The search for long-lived particles with the FASER experiment at the LHC

This thesis documents the results of analyses searching for long-lived particles (LLPs) using the FASER experiment at the Large Hadron Collider. The results of the search for dark photons, $A^{\prime}$, at FASER with coupling to SM particles with the signature $A'\rightarrow e^+e^-$ are presented. This analysis uses proton-proton collision data at a centre of mass energy of $\sqrt{s}=13.6$ TeV corresponding to an integrated luminosity of 27.0 fb$^{-1}$ collected by the FASER experiment in 2022. The search provides sensitivity to dark photons with couplings 4 $\times$ 10$^{-6} < \epsilon <$ 2 $\times$ 10$^{-4}$  and with masses  10 MeV $< m_{A'} <$ 80 MeV, resulting in world-leading exclusion limits for dark photon masses 17 MeV $<m_{A'} <$ 70 MeV and couplings 2 $\times$ 10$^{-5} < \epsilon <$ 1 $\times$ 10$^{-4}$. The results of this analysis are also reinterpreted for the $B-L$ gauge boson model. 

The results of the search for axion-like particles ALPs, $a$, with coupling to the $SU(2)_L$ gauge boson with the decay signature $a \rightarrow \gamma \gamma$ are also presented. This analysis uses proton-proton collision data at a centre of mass energy of $\sqrt{s}=13.6 TeV$ corresponding to an integrated luminosity of 57.7 fb$^{-1}$ collected by the FASER experiment in 2022 and 2023. This search provides sensitivity to ALPs with coupling $10^{-5}$ GeV$^{-1}< g_{aWW} < 10^{-3}$ GeV$^{-1}$ and masses 60 MeV $< m_a <$ 500 MeV, providing world-leading exclusion limits for ALP masses 100 $< m_{a} <$ 250 MeV, with coupling $3 \times 10^{-5}$ GeV$^{-1}< g_{aWW} < 5 \times 10^{-4}$ GeV$^{-1}$. 

Additionally, models where ALPs interact either exclusively with photons or with gluons are considered for interpretation using the selection outlined in the search for ALPs with coupling to the $SU(2)_L$ gauge boson. In the case of the ALP coupling to photons, ALP masses up to $m_a \sim$ 80 MeV are excluded and previously unexplored parameter space around $g_{a\gamma\gamma} \sim 10^{-4}$ GeV$^{-1}$ is probed. In the case of the ALP coupling to gluons, FASER probes unconstrained regions around the $\pi$ mass and $\eta$ meson mass where production rate is enhanced due to resonant mixing. The analysis is also reinterpreted for the $U(1)B$ gauge boson, the up-philic scalar, the Type-I two Higgs doublet model, and the dark photon.

The results of the 2021 calorimeter test beam are also discussed, as well as the planned preshower detector upgrade and the resulting impact on future ALP searches.