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## Photon Production at E cm = 7 TeV

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**S. Gascon-Shotkin Moriond QCD March 15 2012**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**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 Box Born Annihilation Frag 2-Frag 1-Frag HO Box**What direct photon measurements at the**LHC can tell us S. Gascon-Shotkin Moriond QCD March 15 2012 D.d'Enterria & J.Rojo, arXiv:1202.1762 D.d'Enterria & J.Rojo, arXiv:1202.1762 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 R.Ichou & D.d'Enterria, PRD82 (2010) 014015**S. Gascon-Shotkin Moriond QCD March 15 2012**Anatomy of a differential direct photon cross section measurement • Experimental measurement: • We measure the differential isolated cross section in bins of observables qi of width Dqi • 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 e=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**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) : Correlation between 2 variables, set to 1 and systematic assigned Phys. Lett. B 706 (2011) 150-167 Signal Leakage**S. Gascon-Shotkin Moriond QCD March 15 2012**Signal Fraction Measurement techniques: g g + X 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, e 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 Phys. Rev. D 85 (2012) 012003 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**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) Systematics-limited for ET<120 GeV, exp: ~7-20% theoretical: dominated by scale (~10%) 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**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**Comparison with Tevatron Measurements Opposite trends at lowET values (<~50 GeV), possible explanations? Different regimes of xT=2pT/√s ~0.003 (Tevatron)~0.0015 (LHC) Different maximum parton-level Isolation ET : 2GeV CDF, 4-5 GeV LHC Regime dominated by fragmentation Reasonable for such a rapid ET dependence to come from physics? An open question for theorists and experimentalists Phys. Rev. D 84, 052011 Phys. Lett. B 706 (2011) 150-167, ATL-PHYS-PUB-2011-013**S. Gascon-Shotkin Moriond QCD March 15 2012**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. xT=2pT/√s**S. Gascon-Shotkin Moriond QCD March 15 2012**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, DRg,jet > 1.0**S. Gascon-Shotkin Moriond QCD March 15 2012**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**JHEP 01 (2012) 133 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, Dfgg, cosq* (CMS)}, ET >{20, 23 GeV}, |h|<1.44 and |h|<2.5, DR(gg)>0.45 (CMS), ET >{16, 16 GeV}, |h|<1.37 and |h|<2.37, DR(gg)>0.4 mgg : Slight underprediction for ATLAS at lowest values (low Dfgg ,24 NNLO [see later]…) Phys. Rev. D 85 (2012) 012003**S. Gascon-Shotkin Moriond QCD March 15 2012**Differential Cross Sections:gg + X Dfgg : Significant underprediction for all but highest values (<~2.8), corresponds to collinear regime whereno 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**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**JHEP 01 (2012) 133 Differential Cross Sections:gg + X • 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 cosq*=tanh(Dygg)/2 Underprediction for high values seen also by Tevatron experiments JHEP 01 (2012) 133**S. Gascon-Shotkin Moriond QCD March 15 2012**Comparison with Theory: Open question (JHEP 01 (2012) 133) D. de Florian, L. Cieri et al arXiv:1110.2375 LHC-Higgs XS WG YR2 Phys. Rev. D 85 (2012) 012003 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 Dfgg , stay tuned for other observables and addition of box corrections Caveat: Smooth (Frixione) not solid cone isolation, no frag. D0: PLB690 (2010) 108**S. Gascon-Shotkin Moriond QCD March 15 2012**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-1at 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. • Exploit direct measurement of background processes for Hgg search**S. Gascon-Shotkin Moriond QCD March 15 2012**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 Un grand merci to the organisers!**S. Gascon-Shotkin Moriond QCD March 15 2012**Generators/calculators of SM +X processes gamma2MC, NLO Bern, Dixon, Schmidt, hep-ph/0211216, 2002 2gammaNNLO Catani et al, hep-ph/11102375, 2011 RESBOS Balazs, Berger, Mrenna, Yuan, hep-ph/9712471, 1997 DIPHOX Binoth, Guillet, Pilon, Werlen, hep-ph/9911340, 2000 qT SUB : NNLO FIXED ORDER : NLO FIXED ORDER : NLO NLO with NNLL Resummation 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)