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γ +Jet Analysis for the CMS

γ +Jet Analysis for the CMS. Pooja Gupta , Brajesh Choudhary, Sudeep Chatterji, Satyaki Bhattacharya & R.K. Shivpuri University of Delhi, India. OUTLINE. Gamma + jet Physics Data Samples Photon Isolation Studies Analysis Results Systematic Uncertainties. Signal Processes.

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γ +Jet Analysis for the CMS

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  1. γ +Jet Analysis for the CMS Pooja Gupta, Brajesh Choudhary, Sudeep Chatterji, Satyaki Bhattacharya & R.K. Shivpuri University of Delhi, India

  2. OUTLINE • Gamma + jet Physics • Data Samples • Photon Isolation Studies • Analysis • Results • Systematic Uncertainties

  3. Signal Processes Annihilation Diagram Compton Diagram Real NLO Correction Virtual NLO Correction

  4. Background Processes Photons from Bremsstrahlungin jet-jet events Background contributions from EM jets.

  5. Generation Parameters Generator used: PYTHIA (CMKIN_4_4_0) ^ Pre-selection Used : Select only those events which have a direct photon with PT>70 GeV & |η|<2.8

  6. Generation Parameters ^ Two DST Samples :

  7. Preselection Used • Look for the seed particles of electromagnetic objects like photons, electrons and positrons which have PT >5 GeV & |η|<2.7. • Candidate electromagnetic calorimeter trigger tower energies are then estimated by adding energies of all electromagnetic particles found within Δη<0.09 and Δφ < 0.09 from the seed. • Trigger tower candidates that lie within Δη<0.2 and Δφ < 0.2 from each other are suppressed and only those with the highest PT are retained. • The Level-1 single photon electromagnetic trigger is simulated by requiring that one such candidate has transverse energy greater than 20 GeV.

  8. Data Samples ^

  9. Analysis • The histograms were made for all the standard isolation variables suggested for Level 3 single isolated photon trigger after applying the PT > 80 GeV and | η  |<2.6 cuts. • ECAL isolation : the sum of the transverse energy of all the basic clusters lying in the Cone size R around the photon candidate should be less than the threshold value. • HCAL isolation: the sum of the transverse energy of all the particles depositing energies in the HCAL should be less than the threshold value. • Tracker isolation: the number of tracks which have PT > PTthres (which has been taken as 1.0 GeV,1.5 GeV, 2.0 GeV) should be less than the threshold values. The efficiency plots have been done by varying the threshold values for the parameters.

  10. Signal Efficiency vs. Background Rate ΣET in ECAL E_thres =1.0 E_thres =1.0 Barrel only Endcaps only For leading photon with PT > 80 GeV and | η  |<2.6

  11. Signal Efficiency vs. Background Rate ΣET in HCAL H_thres =6.0 H_thres =6.0 Barrel only Endcaps only For leading photon with PT > 80 GeV and | η |<2.6

  12. N_tk =0 N_tk =0 Signal Efficiency vs. Background Rate No. of tracks in cone size R =0.3 & 0.4 Barrel Endcaps For each cone size R, the efficiency and rate are calculated by varying the number of tracks in cone R which have Track PT >1.0 GeV, >1.5 GeV & >2 GeV for leading HLT photon

  13. Next Step • The combinations of detector parameters for Tracker, ECAL & HCAL have been studied. • Isolation parameters with maximum possible background rejection and very high signal efficiency have been chosen. • Tracker information provides a better rejection of the background. • The HCAL information is found to be partially redundant as most of the events where jet fakes a photon are already rejected by either Tracker isolation or ECAL isolation. • Based on the these isolation variables we have devised five schemes to get a better and improved S/B ratio.

  14. Selection Cuts

  15. Comparison of Rates Selection A : These analysis cuts have been used for calculating the HLT rates for +jet and its backgrounds in Physics TDR-I. Rates Reported – Signal: 2.1 Hz & Background : 1.4 Hz

  16. Selection Efficiency

  17. No. of Events/GeV Before any Photon isolation requirement After Photon isolation requirements ∫ L.dt = 1fb-1

  18. Differential Cross-Section Theoretical calculations were provided by Jeff Owens.

  19. Effect of Δφ cut Where Φ(, jet) = 1800 ± Δφ

  20. Errors • Statistical Error: • On Signal Rate ~1% • On Background Rate ~ 5% • Systematic Uncertainties: • Theoretical Uncertainties (Calculated using the recipe provided in CMS NOTE- 2005/013) • Experimental Uncertainties

  21. Systematic Errors

  22. Conclusions • The calculated rates match with the published rates in the Physics TDR Vol-I. We have further optimized the study and have improved the S/B by ~25% while barely losing 2% in signal efficiency. • The isolation conditions reduce the background by three orders of magnitude while keeping the signal efficiency between 70-80%for the low luminosity phase of the LHC running. • We have matched our differential cross-section for the direct photon+jet production results from Pythia based simulation with an independent theoretical calculation and they are found to be in good agreement. • Inclusion of a very wide Δφcut at 400in the analysis leads to a further increase of 15-17%in S/B with no significant loss in signal efficiency.

  23. We express our sincere thanks to • Marco Pieri for providing Background samples and help with software code. • Chris Seez, Ren-Yuan Zhu and Joachim Mnich for their critical comments and helpful suggestions. • Jeff Owens for providing theoretical LO & NLO calculations.

  24. BACKUP SLIDES

  25. Barrel only Endcaps only R= 0.3 R= 0.4 R= 0.5 R= 0.7 Plots of the ΣET in the ECAL for various Cone Sizes

  26. Barrel only Endcaps only R= 0.3 R= 0.4 R= 0.5 R= 0.7 Plots of the Number of Tracks with PT >1.5 GeV for various Cone Sizes

  27. Endcaps only R= 0.3 R= 0.4 R= 0.5 R= 0.7 Plots of the ΣET in the HCAL for various Cone Sizes Barrel only

  28. N_tk =0 N_tk =0 Signal Efficiency vs. Background Rate No. of tracks in cone size R =0.5 & 0.7 Barrel Endcaps For each cone size R, the efficiency and rate are calculated by varying the number of tracks in cone R which have Track PT >1.0 GeV, >1.5 GeV & >2 GeV for leading HLT photon

  29. More on Systematic Errors • Scale Variation: The largest variation from default choice of Q2 was observed when Q2 = s (MSTP 32=4). The variation in signal cross-section with increase in PT is from 4-13% while the background changes by ~27%. • Jet Energy Scale: If the selection criterion for jet is changed to PjetT>60 GeV, the estimated uncertainty is ~7% as the number of events having jets in 40GeV<PjetT<60 GeV bin with single isolated photon is much larger compared to lower PjetT bins. The scaling was done using : • Jet Energy Resolution: The jet energy was smeared using : where ^

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