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  • Direct PhotonsJohn WomersleyFermilab

    CTEQ Summer School, MadisonJune 2002Mehr licht!

  • Hadron-hadron collisionsComplicated byparton distributions a hadron collider is really a broad-band quark and gluon colliderboth the initial and final states can be colored and can radiate gluonsunderlying event from proton remnantsfragmentationpartondistributionpartondistributionJetUnderlyingeventPhoton, W, Z etc.Hard scatteringISRFSR

  • Motivation for photon measurementsAs long as 20 years ago, direct photon measurements were promoted as a way to:Avoid all the systematics associated with jet identification and measurementphotons are simple, well measured EM objectsemerge directly from the hard scattering without fragmentationHoped-for sensitivity to the gluon content of the nucleonQCD Compton process

  • In the meantime . . .Jet measurements have become much better understood

    Lower photon cross sections and ease of triggering on EM objects lead to photon data being at much lower ET than typical jet measurementsTurn out to be susceptible to QCD effects at the few GeV level that

    Photons have not been a simple test of QCD and have not given input to parton distributions, and they continue to challenge our ability to calculate within QCD

  • Photon Signatures of New PhysicsImportant to understand QCD of photon production in order to reliably search forHiggs H is a discovery channel at LHCGauge mediated SUSY breaking 0 G, photon + MET signaturesTechnicolorPhoton + dijet signaturesDiphoton resonancesExtra dimensionsEnhancement ofpp at high masses (virtual gravitons)

  • Photon identificationEssentially every jet contains one or more 0 mesons which decay to photonstherefore the truly inclusive photon cross section would be hugewe are really interested in direct (prompt) photons (from the hard scattering)but what we usually have to settle for is isolated photons (a reasonable approximation)isolation: require less than e.g. 2 GeV within e.g. R = 0.4 cone

    This rejects most of the jet background, but leaves those (very rare) cases where a single 0 or meson carries most of the jets energy

    This happens perhaps 103 of the time, but since the jet cross section is 103 times larger than the isolated photon cross section, we are still left with a signal to background of order 1:1.

  • Event topologySimplest process: pp + jet

    Photon and jet are back-to-back in and balance in ET

    Experimentally we find that at about one third of the photon events have a second jet of significant ETHigher order QCD processesBack to back in parton-parton center of massboosted into lab frame

  • Photon candidate event in D Run 1PhotonRecoil Jet

  • TriggeringThe greatest engineering challenge in hadron collider physicsTo access rare processes, we must collide the beams at luminosities such that there is a hard collision every bunch crossing 396 ns in Run 2 = 2.5 MHz We cannot write to tape (or hope to process offline) more than about 50 events per secondTrigger rejection of 50,000 requiredin real timewith minimal deadtime and high efficiency for physics of interest

  • Photon TriggersExample of how this works in D:

    Level 1 (hardware trigger)Requires ET > threshold in one trigger tower of the EM calorimeter ( = 0.2 0.2)Total accept rate ~ 10 khZ; can allow ~ 1 kHz for electron and photon triggers

    Level 2 (Alpha CPU, processing the trigger tower information)Requires EM fraction cut and isolation cutsRejection ~ 10

    Level 3 (Linux farm, processing the full event readout)Clusters = 0.1 0.1 cells with better resolutionApplies shower shape and isolation cutsRejection ~ 20

  • Thresholds and prescalesRelatively high cross section processes like photons, with steeply falling cross sections, will be accumulated using a variety of thresholds with different prescalesA very simple example:EM cluster > 5 GeVaccept 1 in 1000EM cluster > 10 GeV accept 1 in 50EM cluster > 30 GeV accept allThen paste the cross section together offline:

    ET# events51030 1000 50 1ETCross section51030

  • Signal and BackgroundPhoton candidates: isolated electromagnetic showers in the calorimeter, with no charged tracks pointed at themwhat fraction of these are true photons?



    Experimental techniques in Run 1

    D measured longitudinal shower development at start of shower

    CDF measured transverse profile at start of shower (preshower detector) and at shower maximum 0PreshowerdetectorShower maximumdetector

  • Photon purity estimatorsCDFDEach ET bin fitted as sum of:= photons= background w/o tracks= background w/ tracks

  • Photon sample purityCDFD

  • Angular distributionsThe dominant process producing photons

    Should be quite different from dijet production:Can we test this?

  • Transformation to photon-jet systemLab pseudorapidity of photonLab pseudorapidity of jet* = CM pseudorapidityBOOST of CM relative to labCentral calorimeter coverageBOOST*cos * = tanh *

  • cos * = tanh * CM pseudorapidity * Photon pTLines of minimum and maximum p*p* = pT cosh * Use multiple regions to maximize statistics;paste distribution together using overlapping coverageWant uniform coverage in CM variables while respecting physical limits on detector coverage and trigger pT min pT from trigger min p*

  • Angular distributions

  • Photons as a probe of quark chargeInclusive heavy flavor production sees the quark color charge:

    While photons see the electric charge:Charm (+2/3) should be enhanced relative to bottom (-1/3)

  • CDF photon + heavy flavorUse muon decays; pT of muon relative to jet allows b and c separationCharm/bottom = 2.4 1.2Cf. 2.9 (PYTHIA) 3.2 (NLO QCD)

  • Control sample using same dataset identify 0 (= jet) instead of photon: gg QQ events

    Charm/bottom ~ 0.4

  • An idea for the futureUse tt events to measure the electric charge of the top quarkHow do we know its not 4/3? Baur et al., hep-ph/0106341

  • Photon cross sections at 1.8 TeVD, PRL 84 (2000) 2786

    CDF, submitted to Phys. Rev. DQCD prediction is NLO by Owens et al.

  • (data theory) / theoryD, PRL 84 (2000) 2786

    CDF, submitted to Phys. Rev. DQCD prediction is NLO by Owens et al., CTEQ4M

    Whats going on at low ET?12% normalizationstatistical errors only

  • kT smearingGaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low ET (hep-ph/9808467)

    3 GeV of Gaussian smearingPYTHIA style parton shower(Baer and Reno)Account for soft gluon emissionCDF data 1.25

  • Why would you need to do this?NLO calculation puts in at most one extra gluon emissionIn PYTHIA, find that additional gluons add an extra 2.55 GeV of pT to the system 10 GeV 2.6 GeV kT50 GeV 5 GeV kT

  • Fixed target photon productionEven larger deviations from QCD observed in fixed target (E706)

    again, Gaussian smearing (~1.2 GeV here) can account for the data*

  • Photons at HERAZEUS data agrees well with NLO QCD no need for kT ?Have to include this resolved component

  • ZEUS measurement of photon-jet pT

  • A consistent picture of kTW = invariant mass of photon + jet final state

  • Is this the only explanation?Not necessarily . . . Vogelsang et al. have investigated tweaking the renormalization, factorization and fragmentation scales separately, and can generate shape differences

    This is not theoretically particularly attractive

  • Contrary viewpointsAurenche et al., hep-ph/9811382: NLO QCD (sans kT) can fit all the data with the sole exception of E706 It does not appear very instructive to hide this problem by introducing an extra parameter fitted to the data at each energy


  • Isolated 0 cross sectionsProponents of kT point out that 0 measurements back up the kT hypothesis (plots from Marek Zielinski)WA70 0 data require kT to agree with QCD (unlike WA70 photons)/0 ratio in E706 agrees with theory, in WA70 does not

    Aurenche et al. claim the opposite (hep-ph/9910352)all 0 data below 40 GeV compatible, unlike photon data (E706) seems to indicate that the systematic errors on prompt-photon production are probably underestimated

  • Aurenche et al.vs.E706

  • ResummationPredictive power of Gaussian smearing is small e.g. what happens at LHC? At forward rapidities?The right way to do this should be resummation of soft gluonsthis works nicely for W/Z pT, at the cost of introducing parameters

    Catani et al. hep-ph/9903436ThresholdresummationFixed OrderLaenen, Sterman, Vogelsang, hep-ph/0002078Threshold + recoilresummation:looks promisingThreshold resummation: didnot model E706 data very well

  • Fink and Owens resummed calculationshep-ph/0105276E 706 dataD dataAgreement with data is pretty goodDoes require 2 or 4 non-perturbative parameters to be set

  • Photons at s = 630 GeVAt the end of Run 1, CDF and D both took data at lower CM energy

    Central region data are qualitatively in agreement and show a kT-like excess at low ETCDFD

  • But . . .When the UA2 data (also at 630 GeV) is added, it reinforces the impression of a deficit at large xTWhats happening here?Can I really ignore the data normalization in making all these comparisons with kT?

  • Is it just the PDF?New PDFs from Walter Giele can describe the observed photon cross section at the Tevatron without any kT, and predict the deficit

    CDF (central)D (forward)Blue = Giele/Keller setsGreen = MRS99 setOrange = CTEQ5M and LNot all of Walters PDF sets have this feature: it depends on what data are input


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