Photon plus jet production has been studied by the D0 and CDF experiments in
Run II of the Fermilab Tevatron Collider at a center of mass energy of
sqrt{s}=1.96 TeV. Measurements of the inclusive photon plus jet, di-photon and
photon plus b jet cross section are presented. They are based on integrated
luminosities between 0.2 fb^-1 and 1.1 fb^-1. The results are compared to
perturbative QCD calculations in various approximations.Comment: 4 pages, 4 figures, XLIII Rencontres de Moriond, QCD and High Energy
  Interactions, 200

Sonnenschein, Lars

English

arXiv

Photon plus jet production has been studied by the D0 and CDF experiments in Run II of the Fermilab Tevatron Collider at a center of mass energy of sqrt{s}=1.96 TeV. Measurements of the inclusive photon plus jet, di-photon and photon plus b jet cross section are presented. They are based on integrated luminosities between 0.2 fb^-1 and 1.1 fb^-1. The results are compared to perturbative QCD calculations in various approximations.Photon plus jet production has been studied by the D0 and CDF experiments in Run II of the Fermilab Tevatron Collider at a center of mass energy of sqrt{s}=1.96 TeV. Measurements of the inclusive photon plus jet, di-photon and photon plus b jet cross section are presented. They are based on integrated luminosities between 0.2 fb^-1 and 1.1 fb^-1. The results are compared to perturbative QCD calculations in various approximations

CERN Document Server

Rencontres de Moriond 2008 PHOTON PLUS JET CROSS SECTIONS AT THE TEVATRON L. Sonncnschcin CERN and LPNHE Paris, Universites Paris VI, VII Photon plus jet production has been studied by the 00 and CDF experiments in Run II of the Fermilab Tevatron Collidcr at a center of mass energy of ./S = l.!Jti TeY. Measurements of the inclusive photon plus jet, di-photon and photon plus b jet cross section are presented. They are ba.se<l on i11tegrate<l lumiuoi;ities between 0.2 fb - 1 and 1.1 fb - 1 • The results are compared to perturbative QCD calculations in various approximations. 1 Introduction Photons originating from t.he hard subprocess and produced during fragmentation cont.ribute t.o photon cros.<; sf'ct.ions in hadron~hadron collisions. The contribution of fragment.at.ion photons can be significantly reduced by isolation requirements. Thus, isolated photon cross sections are sensitive tu the dynamics uf the hard subprocess, tu the strung coupling wustant a 5 an<l tu the partuu distribution functions (PDF's) uf the culli<ling hadrons. In particular di-phutun final states provide also a si1?;nature of many interesting physics processes, such as H -+ // production or large extra dimensions. 2 Photon plus jet cross section D0 has measured the photon plus jet cross section2 based on an integrated luminosity of 1.1 fu- 1 • Photon candidate.<; are defined as dusters of electromagm~tic (EM) calorimeter cells within a cone of radius R = 0.2 in the space of pseudorapidity T/ and aximuthal angle cp, if more than 963 of the deteded energy is located in the EM layers of the calorimeter and the probability for a track match is beluw 0.001. As isulatiun criterion the tran~verse energy not CJSsuciated to the photon in a cone of radius R = 0.4 around the photon direction has to be less than 0.07 times the energy of the phutou. Ilackgruuu<ls from cusmics au<l elt.'t:trons frum lV bosun decays are vetoed by a missing transverse energy requirement of $r < 12.5 GeV + 0.36p}. Electromagnetic duster 199 . . Rencontres de Moriond 2008 00 Runll PreWminary • . . L I A9gion 1: yl '91iiCC,SS fWVloii2: y&;.tlnCC, OS Aegk:lr\3: TiiCC, )It.In EC, SS Aegbi 4: 't n CC. jll In EC, OS NLOOCD CTEO 6M µR ~ r=PT°f(y") -· 2-3 -· ' · .. '"'· ..... ~ 5l (; 1.6 00 00 Run II Preliminary L,.= 11 lb·' • ratio or data 1o lheory 1heor scale dependence Prdependent uncenainty -... •• • . . • 0<111<1 0< T\lallll:l,.<08 7_8% is oVEnll ~,.>15GeV normalization uncertainty 50 100 150 200 250 300 IJ\'.(GeV) Figure 1: Differential photon plus jC't cross sect ion as a fund ion of t.he photon transverse momentum in four different kinematic regions (left), the data/theory ratio in one kinematic region. (right). and track information is fed into a neural network (NN) to increase the photon purity further. Central photons (1111 < 1.0) with a transverse momentum above 30 GeV are selected. Jets are defined in the energy scheme by the Run II midpoint cone algorithm1 with a radius of R = 0.7 . The jets with a transverse momentum above 15 GeV are selected in the central (lr1iP.t ! < 0.8) or the forward (1.5 < lr~etl < 2.5) region. Finally, the photon and the leading hadrouic jet have to be separated by 6.R(t, jet) > 0.7. Isolated photons from photon plus jet production come predominantly unaltered from the hard subprocess. Thus, they provide a direct probe of the hard scattering dynamics. At low transverse photon momentum the Compton scattering process qg --+ q/ dominates, which can be exploited to probe the gluon density of the colliding hadrons at low .T where quarks are constrained by HERA data. At higher transverse momenta the quark anti-quark annihilation process qij--+ g~r Lewmes more important. The production of photons by quark fragmentation is highly suppressed by the photon isolation. The differential photon plus jet cross section is measured as a function of the transverse photon momentum. In fig. 1 its p}-weighted average, obtained by a fit in each bin is plotted for four kinematic regions. distinguished by same side (SS) versus opposite side (OS) photon and jet. and distinguished by central (CC) versus forward (EC) jet. In comparison the :'-JLO pQCD prediction of .JETPHOX3 is shown, making use of the CTEQ6.H.-f PDF's and BGP fragmentation functions. The scales have been chosen° to be µR.P.f = p}f(y*) with J (y•) = J(l + exp(-2ly*l))/2 and y* = 0.5(77-Y -rrict). The right plot shows the data/theory ratio in the first region (same side photon and central jet). The scale variation by a factor of two up and down is not. able to describe the data at medium transverse photon momenta. Previous ohsf'rvations of UA2, CDF and D0 show a similar variation. The systematics can be further reduced in considering the ratio of two regions. The ratios of region 1/region 3 and region 2/region 3 exhibit quantitative disagreement Lctween data and theory. 3 Di-photon cross sections Prompt di-photon production has been measured by CDF7 using a data sample of 207 pb- 1. Central photon candidates (177-yl < 0.9) arc selected based on lateral shower profile, preshower hit and no associated track requirements. As isolation criterion the transverse energy not associated to the photon in a cone of radius R. = 0.4 around the photon direction has not to exceed 1 GeV. Leading contributions to di-photon production come from quark-antiquark annihilation and 200 Rencontres de Moriond 2008 ,. Figure 2: Differential di-photon cross section as a function of the invariant di-photon mass m.,., (left) and as a function of the transverse momentum qr of the di-photon system (righl), in comparison to the predictions of DIPHOX (solid line), ResBos (dashed line) and Pythia normalised to data (dotted line). gluon-gluon scattering. A further contribution comes from quark fragmentation into a photon, which is suppressed by photon isolation but relevant in different phase space regions. Fig. 2 shows the differential di-photon cross section as a function of the invariant di-photon mass (left) and the transverse momentum of the di-photon system (right) in comparison to the theoretical predictions of DIPHO:x.8, ResBo~ and LO Pythia1° increased by a factor of two to match the data. While DIPHOX and ResBos contain the ~LO contribution to prompt di-photon production, only DIPHOX has a NLO fragmentation contribution for one or two photons, relevant at low mass, high qr and small D.</J"IT DIPHOX takes also into account the NKLO contribution of the gg ---> TY process, indicated in the inset of the invariant mass distribution. ResBos in turn has only a LO fragmentation contribution but takes into account resummed initial state gluon radiation which is relevant at low qr. Describing the di-photon production in all regions of the phase space simultaneously requires a full ~LO calculation, taking 0( a~) gg ---> 'Y'Y corrections and resummed initial state gluon radiation into account. CDF also searched for gg ---> TY with no other particles produced, i.e. exclusive pj5 ---> p + II + fi. Two candidate events were found11 consistent with theoretical expectations1 2 . 4 Photon plus b jet cross sections Photon plus heavy flavour (HF) jet production is dominated by the process g+ HF---> "(+HF. This can be exploited to set constraints on the HF content of PDF's. The photon plus b jct production cross section has been measured by CDF13 using an inte-grated luminosity of 340 pb- 1. A central photon (l1hl < 1.1) with a transverse energy above 26 GeV and a jet with a transverse energy above 20 GeV within 1111 < 1.5 and a secondary ver-tex are selected. The invariant mass distribution of the secondary vertex of simulated bottom, charm and light quark jet contributions is fitted to the data in order to subtract the charm and light background. This procedure is done separately in each bin of photon transverse energy to determine the differential photon plus b jet cross section as a function of the photon transverse energy. The data is in good agreement with the predictions of LO Pythia and Herwig15 , making use of the CTEQ5L16 PDF's. A similar CDF analysis14 makes use of a dedicated photon plus displaced track trigger, exploiting an integrated luminosity of 208 pb- 1. The transverse energy requirement of the central photon (111"11 < 1.1) has been lowered to 12 GeV which extends the used phase space considerably. As before, a jet with a transverse energy above 20 GeV within lr1I < 1.5 and an associated secondary vertex is required. The invariant mass distribution of the secondary vertex is fitted and the differential cross section as a function of photon transverse energy as well as 201 Rencontres de Moriond 2008 the differential cross section as a function of the jet transverse energy arc determined. The total cross section amounts to 90.5±6.0(stat.) !i1: ~(sys.) pb. The LO prediction of Pythia with CTEQ5L PDF's slightly underestimates the cross section, whose accuracy is already systematics limited, dominated by the tracking efficiency uncertainty. 5 Summary Inclusive photon plus jet, di-photon and photon plus b jet cross sections have been measured by the D0 and CDF experiments in p'fi collisions at a center of mass energy of JS= 1.96 TeV. The differential photon plus jet cross section has been measured in four different kinematic regions and their ratios. Deviations from the prediction of NLO QCD calculations are observed. The structure is similar to previous obserrntions of UA2. CDF and D0. The di-photon im·ariant mass, transverse momentum and azimuthal opening angle depen-dence of the di-photon cross section can not be described simultaneously by a single theoretical prediction. In different regions of phase space, different contributions are relevimt . The different contributions consist of NLO corrections for the direct photon and fragmentation process, initial state soft gluon resummation and NNLO (O(o~)) corrections to the gg ->TT process. A com-putation, combining all separate contributions is needed to describe the di-photon production in all phase space regions. The photon plus b jet cross section has been measured without and with a dedicated trigger, based on a photon plus displaced track requirement. The latter anal~'sis extends the phase space considerably in lowering the photon transverse energy threshold from 25 to 12 GeV. While LO predictions show good agreement with the cross section of the former analysis the~· slightly underestimate the cross section of the latter analysis. Acknowledgements Many thanks to the stuff members at Fermilab, collaborating institutions and the QCD groups of the D0 and CDF experiments. Among others, this work has been supported by the Marie Curie Program, in part under contract number MRTN-CT-2006-035606 and the HEPtools EU Marie Curie Research Training Network under contract number ).1RTN-CT-2006-035505. References 1. G. C. Blazey et al.. D0 note 3750. hep-ex/0005012 (2000). ? D0 collaboration, D0 Note 5369-CONF (2007). 3. S. Catani et al., JHEP 0205, 028 (2002). 4. J. Punplin et al., JHEP 0207, 012 (2002); D. Stump et al., JHEP 0310, 046 (2003) . 5. L. Bourhis, M. Fontannaz and J. Ph. Guillet, Eur. Phys. J., C2, 529 (1998) . 6. Recommendation of Jean-Philippe Guillet and Michel Fontannaz. 7, D. Acosta et al. [CDF collaboration], Phys. Rev. Lett . 95, 022003 (2005) . 8. T. Binoth, J . P. Guillet, E. Pilon and M. \Verlen, Eur. Phys. J . C 16, 311 (2000). 9. C. Balazs, E. L. Berger, S. Mrenna and C. P. Yuan, Phys. Rev. D 57, 6934 (1998). 10. T. Sjostrand et al., Comp. Phys. Comm. 135, 238 (2001). 11. T. Aaltonen et al. [CDF collaboration], Phys. Rev. Lett. 99, 242002 (2007). 12. V. A. Khoze et al., Eur. Phys. J. C 38, 475 (2005). 13. CDF collaboration, CDF Public Note 8377 (2006). 14. CDF collaboration, CDF Public Note 8623 (2007). 15. G. Corcella et al., JHEP 0101 (2001) 010 (hep-ph/0011363); hep-ph/0210213 (2002) . 16. H. L. Lai et al., hep-ph/9903282 (1999). 202 

Photon plus Jet Cross Sections at the Tevatron

UNT Digital Library

PHOTON PLUS JET CROSS SECTIONS AT THE TEVATRONL. SonnenscheinCERN and LPNHE Paris, Universite´s Paris VI, VIIPhoton plus jet production has been studied by the DØ and CDF experiments in Run II ofthe Fermilab Tevatron Collider at a center of mass energy of√s = 1.96 TeV. Measurementsof the inclusive photon plus jet, di-photon and photon plus b jet cross section are presented.They are based on integrated luminosities between 0.2 fb−1 and 1.1 fb−1. The results arecompared to perturbative QCD calculations in various approximations.1 IntroductionPhotons originating from the hard subprocess and produced during fragmentation contribute tophoton cross sections in hadron-hadron collisions. The contribution of fragmentation photonscan be significantly reduced by isolation requirements. Thus, isolated photon cross sections aresensitive to the dynamics of the hard subprocess, to the strong coupling constant αs and tothe parton distribution functions (PDF’s) of the colliding hadrons. In particular di-photon finalstates provide also a signature of many interesting physics processes, such as H → γγ productionor large extra dimensions.2 Photon plus jet cross sectionDØ has measured the photon plus jet cross section2 based on an integrated luminosity of 1.1 fb−1.Photon candidates are defined as clusters of electromagnetic (EM) calorimeter cells within a coneof radius R = 0.2 in the space of pseudorapidity η and azimuthal angle φ, if more than 96% ofthe detected energy is located in the EM layers of the calorimeter and the probability for a trackmatch is below 0.001. As isolation criterion the transverse energy not associated to the photonin a cone of radius R = 0.4 around the photon direction has to be less than 0.07 times the energyof the photon. Backgrounds from cosmics and electrons from W boson decays are vetoed bya missing transverse energy requirement of ET/ < 12.5 GeV + 0.36pγT . Electromagnetic clusterFERMILAB-CONF-08-075 (GeV)γTp0 50 100 150 200 250 300   (pb/GeV) jetηdγ ηd Tγ dp⁄  σ 3 d-410-310-210-110110210310 & jet in CC, SSγ Region 1:    & jet in CC, OSγ Region 2:    in CC, jet in EC, SSγ Region 3:    in CC, jet in EC, OSγ Region 4:    Region 1 Region 2 Region 3 Region 4 -1L = 1.1 fbDØ dataNLO QCD=pT*f(y*)R,F,fµCTEQ 6M, DØ RunII Preliminary (GeV)γTp0 50 100 150 200 250 300 Data / Theory00.20.40.60.811.21.41.6 ratio of data to theory theor. scale dependence dependent uncertaintyT pDØ Run II Preliminary-1 = 1.1 fbintL7.8% is overallnormalization uncertainty < 1γη0 <   < 0.8 lead.jetη0 <   > 15 GeVT lead.jetPFigure 1: Differential photon plus jet cross section as a function of the photon transverse momentum in fourdifferent kinematic regions (left), the data/theory ratio in one kinematic region, (right).and track information is fed into a neural network (NN) to increase the photon purity further.Central photons (|η| < 1.0) with a transverse momentum above 30 GeV are selected. Jets aredefined in the energy scheme by the Run II midpoint cone algorithm1 with a radius of R = 0.7.The jets with a transverse momentum above 15 GeV are selected in the central (|η jet| < 0.8) orthe forward (1.5 < |ηjet| < 2.5) region. Finally, the photon and the leading hadronic jet have tobe separated by ∆R(γ, jet) > 0.7.Isolated photons from photon plus jet production come predominantly unaltered from thehard subprocess. Thus, they provide a direct probe of the hard scattering dynamics. At lowtransverse photon momentum the Compton scattering process qg → qγ dominates, which canbe exploited to probe the gluon density of the colliding hadrons at low x where quarks areconstrained by HERA data. At higher transverse momenta the quark anti-quark annihilationprocess qq¯ → gγ becomes more important. The production of photons by quark fragmentationis highly suppressed by the photon isolation.The differential photon plus jet cross section is measured as a function of the transversephoton momentum. In fig. 1 its pγT -weighted average, obtained by a fit in each bin is plottedfor four kinematic regions, distinguished by same side (SS) versus opposite side (OS) photonand jet and distinguished by central (CC) versus forward (EC) jet. In comparison the NLOpQCD prediction of JETPHOX3 is shown, making use of the CTEQ6.1M4 PDF’s and BGF5fragmentation functions. The scales have been chosen6 to be µR,F,f = pγT f(y∗) with f(y∗) =√(1 + exp(−2|y∗|))/2 and y∗ = 0.5(ηγ −ηjet). The right plot shows the data/theory ratio in thefirst region (same side photon and central jet). The scale variation by a factor of two up and downis not able to describe the data at medium transverse photon momenta. Previous observationsof UA2, CDF and DØ show a similar variation. The systematics can be further reduced inconsidering the ratio of two regions. The ratios of region 1/region 3 and region 2/region 3exhibit quantitative disagreement between data and theory.3 Di-photon cross sectionsPrompt di-photon production has been measured by CDF7 using a data sample of 207 pb−1.Central photon candidates (|ηγ | < 0.9) are selected based on lateral shower profile, preshower hitand no associated track requirements. As isolation criterion the transverse energy not associatedto the photon in a cone of radius R = 0.4 around the photon direction has not to exceed 1 GeV.Leading contributions to di-photon production come from quark-antiquark annihilation andFigure 2: Differential di-photon cross section as a function of the invariant di-photon mass mγγ (left) and asa function of the transverse momentum qT of the di-photon system (right), in comparison to the predictions ofDIPHOX (solid line), ResBos (dashed line) and Pythia normalised to data (dotted line).gluon-gluon scattering. A further contribution comes from quark fragmentation into a photon,which is suppressed by photon isolation but relevant in different phase space regions.Fig. 2 shows the differential di-photon cross section as a function of the invariant di-photonmass (left) and the transverse momentum of the di-photon system (right) in comparison tothe theoretical predictions of DIPHOX8, ResBos9 and LO Pythia10 increased by a factor of twoto match the data. While DIPHOX and ResBos contain the NLO contribution to prompt di-photon production, only DIPHOX has a NLO fragmentation contribution for one or two photons,relevant at low mass, high qT and small ∆φγγ . DIPHOX takes also into account the NNLOcontribution of the gg → γγ process, indicated in the inset of the invariant mass distribution.ResBos in turn has only a LO fragmentation contribution but takes into account resummedinitial state gluon radiation which is relevant at low qT . Describing the di-photon productionin all regions of the phase space simultaneously requires a full NLO calculation, taking O(α3s)gg → γγ corrections and resummed initial state gluon radiation into account.CDF also searched for gg → γγ with no other particles produced, i.e. exclusive pp¯ →p + γγ + p¯. Two candidate events were found11 consistent with theoretical expectations12.4 Photon plus b jet cross sectionsPhoton plus heavy flavour (HF) jet production is dominated by the process g+HF→ γ+HF.This can be exploited to set constraints on the HF content of PDF’s.The photon plus b jet production cross section has been measured by CDF13 using an inte-grated luminosity of 340 pb−1. A central photon (|ηγ | < 1.1) with a transverse energy above26 GeV and a jet with a transverse energy above 20 GeV within |η| < 1.5 and a secondary ver-tex are selected. The invariant mass distribution of the secondary vertex of simulated bottom,charm and light quark jet contributions is fitted to the data in order to subtract the charm andlight background. This procedure is done separately in each bin of photon transverse energy todetermine the differential photon plus b jet cross section as a function of the photon transverseenergy. The data is in good agreement with the predictions of LO Pythia and Herwig15, makinguse of the CTEQ5L16 PDF’s.A similar CDF analysis14 makes use of a dedicated photon plus displaced track trigger,exploiting an integrated luminosity of 208 pb−1. The transverse energy requirement of thecentral photon (|ηγ | < 1.1) has been lowered to 12 GeV which extends the used phase spaceconsiderably. As before, a jet with a transverse energy above 20 GeV within |η| < 1.5 and anassociated secondary vertex is required. The invariant mass distribution of the secondary vertexis fitted and the differential cross section as a function of photon transverse energy as well asthe differential cross section as a function of the jet transverse energy are determined. Thetotal cross section amounts to 90.5±6.0(stat.) +21.7−15.4(sys.) pb. The LO prediction of Pythia withCTEQ5L PDF’s slightly underestimates the cross section, whose accuracy is already systematicslimited, dominated by the tracking efficiency uncertainty.5 SummaryInclusive photon plus jet, di-photon and photon plus b jet cross sections have been measured bythe DØ and CDF experiments in pp¯ collisions at a center of mass energy of√s = 1.96 TeV.The differential photon plus jet cross section has been measured in four different kinematicregions and their ratios. Deviations from the prediction of NLO QCD calculations are observed.The structure is similar to previous observations of UA2, CDF and DØ.The di-photon invariant mass, transverse momentum and azimuthal opening angle depen-dence of the di-photon cross section can not be described simultaneously by a single theoreticalprediction. In different regions of phase space, different contributions are relevant. The differentcontributions consist of NLO corrections for the direct photon and fragmentation process, initialstate soft gluon resummation and NNLO (O(α3s)) corrections to the gg → γγ process. A com-putation, combining all separate contributions is needed to describe the di-photon productionin all phase space regions.The photon plus b jet cross section has been measured without and with a dedicated trigger,based on a photon plus displaced track requirement. The latter analysis extends the phase spaceconsiderably in lowering the photon transverse energy threshold from 25 to 12 GeV. While LOpredictions show good agreement with the cross section of the former analysis they slightlyunderestimate the cross section of the latter analysis.AcknowledgementsMany thanks to the stuff members at Fermilab, collaborating institutions and the QCD groupsof the DØ and CDF experiments. Among others, this work has been supported by the MarieCurie Program, in part under contract number MRTN-CT-2006-035606 and the HEPtools EUMarie Curie Research Training Network under contract number MRTN-CT-2006-035505.References1. G. C. Blazey et al., DØ note 3750, hep-ex/0005012 (2000).2. DØ collaboration, DØ Note 5369-CONF (2007).3. S. Catani et al., JHEP 0205, 028 (2002).4. J. Punplin et al., JHEP 0207, 012 (2002); D. Stump et al., JHEP 0310, 046 (2003).5. L. Bourhis, M. Fontannaz and J. Ph. Guillet, Eur. Phys. J., C2, 529 (1998).6. Recommendation of Jean-Philippe Guillet and Michel Fontannaz.7. D. Acosta et al. [CDF collaboration], Phys. Rev. Lett. 95, 022003 (2005).8. T. Binoth, J. P. Guillet, E. Pilon and M. Werlen, Eur. Phys. J. C 16, 311 (2000).9. C. Balazs, E. L. Berger, S. Mrenna and C. P. Yuan, Phys. Rev. D 57, 6934 (1998).10. T. Sjo¨strand et al., Comp. Phys. Comm. 135, 238 (2001).11. T. Aaltonen et al. [CDF collaboration], Phys. Rev. Lett. 99, 242002 (2007).12. V. A. Khoze et al., Eur. Phys. J. C 38, 475 (2005).13. CDF collaboration, CDF Public Note 8377 (2006).14. CDF collaboration, CDF Public Note 8623 (2007).15. G. Corcella et al., JHEP 0101 (2001) 010 [hep-ph/0011363]; hep-ph/0210213 (2002).16. H. L. Lai et al., hep-ph/9903282 (1999).

Photon plus jet cross sections at the Tevatron

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Photon plus Jet Cross Sections at the Tevatron

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