Pileup measurement and mitigation techniques in CMS

A Perloff

doi:10.1088/1742-6596/404/1/012045

Abstract

When trying to reconstruct an event from a hard-scatter pp collision in CMS, it is of the utmost importance to correctly measure the energy from jets. The jet energy corrections (JEC) correct, on average, the energy of the reconstructed jets back to the energy of the final-state particles that initiated the jets. This effort is hindered by additional energy in the jets coming from other soft pp collisions. The additional energy is termed pileup or offset and comes from everything except the primary vertex (PV) and its underlying event (UE). In this paper, we describe how this pileup energy is measured and parametrized as well as the techniques used to remove this extra energy from the reconstructed jets.

Highlights

During the reconstruction of a proton-proton collision, jets are often reconstructed with a pT that differs from that of the final-state particles that make up the jet
The additional energy is termed pileup or offset and comes from everything except the primary vertex (PV) and its underlying event (UE). We describe how this pileup energy is measured and parametrized as well as the techniques used to remove this extra energy from the reconstructed jets
The CMS collaboration has developed a factorized approach to these jet energy corrections (JEC) which is composed of multiple levels representing corrections for various physics or detector effects and allows enough flexibility in the corrections to be applicable to many types of analyses [1,2,3]

Summary

Introduction

During the reconstruction of a proton-proton (pp) collision (event), jets are often reconstructed with a pT that differs from that of the final-state particles that make up the jet. Software Level Software level pileup mitigation techniques are essential to most analyses The purpose of these offset correction techniques is to measure and remove the extra energy inside of the jets that is not associated with the high-pT vertex and its UE [1,2,3,4; 6]. Uses the JA to correct on a jet-by-jet basis As seen, both of these methods measure the pileup energy almost equivalently, on average [4]. The benefit to applying this additional algorithm to the jet corrections is that it lowers the amount of pileup energy that the AO or HJA methods must compensate for, lowering the multiplicative scale factors.

CMS preliminary

Findings

Conclusion