Performance of the CMS electromagnetic calorimeter and its role in the hunt for the Higgs boson in the two-photon channel

Federico De Guio

doi:10.1088/1742-6596/455/1/012028

Abstract

The electromagnetic calorimeter of CMS (ECAL) is a hermetic, fine grained and homogeneous calorimeter containing 75848 lead-tungstate (PbWO4) crystals, completed by a silicon preshower installed in front of the endcaps. The ECAL sensitivity to decay modes with electromagnetic objects in the final state, such as narrow resonances decaying into two photons, is achieved through its excellent energy and position resolution. The ECAL performance from 2010–2012 is presented in detail and its role in the hunt for the Higgs boson, through the 2-photon decay mode, is discussed.

Highlights

The electromagnetic calorimeter of Compact Muon Solenoid (CMS) (ECAL) is a hermetic, fine grained and homogeneous calorimeter containing 75848 lead-tungstate (PbWO4) crystals, completed by a silicon preshower installed in front of the endcaps
The ambitious physics program conducted by the Compact Muon Solenoid (CMS) [1] collaboration ranges from the investigation of electroweak symmetry breaking through the direct search for the Standard Model (SM) Higgs boson to the search for evidence of new physics at the TeV scale
The design of the CMS electromagnetic calorimeter (ECAL) [2] was driven by the requirements imposed by the search for the Higgs boson in the H → γγ channel, in which a narrow peak in the di-photon invariant mass has to be distinguished from a continuous background [3]

Summary

Introduction

The ambitious physics program conducted by the Compact Muon Solenoid (CMS) [1] collaboration ranges from the investigation of electroweak symmetry breaking through the direct search for the Standard Model (SM) Higgs boson to the search for evidence of new physics at the TeV scale. Silicon avalanche photodiodes (APDs) are used as photodetectors, while vacuum phototriodes (VPTs) have been chosen for the endcaps because of their high radiation hardness The sensitivity of both the crystal and the APD response to temperature changes requires temperature stability. The performance of the components of the calorimeter has been extensively tested with 120 GeV electron beams: the stochastic and the electronic noise contributions to the energy resolution of ECAL have been shown to match the design requirements; the overall resolution was excellent and well below 1% at high energies, with an irreducible constant term of about 0.3% for particles impinging on the centre of the crystals [10]. G: the global ECAL energy scale converts ADC counts into GeV It is determined separately for the ECAL barrel and the endcaps using the the Z → e+e− invariant mass peak;. The instrumental contribution to the Z width (σCB) is extracted from a fit to the invariant mass distribution of a Breit-Wigner convoluted with a Crystal-Ball response function [11]

CMS Preliminary 2011

Findings

Conclusions