Photon production rates have been studied by the D0 and CDF experiments in
Run II of the Fermilab Tevatron Collider. Measurements of the inclusive
isolated photon cross section and the di-photon cross section are presented,
based on integrated luminosities of 0.3 fb-1 and 0.2 fb-1, respectively. The
results are compared to perturbative QCD calculations in various
approximations.Comment: 4 pages 2 figures, presented at XLIrst Rencontres de Moriond on ``QCD
  and High Energy Hadronic Interactions'', La Thuile, Italy, March 18-25, 2006,
  on behalf of the CDF and D0 collaboration

/Fermilab

Wobisch, M.

English

arXiv

Photon production rates have been studied by the D0 and CDF experiments in Run II of the Fermilab Tevatron Collider. Measurements of the inclusive isolated photon cross section and the di-photon cross section are presented, based on integrated luminosities of 0.3 fb{sup -1} and 0.2 fb{sup -1}, respectively. The results are compared to perturbative QCD calculations in various approximations

UNT Digital Library

hep-ex/0606063FERMILAB-CONF-06-214-EPHOTON CROSS SECTIONS AT ECM = 2 TEVM. WOBISCH a(on behalf of the CDF and DØ collaborations)Fermi National Accelerator Laboratory, Batavia, Illinois 60510, U.S.A.Photon production rates have been studied by the DØ and CDF experiments in Run II ofthe Fermilab Tevatron Collider. Measurements of the inclusive isolated photon cross sectionand the di-photon cross section are presented, based on integrated luminosities of 0.3 fb−1 and0.2 fb−1, respectively. The results are compared to perturbative QCD calculations in variousapproximations.1 IntroductionPhoton cross sections in hadron collisions receive contributions from “prompt” photons whichdirectly emerge from the hard subprocess, and from photons which are produced during frag-mentation. The latter are usually accompanied by hadrons, and their contribution can besignificantly reduced by requiring the photon to be isolated from other particles in the event.Isolated photon cross sections are therefore dominated by prompt photons and are directly sen-sitive to the dynamics of the hard subprocess and to the strong coupling constant αs and theparton density functions (PDFs) of the hadrons. Furthermore, di-photon final states are alsosignatures for various “new” physics processes, such as Large Extra Dimensions and for heavynew particles, such as the Higgs boson, decaying into photons.Photon production has been considered an ideal source of direct information on the gluondensity in the proton 1,2. However, it was observed 3 that not all experimental data are consis-tently described by perturbative QCD (pQCD) calculations in fixed order of the strong couplingconstant, αs. On one hand, it was argued that the existing data may not be consistent3. It wasalso suggested that the phenomenological introduction of intrinsic transverse momentum of theincoming partons may help improve the description of the data and reduce the inconsistencies 4.But this ad-hoc procedure still had a significant model dependence and was not seen to be afundamental solution. In recent global PDF fits photon data have been excluded 5,6.aPresented at: XLIrst Rencontres de Moriond on “QCD and High Energy Hadronic Interactions”, La Thuile,Italy, March 18-25, 2006. (GeV)γTp0 50 100 150 200 250 300 (pb/GeV)η/dT/ dpσ2d-310-210-110110210310  data NLO QCD)γT=pfµ=Fµ=Rµ(CTEQ6.1M -1L = 326 pbDØ (GeV)γTp0 50 100 150 200 250 Data / Theory0.40.60.811.21.4 Data / Theory ratio of data to NLO pQCD (JETPHOX) CTEQ6.1M PDF uncertainty scale dependence)γT and 2pγT=0.5pfµ=Fµ=Rµ(DØ300 -1L = 326 pbFigure 1: The isolated photon cross section as a function of the transverse photon momentum pT (left) and theratio of data and the NLO pQCD calculation from JETPHOX (right).In order to rescue the photon data as a source of additional information on the PDFs, it isimportant either to identify critical missing pieces in theory or to clearly establish inconsistenciesof existing data sets. Precision measurements in new kinematic regions are vital for testing theorypredictions. In this article, we present recent results from the DØ and the CDF experiments onthe inclusive isolated photon cross section and the di-photon cross section, measured in proton-antiproton collisions at a center-of-mass energy of√s = 1.96TeV during Run IIa of the FermilabTevatron Collider.2 Isolated Photon Cross SectionAt lowest order, prompt photons are produced via quark-gluon Compton scattering or quark-antiquark annihilation. Additional contributions to the inclusive photon cross section stemfrom the fragmentation of energetic π0 and η mesons. In the DØ analysis 7, photon can-didates are defined as clusters of electromagnetic calorimeter cells within a cone of radius∆R =√∆η2 + ∆φ2 < 0.2, if more than 95% of the energy is deposited in the electromag-netic part of the calorimeter and the probability of a matching track is below 0.1%. Cosmicbackgrounds and electrons from W decays are removed by requiring that the missing transverseenergy in the event is less than 70% of the transverse photon momentum (pT ). Photons withpT > 23GeV are selected in the range of pseudorapidities |η| < 0.9. The isolation criterionrequires that the energy in a cone of radius ∆R < 0.4 not associated with the photon is lessthan 10% of the photon energy. The main background in the measurement are hadronic jetswith a large electromagnetic fraction. A neural net is trained to discriminate between photonsand background using four sensitive variables 7. The photon purity was determined as a functionof pT by fitting the neural net output for Monte Carlo signal and background to the observeddistribution of photon candidates. The photon pT distribution is corrected for energy resolutionin an iterative unsmearing procedure. The largest contributions to the experimental uncertaintyare due to the purity determination and the photon energy calibration.The results are based on 2.7 million photon candidates in an event sample corresponding toan integrated luminosity of Lint ' 0.3 fb−1. The isolated photon cross section is shown in Fig. 1(left). The cross section falls over five orders of magnitude in 23 < pT < 300GeV, the widestpT range ever covered by a single experiment. The pQCD predictions are computed in next-10 20 30 40 50 60 70 80 90 100-210-1101 > 13 GeV2γT > 14 GeV, E1γTE| < 0.91,2γη|)-1CDF II Data (207 pb/2γγ = mRµ = FµDIPHOX CTEQ5M γ γ = mRµ = FµResBos CTEQ5M PYTHIA norm to data30 40 5000.5)2 (GeV/cγγM)2  (pb/GeV/cγγ/dMσd0 5 10 15 20 25 30 35 40-210-1101 > 13 GeV2γT > 14 GeV, E1γTE|<0.91,2γη|-1CDF II Data 207 pb/2π < γ γΦ∆CDF II Data, /2γγ = mRµ = FµDIPHOX CTEQ5M /2π < γ γΦ∆DIPHOX, γγ = mRµ = FµResBos CTEQ5M PYTHIA norm to data (GeV/c)Tq  (pb/GeV/c)T/dqσd0 0.5 1 1.5 2 2.5 3-110110 > 13 GeV2γT > 14 GeV, E1γTE|<0.91,2γη|)-1CDF Run II Data (207 pb/2γγ = mRµ = FµDIPHOX CTEQ5M γ γ = mRµ = FµResBos CTEQ5M PYTHIA norm to data (rad)γγφ∆  (pb/rad)γγφ∆/dσdFigure 2: The CDF measurement of the di-photon cross section as a function of the di-photon mass Mγγ , thevector-sum of the transverse photon momenta qT , and the difference in azimuthal angle ∆φγγ .to-leading order (NLO) in αs using the program JETPHOX8,9 with CTEQ6.1M PDFs 6, BFGfragmentation functions 10, and renormalization, factorization and fragmentation scales set toµR,F,f = pT . The ratio of data and theory is shown in Fig. 1 (right), and it is obvious that NLOtheory gives a good description of the data over the whole pT range. The scale dependence of theNLO calculation is of the same size as the experimental uncertainties. Uncertainties from theproton PDFs are significantly smaller over most of the pT range. PDF sensitivity can thereforeonly be achieved if both experimental and theoretical uncertainties can be significantly reduced.3 Di-Photon Cross SectionThe leading contributions to di-photon production are from quark-antiquark annihilation andfrom gluon-gluon-scattering. Although the latter process is suppressed by a factor of α2s (as thephotons couple to a quark box), its contribution is still significant at small di-photon masses. Inthis kinematic range the PDFs are probed at small momentum fractions, where the gluon densityis much larger than the quark densities. In addition to the prompt contributions, the di-photoncross section also receives contributions where one or two photons are produced in fragmentationprocesses. The CDF analysis 11 selects photon candidates based on the lateral shower profileand a veto against a matching track. The isolation criterion requires the transverse energy ina cone radius of R = 0.4 around the photon direction, not associated with the photon, to bebelow 1 GeV. In total 427± 59 (stat.) di-photon events in 889 di-photon candidates are selectedin the data sample corresponding to Lint ' 0.2 fb−1.The di-photon cross section is presented in Fig. 2 as a function of di-photon mass (top left),the di-photon transverse momentum qT (top right), and the difference in azimuthal angle ∆φγγbetween the two photons (bottom). The results are compared to different approximations ofpQCD. The program DIPHOX 8 includes NLO matrixelements for both the direct contributionand the fragmentation contribution, and the O(α3s) corrections 12 to gg → γγ have been addedhere to the calculation. The program ResBos13 has implemented the fragmentation contributionsonly at LO but it includes resummation of soft initial-state gluon radiation. Also shown arethe results for PYTHIA 14 (LO matrixelements plus parton shower and fragmentation model)which have been scaled by a factor of two. All calculations give a reasonable overall descriptionof the data, except in specific “critical kinematic regions” in which the unique features of thedifferent calculations are probed. The contributions from fragmentation processes are especiallylarge in the regions of small di-photon masses, large qT , and small ∆φγγ . These regions areonly described by DIPHOX which includes the NLO corrections for the fragmentation process.The qT and the ∆φγγ distributions are directly sensitive to higher-order effects, since in lowestorder both are trivial (qT = 0, ∆φγγ = π). The kinematic regions at low qT and large ∆φγγ areespecially sensitive to soft initial-state gluon emissions. Only ResBos describes this phase spacedue to the resummation of initial-state gluon radiation effects. PYTHIA is not only too low bya factor of two, but it also fails in all “critical regions”. For an overall description of di-photonproduction a full NLO calculation (for direct and fragmentation contributions), combined withO(α3s) gg → γγ corrections plus resummed initial-state contributions would be required.4 SummaryThe CDF and DØ experiments have published the first measurements of the single inclusive pho-ton cross section and the di-photon cross section at a center-of-mass energy of√s = 1.96TeV.The new inclusive isolated photon data extend previous measurements and indicate that the√sand the pT dependence of direct photon production can be adequately described in NLO accu-racy15. In the future, PDF sensitivity can only be achieved, if both experimental and theoreticaluncertainties can be significantly reduced. The di-photon mass, qT and ∆φγγ dependence of thedi-photon cross section can not be simultaneously described by a single existing theory calcula-tion. In different regions of phase space, different contributions are required: full NLO accuracy,in the direct and the fragmentation contributions, resummed initial-state contributions and theO(α3s) corrections to gg → γγ. Since all the single ingredients are available, it should not bedifficult to combine these into a powerful di-photon prediction.References1. H. L. Lai et al., Phys. Rev. D 55, 1280 (1997).2. A. D. Martin et al., Eur. Phys. J. C 2, 287 (1998).3. P. Aurenche et al., Eur. Phys. J. C 9, 107 (1999) [arXiv:hep-ph/9811382].4. A. D. Martin et al., Eur. Phys. J. C 4, 463 (1998).5. A. D. Martin et al., Phys. Lett. B 604, 61 (2004).6. J. Pumplin et al., JHEP 0207, 012 (2002) [arXiv:hep-ph/0201195].7. V. M. Abazov et al. [DØ Collab.], accepted by Phys. Lett. B, [arXiv:hep-ex/0511054].8. T. Binoth, J. P. Guillet, E. Pilon and M. Werlen, Eur. Phys. J. C 16, 311 (2000).9. S. Catani, M. Fontannaz, J. P. Guillet and E. Pilon, JHEP 0205, 028 (2002).10. L. Bourhis, M. Fontannaz and J. P. Guillet, Eur. Phys. J. C 2, 529 (1998).11. D. Acosta et al. [CDF Collaboration], Phys. Rev. Lett. 95, 022003 (2005).12. Z. Bern, L. J. Dixon and C. Schmidt, Nucl. Phys. Proc. Suppl. 116, 178 (2003).13. C. Balazs, E. L. Berger, S. Mrenna and C. P. Yuan, Phys. Rev. D 57, 6934 (1998).14. T. Sjostrand et al., Comp. Phys. Comm. 135, 238 (2001).15. P. Aurenche, M. Fontannaz, J. P. Guillet, E. Pilon and M. Werlen, [arXiv:hep-ph/0602133].

Photon cross-sections at ECM = 2-TeV

http://digital.library.unt.edu/ark:/67531/metadc873896/

Photon Cross Sections at Ecm = 2 TeV

Abstract

Similar works

Full text

Available Versions

UNT Digital Library