Study of sub-GeV Dipolar Dark States at SND@LHC within Invisible Bounds on Meson Decays

TL;DR

This study investigates sub-GeV dark states interacting with the Standard Model through electromagnetic dipole operators at SND@LHC. The authors implement a Dirac singlet χ with magnetic and electric dipole moments, analyze production via Drell-Yan and meson decays, and evaluate scattering signals in the SND@LHC target for χ–N and χ–e interactions, while estimating neutrino backgrounds. They compute production spectra using MadGraph, FORESEE, and event generators, and they propagate the flux to derive sensitivity curves, comparing them with direct-detection, beam-dump, fixed-target, and collider constraints, and checking EFT validity. The results show that SND@LHC provides complementary reach to existing searches, particularly in the sub-GeV to few-GeV regime, and HL-LHC data could probe new parameter space accessible by widening the target or scattering length, subject to practical limitations of the TI18 tunnel.

Abstract

Electromagnetic form factors constitute a natural portal for accessing states beyond the Standard Model. In particular, dimension-5 magnetic and electric dipole moment operators offer a minimal and predictive framework for Feebly Interacting Particles (FIPs). In this work, we perform a study of the sensitivity reach of the Scattering and Neutrino Detector (SND@LHC) in the detection of dipolar dark states through photon-mediated interactions with the Standard Model particles. The far-forward region of the LHC provides FIPs with large momenta that scatter off electrons and nuclei inside the target. Production of dark states from meson decays is constrained by invisible decay widths, while the Drell-Yan process offers a production channel in the GeV range. We present sensitivity plots for magnetic and electric dipole moment interactions at SND@LHC and compare them with constraints from direct detection, beam dump, fixed-target, and collider experiments. The validity of the effective theory that describes the dipole model is also studied by considering conservative bounds on the couplings.

Figures (14)

• Figure 1: Left: Schematic layout of the SND@LHC from a top view. Right: Frontal view of the target and HCAL with the collision axis.
• Figure 2: Left: Drell-Yan production cross-section for coupling set to 1 GeV$^{-1}$. Right: Incident $\chi$ and $\bar{\chi}$ at SND@LHC target for coupling set to 1 GeV$^{-1}$. The count has been normalized to both the Run-3 (250 fb$^{-1}$) and Run-4 (3000 fb$^{-1}$) data accumulation.
• Figure 3: Left:$\eta'$ spectrum using EPOS LHC generator. Right:$J/\psi$ spectrum using Pythia 8 generator. The range of polar angles corresponding to SND@LHC geometric acceptance has been shown with two black dotted lines. The color map represents the cross-section in pb per bin on a logarithmic scale.
• Figure 4: We present here the $\chi\bar{\chi}$ pair production cross-section both in the half hemisphere and the annular region described by the pseudorapidity coverage of SND@LHC. Top Left and Top Right: MDM cross-section in forward hemisphere and within 7.2 < $\eta$ < 8.4. Bottom Left and Bottom Right: EDM cross-section in forward hemisphere and within 7.2 < $\eta$ < 8.4.
• Figure 5: Spectrum for $m_\chi$ = 0.1 GeV with coupling set to 1 GeV$^{-1}$. Left: MDM; Right: EDM. The range of polar angles corresponding to SND@LHC geometric acceptance has been shown with two black dotted lines. The color map represents the cross-section in pb per bin on a logarithmic scale.