Energy calibration and resolution of the CMS electromagnetic calorimeter in pp collisions at sqrt(s) = 7 TeV (1306.2016v2)

Abstract: The energy calibration and resolution of the electromagnetic calorimeter (ECAL) of the CMS detector have been determined using proton-proton collision data from LHC operation in 2010 and 2011 at a centre-of-mass energy of sqrt(s)=7 TeV with integrated luminosities of about 5 inverse femtobarns. Crucial aspects of detector operation, such as the environmental stability, alignment, and synchronization, are presented. The in-situ calibration procedures are discussed in detail and include the maintenance of the calibration in the challenging radiation environment inside the CMS detector. The energy resolution for electrons from Z-boson decays is better than 2% in the central region of the ECAL barrel (for pseudorapidity abs(eta)<0.8) and is 2-5% elsewhere. The derived energy resolution for photons from 125 GeV Higgs boson decays varies across the barrel from 1.1% to 2.6% and from 2.2% to 5% in the endcaps. The calibration of the absolute energy is determined from Z to e+e- decays to a precision of 0.4% in the barrel and 0.8% in the endcaps.

Energy Calibration and Resolution of the CMS Electromagnetic Calorimeter in Proton-Proton Collisions

The Compact Muon Solenoid (CMS) electromagnetic calorimeter (ECAL) is a critical component of the CMS detector, designed for precise measurement of electrons and photons to facilitate the exploration of electroweak phenomena, including the search for the Higgs boson. The research paper outlines the calibration and energy resolution procedures of the ECAL, utilizing data from proton-proton collisions at a center-of-mass energy of 7 TeV gathered from the LHC operational periods in 2010 and 2011.

Instrumentation and Methodology

The ECAL, composed of lead tungstate (PbWO$_4$) scintillating crystals, provides high granularity and precision crucial for the identification of photons from Higgs boson decay channels, particularly $\PH\to\Pgg\Pgg$. The calorimeter is divided into a barrel section and two endcaps, each designed with specific photodetectors: Avalanche Photodiodes (APDs) in the barrel and Vacuum Phototriodes (VPTs) in the endcaps. Calibration routines are essential given the radiation environment inside CMS, impacting detector elements over time.

The calibration strategy focused on maintaining energy response stability and included several steps:

• In-situ calibration using decay processes such as $\cPZ\to\Pep\Pem$ provided reference points for energy scale consistency.
• Monitoring environmental factors like temperature and voltage stability to mitigate variations that contribute to the energy resolution constant term.
• Utilization of a laser-based monitoring system to correct crystal transparency changes due to radiation damage.
• Intercalibrations exploiting $\phi$-symmetry, and the $\Pgpz/\Pgh$ resonances provided uniformity across detector channels.

Results

The ECAL achieved an energy resolution better than 2% in the central barrel ($|\eta| < 0.8$) and 2-5% in other regions, as evaluated using electrons from $\cPZ$-boson decays. The energy resolution for photons from 125 GeV Higgs boson decays varied from 1.1% to 2.6% in the barrel and from 2.2% to 5% in the endcaps, with an absolute energy scale precision of 0.4% in the barrel and 0.8% in the endcaps.

Implications and Conclusion

This calibration work was integral to the operations within the CMS, notably facilitating precision measurements needed for bosonic decay channels and constituents crucial to identifying the Higgs boson. The procedures detailed provide an essential reference for continued CMS operations and a basis for future upgrades to counteract evolving radiation damage and improve detector modelling.

Future detector advancements could further refine these calibration techniques, with emphasis on improving the modelling of material interactions and enhancing data-driven energy corrections. Such improvements are anticipated to optimize resolution and enhance the detector's overall performance, leading to more precise measurement capabilities in high-energy physics experiments. The results of this calibration provide a framework that influences the design considerations for new calorimetric systems in future collider experiments.