Alternative angular variables for suppression of QCD multijet events in new physics searches with missing transverse momentum at the LHC

Abstract

We introduce three alternative angular variables-denoted by $\tilde{\omega}_\text{min}$, $\hat{\omega}_\text{min}$, and $\chi_\text{min}$-for QCD multijet event suppression in supersymmetry searches in events with large missing transverse momentum in proton-proton collisions at the LHC at CERN. In searches in all-hadronic final states in the CMS and ATLAS experiments, the angle $\Delta\varphi_i$, the azimuthal angle between a jet and the missing transverse momentum, is widely used to reduce QCD multijet background events with large missing transverse momentum, which is primarily caused by a jet momentum mismeasurement or neutrinos in hadron decays-the missing transverse momentum is aligned with a jet. A related angular variable-denoted by $\Delta\varphi^*_\text{min}$, the minimum of the azimuthal angles between a jet and the transverse momentum imbalance of the other jets in the event-is used instead in a series of searches in all-hadronic final states in CMS to suppress QCD multijet background events to a negligible level. In this paper, before introducing the alternative variables, we review the variable $\Delta\varphi^*_\text{min}$ in detail and identify room for improvement, in particular, to maintain good acceptances for signal models with high jet multiplicity final states. Furthermore, we demonstrate with simulated event samples that $\hat{\omega}_\text{min}$ and $\chi_\text{min}$ considerably outperform $\Delta\varphi^*_\text{min}$ and $\Delta\varphi_i$ in rejecting QCD multijet background events and that $\hat{\omega}_\text{min}$ and $\tilde{\omega}_\text{min}$ are also useful for reducing the total standard model background events.