Enhancing Sensitivities to Long-lived Particles with High Granularity Calorimeters at the LHC

TL;DR

This paper proposes a novel search strategy for long-lived particles at the LHC using the high granularity of the upgraded CMS HGCAL to identify signals and suppress backgrounds, significantly improving sensitivity over previous methods.

Contribution

It introduces a new LLP search method leveraging HGCAL's shower track imaging, enhancing detection sensitivity for Higgs decays into LLP pairs across various lifetimes.

Findings

Projected sensitivity for Higgs decay BR$(h o XX)$ down to 10^{-4}–10^{-6}

HGCAL enables identification of shower tracks to suppress backgrounds

Enhanced coverage of LLP parameter space compared to previous searches

Abstract

The search for long-lived particles (LLP) is an exciting physics opportunity in the upcoming runs of the Large Hadron Collider. In this paper, we focus on a new search strategy of using the High Granularity Calorimeter (HGCAL), part of the upgrade of the CMS detector, in such searches. In particular, we demonstrate that the high granularity of the calorimeter with this upgrade, which allows us to see "shower tracks" in the calorimeter, can play a crucial role in identifying the signal and suppressing the background. We study the potential reach of the HGCAL using a signal model in which the Standard Model Higgs boson decays into a pair of LLPs, $h \to XX$. After carefully estimating the Standard Model QCD and the misreconstructed fake-track backgrounds, we give the projected reach for both a more conservative vector boson fusion trigger and a novel displaced-track-based trigger. Our results show that the best reach for the Higgs decay branching ratio, BR$(h \to XX)$, in the vector boson fusion channel is about $\mathcal{O}(10^{-4})$ with lifetime $c\tau_X \sim 0.1$--$1$ meters, while for the gluon gluon fusion channel it is about $\mathcal{O}(10^{-5}\text{--}10^{-6})$ for similar lifetimes. Alternatively, for an LLP with $c\tau_X \sim 10^3$ meters, the HGCAL based search should be able to probe BR$(h\to XX)$ down to a few $\times 10^{-4}$($10^{-2}$) in the gluon gluon fusion (vector boson fusion) channels, respectively. In comparison with these previous searches, our new search shows enhanced sensitivity in complementary regions of the LLP parameter space.