Discovering dark matter at the LHC through its nuclear scattering in far-forward emulsion and liquid argon detectors

The LHC may produce light, weakly interacting particles that decay to dark matter, creating an intense and highly collimated beam of dark matter particles in the far-forward direction. We investigate the prospects for detecting this dark matter in two far-forward detectors proposed for a future forward physics facility: $\mathrm{FASER}\nu 2$, a 10-ton emulsion detector, and FLArE, a 10- to 100-ton LArTPC. We focus here on nuclear scattering, including elastic scattering, resonant pion production, and deep inelastic scattering, and devise cuts that efficiently remove the neutrino-induced background. In the invisibly decaying dark photon scenario, DM-nuclear scattering probes new parameter space for dark matter masses $5\mathrm{MeV}\lesssim m_{\chi }\lesssim 500\mathrm{MeV}$. When combined with the DM-electron scattering studied previously, $\mathrm{FASER}\nu 2$ and FLArE will be able to discover dark matter in a large swath of the cosmologically favored parameter space with $\mathrm{MeV}\lesssim m_{\chi }\lesssim \mathrm{GeV}$.

Figure 1

The number of neutrinos passing through the detector (top) and interacting in the detector (bottom), for $\mathrm{FASER}\nu 2$ (left), FLArE-10 (center), and FLArE-100 (right) during the HL-LHC era. The detector geometries and locations are described in the text. These results assume 14 TeV $pp$ collisions and an integrated luminosity of $\mathcal{L}=3{\mathrm{ab}}^{-1}$ and are estimated using sibyll 2.3d and the fast neutrino flux simulation introduced in Ref. [51].

Figure 2

Left: the signal-to-background ratio $S/B$ for the elastic scattering signature for FLArE-10 and the two DM benchmark scenarios indicated as a function of the maximum momentum of the outgoing proton $p_p^{\max }$. The expected number of neutrino-induced background events for selected values of $p_p^{\max }$ can be found in Table 2, and we assume the detectability threshold of $E_{k,p}>20\mathrm{MeV}$ for the proton kinetic energy. Right: the projected 90% C.L. exclusion bounds for the elastic scattering signature for $\mathrm{FASER}\nu 2$ with $300\mathrm{MeV}\lesssim p_p\lesssim 1\mathrm{GeV}$ (green), FLArE-10 (red), and FLArE-100 (blue) with the proton energy and momentum cuts indicated. Current bounds exclude the gray-shaded region; see Sec. 7 for details. The thermal relic targets for the Majorana fermion DM and complex scalar DM models are also shown.

Figure 3

Left: the event distribution as a function of the pion energy for $\chi 1{\pi }^0$ signal events and neutrino-induced backgrounds in the liquid argon detectors. The DM results are shown for two benchmark masses $m_{\chi }=m_{A^{\prime }}/3=10\mathrm{MeV}$ (blue) and 100 MeV (yellow) for the complex scalar DM model. They have been obtained with the bdnmc code [36] that takes into account the dominant pion production via production of the $\mathrm{\Delta }$ resonance. We also show the relevant results for neutrino-induced backgrounds from NCRES and NCEL events (brown histogram). This was obtained using the far-forward LHC neutrino energy spectrum and full genie [63, 64] simulations with further resonances and final-state interactions of hadrons taken into account. Right: the colorful solid lines correspond to the projected 90% C.L. exclusion bounds in the DM-nuclei scattering $\chi 1{\pi }^0$ signature for $\mathrm{FASER}\nu 2$ (green), FLArE-10 (red), and FLArE-100 (blue). Current bounds and thermal relic targets are as in Fig. 2.

Figure 4

Expected number of DIS events in the $(E_{\mathrm{had}},p_{T,\mathrm{had}})$ plane for one benchmark Majorana DM scenario (left) and SM NC neutrino background (right) at FLArE-10. Most of the signal events are at low $E_{\mathrm{had}}$ and low $p_{T,\mathrm{had}}$, motivating our choice of cuts. The dashed (solid) box shows the strong (loose) cuts of $1\mathrm{GeV}<E_{\mathrm{had}}<15$ (30) GeV and $1\mathrm{GeV}<p_{T,\mathrm{had}}<1.5$ (2.0) GeV used in our analysis.

Figure 5

The projected 90% C.L. exclusion bounds for the DIS signature in the Majorana fermion DM model at various detectors. For FLArE-10 we show the limits with and without the kinematic cuts, whereas for $\mathrm{FASER}\nu 2$ and FLArE-100 we show only the best limits corresponding to the strong cuts. The thermal relic targets for Majorana fermion DM (black solid) and complex scalar DM (black dashed), and current bounds (gray shaded region) are also shown.

Figure 6

The projected 90% C.L. exclusion bounds for Majorana fermion DM from DM-nucleus elastic scattering, resonant pion production, and DIS (this work), along with DM-electron scattering from Ref. [23] at FLArE-10. In the gray shaded region, we also show the strongest existing constraints from BABAR, NA64, $\mathrm{NO}\nu \mathrm{A}$, E137, and BEBC, as implemented in Refs. [38, 81]. Projected reaches from other experiments are shown in brown for beam dump/collider experiments and in red for missing momentum-type searches. The green contour shows the projected bound on Majorana fermion DM from SuperCDMS; see text for more details.

Figure 7

The projected 90% C.L. exclusion bounds combining all channels for the $\mathrm{FASER}\nu 2$, FLArE-10, and FLArE-100 detectors at the HL-LHC with $3{\mathrm{ab}}^{-1}$ of integrated luminosity. At lower DM mass the DM-electron signature is the best, whereas, at higher masses, DIS provides the most stringent limits. Existing constraints and projected reaches from other experiments are as in Fig. 6.

Figure 8

The projected 90% C.L. exclusion bounds for the FLArE-10 detector combining all channels for the three integrated luminosities indicated. New parameter space will start to be probed even for an integrated luminosity of order $30{\mathrm{fb}}^{-1}$. Existing constraints and projected reaches from other experiments are as in Fig. 6.