[L'angolo del PhD] Alessandro Palma - XXII Ciclo - 2009


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The first part of this work describes how to use Z -> e+e- events in order to calibrate the CMS electromagnetic calorimeter, which makes use of scintillating crystals in order to precisely measure the energy of electrons and photons coming from the proton-proton interactions.
Using the very precise knowledge of the Z mass coming from LEP experiments, it is possible to set the absolute scale of the calorimeter as well as calibrating regions of the calorimeter with various topologies, and finely correct the calorimeter response to electrons. Focus is put on the first weeks of data taking.
The second part of this work concentrates on the misidentification of the electric charge of electrons/positrons in CMS. It will be shown how it is possible to extract the charge misidenti fication rate from the fi rst CMS data, this time relying on the fact that electrons coming from the Z decay are always oppositely-charged.
Measuring this charge misidenti fication rate not only allows to perform a real-time check of the reconstruction quality during data taking, but also has an important role in the study of some physics channels. One of the studies where the charge misidentification has an important in influence is the W+/W- cross section ratio, that represent a test of the Standard Model which does not need a precise knowledge of the machine luminosity, that will be difficult to achieve with the first data.

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[L'angolo del PhD] Alessandro Palma - XXII Ciclo - 2009

  1. 1. Studies on the dielectron spectrum with thefirst data of the CMS experiment at the LHCDottorato di Ricerca in Fisica XXII ciclo – Seminario Finale Alessandro Palma Supervisors: Prof. Egidio Longo Roma, 23 Ottobre 2009 Dr. Riccardo Paramatti Dr. Paolo Meridiani
  2. 2. Outlook of the work   LHC experiments will start data taking before 2010   Precise tests of Standard Model (SM) and search for new TeV-physics   Focus on first months of CMS data (integrated luminosity 100 pb-1)   Important to calibrate detectors with physics events and “re-discover” SM 1. Calibration studies: use Z  ee events to calibrate the CMS e.m. calorimeter •  Absolute energy scale •  Intercalibration of different calorimeter regions 2. Measurement of electron charge misidentification •  Assess and monitor quality of electron reconstruction algorithms •  Crucial ingredient in both SM and beyond-SM physics channels •  Early SM application: measurement of W+/W- cross section ratio   Physics studies developed in the framework of CMS Electroweak Group @CERN23 Ottobre 2009 Alessandro Palma 2
  3. 3. The Large Hadron Collider (LHC) at CERN  First collisions at √s = 7 TeV before 2010, then centre-of-mass energy will be stepped up  Results shown here are for √s = 10 TeV (intermediate step before 14 TeV)23 Ottobre 2009 Alessandro Palma 3
  4. 4. The Compact Muon Solenoid (CMS) experiment23 Ottobre 2009 Alessandro Palma 4
  5. 5. The CMS electromagnetic calorimeter (ECAL) Energy resolution and design performance   75,848 PbWO4 scintillating crystals Constant   Barrel (0 < |η| < 1.479): 61,200 crystals [c = 0.5%]   Endcaps (1.48 < |η| < 2.7): 2 x 7,324 crystals Stochastic Noise [a = 2.7%] [b ≈ 200 MeV] Endcap Test beam resultsEndcap Barrel 23 Ottobre 2009 Alessandro Palma 5
  6. 6. ECAL calibration with Z  ee eventsSimulated Z  ee event in CMS e+e- invariant mass 23 Ottobre 2009 Alessandro Palma 6
  7. 7. Precalibration of the ECAL before startup Barrel 1. Test beam electrons (on 10/36 of Barrel) • Intercalibration at 0.4% level 2. Cosmic ray flux (on all the Barrel) •  Provides intercalibration of 1-2% depending on pseudorapidity 3. Light Yield (LY) lab measures (on all the Barrel) •  Provides intercalibration of 4-5% Endcaps Expected intercalibration at startup: 7-10%23 Ottobre 2009 Alessandro Palma 7
  8. 8. ECAL calibration with Z  ee events Description of the method  In each event “i”, the quadratic mass ratiofolds a weighted average of themiscalibrations of the regions ”j” of thecalorimeter hit by the Z-electrons  The weights are given by the energyfraction carried by region “j”  Event after event, for each region “j” ahistogram is filled with the quadratic massratio with its weight  After a number of events, the histogram isfitted and its peak gives an estimate of theregion miscalibration  The procedure is repeated iteratively 23 Ottobre 2009 Alessandro Palma 8
  9. 9. ECAL calibration with Z  ee events MonteCarlo validation of the method The ECAL regions to study can be defined in different ways, according to:  same η-ring  same manifacturer How to validate the method? 1.  Introduce ad-hoc miscalibration η-rings 2.  Compare miscalib and 1/recalib constants at convergence1/recalib Spread around y=x gives recalibration precision miscalib (improving w/ statistics)23 Ottobre 2009 Alessandro Palma 9
  10. 10. ECAL calibration with Z  ee events Applications of the method in BarrelFind corrections to electron energy in bins of (η, ET) Material budget in front of ECAL Increasing Bremsstrahlung Brings worse energy reconstruction23 Ottobre 2009 Alessandro Palma 10
  11. 11. ECAL calibration with Z  ee events Applications of the method in Endcaps Intercalibration of η-rings of crystals f(x) = p0 + p1/√x ECAL Endcaps Expected miscalibration at LHC startup23 Ottobre 2009 Alessandro Palma 11
  12. 12. Measurement of electron charge misidentification rate from data Electron with wrong reconstructed charge Simulated Z  ee event in CMS23 Ottobre 2009 Alessandro Palma 12
  13. 13. Measurement of electron charge misID Origin of charge misID [1/2] Bremsstrahlung emission followed by conversion “confuses” reco algorithms e+ e+ e- Brem yield pT resolution23 Ottobre 2009 Alessandro Palma 13
  14. 14. Measurement of electron charge misID Origin of charge misID [2/2] If the explanation is correct, for wrongly-reconstructed electrons:o Transverse Impact Parameter (TIP) isexpected to be largero Azimuthal angle φ has worse resolutionand biased determination23 Ottobre 2009 Alessandro Palma 14
  15. 15. Measurement of electron charge misID Description of the method [1/2] Tag & Probe (TP) method applied to Zee events:   one electron (Tag) must pass a stringent track quality selection in order to ensure that its charge is correctly reconstructed   the other electron (Probe) usually passes looser selection   Tag-Probe invariant mass in the range [85,95] GeV/c2 to reduce background   the method measures the charge misID rate on the Probe Probe misID rate = (# of TP events where Tag and Probe have same charge) / (# of TP events) A typical selection for Tag (with efficiency ~10%) is: •  ECAL Barrel only •  ET > 20 GeV •  num. of track-hits > 10 •  χ2 of track < 1.223 Ottobre 2009 Alessandro Palma 15
  16. 16. Measurement of electron charge misID Results: charge misID rate vs reconstructed electron quantities Probe here is a track-isolated electron with ET>20 GeV MisID rate behaves as expected Good agreement with MonteCarlo check on the Probe charge (reco vs gen-level electron) Integrated misID value (1.52 ±0.09)% Statistical error w/ 100 pb-1 data23 Ottobre 2009 Alessandro Palma 16
  17. 17. Measurement of electron charge misID Systematic uncertainties [1/2]•  Invariant mass window   Agreement with MonteCarlo truth checked with various invariant mass windows•  MisID on the Tag electron   If misID for Tag is >0 (Ptag), the method measures the sum of Tag misID + Probe misID   Ptag can be extracted from Tag-Tag events and subtracted•  Probe definition   Stability of the method checked with various “Probe” definitions23 Ottobre 2009 Alessandro Palma 17
  18. 18. Measurement of electron charge misID Systematic uncertainties [2/2]•  Charge symmetryNo significant differences found inmisID results when requesting positiveand negative Tags•  Background level  Background found not significant(S/B ~ 100)  Systematics below 0.1% even whenbackground enhanced by 3 (to accountfor uncertainty in QCD yield) Overall systematics ~0.1% (comparable with stat. @ 100 pb-1)23 Ottobre 2009 Alessandro Palma 18
  19. 19. Application of electron charge misID to W+/W- cross section ratio High-pT isolated Missing transverse electron energy (neutrino)23 Ottobre 2009 Alessandro Palma 19
  20. 20. Application to W+/W- cross section ratio Positive and negative W bosons at CMS   LHC initial p-p state favours W+ production o  integrated W+/W- ratio is > 1  u-type quarks carry more of the proton momentum than d-type o boosted W’s are more often positive23 Ottobre 2009 Alessandro Palma 20
  21. 21. Application to W+/W- cross section ratio Relevance of the measurement: constraining PDFs   PDF investigation: LHC will be able to explore a new region in (x, Q2) plane   Different PDF models give different W+/W- predictions Average ratio W+/W- ≈ 1.423 Ottobre 2009 Alessandro Palma 21
  22. 22. Application to W+/W- cross section ratio W  eν selection   One electron with ET > 15 GeV at online reconstruction (to filter events live to a sustainable rate)   One electron with ET > 30 GeV at offline reconstruction   No 2nd electron with ET > 20 GeV (to reduce Z  ee background)   Track, ECAL, HCAL isolation and tight eleID requirements (to reduce QCD jets) Missing Transverse Energy (MET) distribution of selected events (signal & background): •  gives a flavour of the S/B ratio •  shows how distinctive MET is in W  eν23 Ottobre 2009 Alessandro Palma 22
  23. 23. Application to W+/W- cross section ratio CHAPTER 4. CHARGE MISIDENTIFICATION CORRECTION TO THE W + /W − Number of selected events with 100 pb-1 data CROSS SECTION RATIO Physics channel Selected events W → eν 371937 Z → ττ 1589 W → τν 5018 Z → ee 23650 γ + jets 53138 QCD 76877 tt 1875Table 4.5: Number of signal and background events passing the selection for a statistics of100 pb−1 S/B = 2.3  A robust strategy of background subtraction is under study in CMS asymmetrically, and the final state electron has the same charge of the τ because it   comes from the Background decay. be considered in the following τ → eνe ντ will not Fig. ?? and ?? show the pseudorapidity and transverse energy distribution of the recon-structed positive/negative electrons coming from the decay of positive/negative 23 bosons 23 Ottobre 2009 Alessandro Palma Wrespectively (except for the misID phenomenon that will be covered later on in this chap-ter): the transverse energy distributions are very similar, while the pseudorapidity plot shows
  24. 24. Application to W+/W- cross section ratio W+/W- ratio with misID correctionN+,-:observed misID rate T+,-:true W+/W-W+/W-Correcting for electron charge misIDbecomes important when:1.  W+/W- ratio becomes large2.  misID rate becomes largei.e. at high values of electron |η| 23 Ottobre 2009 Alessandro Palma 24
  25. 25. Application to W+/W- cross section ratio Charge misID rate for W electrons   Apply Tag&Probe method to electrons passing the selection requested for W   Check charge symmetry so that the same misID values can be applied to W+ and W- events Integrated misID rate: (1.27±0.08)%23 Ottobre 2009 Alessandro Palma 25
  26. 26. Application to W+/W- cross section ratio Constraining PDFs without misID correction  W “data” was generated starting from CTEQ6L1 (LO) PDF library  If no misID correction is inserted, at high |η| agreement with MonteCarlo gets weak 2.1 R 2 W data - misID corr. 1.9 CTEQ6L1 (MonteCarlo truth) 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 -2 -1 0 1 223 Ottobre 2009 Alessandro Palma Electron ! 26
  27. 27. Application to W+/W- cross section ratio Constraining PDFs with misID correction   When misID correction is inserted, at high |η| data fit MonteCarlo better 2.1 R 2 W data - misID corr. 1.9 1.8 CTEQ6L1 (MonteCarlo truth) 1.7 1.6 1.5 Integrated W+/W- 1.4 1.3 1.2 1.1 -2 -1 0 1 2 Electron !23 Ottobre 2009 Alessandro Palma 27
  28. 28. Conclusions   An iterative method has been elaborated that allows calibration of regions of the CMS em calorimeter, that with 100 pb-1 data allows: o  Barrel: tuning of (η, ET)-dependent correctiosn to the electron energy o  Endcaps: intercalibration of η-rings at permille level   A “Tag&Probe” method to extract the electron charge misID rate from data has been developed o  Good stability and agreement with MonteCarlo o  Important in Standard Model analyses and beyond   In measuring W+/W- ratio, inserting misID correction is relevant to constraint proton PDFs23 Ottobre 2009 Alessandro Palma 28