the choice of $\varepsilon_0$ and are shown as the purple, dark green and blue shaded regions, respectively. The region favoured by $(g-2)_\mu$ measurements Keshavarzi:2021eqa also does not depend on $\varepsilon_0$ and is shown in light green. The region excluded by the observation of White Dwarf cooling Dreiner:2013tja depends on $\varepsilon_0$ and is shown as shaded brown for $\varepsilon_0=0$, which is the most constraining case in the $m_{Z^\prime}$ range that is considered. The red stars represent two benchmark points we shall use in our NS analysis.
• Figure 2: Existing CMB constraints Planck:2018vygDutta:2022wdi (shaded dark green) and DM direct detection constraints from CRESST-III CRESST:2019jnq, Darkside-50 DarkSide:2018bpj and PandaX-4T PandaX-4T:2021bab (shaded red) for an $L_\mu-L_\tau$ portal dark matter candidate, shown in the $Q_\chi-m_\chi$ plane, assuming $\{g_{\mu\tau} = 1.1 \times 10^{-3}, \, m_{Z^\prime} = 100\,{\rm MeV}\}$ (left panels) and $\{ g_{\mu\tau} = 8.8 \times 10^{-3}, \, m_{Z^\prime} = 5\,{\rm GeV}\}$ (right panels). Because the direct detection constraints depend on the kinetic mixing parameter, we show results for $\varepsilon_0=0$ (upper panels) and $\varepsilon_{IR}=0$ (lower panels). The relic density contours are shown as blue solid lines.
• Figure 3: Projected sensitivities for neutron star captures (shown by orange bands) for a $L_\mu-L_\tau$ portal dark matter candidate in the $Q_\chi-m_\chi$ plane, assuming $\{g_{\mu\tau} = 1.1 \times 10^{-3}, \, m_{Z^\prime} = 100\,{\rm MeV}\}$ (left panels) and $\{ g_{\mu\tau} = 8.8 \times 10^{-3}, \, m_{Z^\prime} = 5\,{\rm GeV}\}$ (right panels). We set $\varepsilon_0=0$ (top panels) and $\varepsilon_{IR}=0$ (lower panels). The dashed and solid orange lines correspond to NSs of mass $1M_\odot$ and $1.9M_\odot$, respectively. The CMB, direct detection constraints, and relic density contours are the same as in Fig. \ref{['fig:NScapLmuLtaufig']}.