Calorimetry in particle physics experiments Unit n. 5 Hadron calorimeters Roberta Arcidiacono
Lecture overview Main technique Few more words on compensation Particle Flow technique Two examples: ZEUS HCAL CMS HF R. Arcidiacono Calorimetria a LHC 2
Main Technique Due to hadronic showers dimensions Hadron calorimeters are necessarily sampling calorimeters The active medium made of similar material as in EM calorimeters: Scintillator (light), gas (ionization chambers, wired chambers), silicon (solid state detectors), noble liquid The passive medium is made of materials with shorter interaction length λ I Iron, Uranium, etc Resolution is worse than in EM calorimeters σ(e) E (35 % 80 %) E R. Arcidiacono Calorimetria a LHC 3
Some material properties Material Z A [g/cm 3 ] X 0 [g/cm 2 ] I [g/cm 2 ] Hydrogen (gas) 1 1.01 0.0899 (g/l) 63 50.8 Helium (gas) 2 4.00 0.1786 (g/l) 94 65.1 Beryllium 4 9.01 1.848 65.19 75.2 Carbon 6 12.01 2.265 43 86.3 Nitrogen (gas) 7 14.01 1.25 (g/l) 38 87.8 Oxygen (gas) 8 16.00 1.428 (g/l) 34 91.0 Aluminium 13 26.98 2.7 24 106.4 Silicon 14 28.09 2.33 22 106.0 Iron 26 55.85 7.87 13.9 131.9 Copper 29 63.55 8.96 12.9 134.9 Tungsten 74 183.85 19.3 6.8 185.0 Lead 82 207.19 11.35 6.4 194.0 Uranium 92 238.03 18.95 6.0 199.0 Example: 80 cm of Uranium can contain the hadronic shower induced by 300 GeV (95%), while 10 cm only are sufficient to contain electrons of the same energy R. Arcidiacono Calorimetria a LHC 4
Main Technique Reminder: energy resolution is mainly limited by large shower fluctuations, which are due to fluctuations in the electromagnetic component ( 0 etc.) of the shower and the invisible energy due to nuclear excitations, muons and neutrinos. The energy sampled is typically a few percent of the total incident energy. A small number of photons have to be detected with good signal to noise ratio. Up to now all sampling hadron calorimeters which are based on the detection of light from plastic scintillators use PMT s. R. Arcidiacono Calorimetria a LHC 5
On compensation... History of compensation Suggested by C. Fabjan and W. Willis that some of the invisible energy can be recuperated using depleted 238 U plates as absorber. The energy loss will be compensated by the emission of soft neutrons and gammas in fission processes of the Uranium. The first compensating 238 U/scintillator calorimeter was employed in the Axial Field Spectrometer @ISR http://cds.cern.ch/record/160036/files/198506232.pdf COMPENSATION = main consideration for the design of calorimeters for high energy collider experiments. R. Arcidiacono Calorimetria a LHC 6
On compensation... Other ways to achieve compensation: Offline compensation: proper longitudinal weighting of the electromagnetic and hadronic components (ex: WA1 calorimeter and H1 calorimeter) In 1987, the compensation effects were fully understood (Wigmans et al.). Compensation can be achieved with absorber materials other than Uranium by tuning, for ex, appropriate sampling fractions. Compensation possibilities: U/sci = 1/1, Pb/sci = 4/1, Fe/sci = 15/1. Crucial role is given to recovery of slow neutrons (10% in had component of cascade or fission neutrons) by hydrogen in the active material. R. Arcidiacono Calorimetria a LHC 7
Hydrogen role in compensation In a Pb/H 2 structure (50/50): slow neutrons release 98% energy to H and 2% to Pb. Sampled Charged particles ~ 2.2%. Hydrogen can amplify the signal thru neutron detection (active material needs to be sensitive to the recoil protons), modifying /e up to a factor 2 L3 collaboration demonstrated this (U+gas hadron calorimeter) Fig: /e ratio as a function of the hydrogen content of the gas mixture used R. Arcidiacono Calorimetria a LHC 8
Particle Flow Analysis A look at future projects... ee colliders At ILC or CLIC several interesting physics channels will appear in multi-jet final states, often accompanied by charged leptons and missing transverse energy. Detector performances will be crucial Z,W di-jets should be identified with an accuracy comparable to their natural decay width R. Arcidiacono Calorimetria a LHC 9
Particle Flow Analysis jet energy resolution of 3-4% over the whole energy range. One of the two approaches followed is the Particle Flow technique (the second one is the dual read-out technique) Particle Flow : Not a New idea. Used in Aleph. Major effort in CMS NOW DRIVING the DESIGN of the new detectors R. Arcidiacono Calorimetria a LHC 10
Particle Flow Calorimetry Particle Flow Calorimetry paradigm: - Charged particles measured in tracker (essentially perfectly) - Photons in ECAL: - Neutral hadrons (ONLY) in HCAL - Only 10 % of jet energy from HCAL Exploit tracker+ecal+hcal to reconstruct the energy (momentum) of each individual particle HCAL ECAL tracker R. Arcidiacono Calorimetria a LHC 11
Particle Flow Analysis Particle flow analysis determines the design of the calorimeters @ LinearColliders Calorimeters drive the design of the apparatus (ECAL ~ HCAL~ $50-100 M) and more than at the LHC, they constitute the heart of the apparatus Need to resolve the energy deposits of each individual particle in hardware and software need extremely high longitudinal and transverse segmentation (high absorber density) and very sophisticated reconstruction software R. Arcidiacono Calorimetria a LHC 12
Particle Flow Analysis Method is based on the combined use of a precision tracker and a highly granular calorimeter. Charged jet fragments can be measured with tracker, neutral component with calorimeter Problem is: proper subtraction of charged component from the calorimeter response. Demonstrated recently: high granularity is irrelevant when jet fragments overlap (like for a leading charged fragment) limit! This method can improve by 30% the performance of poor calorimetry... R. Arcidiacono Calorimetria a LHC 13
HCAL calorimeters Two examples of existing HCAL calorimeters R. Arcidiacono Calorimetria a LHC 14
ZEUS HCAL FCAL RCAL e 27.5 GeV CTD p 820 GeV SOLENOID BCAL R. Arcidiacono Calorimetria a LHC 15
ZEUS HCAL Uranium-Scintillator calorimeter detector 78 modules made up of Scintillator-Uranium plates (EM part + HAD part) Absorber layer ( 238 U) : 3.3 mm thick Scintillator layer: 2.6 mm thick Readout 2 PMTs per cell; imbalance gives position tuned such that e/ = 1 Dead material in barrel region: 1X 0 (0.04λ I ) (solenoid) R. Arcidiacono Calorimetria a LHC 16
1 calorimeter module ZEUS HCAL R. Arcidiacono Calorimetria a LHC 17
ZEUS HCAL Compensation measured with data Single-particle response: ratio of the response of electrons to pions(protons) as a function of the particle energy Strong non linearity below 5 GeV: low energy hadrons do not develop showers and e.m. showers are sampled inefficiently in high-z absorbers. R. Arcidiacono Calorimetria a LHC 18
ZEUS HCAL Hadronic energy resolution: σ(e) E = 35 % E However, since rather coarse sampling frequency, relatively low EM energy resolution σ(e) E =18 % E R. Arcidiacono Calorimetria a LHC 19
CMS Hadron Forward (HF) Covers pseudorapidity range 3< h <5 WHY??? Sampling Čerenkov calorimeter: Steel absorber + quartz fibers as active medium R. Arcidiacono Calorimetria a LHC 20
CMS Hadron Forward (HF) ZOOM in FORWARD region R. Arcidiacono Calorimetria a LHC 21
CMS Hadron Forward Two main objectives: 1. To improve the measurement of the missing transverse energy 2. To enable identification and reconstruction of high energetic very forward jets Half a million quartz fibers viewed with about 2000 phototubes R. Arcidiacono Calorimetria a LHC 22
CMS HF Quartz main characteristics: intrinsically radiation hard at the required level (hundreds of MRads) for all practical purposes, sensitive to the electromagnetic shower components (blind to low energy particles and neutrons, sensitive to electrons/positrons emitting Čerenkov radiation in quartz down to E 190 kev) hadronic shower seen via e.m component, narrower and concentrated in the core of the shower easier jet isolation low but sufficient light yield (< 1 pe/gev) not compensated! by design Čerenkov radiation is fast and correlated with particle trajectory R. Arcidiacono Calorimetria a LHC 23
Quartz fiber signal collection time R. Arcidiacono Calorimetria a LHC 24
HF main characteristics: CMS HF Two types of fibers, long L, 165 cm (10 l I ), and short S, starting from a depth of 22 cm to the end, read-out separately : showers initiated by electrons and photons deposit large fraction of energy in the first 22 cm, while hadrons' induced cascades deposit their energy in both segments L fibers sentitive to EM from all type of particles, S fibers sensitive only to hadronic cascades Second part of the calorimeter (where the S fibers start) has a double sampling density wrt first part partial equalization of the response to hadron and electrons achieved R. Arcidiacono Calorimetria a LHC 25
CMS HF Among the physics cases: Tagging of the vector boson fusion processes with forward jets: Second most dominant production mechanism for Higgs production Higgs decay products R. Arcidiacono Calorimetria a LHC 26