1. S. Gascon-Shotkin Moriond QCD March 15 20121 Photon Production at E cm = 7 TeV Suzanne Gascon-Shotkin Institut de Physique Nucléaire de Lyon Université Claude Bernard Lyon 1 IN2P3-CNRS 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

2. S. Gascon-Shotkin Moriond QCD March 15 20122 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 Diphoton Inclusive Photon/ Photon + Jet Born BoxBox HOBoxHOBox 2-Frag 1-Frag Compton Annihilation Frag

3. S. Gascon-Shotkin Moriond QCD March 15 20123 D.d'Enterria & J.Rojo, arXiv:1202.1762 What direct photon measurements at the LHC can tell us What direct photon measurements at the LHC can tell us R.Ichou & D.d'Enterria, PRD82 (2010) 014015 Compared to the Tevatron, fraction of Compton scattering remains ~80% with increasing 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 probe gluon PDF at unprecedented Et Can reduce gluon NLO PDF error by ~20% Can reduce gluon NLO PDF error by ~20% Leads to ~20% reduction in  (gg → H) PDF uncertainty (D’Enterria & Rojo 12) Leads to ~20% reduction in  (gg → H) PDF uncertainty (D’Enterria & Rojo 12) Direct measurement of background processes to Higgs boson production Direct measurement of background processes to Higgs boson production

4. S. Gascon-Shotkin Moriond QCD March 15 20124 Anatomy of a differential direct photon cross section measurement Experimental measurement: We measure the differential isolated cross section in bins of observables q i of width  q i 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

5. S. Gascon-Shotkin Moriond QCD March 15 20125 Signal Fraction Measurement techniques:  + X Phys. Rev. D 84, 052011  Phys. Lett. B 706 (2011) 150-167 ATLAS: 'ABCD' double-sideband counting method in EM shape and Isolation energy sum (cross-checked with calorimeter isolation template) : 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) Correlation between 2 variables, set to 1 and systematic assigned Signal Leakage

6. S. Gascon-Shotkin Moriond QCD March 15 20126 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 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 Signal Fraction Measurement techniques:  + X ATLAS: ‘Event-weighting’ method (generalisation of that used at Tevatron, see for example Phys. Rev. Lett.107, 102003 (2011).) Signal fraction : 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 Et Iso., n i the observed outcome of the ith event,  a matrix expressing efficiency for an event in true state j’  j, j  jj  to yield observed state j Matrix coefficients obtained Matrix coefficients obtained from data-driven isolation PDFs Xcheck: ABCD + 2d template Xcheck: ABCD + 2d template Phys. Rev. D 85 (2012) 012003 Phys. Rev. D 85 (2012) 012003

7. S. Gascon-Shotkin Moriond QCD March 15 20127 Comparison to pQCD fixed-order calculation: JETPHOX (Aurenche, Belghobsi, Catani, Fontannaz, Guillet, Heinrich, Pilon, Werlen) d  /dE T for |  |<2.5 (CMS) 2.37 (ATLAS), 15(ATLAS) 25(CMS) GeV<E T <400 GeV Correction to theory prediction from underlying event/hadronisation: ~2.5% (CMS) Systematics-limited for E T <120 GeV, exp: ~7-20% theoretical: dominated by scale (~10%) Inclusive Differential Cross Sections:  + X Phys. Rev. D 84, 052011  Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013 ATL-PHYS-PUB-2011-013

8. S. Gascon-Shotkin Moriond QCD March 15 20128 Phys. Rev. D 84, 052011 Slight overprediction at ET<50 GeV for both experiments Inclusive Differential Cross Sections:  + X  Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013 ATL-PHYS-PUB-2011-013

9. S. Gascon-Shotkin Moriond QCD March 15 20129 Opposite trends at low E T values (<~50 GeV), possible explanations? xT=2pT/√s ~0.003 (Tevatron) ~0.0015 (LHC) Different regimes of xT=2pT/√s ~0.003 (Tevatron) ~0.0015 (LHC) Different maximum parton- level Isolation E T : 2GeV CDF, 4-5 GeV LHC Regime dominated by fragmentation Reasonable for such a rapid E T dependence to come from physics? An open question for theorists and experimentalists Comparison with Tevatron Measurements Phys. Rev. D 84, 052011  Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013 ATL-PHYS-PUB-2011-013

10. S. Gascon-Shotkin Moriond QCD March 15 201210 World data summary, obtain power slope of n=4.5 when scale by x T, supports pQCD hypothesis (similar plot shown by K. Okada Tuesday) Inclusive Differential Cross Sections:  + X D.d'Enterria & R.Ichou, HEP-EPS'11 Proceeds. xT=2pT/√s

11. S. Gascon-Shotkin Moriond QCD March 15 201211 Differential Cross Sections: ATLAS  + Jets 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) d  /dE T  for three different rapidity ranges of the leading-pT jet (anti-kt, radius parameter= 0.4), in same- and opposite-sign configurations of   X y jet, |   | 25 GeV, |y jet | 20 GeV,  R ,jet > 1.0 http://arxiv.org/abs/1203.3161 New!New!

12. S. Gascon-Shotkin Moriond QCD March 15 201212 Same 'ABCD' double-sideband method for signal fraction extraction as for  + X Correction to theory prediction from underlying event/pileup/hadronisation: ~0.85-0.92,  1 for high E T Systematics-limited, typical errors, exp: ~3-40%, depending on rapidity/configuration, highest for lowest- E T  photons; theoretical: dominated by scale (~20-40%) Results consistent with those for  + X (data overprediction for E T <~40 GeV). Important test of pQCD: Good data-theory agreement even in high-fragmentation (opp-sign) configurations Differential Cross Sections: ATLAS  + Jets http://arxiv.org/abs/1203.3161

13. S. Gascon-Shotkin Moriond QCD March 15 201213 Comparison to pQCD predictions to order  s   : 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) d  /dQ for Q={m   p t ,  , cos  * (CMS)}, E T >{20, 23 GeV}, |  | 0.45 (CMS), E T >{16, 16 GeV}, |  | 0.4 m  : Slight underprediction for ATLAS at lowest values (low   2  4 NNLO [see later]… ) Differential Cross Sections:  + X JHEP 01 (2012) 133 Phys. Rev. D 85 (2012) 012003 Phys. Rev. D 85 (2012) 012003

14. S. Gascon-Shotkin Moriond QCD March 15 201214   : 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 Differential Cross Sections:  + X Phys. Rev. D 85 (2012) 012003 Phys. Rev. D 85 (2012) 012003 JHEP 01 (2012) 133

15. S. Gascon-Shotkin Moriond QCD March 15 201215 p t , : Familiar ‘shoulder’ (underprediction) seen by both experiments as well as in prior Tevatron measurements, combination of Et threshholds with non-back-to back topology Differential Cross Sections:  + X Phys. Rev. D 85 (2012) 012003 Phys. Rev. D 85 (2012) 012003 JHEP 01 (2012) 133

16. S. Gascon-Shotkin Moriond QCD March 15 201216  + 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 Differential Cross Sections:  + X JHEP 01 (2012) 133 Underprediction for high values seen also by Tevatron experiments JHEP 01 (2012) 133 cos  *=tanh(  y  

17. S. Gascon-Shotkin Moriond QCD March 15 201217 Comparison with Theory: Open question arXiv1201.3084v1 D. de Florian, L. Cieri et al arXiv:1110.2375 (JHEP 01 (2012) 133) D0: PLB690 (2010) 108 LHC-Higgs XS WG YR2 Phys. Rev. D 85 (2012) 012003 Phys. Rev. D 85 (2012) 012003 First theoretical prediction of NNLO direct contributions (de Florian, Cieri, et al 11, cf talk Sunday), to be made public (2  NNLO) Remarkable improvement in CMS data-theory comparison for  , stay tuned for other observables and addition of box corrections Caveat: Smooth (Frixione) not solid cone isolation, no frag.

18. S. Gascon-Shotkin Moriond QCD March 15 201218 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 E T <~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  -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 E CM =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 E CM = 7 and 8 TeV. Reexamine coherence of parton- and reconstruction-level isolation criteria, progress with theorists in understanding of isolation/fragmentation relationship. Exploit direct measurement of background processes for H   search

19. S. Gascon-Shotkin Moriond QCD March 15 201219 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

20. S. Gascon-Shotkin Moriond QCD March 15 201220 Generators/calculators of SM  +X processes RESBOS Balazs, Berger, Mrenna, Yuan, hep-ph/9712471, 1997 DIPHOX Binoth, Guillet, Pilon, Werlen, hep-ph/9911340, 2000 gamma2MC, NLO Bern, Dixon, Schmidt, hep-ph/0211216, 2002 2-frag : DIPHOX only (NLO)‏ Resbos only 1-frag : - LO, effectively in Resbos - NLO in Diphox BORN + FRAG (and NLO corrections)‏ BOX (and NLO corrections)‏ FIXED ORDER : NLO NLO with NNLL Resummation BORN (up to NNLO corrections) 2gammaNNLO Catani et al, hep-ph/11102375, 2011 qT SUB : NNLO