John Womersley
Direct PhotonsDirect Photons
John WomersleyFermilab
CTEQ Summer School, MadisonJune 2002
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John Womersley
Hadron-hadron collisionsHadron-hadron collisions
• 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
fragmentation
partondistribution
partondistribution
Jet
Underlyingevent
Photon, W, Z etc.
Hard scattering
ISR FSR
John Womersley
Motivation for photon measurementsMotivation 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 . . .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
John Womersley
Photon Signatures of New PhysicsPhoton 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 ofpp at high masses (virtual
gravitons)
John Womersley
Photon identificationPhoton 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.
John Womersley
Event topologyEvent 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
jet
Back to backin parton-partoncenter of mass
jet
boosted into lab frame
John Womersley
Photon candidate event in DØ Run 1Photon candidate event in DØ Run 1
Photon
Recoil Jet
John Womersley
TriggeringTriggering
• 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 TriggersPhoton 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
John Womersley
Thresholds and prescalesThresholds 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:
ET
# events
5 10 30
1000 50
1
ET
Crosssection
5 10 30
John Womersley
• Photon candidates: isolated electromagnetic showers in the calorimeter, with no charged tracks pointed at them– what fraction of these are true photons?
• Signal
• Background
Signal and BackgroundSignal and 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
Preshowerdetector
Shower maximumdetector
John Womersley
Photon purity estimatorsPhoton purity estimators
• CDF • DØ
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Each ET bin fitted as sum of:• = photons• = background w/o tracks• = background w/ tracks
John Womersley
Photon sample purityPhoton sample purity
• CDF • DØ
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John Womersley
Angular distributionsAngular distributions
• The dominant process producing photons
• Should be quite different from dijet production:
Can we test this?
John Womersley
Transformation to photon-jet systemTransformation to photon-jet system
Lab pseudorapidity of photon
Lab
pseu
dora
pid
ity o
f je
t
* = CM pseudorapidity
BOOST of CM relative to lab
Central calorimeter coverage
jet
jet
BOOST
*
cos * = tanh *
John Womersley
cos * = tanh *
CM pseudorapidity *
Ph
oto
n p
T
Lines of minimum and maximum p*
p* = pT cosh *
Use multiple regions tomaximize statistics;
paste distribution together using overlapping coverage
Want uniform coverage in CM variables while respecting physical limits on detector coverage and trigger pT
min pT from trigger min p*
John Womersley
Angular distributionsAngular distributions
John Womersley
Photons as a probe of quark chargePhotons 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 flavorCDF 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)
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 futureAn 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
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Photon cross sections at 1.8 TeVPhoton cross sections at 1.8 TeV
• DØ, PRL 84 (2000) 2786 • CDF, submitted to Phys. Rev. D
QCD prediction is NLO by Owens et al.
John Womersley
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(data – theory) / theory(data – theory) / theory
• DØ, PRL 84 (2000) 2786
QCD prediction is NLO by Owens et al., CTEQ4M
What’s going on at low ET?
• CDF, submitted to Phys. Rev. D
±12% normalizationstatistical errors only
John Womersley
““kkTT smearing” smearing”
• Gaussian 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 smearing
PYTHIA style parton shower(Baer and Reno)
Account for soft gluon emission
CDF data 1.25
John Womersley
Why would you need to do this?Why would you need to do this?
• NLO calculation puts in at most one extra gluon emission
In PYTHIA, find that additional gluonsadd an extra 2.5–5 GeV of pT to the system
10 GeV 2.6 GeV “kT”
50 GeV 5 GeV “kT”
John Womersley
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Fixed target photon productionFixed 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
John Womersley
Photons at HERAPhotons at HERA
• ZEUS data agrees well with NLO QCD
– no need for kT ?
ZEUS 96-97
Have to include this “resolved” component
John Womersley
ZEUS measurement of photon-jet pZEUS measurement of photon-jet pTT
John Womersley
A consistent picture of kA consistent picture of kTT
• W = invariant mass of photon + jet final state
John Womersley
Is this the only explanation?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 viewpointsContrary viewpoints
• 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
Ouch!
John Womersley
Isolated Isolated 00 cross sections 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”
John Womersley
Aurenche et al.vs.
E706
John Womersley
ResummationResummation
• 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 parametersCatani et al. hep-ph/9903436
Thresholdresummation
Fixed Order
Laenen, Sterman, Vogelsang, hep-ph/0002078
Threshold + recoilresummation:looks promising
Threshold resummation: didnot model E706 data very well
John Womersley
Fink and Owens resummed Fink and Owens resummed calculationscalculations
• hep-ph/0105276
E 706 data
DØ data
Agreement with data is pretty good
Does require 2 or 4 non-perturbative parameters to be set
John Womersley
Photons at Photons at s = 630 GeVs = 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
CDF
DØ
John Womersley
But . . .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?
John Womersley
Is it just the PDF?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 setsGreen = MRS99 setOrange = CTEQ5M and L
Not all of Walter’s PDF setshave this feature: it depends on what data are input
John Womersley
Anything similar in other final states?Anything similar in other final states?
• b cross section at CDF and at DØ
• Data continue to lie ~ 2 central band of theory
b
B
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central forward
Cross section vs. |y|pT > 5 GeV/c
pT > 8 GeV/c
John Womersley
DØ b-jet cross section at higher pDØ b-jet cross section at higher pTT
Differential cross section Integrated pT > pTmin
from varying the scale from 2μO to μO/2, where μO = (pT
2 + mb2)1/2
New
John Womersley
(data – theory)/theory (data – theory)/theory
John Womersley
b-jet and photon production b-jet and photon production comparedcompared
DØ b-jets (using highest QCD prediction)
0
- 0.5
0.5
1.0
1.5CDF photons 1.33
DØ photons
Data
– T
heory
/Th
eory
Photon or b-jet pT (GeV/c)
John Womersley
Diphoton productionDiphoton 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
John Womersley
DØ diphoton measurementsDØ diphoton measurements
• 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
pT
John Womersley
Latest NLO diphoton calculationLatest 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)
John Womersley
Photons: final remarksPhotons: 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?
John Womersley
Run 2 Missing ERun 2 Missing ETT + di-em Candidate + di-em Candidate
EM1 EM2
ET = 27.4 GeV
= 0.52 = 3.78Loose match with a low-pT track
ET = 26.0 GeV
= 1.54 = 5.86No track match
MET = 34.3 GeV; M(diEM) = 53 GeV
+MET is a signature of gauge-mediated SUSY-breaking
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