Searching for Beyond the Standard Model particles decaying to muon pairs with SND@LHC

Although the Standard Model is a very successful theory, it is not able to explain some of the outstanding problems of modern physics. For these problems, the extension of the SM using Beyond the Standard Model (BSM) physics is required.

The Scattering and Neutrino Detector at the LHC (SND@LHC) is a compact and stand-alone experiment designed for the observation of collider neutrinos. In this thesis, we conduct the first Feebly Interacting Particle (FIP) analysis study at SND@LHC. This analysis was signature driven, focusing on the experimental signature of the studied decay mode (decay to dimuon) more than the specific nature of the decaying particle.

For our benchmark particle, we chose the Dark Higgs, a scalar which interacts with SM particles through mixing with the SM-Higgs. This FIP was simulated using the Forward Experiment Sensitivity Estimator (FORESEE) to have a mass of 251.2MeV and decay vertex in the detector target. This simulation was then converted to an SNDSW-compatible format.

Through the use of both Monte-Carlo background samples and run data samples, several signal selection criteria were devised, culminating in a neutrino background of <0.2 events over 200and a background estimation in the signal region of 0.43 ± 0.43 over 200for the best detector conditions, corresponding to a 7 event background-only exclusion at 5 sigma. Simultaneously, this selection retains 12.78% of signal selection efficiency. Combining these values, we could expect background-only exclusion from, on average, 54 Dark Higgs decays on the target subsystem. Additionally, DH mixing angles above were excluded for our mass range.