GeV-scale dark matter: production at the Main Injector

TL;DR

Dobrescu and Frugiuele propose a GeV-scale dark matter scenario with a GeV-scale leptophobic Z' mediator produced in fixed-target collisions at Fermilab. They quantify DM production via $pN\to Z'\to χ\bar{χ}$, map the resulting DM flux to neutrino near detectors (NOνA, MINOS) while accounting for existing constraints from monojet, invisible quarkonium decays, and BaBar monophoton searches, and explore how to distinguish a DM beam from the neutrino background using angular and energy information as well as hadronic-energy cuts. Their results indicate viable regions in $(M_{Z'}, g_z)$, with potential DM-induced neutral-current event rates up to $\mathcal{O}(10^3-10^4)$ for $N_{\rm POT}=10^{21}$, suggesting that current detectors could probe this light DM portal and that LBNF near-detectors would improve sensitivity. The study emphasizes fixed-target high-intensity experiments as a complementary path to discover or constrain light DM coupled to quarks, and it discusses strategies to infer DM spin and to mitigate backgrounds.

Abstract

Assuming that dark matter particles interact with quarks via a GeV-scale mediator, we study dark matter production in fixed target collisions. The ensuing signal in a neutrino near detector consists of neutral-current events with an energy distribution peaked at higher values than the neutrino background. We find that for a $Z'$ boson of mass around a few GeV that decays to dark matter particles, the dark matter beam produced by the Main Injector at Fermilab allows the exploration of a range of values for the gauge coupling that currently satisfy all experimental constraints. The NO$ν$A detector is well positioned for probing the presence of a dark matter beam, while future LBNF near-detectors would provide more sensitive probes.

Figures (6)

• Figure 1: Constraints on the $U(1)_B$ (left panel) and $U(1)_{ds}$ (right panel) models from monojet collider searches (upper right-hand region), collider bounds on new fermions required to cancel gauge anomalies (upper left-hand corner), and quarkonium decays (regions labelled by $J/\psi$ and $\Upsilon$). The ragged (gray) region in the center of the left panel is due to fluctuations in the BaBar monophoton search.
• Figure 2: CMB constraints for fermonic DM $\psi_{\chi}$ in the $U(1)_B$ (left panel) and $U(1)_{ds}$ (right panel) models. The region above the solid (blue) line is viable if $\psi_{\chi}$ is asymmetric DM or a subdominant DM component. The region below the dashed (red) line is also CMB safe since the $s$-wave annihilation into quarks is small; the correct relic abundance is obtained for example via $p$-wave annihilation into new light scalars. The shaded region requires a more complex hidden sector (asymmetric DM with the symmetric component depleted by annihilation into new states).
• Figure 3: Number of $\chi$ or $\bar{\chi}$ DM particles produced for $N_{\rm POT} = 10^{21}$ protons of 120 GeV scattering off a fixed target which is thick enough to stop all incoming protons. The two lines are predicted in the $U(1)_B$ (solid blue line) and $U(1)_{ds}$ models (dashed red line, for $A_T = 2 Z_T$) with $g_z=0.1$. The $Z'\to \chi \bar{\chi}$ branching fraction used here is 100%, corresponding to $z_\chi \gg 1$; for smaller $z_\chi$ the branching fraction depends on $M_Z'$, $m_\chi$ and the $\chi$ spin (see Section 2).
• Figure 4: Polar angle distribution in the lab frame of the DM particles produced in the $pN \to Z' \to \chi\bar{\chi}$ process, for $M_{Z'} = 3$ GeV (dashed lines) or $M_{Z'} = 5$ GeV (solid lines), when $\chi$ is a Dirac fermion or a complex scalar.
• Figure 5: Energy distribution of fermonic DM particles produced in the absorber and passing through the NO$\nu$A or MINOS near detectors for $M_{Z'}= 3$ GeV.