Constraints on Light WIMP candidates from the Isotropic Diffuse Gamma-Ray Emission

TL;DR

This paper assesses how the Fermi-LAT isotropic gamma-ray background constrains light WIMPs in the few-GeV range, focusing on two minimal models: a Higgs-portal scalar singlet and a Z′-portal Dirac fermion. By computing the extragalactic DM annihilation flux and exploring structure-formation boosts, optical depth, and gamma-ray spectra, the authors derive 95% CL limits that depend sensitively on halo properties such as M_min and c_vir. They find that the scalar Higgs-portal is strongly constrained by IGRB data under some halo assumptions, potentially excluding the WMAP-favored region, while the Z′-portal fermion faces weaker constraints. The work highlights significant astrophysical uncertainties in indirect detection and emphasizes the complementarity of gamma-ray constraints with direct-detection hints for light DM.

Abstract

Motivated by the measurements reported by direct detection experiments, most notably DAMA, CDMS-II, CoGeNT and Xenon10/100, we study further the constraints that might be set on some light dark matter candidates, M_DM ~ few GeV, using the Fermi-LAT data on the isotropic gamma-ray diffuse emission. In particular, we consider a Dirac fermion singlet interacting through a new Z' gauge boson, and a scalar singlet S interacting through the Higgs portal. Both candidates are WIMP (Weakly Interacting Massive Particles), i.e. they have an annihilation cross-section in the pbarn range. Also they may both have a spin-independent elastic cross section on nucleons in the range required by direct detection experiments. Although being generic WIMP candidates, because they have different interactions with Standard Model particles, their phenomenology regarding the isotropic diffuse gamma-ray emission is quite distinct. In the case of the scalar singlet, the one-to-one correspondence between its annihilation cross-section and its spin-independent elastic scattering cross-section permits to express the constraints from the Fermi-LAT data in the direct detection exclusion plot, sigma_n^0--M_DM. Depending on the astrophysics, we argue that it is possible to exclude the singlet scalar dark matter candidate at 95 % CL. The constraints on the Dirac singlet interacting through a Z' are comparatively weaker.

Figures (9)

• Figure 1: Contour plot of the optical depth with $\tau(E',z,z')=1$, where $E'/(1+z')=E/(1+z)$. Here $E$ is the energy observed at redshift $z$, so that $E^\prime$ is the emission energy at redshift $z^\prime$. The different lines correspond to the different observers. For each, they give the maximal possible redshift at emission: $z= 0$ dotted line, $z=10$ dashed curve, $z=100$ long dashed line and $z=1000$ solid line. The red (gray) region denotes the Fermi-LAT measured energy range. The triangular region gives the relevant (for observations today) redshifts for a DM candidate of mass $M_{DM} = 20$ GeV.
• Figure 2: Example of $E^2 dN_{\gamma}/dE$ gamma-ray spectra from the annihilation of a 10 GeV dark matter particle, as a function of the photon energy $E$. The solid blue line represents a pure annihilation into $b\bar{b}$ ($BR_{b\bar{b}}=100\%$), the green dashed curve is for a pure annihilation into $\tau^+\tau^-$ ($BR_{\tau^+\tau^-}=100\%$), the red long dashed to a "Higgs-like" annihilation ($BR_{\tau^+\tau^-}\sim 9\%$ and $BR_{b\bar{b}} \sim 83\%$), while the black dot-dashed line refers to a"Z-like" annihilation ($BR_{l^\pm}\sim 3\%$ and $BR_{q\bar{q}}\sim 13\%$).
• Figure 3: On the left, boost factor $1+{\rm B}(z)$ as a function of the redshift $z$. On the right, zoom at small redshift values of the total enhancement factor $\mathcal{B}^2(z)/h(z)(1+z)^3$. A NFW density profile for the dark matter is assumed. The thin green lines are given for the $C_{\rm WMAP}$ concentration parameter Maccio:2008xb, while the thick black lines are for the $C_{\rm PL}$ concentration parameter Bullock:1999he. The long dashed lines refer to $M_{\rm min}= 10^{-8} M_{\odot}$, the solid to $M_{\rm min}= 10^{-6} M_{\odot}$ and the short dashed to $M_{\rm min}= 10^{-4} M_{\odot}$.
• Figure 4: Diffuse photon emission from a 10 GeV dark matter particle for $BR_{b\bar{b}}=100\%$ on the left, and with annihilation into $BR_{\tau^+\tau^-}=100\%$ on the right. For the $b\bar{b}$ channel ${<\sigma v>_{ann}}= 2.6 \ 10^{-26} \rm cm^{3} \rm s^{-1}$, while for the $\tau^+\tau^-$ case the thermal cross-section is fixed to $3.3 \ 10^{-26} {\rm cm^{3} s^{-1}}$. The color code is as in Fig. \ref{['fig:boost']}. The red points (below) are the measurements of the diffuse emission by Fermi-LAT Abdo:2010nz and the blue points (above) are those by EGRET Sreekumar:1997unStrong:2004ry.
• Figure 5: Fermi-LAT upper bounds in the plane ${<\sigma v>_{ann}}$ versus the DM mass $M_{DM}$ at $95\%$ C.L. for two different annihilation channels: on the left $BR_{b\bar{b}}=100\%$ and on the right $BR_{\tau^+\tau^-}=100\%$. The horizontal red (gray) region corresponds to the WMAP-7yrs $5-\sigma$ for the thermal annihilation cross-section. The vertical dotted line indicates the energy threshold for $b, \bar{b}$ and $\tau^{\pm}$ production, on the left and right respectively. The color code is as in Fig. \ref{['fig:boost']}.