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Direct Photons

Direct Photons. Mehr licht!. John Womersley Fermilab CTEQ Summer School, Madison June 2002. Hadron-hadron collisions. Photon, W, Z etc. Complicated by parton distributions — a hadron collider is really a broad-band quark and gluon collider

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Direct Photons

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  1. Direct Photons Mehr licht! John Womersley Fermilab CTEQ Summer School, Madison June 2002

  2. Hadron-hadron collisions Photon, W, Z etc. • Complicated by • parton distributions — a hadron collider is really a broad-band quark and gluon collider • both the initial and final states can be colored and can radiate gluons • underlying event from proton remnants parton distribution Underlying event Hard scattering FSR parton distribution ISR fragmentation Jet

  3. Motivation for photon measurements • As long as 20 years ago, direct photon measurements were promoted as a way to: • Avoid all the systematics associated with jet identification and measurement • photons are simple, well measured EM objects • emerge directly from the hard scattering without fragmentation • Hoped-for sensitivity to the gluon content of the nucleon • “QCD Compton process” 

  4. 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 measurements • Turn 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

  5. Photon Signatures of New Physics • Important to understand QCD of photon production in order to reliably search for • Higgs • H   is a discovery channel at LHC • Gauge mediated SUSY breaking • 0   G, photon + MET signatures • Technicolor • Photon + dijet signatures • Diphoton resonances • Extra dimensions • Enhancement ofpp   at high masses (virtual gravitons)

  6. Photon identification • Essentially every jet contains one or more 0 mesons which decay to photons • therefore the truly inclusive photon cross section would be huge • we 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 jet’s energy • This happens perhaps 10–3 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.

  7. jet jet   Event topology • Simplest 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 ET • Higher order QCD processes Back to backin parton-partoncenter of mass boosted into lab frame

  8. Photon candidate event in DØ Run 1 Recoil Jet Photon

  9. Triggering • The greatest engineering challenge in hadron collider physics • To 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 second • Trigger rejection of 50,000 required • in real time • with minimal deadtime • and high efficiency for physics of interest

  10. Photon Triggers • Example of how this works in DØ: • 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 cuts • Rejection ~ 10 • Level 3 (Linux farm, processing the full event readout) • Clusters    = 0.1  0.1 cells with better resolution • Applies shower shape and isolation cuts • Rejection ~ 20

  11. Thresholds and prescales • Relatively high cross section processes like photons, with steeply falling cross sections, will be accumulated using a variety of thresholds with different prescales • A very simple example: • EM cluster > 5 GeV accept 1 in 1000 • EM cluster > 10 GeV accept 1 in 50 • EM cluster > 30 GeV accept all • Then “paste” the cross section together offline:  1000  50 Crosssection  1 # events ET ET 30 30 10 10 5 5

  12. Signal and Background • Photon candidates: isolated electromagnetic showers in the calorimeter, with no charged tracks pointed at them • what fraction of these are true photons? • Signal • Background • Experimental techniques in Run 1 • DØ measured longitudinal shower development at start of shower • CDF measured transverse profile at start of shower (preshower detector) and at shower maximum   0  Preshower detector Shower maximum detector

  13. CDF DØ Photon purity estimators • Each ET bin fitted as sum of: • = photons • = background w/o tracks • = background w/ tracks

  14. CDF DØ Photon sample purity

  15. Angular distributions • The dominant process producing photons • Should be quite different from dijet production: Can we test this?

  16. jet jet   Transformation to photon-jet system Central calorimeter coverage BOOST of CM relative to lab BOOST Lab pseudorapidity of jet cos * = tanh * * = CM pseudorapidity * Lab pseudorapidity of photon

  17. Want uniform coverage in CM variables while respecting physical limits on detector  coverage and trigger pT cos * = tanh * Lines of minimum and maximum p* p* = pT cosh * Photon pT min pT from trigger  min p* Use multiple regions tomaximize statistics; paste distribution together using overlapping coverage CM pseudorapidity *

  18. Angular distributions

  19. Photons as a probe of quark charge • Inclusive 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)

  20. CDF photon + heavy flavor • Use muon decays; pT of muon relative to jet allows b and c separation Charm/bottom = 2.4  1.2 Cf. 2.9 (PYTHIA) 3.2 (NLO QCD)

  21. Control sample using same dataset • identify 0 (= jet) instead of photon: gg  QQ events Charm/bottom ~ 0.4

  22. An idea for the future • Use tt events to measure the electric charge of the top quark • How do we know it’s not 4/3? • Baur et al., hep-ph/0106341

  23. DØ, PRL 84 (2000) 2786 CDF, submitted to Phys. Rev. D Photon cross sections at 1.8 TeV QCD prediction is NLO by Owens et al.

  24. DØ, PRL 84 (2000) 2786 CDF, submitted to Phys. Rev. D (data – theory) / theory ±12% normalization statistical errors only QCD prediction is NLO by Owens et al., CTEQ4M What’s going on at low ET?

  25. “kT smearing” • Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low ET (hep-ph/9808467) Account for soft gluon emission CDF data  1.25 PYTHIA style parton shower (Baer and Reno) 3 GeV of Gaussian smearing

  26. Why would you need to do this? • NLO calculation puts in at most one extra gluon emission 10 GeV  2.6 GeV “kT” 50 GeV  5 GeV “kT” In PYTHIA, find that additional gluonsadd an extra 2.5–5 GeV of pT to the system

  27. Fixed target photon production • Even larger deviations from QCD observed in fixed target (E706) • again, Gaussian smearing (~1.2 GeV here) can account for the data

  28. ZEUS 96-97 Photons at HERA • ZEUS data agrees well with NLO QCD • no need for kT ? Have to include this “resolved” component

  29. ZEUS measurement of photon-jet pT

  30. A consistent picture of kT • W = invariant mass of photon + jet final state

  31. 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

  32. Aurenche 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” E706 Contrary viewpoints Ouch!

  33. Isolated 0 cross sections • Proponents 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”

  34. Aurenche et al. vs. E706

  35. Resummation • Predictive 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 gluons • this works nicely for W/Z pT, at the cost of introducing parameters Laenen, Sterman, Vogelsang, hep-ph/0002078 Catani et al. hep-ph/9903436 Threshold + recoil resummation: looks promising Threshold resummation Fixed Order Threshold resummation: did not model E706 data very well

  36. Fink and Owens resummed calculations • hep-ph/0105276 DØ data E 706 data Agreement with data is pretty good Does require 2 or 4 non-perturbative parameters to be set

  37. Photons at s = 630 GeV • At the end of Run 1, CDF and DØ both took data at lower CM energy • Central region data are qualitatively in agreement and show akT-like excess at low ET DØ CDF

  38. But . . . • When the UA2 data (also at 630 GeV) is added, it reinforces the impression of a deficit at large xT What’s happening here? Can I really ignore the datanormalization in making allthese comparisons with kT?

  39. Is it just the PDF? • New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any kT, and predict the “deficit” CDF (central) DØ (forward) Blue = Giele/Keller sets Green = MRS99 set Orange = CTEQ5M and L Not all of Walter’s PDF setshave this feature: it depends on what data are input

  40. Anything similar in other final states? • b cross section at CDF and at DØ • Data continue to lie ~ 2  central band of theory central forward b Cross section vs. |y| pT > 5 GeV/c pT > 8 GeV/c B

  41. DØ b-jet cross section at higher pT Differential cross section Integrated pT > pTmin New from varying the scale from 2μO toμO/2, where μO = (pT2 + mb2)1/2

  42. (data – theory)/theory

  43. 1.5 DØ b-jets (using highest QCD prediction) CDF photons  1.33 DØ photons 1.0 0.5 Data – Theory/Theory 0 - 0.5 Photon or b-jet pT (GeV/c) b-jet and photon production compared

  44. Diphoton production • Rate is very small: few hundred events in Run I (pT > 12 GeV) • But interesting because • final state kinematics can be completely reconstructed (mass, pT and opening angle of  system) • background to H   at LHC • NLO calculations available

  45. DØ diphoton measurements  pT • Find that we need NLO QCD to model the data at large pT (small ), but NLO calculation is divergent at pT = 0 ( = ) • Need a resummation approach (RESBOS) or showering Monte Carlo (PYTHIA) or ad hoc few-GeV kT smearing pT ~ 3 GeV

  46. Latest NLO diphoton calculation • Binoth, Guillet, Pilon and Werlen, hep-ph/0012191 Shoulder at 30 GeV in calculation is a real NLO effect (contribution opens up with both photons on same side of the event)

  47. Photons: final remarks • For many years it was hoped that direct photon production could be used to pin down the gluon distribution through the dominant process: • Theorist’s viewpoint (Giele): “... discrepancies between data and theory for a wide range of experiments have cast a dark spell on this once promising cross section … now drowning in a swamp of non-perturbative fixes” • Experimenter’s viewpoint: it is an interesting puzzle, and we like solving interesting puzzles • data  NLO QCD • kT remains a controversial topic • experiments may not all be consistent • resummation looks quite good, but how predictive is it? • what is the role of the PDF’s?

  48. Run 2 Missing ET + di-em Candidate +MET is a signature of gauge-mediated SUSY-breaking

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