(YSF) Search for SUSY with a Highly Compressed Mass Spectrum in the Soft Single Lepton Channel with the CMS Experiment

SUSY models with compressed mass spectra are highly motivated by theoretical considerations, such as dark matter constraints and naturalness. The search targets simplified models in which the signal consists of top squark (stop) pair-production, followed by 4-body decays or decays via an intermediate chargino. The models assume the neutralino as the lightest supersymmetric particle (LSP) and a mass gap between the stop and the LSP that is smaller than the W-boson mass. The difficulty with resulting soft decay products can be mitigated by requiring the presence of an initial-state radiation jet, which boosts the system. The results are based on a sample of proton-proton data from Run II of the Large Hadron Collider, recorded with the CMS detector at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The data is analysed using two approaches a sequential selection based on discriminating variables and a multivariate technique. No significant deviation from the Standard Model is observed and limits are set on the stop production cross section in the plane of the top squark vs. LSP masses, excluding top squark masses up to 560 GeV. Furthermore, a statistical combination with the results of the all-hadronic channel is presented, yielding the most stringent limits in this region of SUSY parameter space with 2016 data.