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

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 2 OUTLINE Gamma + jet Physics Data Samples Photon Isolation Studies Analysis Results Systematic Uncertainties

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 3 Signal Processes Real NLO Correction Virtual NLO Correction Annihilation Diagram Compton Diagram

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 4 Background contributions from EM jets. Photons from Bremsstrahlung in jet- jet events Background Processes

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 5 ParameterDirect Photon Signal Process MSUB = 14 (Gluon Compton Scattering) MSUB = 29 (Annihilation Process) √s14 TeV PDF UsedCTEQ5L p T Range >50 GeV (in different bins) Generation Parameters Generator used: PYTHIA (CMKIN_4_4_0) Pre-selection Used : Select only those events which have a direct photon with P T  >70 GeV & |η  |<2.8 ^

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 6 Generation Parameters ParameterBackground ProcessMSEL = 1 (QCD Jets) √s14 TeV PDF UsedCTEQ5L p T Range >50 GeV (in different bins) Data Sample NamePre-selection Used jm03b_qcdNo eg03_jets_1eYes Two DST Samples : ^

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 7 Preselection Used  Look for the seed particles of electromagnetic objects like photons, electrons and positrons which have P T >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 P T are retained.  The Level-1 single photon electromagnetic trigger is simulated by requiring that one such candidate has transverse energy greater than 20 GeV.

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 8 Data Samples p T (in GeV) σ Generated (in pb) σ Preselected (in pb) Number of Simulated Events Integrated Luminosity (in pb -1 )  +jet E E  +jet E E  +jet E E  +jet E E  +jet > E E eg03_jets_1e_ E E eg03_jets_1e_170up> E E jm03b_qcd_50_ E jm03b_qcd_80_ E jm03b_qcd_120_ E jm03b_qcd_ 170_ E jm03b_qcd_ 230_ E ^

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 9 Analysis The histograms were made for all the standard isolation variables suggested for Level 3 single isolated photon trigger after applying the P T  > 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 P T > P T thres (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.

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 10 Signal Efficiency vs. Background Rate Barrel only Endcaps only For leading photon with P T  > 80 GeV and | η  |<2.6 ΣE T in ECAL E_thres =1.0

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 11 Signal Efficiency vs. Background Rate Barrel only Endcaps only For leading photon with P T  > 80 GeV and | η  |<2.6 ΣE T in HCAL H_thres =6.0

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 12 N_tk =0 Signal Efficiency vs. Background Rate For each cone size R, the efficiency and rate are calculated by varying the number of tracks in cone R which have Track P T >1.0 GeV, >1.5 GeV & >2 GeV for leading HLT photon No. of tracks in cone size R =0.3 & 0.4 Barrel Endcaps

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 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.

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 14 Selection Cuts Cuts Cone Size R P T  (GeV) | || | ECAL Isolation (GeV) HCAL Isolation (GeV) No. of tracks Threshold Track P T (GeV) A0.380<2.6 <1.5 <6 Barrel <4 Endcap 01.5 B0.380<2.6<1.5 <6 Barrel <5 Endcap 01.0 C0.480<2.6<2.0 <7 Barrel <5 Endcap 01.5 D0.480<2.6<2.0 <6.5 Barrel <5 Endcap 01.5 E0.580<2.6<2.5 <8Barrel <6 Endcap 01.5

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 15 Comparison of Rates Selection Cuts Signal Rate (Hz) Background Rate (Hz) S/B A B C D E 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

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 16 Selection cut  + jet jet-jet background L1+HLT61860 (81.3%) (7.5%) |   | < (95.3%) (94.6%) Photon Isolation in R=0.4 No track with P T >1.5 GeV53312 (90.4%)6414 (3.2%) ECAL Isolation E_iso< (94.3%)1420 (22.1%) HCAL Isolation H_iso<7.0(Barrel) H_iso<5.0 (Endcaps) (84.5%)507 (35.7%) Jet P T >40 GeV & |  jet | < (99.6%)501 (98.8%) Overall Efficiency55.9%0.02% Selection Efficiency

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 17 No. of Events/GeV After Photon isolation requirements ∫ L.dt = 1fb -1 Before any Photon isolation requirement

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 18 Differential Cross-Section Theoretical calculations were provided by Jeff Owens.

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 19 Effect of Δφ cut Where Φ( , jet) = ± Δφ

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 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

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 21 Systematic Errors Source Uncertainty on Signal Rate (in %) Uncertainty on Bkgd Rate (in %) PDF Uncertainty ~3%~5% Scale Variation ~7% ~27 % Luminosity ~10% Photon Trigger ~1% Photon Energy Scale ~0.5% Jet Energy Scale ~0.4% ~0.7% Jet Energy Resolution ~0.3% ~1.0% Preselection Efficiency ~3%

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 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 Δφ cut at 40 0 in the analysis leads to a further increase of 15-17% in S/B with no significant loss in signal efficiency.

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 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.

BACKUP SLIDES

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 25 Plots of the ΣE T in the ECAL for various Cone Sizes Barrel only R= 0.3 Endcaps only R= 0.4R= 0.5R= 0.7

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 26 Plots of the Number of Tracks with P T >1.5 GeV for various Cone Sizes Barrel only R= 0.3 Endcaps only R= 0.4R= 0.5R= 0.7

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 27 Plots of the ΣE T in the HCAL for various Cone Sizes Barrel only R= 0.3 Endcaps only R= 0.4R= 0.5R= 0.7

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 28 Signal Efficiency vs. Background Rate No. of tracks in cone size R =0.5 & 0.7 Barrel Endcaps N_tk =0 For each cone size R, the efficiency and rate are calculated by varying the number of tracks in cone R which have Track P T >1.0 GeV, >1.5 GeV & >2 GeV for leading HLT photon

Pooja Gupta India-CMS Meeting Visva-Bharati, Santiniketan 29 More on Systematic Errors Scale Variation: The largest variation from default choice of Q 2 was observed when Q 2 = s (MSTP 32=4). The variation in signal cross-section with increase in P T is from 4-13% while the background changes by ~27%. Jet Energy Scale: If the selection criterion for jet is changed to P jet T >60 GeV, the estimated uncertainty is ~7% as the number of events having jets in 40GeV<P jet T <60 GeV bin with single isolated photon is much larger compared to lower P jet T bins. The scaling was done using : Jet Energy Resolution: The jet energy was smeared using : where ^