Search for higgsinos in compressed mass spectra using low-momentum tracks in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

This paper reports on two searches for higgsinos with compressed mass spectra using ATLAS data, employing low-momentum track techniques to identify decay products, and sets new limits on higgsino masses in this challenging parameter space.

Contribution

The study introduces novel search strategies for compressed higgsino spectra using low-momentum tracks and neural network-based lepton tagging, extending previous experimental limits.

Findings

No significant excess observed over Standard Model predictions.

Excluded chargino masses below 126 GeV for certain mass splittings.

First ATLAS constraints in this compressed higgsino parameter space.

Abstract

This paper presents two searches for the electroweak production of higgsinos with compressed mass spectra using 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events are required to feature an energetic jet, large missing transverse momentum, and at least one low-momentum charged particle that serves as a candidate higgsino decay product. In the first search, targeting higgsino mass splittings in the range of 0.3-1 GeV, the higgsinos are expected to predominantly decay into pions that are identified as low-momentum charged particles with large transverse impact parameters due to the long higgsino lifetime ($c\tau\approx\mathcal{O}$(0.1-1 mm)). The second search targets larger mass splittings in the range of 1-3 GeV, where the higgsinos are expected to decay promptly into low-momentum leptons, one of which is identified by dedicated low-momentum electron or muon taggers based on neural networks utilising tracking and calorimeter information. No significant excess above the Standard Model prediction is observed in either search and the results are used to set lower limits on the masses of the higgsino-like charginos and neutralinos within a simplified model. Together, these searches exclude chargino masses below 126 GeV at 95% confidence level for mass splittings between the chargino and lightest neutralino in the range of 0.3-2 GeV, representing the first ATLAS constraints in this parameter space and surpassing the limits previously set by the LEP experiments.