Illuminating Scalar Dark Matter Co-Scattering in EFT with Monophoton Signatures

TL;DR

This work investigates dark matter production in an EFT extended by two $Z_2$-odd singlet fermions $N_{1,2}$ and a real scalar $χ$, with $χ$ as the DM candidate. DM relic density is set by dimension-5 operators, with a nearly degenerate dark sector enabling co-scattering or co-annihilation; the authors solve coupled Boltzmann equations and study two benchmarks—one dominated by co-annihilation and the other by co-scattering. Collider constraints arise from ATLAS monophoton searches, focusing on the dipole-induced production $pp\to N_1N_2$ and the decay $N_2\to N_1γ$, leading to limits on the off-diagonal dipole coupling $c_3'$, and HL-LHC projections are explored. A broad parameter scan reveals viable regions with $M_{N_1} \lesssim 600$ GeV and $M_{N_2} \lesssim 1$ TeV, and demonstrates that HL-LHC can probe portions of the co-scattering-dominated landscape, highlighting the testability of this non-standard DM production mechanism at future colliders.

Abstract

We investigate the co-scattering mechanism for dark matter production in an EFT framework which contains new $Z_2$-odd singlets, namely two fermions $N_{1,2}$ and a real scalar $χ$. The singlet scalar $χ$ is the dark matter candidate. The dimension-5 operators play a vital role to set the observed DM relic density. We focus on a nearly degenerate mass spectrum for the $Z_2$ odd particles to allow for a significant contribution from the co-scattering or co-annihilation mechanisms. We present two benchmark points where either of the two mechanisms primarily set the DM relic abundance. The main constraint on the model at the LHC arise from the ATLAS mono-$γ$ search. We obtain the parameter space allowed by the observed relic density and the mono-$γ$ search after performing a scan over the key parameters, the masses $M_{N_{1,2}}, M_χ$ and couplings $c_3^\prime, y^\prime_{11,22}$. We find the region of parameter space where the relic abundance is set primarily by the co-scattering mechanism while being allowed by the LHC search. We also determine how the model can be further probed at the HL-LHC via the mono-$γ$ signature.

Figures (13)

• Figure 1: Schematic diagrams representing different WIMP $\chi$ dilution mechanisms in the early Universe for our model.
• Figure 2: Feynman diagrams for the conversion processes that depend on the Yukawa coupling $Y^\prime$. The inverse decay shown in the bottom line usually dominates.
• Figure 3: Feynman diagrams for the pair annihilation and co-annihilation process of $N_{1,2}$ that depend on the coupling $c_3^\prime$. Note that due to the choice $(c^{\prime}_{3})_{ij}=\epsilon_{ij}(c^{\prime}_{3})_{ij}$, we have both co-annihilation and annihilation processes between $N_{1}$ and $N_{2}$.
• Figure 4: Evolution of the $\chi$ and $N(=N_{1}+N_{2})$ abundances in the early universe for the benchmark point (a) BP1 and (b) BP2. The observed DM relic density ($\Omega_{\chi}h^{2} \simeq0.12$) is satisfied for both benchmark points.
• Figure 5: The ratio of interaction rates to the Hubble rate $\Gamma/H$ for various processes as a function of $x=M_{\chi}/T$ for the benchmark point (a) BP1 and (b) BP2. The horizontal light blue line depicts $\Gamma/H$=1.