Devanshu Panchal, UT-Austin

"Detoured and Delayed: Delving into the Depths of Displaced Vertices"

Abstract: The Standard Model (SM) of particle physics describes the elementary particles, which are the building blocks of matter, and their interactions with three of the four fundamental forces . Predictions of the SM have been substantiated by experimental results over decades, with the highlight being the 2012 discovery of the Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider (LHC). However, the absence in the SM of an explanation for gravity, dark matter, and many other phenomena all point to the need for new fundamental physics beyond the SM.

Supersymmetry (SUSY) is a well-motivated theoretical extension to the SM that offers possible answers to these remaining questions. In the gauge-mediated SUSY breaking (GMSB) models, the sparticles exist at energy scales which are accessible at the LHC. Thus, the ATLAS and the CMS experiments at the LHC have broad search programs for these sparticles. In this talk, I will describe a GMSB model where the next-to-lightest sparticle (NLSP) is long-lived such that it travels undetected through the ATLAS inner detectors before decaying into the SM Higgs or Z boson. The Higgs boson then decays into a pair of photons, and the Z boson decays into an electron-positron pair. Therefore, the final-state signature consists of the detection of a pair of photons or an electron-positron pair in the calorimeter.

In the GMSB model under study, where the NLSP is long-lived, the final-state photons or the electron-positron pair will have a non-zero time-of-arrival to the calorimeter. These final-state particles will also be displaced such that they do not point back to the primary collision point. Therefore, the timing and pointing information from the calorimeter will be used to search for delayed and displaced diphoton or dielectron objects produced from the decay of the long-lived SUSY particle. The search makes use of proton collision data collected by the ATLAS experiment during the Run 2 data-taking period from 2015 till 2018, which corresponds to an integrated luminosity of 139 inverse fb. No significant excess is observed over the no-SUSY hypothesis, and the search sets strong limits on the allowed lifetime and mass of the NLSP in the GMSB model under study.