Ratio of Three over Two Jet Cross Sections: Update 36 pb -1 P.Kokkas, I.Papadopoulos, C.Fountas University of Ioannina, Greece QCD High p T Meeting 17 Dec 2010

Motivation 2 D0 PRL 86, p1955 (2001) Motivation: Measure the ratio R 32 and compare with pQCD predictions with goals: Extend the phase space of the measurement in a regime that goes above the Tevatron. Measure  S : Comparisons of the measured ratio at hadron level with the predictions of pQCD (parton level), after accounting for hadronisation corrections uncertainty, will measure the QCD coupling constant  S at a scale never measured before. First CMS result with 76nb -1 consistent with the predictions of PYTHIA 6 and MadGraph (PAS QCD ) The measured ratio rises, due to phase space, with H T. Above H T ≈0.7 TeV it reaches a plateau which is most sensitive to α s. Two sources of systematic uncertainties were considered: Uncertainties due to absolute (10%) and eta- dependent (2%xη) Jet Energy Scale lead to R 32 uncertainties of 5%. Systematic uncertainty due to difference in shape between data and MC is 5%.

Data - Triggers 3 ErasRunsLumiData & JSON file & Relevant HLT_Jet Triggers Era_ nb -1 /JetMETTau/Run2010A-Nov4ReReco_v1/RECO Cert_ _7TeV_Nov4ReReco_Collisions10_JSON.txt HLT_Jet30U, HLT_Jet50U Era_ pb -1 /JetMET/Run2010A-Nov4ReReco_v1/RECO Cert_ _7TeV_Nov4ReReco_Collisions10_JSON.txt HLT_Jet30U, HLT_Jet50U Era_ pb -1 /Jet/Run2010B-Nov4ReReco_v1/RECO Cert_ _7TeV_Nov4ReReco_Collisions10_JSON.txt HLT_Jet30U, HLT_Jet50U, HLT_Jet70U Era_ pb -1 /Jet/Run2010B-Nov4ReReco_v1/RECO Cert_ _7TeV_Nov4ReReco_Collisions10_JSON.txt HLT_Jet30U, HLT_Jet50U, HLT_Jet70U_v2, HLT_Jet100U_v2 Era_ pb -1 /Jet/Run2010B-Nov4ReReco_v1/RECO Cert_ _7TeV_Nov4ReReco_Collisions10_JSON.txt HLT_Jet30U_v3, HLT_Jet50U_v3, HLT_Jet70U_v3, HLT_Jet100U_v3, HLT_Jet140U_v3 Data were spitted to 5 eras according to different luminosities and trigger conditions. Below in the Table we present for each era: The run range The total Integrated Luminosity The latest Nov4ReReco and JSON file used The triggers involved in the analysis (prescaled in red, unprescaled in green).

Hadron Level Various MC 4 The following MC Samples (available up to now) were also used: Pythia6 TuneZ2 Fall10: QCD_Pt-xxtoxx_TuneZ2_7TeV-pythia6/Fall10-START38_V12-v1/GEN-SIM-RECO Pythia8 Tune1 Fall10: QCD_Pt_xxtoxx_Tune1_7TeV_pythia8/Fall10-START38_V12-v1/GEN-SIM-RECO Madgraph TuneD6T Fall10 : QCD_TuneD6T_HT-xxx_7TeV_madgraph/Fall10-START38_V12-v1/GEN-SIM-RECO Alpgen TuneZ2 Fall10: QCDXJets_Pt-xxtoxx_TuneZ2_7TeV_alpgen/Fall10-START38_V12-v1/GEN-SIM-RECO Pythia6 D6T Spring10: QCDDiJet_Ptxxtoxx/Spring10-START3X_V26_S09-v1/GEN-SIM-RECO On the right we see the predictions of above MCs at hadron level. The new MCs : Herwig++ Tune23 Fall10 (Flat) Pythia6 D6T Fall10 (Flat or slices) will also be added in the next weeks (Not available for analysis at some QCD T2). P.Kokkas, Univ. of Ioannina

Analysis JEC-Selection 5 Analysis with CMSSW_3_8_6 Jets were reconstructed using the antikt (R=0.5) clustering algorithm. PF + Calo jets were corrected for energy loss and effects due to non-linear response of calorimeter Relative (corrects for η dependence) (Data & MC) [Spring10_L2Relative_AK5PPF, Spring10_L2Relative_AK5Calo ] Absolute (corrects for the p T dependence) (Data & MC) [Spring10_L3Absolute_AK5PF, Spring10_L3Absolute_AK5Calo] Residual corrections for η>1.5 only to Data [Spring10DataV1_L2L3Residual_AK5PF, Spring10DataV1_L2L3Residual_AK5Calo] On line Selection: Technical bit 0, and halo bits (None of Technical bits 36,37,38,39) NOT USED! Select events with HLT single Jet triggers: HLT Jet30U, HLT Jet50U etc Off line Selection: Primary Vertex: |PVz| 4 Jet selection: p T ≥ 50 GeV and |y|≤2.5 “Loose” Jet ID for PF and Calo jets. P.Kokkas, Univ. of Ioannina

Trigger Efficiencies 6 HLT Jet70U HLT Jet50U HLT Jet30U HLT Jet140U_v3 HLT Jet100U Trigger Efficiencies were studied in detail for all data eras. Below the efficiencies of main triggers versus H T in the case of nJets ≥2 and nJets ≥3.

Combining Triggers 7 HLT Jet TriggerH T nJets≥2 (GeV) H T nJets≥3 (GeV) HLT Jet30U HLT Jet50U HLT Jet70U HLT Jet100U HLT Jet140U HLT Jet Trigger H T (GeV)Lumi HLT Jet30U nb -1 HLT Jet50U pb -1 HLT Jet70U pb -1 HLT Jet100U pb -1 HLT Jet140U pb -1 The table on the right shows the H T values where every trigger becomes fully efficient. The table below-left shows for every trigger used : the H T range and the corresponding Integrated Luminosity. The figure below-right shows how we combine the triggers to extract our measurement. P.Kokkas, Univ. of Ioannina

Data stability : Era1 (PF Jets) 8 nJets≥2 (HLT_Jet50U) nJets≥3 (HLT_Jet50U) (nJets≥3)/(nJets≥2) (HLT_Jet50U) To control the Data Stability over the whole period we produced for each era the following plots: Number of events per nb -1 for of nJets ≥2 and nJets ≥3 (Luminosity dependent). Ratio events (nJets ≥3)/(nJets ≥2) (Luminosity independent).

Data stability : Era2 (PF Jets) 9 nJets≥2 (HLT_Jet50U) nJets≥3 (HLT_Jet50U) (nJets≥3)/(nJets≥2) (HLT_Jet50U) P.Kokkas, Univ. of Ioannina

Data stability : Era3 (PF Jets) 10 nJets≥2 (HLT_Jet70U)nJets≥3 (HLT_Jet70U) (nJets≥3)/(nJets≥2) (HLT_Jet70U) P.Kokkas, Univ. of Ioannina

Data stability : Era4 (PF Jets) 11 nJets≥2 (HLT_Jet100U)nJets≥3 (HLT_Jet100U) (nJets≥3)/(nJets≥2) (HLT_Jet100U) P.Kokkas, Univ. of Ioannina

Data stability : Era5 (PF Jets) 12 nJets≥2 (HLT_Jet140U)nJets≥3 (HLT_Jet140U) (nJets≥3)/(nJets≥2) (HLT_Jet140U) P.Kokkas, Univ. of Ioannina No bad runs were spotted for any era. No slope was observed for any era.

Data over MC : HLT_Jet30U (PF Jets : H T > 200 GeV) 13 nJets ≥2 nJets ≥3 PYTHIA6 TuneZ2 Fall10 MC normalized to the total number of inclusive DiJet events for H T >200GeV.

Data over MC : HLT_Jet50U (PF Jets : H T > 310 GeV) 14 nJets ≥2 nJets ≥3 PYTHIA6 TuneZ2 Fall10 MC normalized to the total number of inclusive DiJet events for H T >310GeV.

Data over MC : HLT_Jet70U (PF Jets : H T > 400 GeV) 15 nJets ≥2 nJets ≥3 PYTHIA6 TuneZ2 Fall10 MC normalized to the total number of inclusive DiJet events for H T >400GeV.

Data over MC : HLT_Jet100U (PF Jets : H T > 560 GeV) 16 nJets ≥2 nJets ≥3 PYTHIA6 TuneZ2 Fall10 MC normalized to the total number of inclusive DiJet events for H T >560GeV.

Data over MC : HLT_Jet140U (PF Jets : H T > 800 GeV) 17 nJets ≥2 nJets ≥3 PYTHIA6 TuneZ2 Fall10 MC normalized to the total number of inclusive DiJet events for H T >800GeV.

Data over MC – R 32 p T, y and phi Data distributions have been compared with PYTHIA6 TuneZ2 Fall10 MC. No major discrepancies have been observed. However there are discrepancies in all kinematic distributions which must be addressed and a systematic error must be assigned to it. Shown bellow is the measured ratio with PF Jets. Multiplicative Correction factor to go to hadron level. P.Kokkas, Univ. of Ioannina18 R 32 with PF Jets Multiplicative Correction factor ≈1-2%

Ratio at Hadron level 19 The measured ratio rises, due to phase space, with H T. Above H T ≈0.8 TeV it reaches a plateau which is most sensitive to α s. Errors are only statistical. PYTHIA6 and MadGraph show better agreement with data however the full systematic error is not yet known. It appears that PYTHIA8 and Alpgen do not agree with the measurement.

Ratio at Hadron level P.Kokkas, Univ. of Ioannina20 Measured ratio seems to be closer to Madgraph (systematics pending).

Ratio at Hadron Level 21 Analysis with Calo Jets gives very similar result.

New result & ICHEP P.Kokkas, Univ. of Ioannina22 The new measurement with 36 pb -1 is in agreement with the previous CMS public result (ICHEP result.)

Summary P.Kokkas, Univ. of Ioannina23 Summary: The ratio of the inclusive three-jet over two-jet cross sections as a function of the total jet transverse energy, H T, has been measured with the CMS detector at the LHC for proton-proton center-of-mass energy 7 TeV using an integrated luminosity of 36 pb -1. Measurements have been performed for jets with p T ≥50 GeV in the rapidity range |y| ≤2.5. The measured ratio rises, due to phase space, with H T. Above H T ≈0.8 TeV it reaches a plateau which is most sensitive to α s. Study of Systematic uncertainties is on the way: Uncertainties due Jet Energy Scale (PAS JME ) are expected to be ≈1%. Systematic uncertainties due to difference in shape between data and MC and pileup are under investigation. PYTHIA6 and MadGraph show better agreement with data however the full systematic error is not yet known. It appears that PYTHIA8 and Alpgen do not agree with the measurement.

Spare 24P.Kokkas, Univ. of Ioannina

Data over MC 25 Data over MC ratios PF Jets P.Kokkas, Univ. of Ioannina

26 Data - MC : HLT_Jet30U (PF Jets : H T > 200 GeV)

27 Data - MC : HLT_Jet50U (PF Jets : H T > 310 GeV)

P.Kokkas, Univ. of Ioannina28 Data - MC : HLT_Jet70U (PF Jets : H T > 400 GeV)

P.Kokkas, Univ. of Ioannina29 Data - MC : HLT_Jet100U (PF Jets : H T > 560 GeV)

P.Kokkas, Univ. of Ioannina30 Data - MC : HLT_Jet140U (PF Jets : H T > 800 GeV)

Method of extracting R 32 P.Kokkas, Univ. of Ioannina31 Inclusive jet cross section Ratio R 32 A B measurement The factor AxB has been used to correct the Calo Level ratio to stable particle level (Hadron Level).

Method of extracting R 32 P.Kokkas, Univ. of Ioannina32 1/ε 3 (1/efficiency) nJets≥3 C Smear3 Smearing correction nJets≥3 ε 2 (efficiency) nJets≥2 1/C Smear2 Smearing correction nJets≥2 With This MC driven method of extracting R 32 from the data does not depend upon the absolute predictions of the MC programs. It does depend on how well the MC distributions describe the shape of the data distributions. As we have seen in the previous slides PYTHIA6 describes well the shapes of the data distributions, hence we are entitle to use them to derive the AxB correction factor to the data.

Ratio PF-Calo Level 33P.Kokkas, Univ. of Ioannina

Ratio for H T <1TeV For H T <1TeV the measured ratio is closer to Madgraph (systematics pending). P.Kokkas, Univ. of Ioannina34