Report

Photon Production at Ecm= 7 TeV Suzanne Gascon-Shotkin Institut de Physique Nucléaire de Lyon Université Claude Bernard Lyon 1 IN2P3-CNRS on behalf of the CMS and ATLAS Collaborations Rencontres de Moriond QCD/High-Energy Interactions March 10-17 2012 La Thuile Valle d’Aosta S. Gascon-Shotkin Moriond QCD March 15 2012 1 Outline What direct photon measurements at the LHC can tell us Anatomy of a differential direct photon cross section measurement Signal fraction measurement techniques Differential cross sections, isolated inclusive photons Comparison with Tevatron measurements Differential cross sections, isolated photon + jets (ATLAS) Differential cross sections, isolated inclusive diphoton pairs Comparison with theoretical predictions and open questions Conclusions and perspectives Inclusive Photon/ Photon + Jet Compton Diphoton Born Box HO Box Annihilation 2-Frag Frag 1-Frag S. Gascon-Shotkin Moriond QCD March 15 2012 2 What direct photon measurements at the LHC can tell us Compared to the Tevatron, fraction of Compton scattering remains ~80% with increasing Et_gamma (isolated photons) [Ichou, d’Enterria 10] Can probe gluon PDF at unprecedented Et Can reduce gluon NLO PDF error by ~20% Leads to ~20% reduction in s (gg → H) PDF uncertainty (D’Enterria & Rojo 12) Direct measurement of background processes to Higgs boson production D.d'Enterria & J.Rojo, arXiv:1202.1762 R.Ichou & D.d'Enterria, PRD82 (2010) 014015 D.d'Enterria & J.Rojo, arXiv:1202.1762 S. Gascon-Shotkin Moriond QCD March 15 2012 3 Anatomy of a differential direct photon cross section measurement d ns Isolated n i 1 dqi 1 n i 1 qi N U L Experimental measurement: We measure the differential isolated cross section in bins of observables qi of width qi From a set of identified isolated photon candidates we estimate N, the number of signal photons or photon pairs in each bin To arrive at the true value of N in each bin: We account for bin-to-bin migrations arising from detector resolution (unfolding factors U) We correct N for for =Trigger, reconstruction and photon identification efficiencies Comparison with (usually parton-level) theoretical predictions: Impose a parton-level isolation Correct for effect on isolation efficiency due to underlying event and hadronisation (or assign a systematic error) All measurements presented here were performed on the 2010 dataset with ~36 pb-1 of integrated luminosity S. Gascon-Shotkin Moriond QCD March 15 2012 4 Signal Fraction Measurement techniques: g + X Phys. Rev. D 84, 052011 CMS: Combined photon Isolation and Converted photon Et/pt templates (MC w/ data validation), 2-comp. (un)binned EML fit, final results combined with Best Linear Unbiased Estimator method (Lyons et al) ATLAS: 'ABCD' double-sideband counting method in EM shape and Isolation energy sum (cross-checked with calorimeter isolation template) : Phys. Lett. B 706 (2011) 150-167 Correlation between 2 variables, set to 1 and systematic assigned S. Gascon-Shotkin Moriond QCD March 15 2012 Signal Leakage 5 Signal Fraction Measurement techniques: g g + X d 4s Isolated dmgg dqTgg dgg d cosgg * Phys. Rev. D 85 (2012) 012003 1 mgg qTgg ( gg ) cosgg* N gg unfolded L ATLAS: ‘Event-weighting’ method (generalisation of that used at Tevatron, see for example Phys. Rev. Lett.107, 102003 (2011).) Signal fraction : where j={PP,PF,FP,FF} wrt selection criterion on EtIso. , ni the observed outcome of the ith event, a matrix expressing efficiency for an event in true state j’{gg, gj, jg, jj} to yield observed state j Matrix coefficients obtained from data-driven isolation PDFs Xcheck: ABCD + 2d template CMS: Electromagnetic Calorimeter Isolation template, signal sample (random cones) and background sample (photon candidates with a single clean track in the isolation cone) entirely from data, validated with additional data samples (W, Z and 2 clean-track sample [shown]). Binned maximum likelihood fit to the isolation distributions of the two photons S. Gascon-Shotkin Moriond QCD March 15 2012 6 Inclusive Differential Cross Sections: g + X Comparison to pQCD fixed-order calculation: JETPHOX (Aurenche, Belghobsi, Catani, Fontannaz, Guillet, Heinrich, Pilon, Werlen) ds/dET for |h|<2.5 (CMS) 2.37 (ATLAS), 15(ATLAS) 25(CMS) GeV<ET<400 GeV Correction to theory prediction from underlying event/hadronisation: ~2.5% (CMS) Phys. Lett. B 706 (2011) 150-167, Systematics-limited for ET<120 GeV, exp: ~7-20% ATL-PHYS-PUB-2011-013 theoretical: dominated by scale (~10%) Phys. Rev. D 84, 052011 S. Gascon-Shotkin Moriond QCD March 15 2012 7 Inclusive Differential Cross Sections: g + X Slight overprediction at ET<50 GeV for both experiments Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013 Phys. Rev. D 84, 052011 S. Gascon-Shotkin Moriond QCD March 15 2012 8 Comparison with Tevatron Measurements Opposite trends at low ET values (<~50 GeV), possible explanations? Different regimes of xT=2pT/√s ~0.003 (Tevatron) ~0.0015 (LHC) Different maximum partonlevel Isolation ET : 2GeV CDF, 4-5 GeV LHC Phys. Rev. D 84, 052011 Regime dominated by fragmentation Reasonable for such a rapid ET dependence to come from physics? Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013 S. Gascon-Shotkin Moriond QCD March 15 2012 An open question for theorists and experimentalists 9 Inclusive Differential Cross Sections: g + X World data summary, obtain power slope of n=4.5 when scale by xT, supports pQCD hypothesis (similar plot shown by K. Okada Tuesday) D.d'Enterria & R.Ichou, HEP-EPS'11 Proceeds. S. Gascon-Shotkin Moriond QCD March 15 2012 xT=2pT/√s 10 Differential Cross Sections: ATLAS g + Jets New! http://arxiv.org/abs/1203.3161 Motivation: Different photon-jet angular configurations correspond to regions with different relative fragmentation contribution, also different values of {x,Q2} Comparison to pQCD fixed-order calculation: JETPHOX, PYTHIA (Sjostrand et al) SHERPA (Krauss et al) ds/dETg for three different rapidity ranges of the leading-pT jet (anti-kt, radius parameter= 0.4), in same- and opposite-sign configurations of hg X yjet, |hg|<1.37, ETg>25 GeV, |yjet|<4.4, pTjet>20 GeV, Rg,jet > 1.0 S. Gascon-Shotkin Moriond QCD March 15 2012 11 Differential Cross Sections: ATLAS g + Jets http://arxiv.org/abs/1203.3161 Same 'ABCD' double-sideband method for signal fraction extraction as for g + X Correction to theory prediction from underlying event/pileup/hadronisation: ~0.85-0.92, 1 for high ET Systematics-limited, typical errors, exp: ~3-40% , depending on rapidity/configuration, highest for lowest- ETg photons; theoretical: dominated by scale (~20-40%) Results consistent with those for g + X (data overprediction for ET<~40 GeV). Important test of pQCD: Good data-theory agreement even in high-fragmentation (opp-sign) configurations S. Gascon-Shotkin Moriond QCD March 15 2012 12 Differential Cross Sections: gg + X Comparison to pQCD predictions to order asa2: DIPHOX (Binoth, Guillet, Pilon, Werlen) with gamma2MC (Bern,Dixon, Schmidt) [CMS], DIPHOX and ResBos (Balazs, Berger, Mrenna, Nadolsky, Schmidt, Yuan) [ATLAS]. Correction for UE/hadronisation: ~5% (CMS) ds/dQ for Q={mgg, ptgg, gg, cosq* (CMS)}, ET >{20, 23 GeV}, |h|<1.44 and |h|<2.5, R(gg)>0.45 (CMS), ET >{16, 16 GeV}, |h|<1.37 and |h|<2.37, R(gg)>0.4 mgg : Slight underprediction for ATLAS at lowest values (low gg ,24 NNLO [see later]… Phys. Rev. D 85 (2012) 012003 ) JHEP 01 (2012) 133 S. Gascon-Shotkin Moriond QCD March 15 2012 13 Differential Cross Sections: gg + X gg : Significant underprediction for all but highest values (<~2.8), corresponds to collinear regime where no LO terms contribute, very sensitive to predictions of HO terms Phys. Rev. D 85 (2012) 012003 JHEP 01 (2012) 133 S. Gascon-Shotkin Moriond QCD March 15 2012 14 Differential Cross Sections: gg + X ptgg, : Familiar ‘shoulder’ (underprediction) seen by both experiments as well as in prior Tevatron measurements, combination of Et threshholds with non-back-to back topology Phys. Rev. D 85 (2012) 012003 JHEP 01 (2012) 133 S. Gascon-Shotkin Moriond QCD March 15 2012 15 Differential Cross Sections: gg + X cosq*=tanh( ygg)/2 gg + X Uncertainties: Experimental: Statistical error ~20% Systematics typically order of 10% dominated by knowledge of signal/background template shapes (CMS), definition of control sample (ATLAS) Theoretical: 10-20% dominated by scale uncertainties JHEP 01 (2012) 133 Underprediction for high values seen JHEP 01 (2012) 133 also by Tevatron experiments S. Gascon-Shotkin Moriond QCD March 15 2012 16 Comparison with Theory: Open question (JHEP 01 (2012) 133) D. de Florian, L. Cieri et al arXiv:1110.2375 Phys. Rev. D 85 (2012) 012003 D0: PLB690 (2010) 108 LHC-Higgs XS WG YR2 arXiv 1201.3084v1 First theoretical prediction of NNLO direct contributions (de Florian, Cieri, et al 11, cf talk Sunday), to be made public (2gNNLO) Remarkable improvement in CMS data-theory comparison for gg , stay tuned for other observables and addition of box corrections Caveat: Smooth (Frixione) not solid cone isolation, no frag. S. Gascon-Shotkin Moriond QCD March 15 2012 17 Conclusion and Perspectives Direct photon measurements at the LHC performed with the 2010 dataset corresponding to ~36 pb-1 of integrated luminosity are starting to be used to constrain gluon and gluon fusion Higgs boson production PDF uncertainties Differential cross sections, isolated inclusive photons Measurements are already systematics-limited Data in agreement with theory predictions and Tevatron measurements except for ET <~50 GeV, where there are opposite tendencies, an open question New ATLAS photon + jet differential cross-section measurement in three different ranges of leading-pT jet y, results consistent with inclusive measurement, agreement with theory in all g-jet configurations, even those with larger fragmentation contributions. Differential cross sections, isolated inclusive diphoton pairs Measurements consistent with those of the Tevatron experiments Becoming systematics-limited New NNLO calculation of direct contributions seems to resolve a large part of collinear-regime discrepancies, need to take into account fragmentation and HO box Perspectives: 2011 ~5fb-1 additional, 2012 ~15fb-1 at ECM =8 TeV Possible measurements: Differential cross sections: Isolated inclusive, photon + jets, inclusive diphoton pair , diphoton + jets, photon + hf, all with additional observables. Ratios between cross-sections at ECM = 7 and 8 TeV . Reexamine coherence of parton- and reconstruction-level isolation criteria, progress with theorists in understanding of isolation/fragmentation relationship. S. Gascon-Shotkin Moriond QCD March 15 2012 18 Exploit direct measurement of background processes for Hgg search Acknowledgements Thanks to: L. Carminati , V. Cerulli , V. Chetluru, L. Cieri , D. D'Enterria, D. de Florian, P. Gras , M. Grazzini, R. Ichou, J.-P. Guillet, K. Kousouris, S. Linn, D. Lopez Mateos, Y. Ma, L. Malgeri, G. Marchiori, J. Owens, E. Pilon, A. de Roeck, J. Rojo, S.-S. Yu S. Gascon-Shotkin Moriond QCD March 15 2012 19 Generators/calculators of SM gg+X processes DIPHOX RESBOS gamma2MC, NLO Bern, Dixon, Schmidt, Balazs, Berger, Mrenna, Binoth, Guillet, Pilon, Werlen, Yuan, hep-ph/9712471, 1997 hep-ph/0211216, 2002 hep-ph/9911340, 2000 2gammaNNLO Catani et al, hep-ph/11102375, 2011 qT SUB : NNLO FIXED ORDER : NLO NLO with NNLL Resummation FIXED ORDER : NLO BORN + FRAG (and NLO corrections) BOX (and NLO corrections) Resbos only BORN (up to NNLO corrections) 1-frag : - LO, effectively in Resbos - NLO in Diphox 2-frag : DIPHOX only (NLO) S. Gascon-Shotkin Moriond QCD March 15 2012 20