Closing in on singly charged scalars

TL;DR

This paper explores the current experimental limits and future detection prospects for a hypothetical singly charged scalar particle, focusing on collider signatures, decay modes, and improved search strategies at the LHC.

Contribution

It provides new insights into the allowed mass range of singly charged scalars and demonstrates the effectiveness of machine learning techniques in future collider searches.

Findings

SCS masses above ~185 GeV are still allowed at 95% CL.

A lower-mass window of ~80-125 GeV remains viable.

Boosted decision trees significantly enhance search sensitivity.

Abstract

We investigate current experimental constraints and future search prospects for a hypothetical spin-zero particle that carries unit electric charge: a singly charged scalar (SCS). In addition to providing useful benchmarks for collider searches, SCS particles are also well-motivated ingredients in relatively minimal dark sectors. We focus on scenarios in which the SCS decays promptly at colliders to a lepton plus either a neutrino or an invisible dark-sector particle of negligible mass. A promptly decaying SCS can easily have appreciable branching ratios to more than one lepton flavor while remaining consistent with constraints on lepton flavor violation. This broadens the allowed range of SCS masses to extend well beyond those for right-handed selectrons, smuons, or staus. For particular benchmark SCS branching ratios, we find that SCS masses above ~185 GeV and in a lower-mass window ~80-125 GeV are still allowed at 95% confidence level. We carry out Monte Carlo simulations to explore the potential of a boosted-decision-tree-based analysis to probe the surviving SCS parameter space in future searches at the (HL-)LHC, finding a significant increase in sensitivity relative to cut-based analyses both in the lower-mass window and at higher SCS masses.