Search for long-lived charged particles using the CMS detector in Run-2

TL;DR

This work targets Heavy Stable Charged Particles (HSCPs) predicted by beyond-Standard-Model theories by exploiting distinctive ionization signatures in the CMS silicon tracker. It introduces a data-driven background strategy that uses independent pixel and strip observables to predict backgrounds, complemented by a mass proxy estimator from a Bethe-Bloch–inspired harmonic mean. The analysis, based on Run-2 data from 2017–2018, sets competitive exclusion limits on ten HSCP scenarios, including gluino and stop R-hadrons, various stau and tau' configurations, and Z$'$-mediated production, with no significant excess observed. The results provide the strongest mass exclusions to date for several models and demonstrate a robust, signature-driven approach to HSCP searches with strong potential for future runs.

Abstract

Long-lived charged particles are predicted by various theories beyond the Standard Model, leading to unique signatures that could reveal new physics. At the LHC, the CMS detector enables searches for these massive particles, identifiable by their characteristic ionization patterns. Using data collected during 2017-2018, we search for signals of anomalous ionization in the silicon tracker. We present a novel approach to background prediction, utilizing the distinct ionization measurements of the silicon pixel and strip detectors as independent variables. We interpret the results within several models including those with staus, stops, gluinos, and multiply charged particles as well as a new model with decays from a Z' boson

Figures (12)

• Figure 1: Flow chart for possible new directions in searches.
• Figure 2: The $F_{\text{i}}^{\text{Pixels}}$ vs. $G_{\text{i}}^{\text{Strips}}$ distributions for the SM background (QCD, W+jet, $\mathrm{t}\bar{\mathrm{t}}$ ) simulation (left), and an 1800 GeV gluino R-hadron simulation (right), for events after pre-selection.
• Figure 3: The distribution of $I_h$ vs. HSCP candidate $p$. Different colors correspond to different signal simulations. Data is shown in a grey histogram, normalized to unit area. Based on Eq. \ref{['eq:MassFromHarmonicEstimator']} we show the distribution for a particle mass of 557 and 2000 GeV on the left, and 1400 and 3000 GeV on the right using the dashed lines.
• Figure 4: Event display of a boosted J/$\Psi$ particle decaying into two overlapping muons.
• Figure 5: Trigger efficiency as a function of generated $\beta$ for gluino (left) and stau (right) in bins of pseudorapidity.