Search for new physics in final states with semivisible jets or anomalous signatures using the ATLAS detector

Abstract

A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with anemphasis on signatures of a strongly coupled hidden dark sector accessed via resonant production of a Z0mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 isused, consisting of proton-proton collisions at ffisffiffip1/4 13 TeV and corresponding to an integrated luminosityof 140 fb-1. The Z0 mediator is considered to decay to two dark quarks, which each hadronize and decay toshowers containing both dark and Standard Model particles, producing a topology of interacting andnoninteracting particles within a jet known as "semivisible." Machine learning methods are used to selectthese dark showers and reject the dominant background of mismeasured multijet events, including ananomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonancesearch is performed by fitting the transverse mass spectrum based on a functional form backgroundestimation. No significant excess over the expected background is observed. Results are presented as limitson the production cross section of semivisible jet signals, parametrized by the fraction of invisible particlesin the decay and the Z0 mass, and by quantifying the significance of any generic Gaussian-shaped masspeak in the anomaly region.