Timing information at HL-LHC: complete determination of masses of dark matter and long lived particle

A long-standing kinematic challenge in data analysis at hadron colliders is the determination of the masses of invisible particles. This issue is particularly relevant in searches for evidence of dark matter production, which remains one of the prominent targets of future collider experiments. In this paper, we show that the additional information from the precision timing measurements, provided by planned detector upgrades during the high- luminosity run of the LHC (HL-LHC), allows for previously unrealizable measurements of invisible particle kinematics. As a concrete example, we focus on the signal of pair produced long-lived particles (LLP1,2), each decaying with a displaced vertex to visible (V1,2) and invisible (I1,2) final state particles, pp → LLP1 + LLP2→ (V1 + I1) + (V2 + I2). We explicitly show that the complete kinematics of the invisible particles in such events can be determined with the addition of timing information, and evaluate the precision with which the masses of new long-lived and invisible particles can be determined.