Abstract
At the Large Hadron Collider (LHC), several far detectors such as FASER and MATHUSLA have been proposed to target the long-lived particles (LLPs) featured with displaced vertices. Naturally one question arises as to the feasibility of installing similar far detectors at future lepton colliders like the CEPC and FCC-ee. Because of the different kinematics of final state particles and the freedom to locate both the experiment hall and the detectors, the future lepton collider with an additional far detector may play a unique role in searching for LLPs. In this study, we consider various locations and designs of far detectors at future $e^- e^+$ colliders and investigate their potentials for discovering LLPs in the physics scenarios including exotic Higgs decays, heavy neutral leptons, and the lightest neutralinos. Our analyses show that the kinematical distinctions between the lepton and hadron colliders render the optimal positions of far detectors lying at the direction perpendicular to the collider beams at future $e^- e^+$ colliders, in contrast to the LHC where a boost in the forward direction can be exploited. We also find that when searching for LLPs, such new experiments with far detectors at future lepton colliders may extend and complement the sensitivity reaches of the experiments at the future lepton colliders with usual near detectors, and the present and future experiments at the LHC. In particular, we find that, for the theory models considered in this study, a MATHUSLA-sized far detector would give a modest improvement compared to the case with a near detector only at future lepton colliders.
Highlights
In many theories beyond the Standard Model (BSM), new particles are predicted to have a relatively long lifetime; see for instance Refs. [1,2,3,4,5] for reviews of different models of long-lived particles (LLPs)
Our analyses show that the kinematical distinctions between the lepton and hadron colliders render the optimal positions of far detectors lying at the direction perpendicular to the collider beams at future e−eþ colliders, in contrast to the Large Hadron Collider (LHC) where a boost in the forward direction can be exploited
We focus on the heavy neutral leptons (HNLs) produced from Z–decays with paffiffi general e−eþ collider running at the Z–pole, and provide sensitivity predictions for both the far detectors FD1–FD8 and near detectors at the Circular Electron Positron Collider (CEPC) and
Summary
In many theories beyond the Standard Model (BSM), new particles are predicted to have a relatively long lifetime; see for instance Refs. [1,2,3,4,5] for reviews of different models of long-lived particles (LLPs). New particles become long-lived for various reasons including feeble couplings with the standard model (SM) particles, phase space suppression, and heavy mediators Such LLPs, after being produced, travel a macroscopic distance before decaying into other SM and/or new particles. Current LHC experiments with a detector located at the interaction point (which we call “near detector” or abbreviate as “ND” in this article) have sensitivities to LLPs in some parameter space [6,7,8,9], a class of new experiments at the LHC with an additional detector located far from the interaction point (which we call “far detector” or abbreviate as “FD” in this article) have been proposed and shown to have potential sensitivity reaches beyond those of the current LHC experiments in a variety of BSM models These proposed experiments include MATHUSLA [4,10], CODEX-b [11], FASER [12] and AL3X [13], which suggest to install an additional detector at a position Oð5–500Þ m away from different interaction points (IPs) of the LHC.
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