Leptophilic dark matter in $U(1)_{L_{i}-L_{j}}$ models: a solution to the Fermi-LAT Galactic Center Excess consistent with cosmological and laboratory observations

Abstract

The particle origin of dark matter (DM) remains elusive despite decades of direct, indirect, and collider searches. Several groups have reported a $γ$-ray excess toward the Galactic Centre, commonly referred to as the Galactic Centre Excess (GCE). Its spectrum is consistent with annihilation of weakly interacting massive particles (WIMPs) of mass $\mathcal{O}(10-100)$ GeV and a thermal-relic cross section. Although many concrete WIMP models reproduce the GCE spectrum, most are now excluded by direct detection experiments that are approaching the neutrino floor. We investigate a class of anomaly-free extensions of the Standard Model featuring gauged differences of lepton number, $U(1)_{L_i-L_j}$, and gauged baryon minus lepton number, $U(1)_{B-L}$. We show that these models can reproduce the GCE while remaining compatible with the observed relic abundance. We then impose collider and direct detection constraints, accounting for both tree-level and loop-induced kinetic mixing. The $L_μ-L_e$ model gives the best fit to the GCE: a DM mass of $m_χ\sim 40-50$ GeV remains consistent with the muon and electron magnetic moment anomalies, $(g-2)_{μ,e}$, as well as current collider and direct detection limits, for mediator masses in the range $m_{A'}\sim 70-86$ GeV and a DM-mediator coupling of $(1-5)\times10^{-2}$. By contrast, the $L_e-L_τ$ and $L_μ-L_τ$ models yield poorer fits; satisfying both the relic density and experimental bounds forces the DM mass to lie very close to resonance (i.e., approximately half the mediator mass). Finally, while the $B-L$ model also matches the GCE well, its parameter space is almost entirely ruled out by strong direct detection limits, except for the narrow resonance region where $m_χ$ should be equal to $m_{A'}/2$ requiring a fine-tuning at the few-percent level.

Figures (7)

• Figure 1: Loop–induced kinetic mixing for the scattering of the DM particle $\chi$ with a quark $q$ or the $\ell_k$ lepton family: the total form factor $\epsilon_{\text{TOT}}(Q)$ (left panel) decomposes into the tree-level insertion $\epsilon$ (middle panel) plus the $\ell_i$ and $\ell_j$ loop vacuum polarisation (right panel).
• Figure 2: Momentum dependent kinetic mixing $\epsilon_{\rm{TOT}}(Q)$ divided by the coupling constant $g_X$ as a function of momentum transfer $Q$. The solid blue and gold curves correspond to the boundary condition $\epsilon_\textrm{IR} = 0$ and $\epsilon_\textrm{UV} = 0$, respectively. The vertical, dashed black lines indicate the values of $Q$ for which we match the leptons masses and the shaded red (orange) region represents the WIMP detection window in Lz ( Xenonn T), for DM masses between 10 and 1000 GeV.
• Figure 3: Upper panel: Nuclear recoil rate $R$ at Lz in terms of the transfer momentum $Q$ for the $L_\mu-L_e$ model in the case $\epsilon_\text{UV}=0$. Lower panel: Variation of $(\epsilon_{\rm{TOT}}/g_X)^2$ in the $L_\mu-L_e$, assuming three cases for the tree-level component: $\epsilon_\text{UV}=0$ (blue), $\epsilon_\text{IR}=0$ (orange), and $\epsilon_\text{RES}=0$ (green).
• Figure 4: Flux of positrons from DM in the $L_e-L_\mu$ model for the best fit we obtain when fitting the GCE (dotter red curve). We also show the flux from the secondary production as reported in Ref. DiMauro:2023oqx (green dashed curve) and total positron flux (blue solid) compared to the Ams-02 data PhysRevLett.122.041102.
• Figure 5: Best-fit of the four models considered in this work to the Di Mauro+21 and Cholis+22 GCE datasets. In each figure we report the GCE data and the theoretical predictions for the prompt (blue curve), ICS (red curve) and total $\gamma$-ray flux (black curve).