Direct Photons John Womersley Fermilab CTEQ Summer School

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

Hadron-hadron collisions Photon, W, Z etc. parton distribution Underlying event Hard scattering parton distribution ISR FSR fragmentation • 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 John Womersley Jet

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” John Womersley

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 Ge. V 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 John Womersley

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) John Womersley

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 Ge. V 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. John Womersley

Event topology • Simplest process: pp + jet Back to back in parton-parton center of mass boosted into lab frame 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 John Womersley

Photon candidate event in DØ Run 1 Recoil Jet Photon John Womersley

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 John Womersley

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) – Total accept rate ~ 10 kh. Z; can allow ~ 1 k. Hz 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 cells with better resolution – Applies shower shape and isolation cuts – Rejection ~ 20 John Womersley

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 Ge. V accept 1 in 1000 – EM cluster > 10 Ge. V accept 1 in 50 – EM cluster > 30 Ge. V accept all Then “paste” the cross section together offline: 1000 50 # events 5 10 John Womersley 30 Cross section 1 ET 5 10 30 ET

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 Experimental techniques in Run 1 • Background 0 • CDF measured transverse profile at start of shower (preshower detector) and at shower maximum Preshower detector John Womersley • DØ measured longitudinal shower development at start of shower Shower maximum detector

Photon purity estimators • CDF • DØ Each ET bin fitted as sum of: 1. = photons 2. = background w/o tracks 3. = background w/ tracks John Womersley

Photon sample purity • CDF John Womersley • DØ

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

Transformation to photon-jet system Central calorimeter coverage BOOST of CM relative to lab jet Lab pseudorapidity of jet BOOST cos * = tanh * jet * = CM pseudorapidity * Lab pseudorapidity of photon John Womersley

Want uniform coverage in CM variables while respecting physical limits on detector coverage and trigger p. T cos * = tanh * Photon p. T Lines of minimum and maximum p* p* = p. T cosh * min p. T from trigger min p* CM pseudorapidity * John Womersley Use multiple regions to maximize statistics; paste distribution together using overlapping coverage

Angular distributions John Womersley

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) John Womersley

CDF photon + heavy flavor • Use muon decays; p. T of muon relative to jet allows b and c separation Charm/bottom = 2. 4 1. 2 Cf. 2. 9 (PYTHIA) 3. 2 (NLO QCD) John Womersley

• Control sample using same dataset – identify 0 (= jet) instead of photon: gg QQ events Charm/bottom ~ 0. 4 John Womersley

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 John Womersley

Photon cross sections at 1. 8 Te. V • DØ, PRL 84 (2000) 2786 • CDF, submitted to Phys. Rev. D QCD prediction is NLO by Owens et al. John Womersley

(data – theory) / theory • DØ, PRL 84 (2000) 2786 • CDF, submitted to Phys. Rev. D ± 12% normalization statistical errors only QCD prediction is NLO by Owens et al. , CTEQ 4 M What’s going on at low ET? John Womersley

“k. T smearing” • Gaussian smearing of the transverse momenta by a few Ge. V can model the rise of cross section at low ET (hep-ph/9808467) Account for soft gluon emission CDF data 1. 25 3 Ge. V of Gaussian smearing John Womersley PYTHIA style parton shower (Baer and Reno)

Why would you need to do this? • NLO calculation puts in at most one extra gluon emission 10 Ge. V 2. 6 Ge. V “k. T” 50 Ge. V 5 Ge. V “k. T” In PYTHIA, find that additional gluons add an extra 2. 5– 5 Ge. V of p. T to the system John Womersley

Fixed target photon production • Even larger deviations from QCD observed in fixed target (E 706) • again, Gaussian smearing (~1. 2 Ge. V here) can account for the data John Womersley

Photons at HERA • ZEUS data agrees well with NLO QCD – no need for k. T ? ZEUS 96 -97 Have to include this “resolved” component John Womersley

ZEUS measurement of photon-jet p. T John Womersley

A consistent picture of k. T • W = invariant mass of photon + jet final state John Womersley

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 John Womersley

Contrary viewpoints • Aurenche et al. , hepph/9811382: NLO QCD (sans k. T) can fit all the data with the sole exception of E 706 “It does not appear very instructive to hide this problem by introducing an extra parameter fitted to the data at each energy” Ouch! John Womersley E 706

Isolated 0 cross sections • Proponents of k. T point out that 0 measurements back up the k. T hypothesis (plots from Marek Zielinski) – WA 70 0 data require k. T to agree with QCD (unlike WA 70 photons) – / 0 ratio in E 706 agrees with theory, in WA 70 does not • Aurenche et al. claim the opposite (hep-ph/9910352) – all 0 data below 40 Ge. V compatible, unlike photon data (E 706) – “seems to indicate that the systematic errors on prompt-photon production are probably underestimated” John Womersley

Aurenche et al. vs. E 706 John Womersley

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 p. T, at the cost of introducing parameters Catani et al. hep-ph/9903436 Laenen, Sterman, Vogelsang, hep-ph/0002078 Threshold + recoil resummation: looks promising Threshold resummation: did not model E 706 data very well John Womersley Fixed Order

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 John Womersley

Photons at s = 630 Ge. V • At the end of Run 1, CDF and DØ both took data at lower CM energy DØ CDF • Central region data are qualitatively in agreement and show a k. T-like excess at low ET John Womersley

But. . . • When the UA 2 data (also at 630 Ge. V) is added, it reinforces the impression of a deficit at large x. T What’s happening here? Can I really ignore the data normalization in making all these comparisons with k. T? John Womersley

Is it just the PDF? • New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any k. T, and predict the “deficit” CDF (central) Blue = Giele/Keller sets Green = MRS 99 set Orange = CTEQ 5 M and L John Womersley DØ (forward) Not all of Walter’s PDF sets have this feature: it depends on what data are input

Anything similar in other final states? • b cross section at CDF and at DØ central forward b B Cross section vs. |y| p. T > 5 Ge. V/c p. T > 8 Ge. V/c • Data continue to lie ~ 2 central band of theory John Womersley

DØ b-jet cross section at higher p. T Differential cross section Integrated p. T > p. Tmin New from varying the scale from 2μO to μO/2, where μO = (p. T 2 + mb 2)1/2 John Womersley

(data – theory)/theory John Womersley

b-jet and photon production compared 1. 5 DØ b-jets (using highest QCD prediction) CDF photons 1. 33 Data – Theory/Theory DØ photons 1. 0 0. 5 0 - 0. 5 Photon or b-jet p. T (Ge. V/c) John Womersley

Diphoton production • • • Rate is very small: few hundred events in Run I (p. T > 12 Ge. V) But interesting because – final state kinematics can be completely reconstructed (mass, p. T and opening angle of system) – background to H at LHC NLO calculations available John Womersley

DØ diphoton measurements p. T ~ 3 Ge. V • • Find that we need NLO QCD to model the data at large p. T (small ), but NLO calculation is divergent at p. T = 0 ( = ) Need a resummation approach (RESBOS) or showering Monte Carlo (PYTHIA) or ad hoc few-Ge. V k. T smearing John Womersley

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

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 – k. T 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? John Womersley

Run 2 Missing ET + di-em Candidate +MET is a signature of gauge-mediated SUSY-breaking EM 1 ET = 27. 4 Ge. V = 0. 52 = 3. 78 Loose match with a low-p. T track EM 2 ET = 26. 0 Ge. V = 1. 54 = 5. 86 No track match MET = 34. 3 Ge. V; M(di. EM) = 53 Ge. V John Womersley