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 $\Delta {\phi }_i$, 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 $\Delta {\phi }_{\text{min}}^{\ast }$, 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 $\Delta {\phi }_{\text{min}}^{\ast }$ 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 $\Delta {\phi }_{\text{min}}^{\ast }$ and $\Delta {\phi }_i$ 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.

32 pages, 18 figures