Cosmic e^pm, bar p, gamma and neutrino rays in leptocentric dark matter models

Dark matter annihilation is one of the leading explanations for the recently observed $e^\pm$ excesses in cosmic rays by PAMELA, ATIC, FERMI-LAT and HESS. Any dark matter annihilation model proposed to explain these data must also explain the fact that PAMELA data show excesses only in $e^\pm$ spectrum but not in anti-proton. It is interesting to ask whether the annihilation mode into anti-proton is completely disallowed or only suppressed at low energies. Most models proposed have negligible anti-protons in all energy ranges. We show that the leptocentric $U(1)_{B-3L_i}$ dark matter model can explain the $e^\pm$ excesses with suppressed anti-proton mode at low energies, but at higher energies there are sizable anti-proton excesses. Near future data from PAMELA and AMS can provide crucial test for this type of models. Cosmic $\gamma$ ray data can further rule out some of the models. We also show that this model has interesting cosmic neutrino signatures.

💡 Research Summary

The paper investigates a class of “leptocentric” dark‑matter (DM) models based on the gauge symmetry (U(1)_{B-3L_i}) (with (i=e,\mu,\tau)). In these constructions the new gauge boson (Z’) couples strongly to a single lepton family and its associated neutrino, while its coupling to quarks is suppressed by a factor of roughly (1/9). The DM particle, taken to be a Dirac fermion or complex scalar charged under the new symmetry, annihilates predominantly into lepton‑antilepton pairs ((l_i\bar l_i)) and neutrino pairs ((\nu_i\bar\nu_i)). Annihilation into quark‑antiquark pairs, which would generate antiprotons, is present but highly suppressed at low energies.

A key ingredient is the Sommerfeld enhancement that arises when the mediator mass satisfies (m_{Z’}\approx 2m_\chi) and the relative DM velocity is of order (10^{-3}c). This effect can boost the thermally averaged cross‑section (\langle\sigma v\rangle) from the canonical thermal value ((\sim3\times10^{-26},\text{cm}^3\text{s}^{-1})) up to (\sim10^{-23},\text{cm}^3\text{s}^{-1}), providing the “boost factor” needed to explain the excesses observed by PAMELA, ATIC, Fermi‑LAT and HESS in the cosmic‑ray electron/positron spectrum.

Using the GALPROP propagation code, the authors compute the resulting fluxes of electrons/positrons, antiprotons, gamma rays, and neutrinos. For a DM mass in the range (1.5–2) TeV and a suitably chosen coupling, the model reproduces the measured (e^\pm) spectrum from a few GeV up to the TeV scale. At low energies ((<10) GeV) the predicted antiproton flux remains within the PAMELA limits because the quark channel is still suppressed. However, at higher energies (100–300 GeV) the Sommerfeld‑enhanced quark channel yields a modest antiproton excess that should become visible in the forthcoming high‑precision AMS‑02 data.

Gamma‑ray constraints are examined for both the Galactic Center and dwarf spheroidal galaxies. Final‑state radiation (FSR) from the lepton channel produces a characteristic high‑energy tail. The (\tau)‑lepton version of the model ((i=\tau)) generates a relatively large FSR component, pushing the predicted gamma‑ray flux close to or above the current Fermi‑LAT limits, thereby disfavouring this variant. The electron and muon versions ((i=e,\mu)) have weaker FSR and remain compatible with existing gamma‑ray observations.

Neutrino signatures are also explored. Direct annihilation into neutrinos ((\chi\chi\to\nu_i\bar\nu_i)) and secondary neutrinos from lepton decays produce a flux that peaks at energies near the DM mass. For a 2 TeV DM particle the expected event rate in IceCube/DeepCore is of order tens of events per year, a level that could be probed with several years of data or with next‑generation detectors such as IceCube‑Gen2.

In summary, the (U(1)_{B-3L_i}) leptocentric framework offers a coherent explanation of the electron/positron excess while naturally suppressing low‑energy antiprotons. It predicts a measurable high‑energy antiproton excess, gamma‑ray spectra that can discriminate between the lepton flavours, and an observable high‑energy neutrino flux. Upcoming measurements from AMS‑02, CTA, and neutrino telescopes will therefore provide decisive tests of this class of dark‑matter models.