Heavy Neutral Leptons at FASER

TL;DR

FASER has strong potential to detect heavy neutral leptons with very small mixing angles, especially from B-meson decays, complementing other LHC searches and contributing to understanding dark matter and baryogenesis.

Contribution

This paper demonstrates FASER's sensitivity to heavy neutral leptons with lower mixing angles than current bounds, highlighting its role in new physics searches.

Findings

FASER can probe heavy neutral leptons with mixing angles an order of magnitude lower than current bounds.

FASER's sensitivity is comparable to SHiP for B-meson produced heavy neutral leptons.

Searches at FASER complement high-$p_T$ LHC experiments and inform dark matter and baryogenesis studies.

Abstract

We study the prospects for discovering heavy neutral leptons at ForwArd Search ExpeRiment (FASER), the newly proposed detector at the LHC. Previous studies showed that a relatively small detector with ~10 m length and ~1 m cross sectional area can probe large unconstrained parts of parameter space for dark photons and dark Higgs bosons. In this work we show that FASER will also be sensitive to heavy neutral leptons that have mixing angles with the active neutrinos that are up to an order of magnitude lower than current bounds. In particular, this is true for heavy neutral leptons produced dominantly in $B$-meson decays, in which case FASER's discovery potential is comparable to the proposed SHiP detector. We also illustrate how the search for heavy neutral leptons at FASER will be complementary to ongoing searches in high-$p_T$ experiments at the LHC and can shed light on the nature of dark matter and the process of baryogenesis in the early Universe.