Galactic center gamma-ray excess through a dark shower

The reported excess of $\gamma$ rays, emitted from an extended region around the galactic center, has a distribution and rate suggestive of an origin in dark matter (DM) annihilations. The conventional annihilation channels into standard model (SM) $b$ quarks or $\tau$ leptons may, however, be in tension with various experimental constraints on antiproton and positron fluxes. We present a framework that is free from such constraints. The key idea is that the mediators between the dark matter and the SM are themselves part of a strongly coupled sector: a hidden valley. In this scenario, the dark matter particles annihilate only into hidden quarks that subsequently shower and hadronize. Hidden quark effective couplings to SM hypercharge allow the lightest hidden bound states to subsequently decay into SM photons, producing the observed photon energy spectrum. Associated production of SM fermions is, in contrast, suppressed by electroweak, loop or helicity effects. We find that, generically, $\sim 10\mathrm{GeV}$ DM and a confinement scale $\sim 1\mathrm{GeV}$ provide a good fit to the observed spectrum. An SU(2) hidden confining group is preferred over SU(3) or higher-rank gauge groups, up to uncertainties in the extraction of the astrophysical background. An explicit realization of this framework is also presented, and its phenomenology is discussed in detail, along with pertinent cosmological, astrophysical and collider bounds. This framework may be probed by model-independent searches, including future beam-dump experiments.

Figure 1

Schematic amplitude for GeV DM, $\chi$, annihilation through a hidden $Z^{\prime }$ to hidden quarks, $\mathcal{q}$, followed by showering to hidden pseudoscalars, ${\eta }_v^{\prime }$. The shower may include other light (meta)stable bound states, ${\omega }_v$ and ${\mathrm{\Delta }}_v$ (defined below). The ${\eta }_v^{\prime }$’s subsequently dominantly decay to photons, as may other metastable states.

Figure 2

The $\gamma$-ray spectrum at the $\mathrm{SU}(2{)}_{\text{V}}$ best-fit point, compared to the $\gamma$-ray excess reported in Ref. [8]. All rates are normalized to the flux at 5° from the GC, using a generalized Navarro-Frenk-White halo profile with inner slope $\gamma =1.26$.

Figure 3

Goodness-of-fit contour heat maps for production of the GC $\gamma$-ray excess by a hidden valley shower with gauge groups $\mathrm{SU}(N{)}_{\text{V}}$, $N=2$ (left) and $N=3,4$ (right solid and dashed respectively). The cross section is allowed to float at each point. $N=2$ annihilation cross section contours are also shown for $⟨\sigma v⟩=6,8,10\times 10^{-27}{\mathrm{cm}}^{-3}{\mathrm{s}}^{-1}$ (gray dashed; left to right respectively). The best fit occurs at $N=2$, $m_{\chi }\simeq 10\mathrm{GeV}$, and ${\mathrm{\Lambda }}_v\simeq 1\mathrm{GeV}$, with a cross section $⟨\sigma v⟩=7.8\times 10^{-27}{\mathrm{cm}}^{-3}{\mathrm{s}}^{-1}$.

Figure 4

Speculative spectrum of DM, $v$ leptons, light (meta)stable and heavy bound states. Certain dominant decay modes are shown with grey arrows.

Figure 5

ALP beam-dump [63] (green), SN1987 A [64] (orange, dashed line) and BBN (blue) bounds for the ${\eta }_v^{\prime }$, with SN temperature $T\simeq 30\mathrm{MeV}$. Also shown is the displaced vertex region $L_{\mathrm{dec}}\lesssim 1\mathrm{m}$ for photon energies $\sim 10\mathrm{GeV}$ (dotted line), and benchmark values (5) for the $\{m_{{\eta }_v^{\prime }},ϵ,N\}$ parameter space (red diamond).