Calorimeter Calibration and Jet Energy Scale

1. Jean-Francois Arguin November 28th, 2005 Physics 252B, UC Davis Calorimeter Calibration and Jet Energy Scale

2. Outline Quick remainder of calorimetry Calibration before the experiment starts: test beam Calibration when the experiment is running: Hardware calibration Collider data Measuring jets at high-energy colliders Example of a physics measurement: top quark mass

3. Basics of Calorimetry Incident particle creates a shower inside material Shower can be either electromagnetic or hadronic Energy is deposited in material through ionization/excitation

4. Basics of Calorimetry II Basic principle of calorimetry: deposited energy is proportional to incident energy Calorimeter calibration translate detector response to incident energy Great feature of showers for detector use: length is proportional to logE

5. Electromagnetic showers Created by incident photon and electron electrons emit bremstrahlung photons undergo pair production Length of shower expressed in term of X0 X0 depends on material 95% containment requires typically about 20X0

6. Created by incident charged pion, kaon, proton, etc Typical composition: 50% EM (e.g. ) 25% Visible non-EM energy 25% invisible energy (nuclear break-ups) Requires longer containment (expressed in λ) Hadronic showers

7. Calorimeter detectors Detector hardware must: Favor shower development Collect deposited energy Can do both at the same time (e.g. BaBar/Belle crystal calorimeters) Or have calorimeters with alternating passive and sensitive material Example of electron shower with lead absorber:

8. Sampling calorimetry (Ex.: CDF) • Scintillators (sensitive material) emit lights with passage of ionizing particles • Collect light deposited in sensitive material using wavelength shifter (WLS) • WLS →photomultipliers that convert light into electric signal

9. CDF Calorimeters Segmentation • Calorimeter is segmented into towers that are read-out independently • Lead (iron) interspersed with scintillators for EM (HAD) calorimeters • Each central tower covers • Each tower has an EM calorimeter followed by an HAD calorimeter

10. CDF Calorimeters • Three regions: central, wall and plug • Use “projective” geometry • Designed to measure electrons, photons, quarks, gluons, hadrons, neutrinos • Note: design of calorimeter performed with a simulation of the most important processes you plan to measure (ex.: Higgs at LHC)

11. Construction • Go on and built the thing after it is designed! • Many institutions in the world participate

12. First calibration: test beam Take one calorimeter “wedge”, send beam of particles with known energy Obtain correspondence detector response → energy in GeV A few towers only submitted to test beam Set absolute scale for all towers Relative scale for other towers obtained later • Wedge getting ready to receive beam:

13. How does the test beam works (Ex.: plug calorimeter) Why Muons? • Performed at Fermilab meson beam facilities • Beams characteristics: • Various types for EM and HAD showers: electrons, pions, muons • Various energy: 5-120 GeV (electrons), 5-220 GeV (pions) • Beams can be contaminated → bias the calibration constants • E.g. use Cherenkov detector in front of calorimeter to identify proton contamination in pion beam

14. Calorimeter response linearity • Extract calibration constant for many energy point • Can test linearity of calorimeter • Can add “artificial” material in front of calorimeter to simulate tracker+magnet material • Send pions and electrons to hadronic calorimeter Why sending electrons in hadron calorimeter?

15. Performance determined from test beam • From RMS of tower response to same beam energy → measure calorimeter resolution • Can test tower transverse uniformity (influences resolution) • Stochastic term resolution: • EM: • HAD:

16. Final detector assembly: getting ready for physics!

17. The Tevatron • Proton-antiproton collisions at • Most energetic collider in the world • Collisions every 0.4 μs • Circumference of 6.3 km

18. The CDF Detector CDF II: general purpose solenoidal detector • 7 layers of silicon tracking • Vertexing, B-tagging • COT: drift chamber • coverage • Resolution: • Muon chambers • Proportional chamber interspersed with absorber • Provide muon ID up-to • Calorimeters • Central, wall, plug calorimeter

19. Calibration when the detector is installed • Only a few towers saw test beam, how to calibrate the whole thing?? • Test beam sets the absolute scale as a function energy • Two solutions: • Hardware calibrations • Physics calibration (using collider data) • These calibrations need to: • Cross-check absolute scale (e.g. test beam not 100% realistic) • Track detector response through time • Expected degradation of scintillator and PMT • PMT sensitive to temperature • Uniform response through all towers

20. Hardware calibration Can use radioactive sources that have very well defined decay energy Cobalt 60 (2.8 MeV) Cesium 137 (1.2 MeV) Source calibration can be performed between colliders run Sources are movable and can expose one tower at a time Check uniformity over all towers and over time Sources are sensitive to both scintillator and PMT responses

21. Laser calibrations • The lasers are connected directly to PMTs • Skip scintillator/WLS steps • Used to uniformize PMTs response over towers and time

22. Physics calibrations • Use real collider data • For calibration, you have to have some “known” and some “unknown” (the calorimeter response) • Examples of “known” information: • Mass of a well-known particles • Ex.: Z→ee (Z mass measured at LEP) • Energy deposited by muons over a given length • Muon sample • Energy measured in tracker (assuming tracker in calibrated) • Redundant to energy measured in calorimeter for electrons

23. Example: Z boson mass Z mass peak: • Z mass measured with great accuracy at LEP using beam energy • Background is very small for Z→ee • Sample is relatively small, but good enough

24. Example: E/p of electrons • Used for relative scale over towers • Cannot be used in forward region (no tracker) • In plug: rely on sourcing and lasers

25. Example: muons for HAD calorimeters • Muon calibrate detector response to ionizing energy • Use muon from J/ψ for identification (mass not used like Z boson) • Again, not used for PHA (rely on sourcing, laser)

26. Physics with photons/electrons Search for new physics: Z' candidate: • Calorimeter calibration not the only issue • Electron/photon physics also rely on tracking • Removal of background • E.g. remove pion background by studying shower shape Precision measurement: W mass:

27. What are jets? Why not using tracker (has better resolution)? • Jets are a collimated group of particles that result from the fragmentation of quarks and gluons • They are measured as clusters in the calorimeter • momentum of cluster of towers is correlated with the momentum of the original quark and lepton

28. Phenomenology of jets • Quark/gluon produced from ppbar interaction • Fragmentation into hadrons • Jets clustering algorithm: • Adds towers inside cone • Fraction of energy is out-of-cone • Underlying event contributes

29. Jet versus calorimeter energy scale • Jets are complicated processes • Previous calorimeter calibrations are not sufficient to get calibrated jet energy • More work needs to be done!! • Jet energy scale is crucial for many important measurements: • Top quark mass (used to constrain Higgs boson) • Jet cross-sections (comparison to QCD predictions) • Measurements often performed by comparing real data with simulations • Need to get both physics and detector simulation right

30. Relative energy scale • Relative energy scale: • Use QCD dijet events • Should have equal transverse momentum • Jet energy measurement depend on location in detector • True even after all previous calibrations! • How come? • Jets are wide • Some regions of CDF calorimeter are not instrumented

31. Absolute energy scale Solution: • Get the average energy scale Simulate an “average” particles configuration inside jet • Use test beam information to get calibration factor for single particles • Response to single pion non-linear (in test beam) • However, jets are identified as one single objects • For a 50 GeV jet: calibration is not the same whether: • One 50 GeV pion • 10 times 5 GeV pions

32. Out-of-cone energy • Cone of fixed radius used to identify jets • Need to correct for fraction of energy out-of-cone (typically 15%) • This is mostly physics related • How well is the physics generator representing fragmentation?

33. Underlying event energy • Proton/antiproton remnants splash energy in calorimeter • Spoils jet energy measurement • Depends on the number of ppbar interaction per event • Extracted from “minimum bias” events • Small effect: ~0.4 GeV per jet

34. Final jet energy scale uncertainty • Estimate of jet energy scale uncertainty is important to estimate systematic uncertainties of measurements • Dominated by out-of-cone (low-pT) and absolute energy scale (high-pT) • Ranges from 10% to 3% energy uncertainties

35. Example physics measurement: top quark mass • Top produced in pairs at Tevatron • Top decays to W boson and b-quark 100% of time in SM • Typical event selections: • Well-identified electron(s) or muon(s) • Large missing ET • Several reconstructed jets identified in calorimeters • Note: 4 jets in final state!

36. Identification of b-quark jets • Complicated final state: • Which jets come from which parton? • Can identify b-quark jets using one characteristic: • Long b-quark lifetime • Note: lots of semileptonc B-hadrons decay (involving neutrino) • Require special b-jets calibration

37. Top mass reconstruction • Event-by-event kinematic fitter (assumes event is ttbar) • Attempts all jet-parton assignments • Assign b-tag jets to b-quarks • The one most consistent with ttbar hypothesis is kept More correct combinations with b-tags!

38. The strategy • Construct reconstructed top mass distributions for many true top mass • So-called “templates” • Compare distribution reconstructed in data with templates • Using likelihood fit • Account for background contamination • Dominated by W+jets production

39. The measurement (spring 2005) • Using 138 candidate ttbar events, fit yields: Mtop= 173.2 +2.9/-2.8 (stat.) +/- 3.4 (syst.) GeV/c2 • By shifting by JES uncertainty defined before: Mtop changes by 3.1 GeV/c ! • JES uncertainty limiting factor for Mtop measurement

40. Improvement: W→jj calibration • Inside ttbar events, invariant mass of two jets from W boson decay should equal MW • Can use W→jj decays to further constraint JES • Use same data for measurement and calibration… cheating?? • No: Mjj (almost) independent of Mtop • Remaining correlations are accounted for

41. The measurement(adding W→jj information) • Using same dataset as previously: Mtop= 173.5 +2.7/-2.6 (stat.) +/- 2.8 (syst.) GeV/c2 • Total Mtop uncertainty improved by 10% • JES uncertainty decreased by 20% • Good prospect for future

42. Impact of Mtop measurement • Mtop, MW connected to Higgs boson mass through radiative corrections MH< 186 GeV/c2 @ 95%C.L. • Can constrain mass of supersymmetric particles

43. Conclusion • Detector calibration needed to translate detector response in energy • Various techniques used for calorimetry: • Test beam • Radioactive sources • Lasers • Collider data • Calorimeter can be used to measure: • Electrons, photons, jets, missing ET • Good calorimeter and jet calibration needed for measurements like top quark mass