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