Calorimetry in particle physics experiments

Transcription

1 Calorimetry in particle physics experiments Unit n. 8 Calibration techniques Roberta Arcidiacono

2 Lecture overview Introduction Hardware Calibration Test Beam Calibration In-situ Calibration (EM calorimeters) R. Arcidiacono Calorimetria a LHC 2

3 Introduction The goal of the calibration strategy is to achieve the most accurate energy measurement for the particles absorbed by the calorimeter. ADC counts MeV conversion factor Relationship between the energy deposited and the resulting calorimeter signal R. Arcidiacono Calorimetria a LHC 3

4 Introduction In general, one can always parametrize the energy deposited inside one em or hadronic shower as E p = G F p i c i s i A i A i Single channel amplitude s i Single channel time dependent correction for response variations c i Intercalibration coefficient (IC): relative single channel response F p G Particle energy correction (geometry, clustering, etc ) Global scale calibration R. Arcidiacono Calorimetria a LHC 4

5 Introduction The calibration strategy includes: Calibration of all hardware components: electronic chain, detector modules Specific calibration systems designed for the read-out chain Specific Test Beams studies carried on with all or part of the calorimeter modules Continuous monitoring of the calibration constants throughout the lifetime of the calorimeter, in the experimental set-up (in-situ calibration) R. Arcidiacono Calorimetria a LHC 5

6 Hardware calibration Used to equalize and monitor the cell-to-cell response of the detector and associated electronics, and to track the time-variations of the response The electronics calibration system injects a known pulse at the input of the readout chain (typically at PA level). Channel-to-channel dispersions as small as 0.2% can be achieved (precision and stability of the calibration system are essential) However, this system does not allow a calibration of the detector response, for which other devices (e.g. lasers, radioactive sources) are used that inject a well-known light or charge signal into the active elements of the detector R. Arcidiacono Calorimetria a LHC 6

7 Hardware calibration For Gain Switching PA: calibration system plays a crucial role in monitoring the gain stability and the gain offset values Calibration pulses are typically issued regularly during data taking, in allocated time slots without physics ex: in SPS cycles: calibration time is right after the 3s physics spill - extra 0.5 s every cycle (14 s); ex: in LHC cycles: using LHC gaps (1 of ~ 3ms every ~ 3200 Bunch crossings) radioactive sources used to calibrate hadron calorimeters or calorimeter designed for low energies, or to track transparency changes in crystals ( light transmission curve) Radioactive sources have very well defined decay energy [ Cobalt 60 (2.8 MeV) Cesium 137 (1.2 MeV) ] R. Arcidiacono Calorimetria a LHC 7

8 Test Beam calibration Usually some (or ALL) calorimeter modules are exposed to test beams (like e-/pion beam/muons) before being installed in the final detector Among the reasons: Commission the hardware/read-out/software systems around a detector; study detector performance compute a first set of inter-calibration constants set the preliminary absolute energy scale for electrons and pions, given that the incident beam energy is well known. R. Arcidiacono Calorimetria a LHC 8

9 In-situ calibration In-situ calibration is performed with physics samples. Every calorimeter needs to be calibrated (re-calibrated) after installation in the experimental hall. The experimental environment (ex. presence of material in front of the calorimeter) is different from test beam environment, and it is not seen by the hardware calibration. Also, calorimeter response to jets and the missing transverse energy cannot be measured at the test beam where only single particles are available. Finally, calibration stability has to be monitored. R. Arcidiacono Calorimetria a LHC 9

10 In-situ calibration (2) In-situ calibration allows correction of residual non uniformities, to follow the detector response variations with time, and to set the final absolute energy scale under experimental conditions. Well-known control physics samples (having high branching ratios) are used, such as: 0 /, Ke3 (K L e ) J/ψ,Z ee, W e (collider) W jj (collider) or, when possible (fixed-target), calibration electron beams R. Arcidiacono Calorimetria a LHC 10

11 Muons for HCAL as well... Energy deposited by muons over a given length is a well-known quantity (MIPS) Muon calibrate detector response to ionization energy Muons are also perfect to calibrate calorimeters with a longitudinal segmentation, with EM and HAD parts use of muons from J/ψ events, to have a sample of a well identified energy R. Arcidiacono Calorimetria a LHC 11

12 Setting the energy scale Issue of setting the absolute energy scale: Hadron colliders - see next e+e- colliders precise knowledge of the center-of-mass energy provides useful constraints and renders this operation easier fixed target experiments use well-known particles decays R. Arcidiacono Calorimetria a LHC 12

13 Absolute Energy Scale: Hadron colliders The electromagnetic absolute energy scale at hadron colliders is set mainly by using well-known resonances such as 0 /, J/ ee, ee in the low-energy range and Z ee at higher energies. Resonance Mass error or = ħ/ MeV (8.4 ± 0.6) s MeV 1.30 ± 0.07 kev J/ MeV 93.2 ± 2.1 kev Z GeV GeV W GeV GeV R. Arcidiacono Calorimetria a LHC 13

14 Absolute Energy Scale: Hadron colliders Another method: transfer the energy scale from the tracker to the electromagnetic calorimeter by measuring the E/p ratio for isolated electrons (E from calorimeter, p from tracker) The tracker momentum scale in the inner tracker is calibrated by using isolated muons, (ex: from Z decays) For the electron momentum scale, Monte Carlo simulation of the tracker material distribution is used to compute the electron energy losses (bremsstrahlung) and hence obtain the initial electron momentum. Finally, the momentum scale is transferred to the calorimeter by adjusting the E/p distribution for electrons to 1. R. Arcidiacono Calorimetria a LHC 14

15 Ex: D0 calorimeter calibration Energy scale is calibrated by using Z ee events: E true = E meas + where E meas is the electron energy measured in the calorimeter and the parameters and are varied until the reconstructed Z mass peak agrees with the nominal value. In RUN I, Z peak was ;5% below the nominal mass. Wrong initial scale because no module of the final D0 central calorimeter was calibrated with test beams indicates the importance of performing test beam measurements, to keep the energy correction factors minimal. R. Arcidiacono Calorimetria a LHC 15

16 Ex: D0 and CDF The E/p ratio for isolated electrons from W decays as obtained from the CDF run-ib data (Abe et al., 1995; Kim, 1999). The di-electron mass spectrum reconstructed in the D0 central calorimeter before the final energy scale calibration, Zee data sample (Abbott et al.,1998). Both D0 and CDF achieved a precision on the absolute electron energy scale of ~0.1%. R. Arcidiacono Calorimetria a LHC 16

17 Ex: D0 and CDF Absolute Energy scale precision was limited by: the statistics of the physics samples used to calibrate the mass peak or the E/p peak. systematic uncertainties: dominant sources are the incomplete knowledge of the dead material, calorimeter response non-linearities, the knowledge of the mass of the resonance used, and radiative Z decays ( Z eeg with low energy undetected photons). R. Arcidiacono Calorimetria a LHC 17

18 Ex: effect of calibration in CMS ECAL R. Arcidiacono Calorimetria a LHC 18

19 Measurement of Jets Jet is a collimated group of particles that result from the fragmentation of quarks and gluons measured as clusters by the calorimeters Previous calorimeter calibrations are not sufficient to get calibrated jet energy R. Arcidiacono Calorimetria a LHC 19

20 Jet Energy Scale The setting of the energy scale of the jet, inferring the original parton energy from the measured jet debris, is more complex than the setting of the electron scale: there are more numerous (and more difficult to control) sources of uncertainties Samples used at hadron colliders: events with associated production of a single jet with a photon or a boson, like Z ll If there is only one jet and one boson in the event, then the boson and the jet must have equal and opposite momenta in the plane transverse to the beam ( E T = 0 ) The transverse momentum of the photon or Z particle can be determined with high precision R. Arcidiacono Calorimetria a LHC 20

21 Jet energy calibration Jet energy measurement depend on location in detector and relative energy scale Another sample: QCD dijet events, should have equal transverse momentum Response to single pion nonlinear (in test beam) For a 50 GeV jet: calibration is not the same whether: One 50 GeV pion 10 times 5 GeV pion Solution: Get the average energy scale Simulate an average particles configuration inside jet R. Arcidiacono Calorimetria a LHC 21

22 Jet energy scale Corrections: Out-of-cone energy Cone of fixed radius used to identify jets Need to correct for fraction of energy out-of-cone (typically 15%) Underlying event Spoils jet energy measurement Depends on the number of primary interactions per event Extracted from minimum bias events R. Arcidiacono Calorimetria a LHC 22

23 Final JES uncertainty Dominated by out-of-cone (low-pt ) and absolute energy scale (high-p T ) Ranges from 10% to 3% W jj calibration In ttbar events, invariant mass of two jets from W boson decay should be equal to M W Can use W jj decays to further constraint JES R. Arcidiacono Calorimetria a LHC 23