Long-lived particle reconstruction downstream of the LHCb magnet

Charged-particle trajectories are usually reconstructed with the LHCb detector using combined information from the tracking devices placed upstream and downstream of the 4 Tm dipole magnet. Trajectories reconstructed using only information from the tracker downstream of the dipole magnet, which are referred to as T tracks, have not been used for physics analysis to date due to their limited momentum resolution. The challenges of the reconstruction of long-lived particles using T tracks for use in physics analyses are discussed and a solution is proposed. The feasibility of the experimental technique is demonstrated by reconstructing samples of long-lived $\it \Lambda$ and $K_S^0$ hadrons decaying between 6.0 and 7.6 metres downstream of the proton-proton collision point. The long-lived hadrons are selected using a data sample recorded between 2015 and 2018, corresponding to an integrated luminosity of about 6 fb$^{-1}$. These results open an opportunity to further extend the physics reach of the LHCb experiment.