Constraining the monochromatic gamma-rays from dark matter annihilation by the LHC

The installation of forward detectors in CMS and ATLAS turn the LHC into an effective photon-photon collider. The elastic scattering of the beam protons via the emission of photons, which can be identified by tagging the intact protons in the forward detectors, provides a powerful diagnostic of the central production of new particles through photon-photon annihilation. In this paper we study the central production of dark matter particles and the potential of the LHC to constrain the cross section of this process. By virtue of the crossing symmetry, this limit can immediately be used to constrain the production of monochromatic gamma rays in dark matter annihilation, a smoking gun signal under investigation in indirect dark matter searches. We show that with the integrated luminosity $\mathcal{L}=30{\mathrm{fb}}^{-1}$ in the LHC at center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$, for dark matter masses $\sim (50–600)\mathrm{GeV}$, a model-independent constraint on the cross section of dark matter annihilation to monochromatic gamma rays at the same order of magnitude as the current Fermi-LAT and the future limits from CTA can be obtained.

Figure 1

(a) The spectrum of photons (log-log plot) as a function of $x=E_{\gamma }/E_p$. (b) Photon-photon luminosity (log-linear plot) as a function of center of mass energy $W_{\gamma \gamma }$. The color codes are explained in the main text.

Figure 2

The Feynman diagram for $pp\to p+\gamma \gamma +p\to p+\chi \overline{\chi }+p$ at the LHC.

Figure 3

The distribution of the $p_{\mathrm{T}}$ of protons after bremsstrahlung emission, for a proton beam with $\sqrt{s}=13\mathrm{TeV}$.

Figure 4

The expected upper limit on the cross section of $pp\to p+\gamma \gamma +p\to pp+\cancel{E}$, at 95% C.L., in terms of the two-photon invariant mass. The solid and dashed curves correspond to the LHC luminosity $\mathcal{L}=30{\mathrm{fb}}^{-1}$ and $\mathcal{L}=100{\mathrm{fb}}^{-1}$, respectively. The red, blue, and green curves show the expected sensitivity for no cut on $p_{\mathrm{T}}$, $p_{\mathrm{T}}^{\mathrm{cut}}=0.2\mathrm{GeV}$, and $p_{\mathrm{T}}^{\mathrm{cut}}=0.4\mathrm{GeV}$, respectively.

Figure 5

The sensitivity of the LHC, at 95% C.L. and $\mathcal{L}=30{\mathrm{fb}}^{-1}$, to the cross section of dark matter annihilation to monochromatic gamma rays, $⟨\sigma v{⟩}_{\gamma \gamma }\equiv ⟨\sigma (\chi \overline{\chi }\to \gamma \gamma )v⟩$, for dark matter masses $m_{\mathrm{DM}}\sim (50–600)\mathrm{GeV}$ and for three different assumptions on $p_{\mathrm{T}}^{\mathrm{cut}}$ (we have assumed $v\simeq {10}^{-3}c$ in the calculation of LHC sensitivity). The red curve shows the sensitivity of the LHC without applying any cut on the transverse momentum of forward protons, while the blue and green curves show the sensitivity by applying the cuts $p_{\mathrm{T}}^{\mathrm{cut}}=0.2\mathrm{GeV}$ and $p_{\mathrm{T}}^{\mathrm{cut}}=0.4\mathrm{GeV}$, respectively. The black and brown curves show the upper limits on $⟨\sigma v{⟩}_{\gamma \gamma }$ from the Fermi-LAT experiment assuming cuspy NFW and an isothermal dark matter profile, respectively [65]. The orange and dark-red curves show the sensitivity of CTA, respectively, for nominal [68] and updated [69] performances (see [70]). The dashed gray line shows the expected $⟨\sigma v{⟩}_{\gamma \gamma }$ for a thermal dark matter scenario with $⟨\sigma v{⟩}_{\gamma \gamma }/⟨\sigma v⟩={10}^{-3}$.

Figure 6

The same as Fig. 5 but for $\mathcal{L}=100{\mathrm{fb}}^{-1}$.