Electron and Photon Reconstruction and Performance in CMS L. Finco University of Nebraska – Lincoln LPC Physics Forum 18th of February 2021 Outline 2 Calorimeters at Colliders ECAL Detector at CMS Electrons and Photons in Run 2 … and Beyond 3 Calorimeters at Colliders Definition of Calorimeter 4 • In particle physics a calorimeter is a detector measuring the energy carried by an incoming particle • Instrumented blocks of matter in which the particle interacts and deposits all its energy in the form of a cascade of particles • The particle energy is measured in eV (MeV-GeV-TeV 106, 109, 1012 eV) • 1 eV = energy acquired by one electron accelerated by 1 V • The temperature effect of a 100 GeV particle in 1 litre of water -12 (at 20 °C) is ΔT = 3.8×10 K https://www.mpp.mpg.de/~menke/elss/home.shtml General Features of Caloremeters 5 • Calorimetry is a “destructive” method • The energy is measured by total absorption of the particle • A calorimeter converts the energy of an incident particle to a detectable signal, proportional to the incoming energy • Calorimeters can measure the energy of electromagnetic radiation (electrons, photons) or hadronic particles (charged pions, protons, neutrons) • The charged component of the particles' cascade deposits energy in the active part of the calorimeter and it is detected in the form of charge or light • This talk is mainly focused on electromagnetic calorimeters https://www.mpp.mpg.de/~menke/elss/home.shtml Particles’ Life through a Detector 6 https://cds.cern.ch/record/2120661 Particle-Matter Interactions 7 • In matter electrons and photons loose energy interacting with nuclei and atomic electrons • Main photon interactions with matter: • Photoelectric effect • Compton scattering • Pair production • Main electron interactions with matter: • Ionization PDG 2018 • Bremsstrahlung • Čerenkov radiation • Multiple scattering PDG 2018 Particle-Matter Interactions 8 • In matter electrons and photons loose energy interacting with nuclei and atomic electrons • Main photon interactions with matter: Pair production • Photoelectric effect • Compton scattering • Pair production • Main electron interactions with matter: • Ionization PDG 2018 • Bremsstrahlung • Čerenkov radiation • Multiple scattering Bremsstrahlung At colliders interesting electrons and photons have usually E > 1 GeV PDG 2018 Particle-Matter Interactions 9 • In matter electrons and photons loose energy Pair production interacting with nuclei and atomic electrons • Main photon interactions with matter: • Photoelectric effect • Compton scattering • Pair production • Main electron interactions with matter: • Ionization Courtesy of S. Harper • Bremsstrahlung • Čerenkov radiation Bremsstrahlung • Multiple scattering These processes are the basis of the electromagnetic shower Courtesy of S. Harper Particle-Matter Interactions 10 Pair production Courtesy of S. Harper Bremsstrahlung These processes are the basis of the electromagnetic shower Courtesy of S. Harper Electromagnetic Shower 11 • Electrons and photons create a cascade (shower) of particles • Number of particles N • Is proportional to the energy of the incoming particle • Increases until the energy of the electron component falls below Ec (critical energy ~ 5-10 MeV) • With E < Ec a slow decrease in number of particles occurs as electrons are stopped and photons absorbed • Longitudinal extension of the shower is proportional to ln(E/ Ec) Electromagnetic Shower 12 Determined by radiation length Determined by Moliere radius Moliere by Determined • The evolution of an electromagnetic shower through a material is dictated by: • Radiation length (X0) : • Mean distance in which an electron will lose all but 1/e of its energy • 0.78 of mean free path of a photon (photon showers will start later) • Moliere radius (MR): • 90% of shower’s energy is contained within a cylinder of radius = 1 MR • 2 MR contain 95% of its energy Energy Resolution of EM Calorimeters 13 Intrinsic energy resolution: 퐸 ∝ 푁 휎 푁 ∝ 푁 휎(퐸) 1 ∝ 퐸 퐸 Energy resolution of real detectors 휎(퐸) 푆 푁 = ⨁ ⨁퐶 퐸 퐸 퐸 S: stochastic term from Poisson-like fluctuations N: noise term from electronics and Example using real data pile-up C: constant term Calorimeter’s Design Characteristics 14 • Size of the detector: the longitudinal depth of the detector is dictated by X0 and is driven by the required resolution • Small X0 allows to reduce calorimeter size (compactness) • Cell size: typically chosen such that 70-80% of energy of a centrally incoming particle is deposited in the cell, while having energy in the neighbouring cells large enough to measure the centre of gravity coordinates • Related to MR • Granularity: size of detector elements in the transverse and longitudinal direction, which determines the ability to resolve two showers induced by nearby particles as distinct. It depends on the shower size, distance from the interaction point and transverse size of detector elements/cells 15 ECAL Detector at CMS Large Hadron Collider 16 Experimental Conditions at the LHC 17 6.5 TeV beam energy 1011 protons/bunch 1034 cm-2 s-1 machine luminosity 40.000.000 bunch crossing/second More than 30 events/crossing CMSPublic/LumiPublicResults LHC Requirements for Calorimeters 18 • Fast response (25 ns or faster) and high granularity, to reduce pile-up induced noise • Radiation-hard detectors and electronics • Hermetic and cover the full azimuthal angle and rapidity range, to tag very forward jets and well measure the missing energy • Excellent electromagnetic energy resolution • To detect the two photon decay of an intermediate mass Higgs (golden channel together with H→ZZ →4l) • 푚훾훾 = 2퐸1퐸2(1 − cos 휃) • Uncertainty on 푚훾훾 determined by uncertainty on photon energy and direction Courtesy of C. Mariotti LHC Requirements for Calorimeters 19 • Fast response (25 ns or faster) and high granularity, to reduce pile-up induced noise • Radiation-hard detectors and electronics • Hermetic and cover the full azimuthal angle and rapidity range, to tag very forward jets and well measure the missing energy • Excellent electromagnetic energy resolution • To detect the two photon decay of an intermediate mass Higgs (golden channel together with H→ZZ →4l) • 푚훾훾 = 2퐸1퐸2(1 − cos 휃) • Uncertainty on 푚훾훾 determined by uncertainty on photon energy and direction JHEP 11 (2018) 185 Compact Muon Detector 20 https://cms-docdb.cern.ch/cgi-bin/PublicDocDB/RetrieveFile?docid=13631&filename=cms_160312_06.png CMS Electromagnetic Calorimeter 21 휃 휂 ≡ ln(tan ) 2 • Lead Tungstate (PbWO4) homogenous crystal calorimeter • Barrel (EB): • 36 super-modules, each 1700 crystals Wikipedia • |η|< 1.48 • Endcaps (EE): • 2 endcap sides, each 7324 crystals • 1.48<|η|<3.0 • Preshower (ES): Barrel • Sampling calorimeter Endcaps (lead, silicon strips) Preshower • 1.65<|η|<2.6 CERN/LHCC 97–33CMS TDR 4 CMS Electromagnetic Calorimeter 22 Barrel Endcaps Preshower CERN/LHCC 97–33CMS TDR 4 CMS ECAL Crystals 23 • Incident electron/photon generates EM shower in the heavy PbWO4 material • Charged particles in the shower produce scintillation light proportional to incident particle energy • Scintillation light detected by photodetectors with internal amplification • ECAL is made of 75848 PbWO4 crystals Parameter Value Radiation length 0.89 cm 2.9 cm Moliere radius 2.2 cm Rad. hardness excellent Refractive index 2.3 Peak emission 440 nm % of light in 25 ns 80% Light yield ~10 p.e./MeV ECAL Energy Resolution 24 ECAL “standalone” energy resolution measured at the test beam (3x3 arrays of barrel crystals) • No magnetic field • No material in front of the ECAL • Negligible inter-calibration contribution in the constant term http://cds.cern.ch/record/1000388 25 Electrons and Photons Performance in Run 2 Electrons and Photons in CMS 26 • Electrons and photons are grouped together because they both create electromagnetic showers • The energy of e/γ object is measured in the electromagnetic calorimeter CMS-PHO-EVENTS-2021-002 ECAL Energy Reconstruction 27 • Energy in the ECAL observed as a pulse as the photons from the shower arrive over time • This pulse shape is converted to an energy (E ∝ A, signal channel amplitude) • In case of out-of-time pile-up events, multiple pulses from different bunch 10.1088/1748-0221/15/10/P10002 crossings are generated • They lead to an apparent increased amplitude measurement • The amplitudes of different pulses are resolved by fitting the multiple pulse shapes simultaneously 10.1088/1748-0221/15/10/P10002 Electron and Photon Reconstruction 28 E • First step is to construct individual particles from the reconstructed energy deposits (clustering) • The clustering procedure looks for local maxima above a given threshold (1 GeV) • Energies between overlapping adjacent clusters are shared Clustering of Crystals Energy 29 Unclustered energy Energy shared by two clusters E Clusters corresponding to electrons/photons • Found 5 clusters in this region of the ECAL • Each one represents the energy deposits of a particle (electron/photon) • In practice not this clean: often clusters have a very large number of crystals with small energy fraction Superclusters 30 Supercluster Incompatible with the supercluster E • An electron or photon when it arrives to ECAL, may be accompanied by multiple secondary particles created through interactions with matter on the way to ECAL • Superclustering (clustering on clusters) aims to combine the individual electrons and photons into a single object, corresponding to the original electron/photon • It starts by taking the highest energy cluster (in this