Chasing new physics in stacks of soft tracks

In this paper, we introduce a new variable ξ , namely, the number of tracks associated with the primary vertex, which are not parts of reconstructed objects such as jets, isolated leptons, etc. We demonstrate its usefulness in the context of new physics searches with compressed spectrum in the channel monojet+missing transverse momentum (E_T). In models such as in compressed supersymmetry, events are often characterized by a rather large number of soft partons from the cascade decays, none of which results in reconstructed objects. We find that ξ , binned in p_T, can discriminate these new physics events from events due to Z +jets , that is, the main background in the channel monojet+E_T. The information contained in soft tracks is largely uncorrelated with traditional variables such as the effective mass, E_T, p_T of the jet, etc., and, therefore, can be combined with these to increase the discovery potential by more than 200% (depending on the spectra, of course). In fact we find that simple cuts on ξ (p_T), when combined with cuts on the effective mass, outperforms sophisticated optimized Multivariate Data Analysis using all conventional variables. One can model the background distribution of ξ (p_T) in an entirely data-driven way and make these robust against pileup by identifying the primary vertex.