Gamma Ray Line Constraints on Effective Theories of Dark Matter

TL;DR

This work addresses how gamma-ray line observations constrain effective theories of dark matter interactions with the Standard Model. It develops an EFT with SM plus a SM-singlet WIMP (allowed spins: real/complex scalar, Majorana/Dirac fermion) and a comprehensive set of higher-dimensional operators coupling to quarks and gluons. By computing loop-induced annihilation rates into gamma gamma and gamma Z and comparing to Fermi LAT line-search data, it maps observational limits onto bounds on the operator suppression scales. The results show that gamma-ray line searches provide complementary and sometimes stronger constraints than collider or direct-detection bounds, underscoring the value of indirect searches within an EFT framework for probing WIMP properties across operator classes and masses.

Abstract

A monochromatic gamma ray line results when dark matter particles in the galactic halo annihilate to produce a two body final state which includes a photon. Such a signal is very distinctive from astrophysical backgrounds, and thus represents an incisive probe of theories of dark matter. We compare the recent null results of searches for gamma ray lines in the galactic center and other regions of the sky with the predictions of effective theories describing the interactions of dark matter particles with the Standard Model. We find that the null results of these searches provide constraints on the nature of dark matter interactions with ordinary matter which are complementary to constraints from other observables, and stronger than collider constraints in some cases.

Figures (9)

• Figure 1: Representative Feynman diagram for the loop level annihilation of two DM particles $\chi$ to a photon and a second vector boson, either another photon or a $Z$ boson, through an operator coupling the DM to SM quarks (represented as the shaded circle).
• Figure 2: Graph of the parameter space for spin-independent direct detection of WIMPs, in terms of the WIMP mass and cross section for scattering on a nucleon, including current bounds from direct detection, Tevatron data, and the Fermi line search (solid lines, as labelled), and projected lines for direct detection experiments and the LHC (dashed lines, as labelled). The bounds on complex scalars are shown. The bounds for the operator C3 are above the axes displayed here. Fermi bounds on real scalars are obtained by improving the corresponding bounds for the complex scalars by a factor of 2. Fermi bounds on fermion operators are weak and outside the scale of this plot.
• Figure 3: Graph of the parameter space for spin-dependent direct detection of WIMPs, in terms of the WIMP mass and cross section for scattering on a proton or neutron, including current bounds from direct detection, Tevatron data, and the Fermi line search (solid lines, as labelled), and projected lines for direct detection experiments and the LHC (dashed lines, as labelled). Fermi bounds on Majorana fermion with M5 interaction are obtained by improving the D8 bounds for the Dirac fermion by a factor of 2.
• Figure 4: On the left, a graph of the parameter space for indirect detection of gamma ray lines for a Majorana WIMP interacting primarily through operator M6, in terms of the WIMP mass and cross section annihilation into $\gamma \gamma$. Shown are the Fermi line search bounds, bounds from Tevatron data, and the direct detection experiments (solid lines, as labelled). On the right, a comparison between the Fermi line search bounds and the MiDM Chang:2010en model parameter space. The shaded regions correspond to the $90\%$ ($99\%$) fits of the MiDM model to explain the DAMA signal .
• Figure 5: The lower limits from current Fermi line searches Abdo:2010nc (long dashed lines with data points shown) and an estimate of the reach of future searches at lower energies (short dashed curve) on the suppression scale of new physics $M_*$ leading to interactions with the SM for the Dirac WIMP operators D1-D4. Note that the constraints on D1 and D3 are significantly weaker than the others, because these operators lead to cross sections which are velocity-suppressed. For comparison we also show the current bounds from Tevatron and future reach of LHC Goodman:2010yf (solid and short dashed curves, respectively), as well as the value of $M_*$ leading to the correct thermal relic abundance in the absence of other interactions (dash-dotted curves).