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.
Highlights
In the past years, the LHC has collected an impressive amount of data and placed constraints on a multitude of models for new physics
Up to now, there has been no discovery of any elementary beyond the standard model (BSM) particle, which has motivated the community to consider a broader range of different physics signatures
The expected signal from sterile neutrinos decaying in Forward Search Experiment (FASER) consists of two simultaneous high-energy charged tracks, which originate from a vertex inside the detector and the combined momentum of which points into the direction of the interaction point (IP)
Summary
The LHC has collected an impressive amount of data and placed constraints on a multitude of models for new physics. A more detailed study of the LHC infrastructure around the ATLAS IP revealed an attractive location for FASER in the currently unused side tunnel TI18 in the distance about 480 m away from the IP. It is a former service tunnel used by large electron-positron (LEP) collider as a connection between the super proton synchrotron and the main tunnel. The expected signal of new physics in this case can be enhanced with respect to the far location in models in which the lifetime of the LLPs is too small to reach the far location This is, e.g., true for a dark photon search if its kinetic mixing with the SM photon is not suppressed too much as discussed in Ref. A more detailed discussion of fragmentation functions for D and B mesons can be found in Appendix
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