CMS event reconstruction status in Run 2

Andrea Perrotta

doi:10.1051/epjconf/201921402015

Abstract

LHC Run2 began in April 2015 with the restart of the collisions in the CERN Large Hadron Collider. In the perspective of the offline event reconstruction, the most relevant detector updates appeared in 2017: the upgrade of the Pixel Detector, with the insertion of an additional layer closer to the beams, and the improved photodetectors and readout chips for the Endcap Hadron Calorimeter, which allow a finer longitudinal segmentation. The long shutdown between Run1 and Run2 was instrumental in the optimization of the reconstruction code and for the introduction of new algorithms to mitigate sensitivity to increased pileup, especially to out-of-time contribution with the advent of 25 ns separation between collisions compared to 50 ns in Run1. Such an optimization continued in the following years, when the reconstruction code of CMS evolved together with the improving of the performance of the LHC. The current status of the reconstruction software of the CMS experiment is described here, with emphasis on some of the recently integrated developments.

Highlights

The offline event reconstruction for the CMS experiment is based on the concept of Particle Flow (PF) [1]
Since the beginning of LHC Run2 in April 2015, the subdetectors of the CMS experiment started getting updated in view of the higher luminosities and harsher radiation environment as expected for the prosecution of Run2, or even for Run3
The two CMS detector updates most relevant for the offline reconstruction were both integrated in 2017: the replacement of the Pixel Detector with an upgraded one which has an additional layer closer to the beams, which allows a more precise determination of the impact and other track parameters, and the replacement of photodetectors and readout chips in the Endcap Hadron Calorimeter with improved ones which provide a

Summary

Introduction

The offline event reconstruction for the CMS experiment is based on the concept of Particle Flow (PF) [1]. Particles traversing the sensitive areas of the detector release an electric signal which is digitized by the acquisition system, and locally reconstructed in the different subdetector units (Inner Tracker, Electromagnetic and Hadronic calorimeters, Muon detectors) to provide the building blocks for the subsequent overall event description. The long shutdown between Run and Run was instrumental in the optimization of the reconstruction code and for the introduction of new algorithms to mitigate sensitivity to the increased pileup, especially to the out-of-time contribution which became quite relevant when the time separation between collisions was reduced to 25 ns from the previous 50 ns of Run1 Such an optimization continued in the following years, when the reconstruction code of CMS evolved together with the improving of the performance of the LHC. The computational performance of the offline recostruction code of CMS is addressed in another contribution to this conference [2]

Tracking

Electrons and Photons

Hadron Calorimeter

Conclusions