Calorimetry in particle physics experiments Universita' degli Studi di Torino Scuola di Dottorato Roberta Arcidiacono Universita' degli Studi del Piemonte Orientale INFN Torino
Program 1.The relevance of Calorimetry in HEP 2.The physics of calorimetry 3.Detector response: energy resolution 4.Electromagnetic calorimeters 5.Hadron calorimeters 6.Calorimeter design principles 7.Front-end and Trigger electronics R. Arcidiacono Calorimetry 2
Program (2) 8.Calibration techniques 9.Some examples Course duration: 16 hours 4 credits (~12 h frontal 4 h homework assignment) R. Arcidiacono Calorimetry 3
Calorimetry in particle physics experiments Unit n.1 The relevance of Calorimetry in HEP R. Arcidiacono Calorimetry 4
Lecture overview Definition of calorimeter Introduction on Calorimetry Why are calorimeters important in high energy experiments? R. Arcidiacono Calorimetry 5
Defi nition of Calorimeter Calorimeter object used in Calorimetry to measure transfer of heat (energy) in chemical reactions or physical changes in particle physics is a detector which measures the energy carried by a particle instrumented blocks of matter in which the particle to be measured interacts and deposits all its energy in the form of a cascade of particles whose energy decreases progressively down to the threshold of ionization/excitations detectable by the media R. Arcidiacono Calorimetry 6
Defi nition of Calorimeter In Thermodynamics: thermically hermetic box with a substance of which we want to measure the temperature 1 calorie (4.185 J): energy necessary to increase of one degree the temperature of 1 gr of water at 15 C In particle physics we measure ev (MeV-GeV-TeV 10 6, 10 9 10 12 ev) 1 ev = energy acquired by one electron accelerated by a d.d.p of 1 V R. Arcidiacono Calorimetry 7
Main property of a calorimeter ENERGY MEASURED by TOTAL ABSORPTION Deposited energy detectable signal is PROPORTIONAL to the incoming energy The measurement is DESTRUCTIVE calorimeters can measure the energy of electromagnetic particles (electrons, photons) or hadronic particles ( charged pions, protons, neutrons) It is the charged component of the particles' cascade which deposits energy in the active part of the calorimeter, detected then in the form of charge or light R. Arcidiacono Calorimetry 8
Generic detector structure R. Arcidiacono Calorimetry 9
Introduction Three historical motivations to conceive calorimeters: 1. energy measurement (direction, time) for neutral particles π 0, n, η, K 0... bubble chambers/magnetic spectrometers could not do the job use cases: spectroscopy of exited states which decay into X + nπ 0 kinematic closure of event if possible (four-momentum conservation) - precise measurement of the event energy flow R. Arcidiacono Calorimetry 10
Introduction This last point particularly important at e+e- collider machines, where the annihilation process produces particles at relatively large angles with a total energy equal to the centre-of-mass collision energy (energy of the collision is very well known unlike in the hadronic colliders) Total Energy Measurement method Calorimeters essential to measure (hopefully) the total energy (and identify so the missing energy) R. Arcidiacono Calorimetry 11
Introduction 2. neutrino experiments Late 60's, early 70's: advent of intense high energy neutrino beam: need to have highly massive detectors acting as target as well - to study their interaction (cross section very small 10 10 smaller than hadronic cross-sections) need to study the inelastic processes throughout the target massive detector should be uniformly sensitive to reaction products! Among the firsts: Gargamelle, CDHS R. Arcidiacono Calorimetry 12
Gargamelle Bubble chamber built during the late 1960s, and was designed principally for the detection of the neutrinos. Contains a liquid under pressure, which reveals the tracks of electrically charged particles as trails of tiny bubbles when the pressure is reduced. Neutrinos interact very rarely, so Gargamelle was designed not only to be as big as possible, but also to work with a dense liquid - Freon (CF3Br) - in which neutrinos would be more likely to interact. The final chamber was a cylinder, 4.8 m long and 1.85 m wide, with a volume of 12 cubic meters. R. Arcidiacono Calorimetry 13
Gargamelle Neutrino scattering from electron e e Z 0 n n EVIDENCE OF NEUTRAL CURRENTS such as the historic exposure on the right. The first direct evidence for a weak neutral current An electron scattered from a neutrino CERN 1973 R. Arcidiacono Calorimetry 14
CDHS CERN-Dortmund- Heidelberg-Saclay neutrino experiment Led by Jack Steinberger* Designed to study deep inelastic neutrino interactions in iron. The detector had a mass of 1250 tons and combined the functions of a muon spectrometer and hadron calorimeter. It consisted of 19 magnetized iron modules, separated from each other by wire drift chambers. December 1976-1984. * Nobel Prize in 1988 (with Lederman & Schwartz): conception/realization of high energy neutrino beam ( discovery of muonic neutrino) R. Arcidiacono Calorimetry 15
Introduction 3. jets physics as a consequence of asymptotic freedom (QCD), the particles constituents behave like free particles in collisions with very high momentum transfer in High Q 2 reactions -> constituents collision as a consequence of confinement, the constituents hadronize producing JETS The jet retains the direction and energy of the parent parton Jet study becomes important @ colliders Moving from description of the single particles to measurement of global characteristics (energy flow, missing energy, jets) R. Arcidiacono Calorimetry 16
Introduction LEP e+e- Z q qbar R. Arcidiacono Calorimetry 17
Introduction Moreover: parton-parton cross-section << total cross-section Need to trigger and measure groups of hadrons (neutral and charged) coming from the hadronization process Interest is on High P T jets Trigger: not useful to try to trigger on the P T of a single isolated hadron (which is typically not so high). Need to trigger on jet P T Measure: in many physics cases it is essential to measure all the collision energy flow: need hermetic detector CALORIMETERS!! R. Arcidiacono Calorimetry 18
Introduction Calorimeters play a fundamental role in measuring jets characteristics (too complicated to measure at the individual particle level) They measure energy of ALL interacting particles, particularly useful @ high energy when particle density is high; can provide simple and fast trigger R. Arcidiacono Calorimetry 19
Introduction The visible final state is constituted by stable hadrons (cτγ >> meters) π+,π-, k+,k-, k 0, p+ p-,n nbar, π 0 (photons) In the hadronization process of heavy quarks also heavy hadrons are produced. They decay weakly (few mm) and only the decay products are detected. 30% of jet energy is due to em particles as well (from π 0 ). R. Arcidiacono Calorimetry 20
Why calorimeters are so appealing Calorimeter energy resolution typically improves with energy as 1/ E magnetic spectrometer calorimeters E / E 1/ E spectrometers p / p p R. Arcidiacono Calorimetry 21
Why calorimeters are so appealing They are sensitive to all type of particles, charged and neutrals magnetic spectrometers Versatile: can measure energy, position, direction, time and even particle ID Can be used in trigger systems signal is fast and easy to process Shower length increases as log E detector thickness increases logarithmically with the energy of the particles magnetic spectrometers R. Arcidiacono Calorimetry 22