Direct photons
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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

Direct Photons

Mehr licht!

John Womersley

Fermilab

CTEQ Summer School, Madison

June 2002


Hadron hadron collisions

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


Motivation for photon measurements

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”


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


Photon signatures of new physics

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)


Photon identification

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.


Event topology

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


Photon candidate event in d run 1

Photon candidate event in DØ Run 1

Recoil Jet

Photon


Triggering

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


Photon triggers

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


Thresholds and prescales

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 GeVaccept 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


Signal and background

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


Photon purity estimators

CDF

Photon purity estimators

  • Each ET bin fitted as sum of:

  • = photons

  • = background w/o tracks

  • = background w/ tracks


Photon sample purity

CDF

Photon sample purity


Angular distributions

Angular distributions

  • The dominant process producing photons

  • Should be quite different from dijet production:

Can we

test this?


Transformation to photon jet system

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


Direct photons

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 *


Angular distributions1

Angular distributions


Photons as a probe of quark charge

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)


Cdf photon heavy flavor

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)


Direct photons

  • Control sample using same dataset

    • identify 0 (= jet) instead of photon: gg  QQ events

Charm/bottom ~ 0.4


An idea for the future

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


Photon cross sections at 1 8 tev

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.


Data theory theory

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?


K t smearing

“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


Why would you need to do this

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


Fixed target photon production

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


Photons at hera

ZEUS 96-97

Photons at HERA

  • ZEUS data agrees well with NLO QCD

    • no need for kT ?

Have to include this “resolved” component


Zeus measurement of photon jet p t

ZEUS measurement of photon-jet pT


A consistent picture of k t

A consistent picture of kT

  • W = invariant mass of photon + jet final state


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


Contrary 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

Contrary viewpoints

Ouch!


Isolated 0 cross sections

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”


Direct photons

Aurenche et al.

vs.

E706


Resummation

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


Fink and owens resummed calculations

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


Photons at s 630 gev

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

CDF


Direct photons

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?


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 sets

Green = MRS99 set

Orange = CTEQ5M and L

Not all of Walter’s PDF setshave this feature: it depends on what data are input


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

central

forward

b

Cross section vs. |y|

pT > 5 GeV/c

pT > 8 GeV/c

B


D b jet cross section at higher p t

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


Direct photons

(data – theory)/theory


Direct photons

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


Diphoton production

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


D diphoton measurements

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


Latest nlo diphoton calculation

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)


Photons final remarks

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?


Run 2 missing e t di em candidate

Run 2 Missing ET + di-em Candidate

+MET is a signature of gauge-mediated SUSY-breaking


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