introduction / motivation photon identification results inclusive prompt photons
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Prompt Photon Production at HERA and LEP ICHEP 2004, Beijing Thomas Kluge, DESY on behalf of the H1, OPAL and ZEUS Collaborations. Introduction / Motivation Photon Identification Results Inclusive Prompt Photons Prompt Photons together with a Jet Conclusion / Summary. prompt photon. - PowerPoint PPT PresentationTRANSCRIPT
Prompt Photon Production at HERA and LEPPrompt Photon Production at HERA and LEPICHEP 2004, Beijing
Thomas Kluge, DESYon behalf of the H1, OPAL and ZEUS Collaborations
Prompt Photon Production at HERA and LEPPrompt Photon Production at HERA and LEPICHEP 2004, Beijing
Thomas Kluge, DESYon behalf of the H1, OPAL and ZEUS Collaborations
• Introduction / MotivationIntroduction / Motivation• Photon IdentificationPhoton Identification• Results Results
– Inclusive Prompt PhotonsInclusive Prompt Photons– Prompt Photons together with a JetPrompt Photons together with a Jet
• Conclusion / SummaryConclusion / Summary
• Introduction / MotivationIntroduction / Motivation• Photon IdentificationPhoton Identification• Results Results
– Inclusive Prompt PhotonsInclusive Prompt Photons– Prompt Photons together with a JetPrompt Photons together with a Jet
• Conclusion / SummaryConclusion / Summary
“Test QCD by measuring photons”“Test QCD by measuring photons”
prompt photonprompt photon
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 2
Prompt Photons and QCDPrompt Photons and QCD
soft QCD
parametrised in p.d.f.
hard QCD
matrix elements up to NLO in s
soft QCD
hadronisation models
An example (HERA)
e
p
q
prompt photonprompt photon
Prompt photon: small hadronisation uncertainty, good energy measurement!
Two signatures are investigated1) isolated photon (inclusive)2) isolated photon + at least one jet (ET > 3-5 GeV)
Prompt photon: small hadronisation uncertainty, good energy measurement!
Two signatures are investigated1) isolated photon (inclusive)2) isolated photon + at least one jet (ET > 3-5 GeV)
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 3
Production of Prompt PhotonsProduction of Prompt Photons
marks a remnant, made of low pT particles
direct/single resolved (resolved/double resolved) both present in data …there are more (higher order) diagrams
marks a remnant, made of low pT particles
direct/single resolved (resolved/double resolved) both present in data …there are more (higher order) diagrams
e
e
q
e
e
q
photon photon collisions at LEPphoton photon collisions at LEPelectron proton collisions at HERAelectron proton collisions at HERA
e
p
q
e
p
q
resolved photoproduction
direct photoproduction & deep inelastic scattering single resolved process
double resolved process
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 4
Some details on photon identification….Some details on photon identification….
1.1. Basic Basic selection: electromagnetic cluster, with selection: electromagnetic cluster, with no associated trackno associated trackProblem: large background of Problem: large background of 00->-> and and --
>>
2.2. Require isolation cone: veto energy nearby the Require isolation cone: veto energy nearby the photon candidate photon candidate Problem: sensitive to energy deposits from Problem: sensitive to energy deposits from
multiple interactions (m.i.)multiple interactions (m.i.)
3.3. Perform a shower shape analysis: single Perform a shower shape analysis: single photons are more compactphotons are more compact-> Fit signal/background ratio for any result bin -> Fit signal/background ratio for any result bin
(independent of MC background rate!)(independent of MC background rate!)
1.1. Basic Basic selection: electromagnetic cluster, with selection: electromagnetic cluster, with no associated trackno associated trackProblem: large background of Problem: large background of 00->-> and and --
>>
2.2. Require isolation cone: veto energy nearby the Require isolation cone: veto energy nearby the photon candidate photon candidate Problem: sensitive to energy deposits from Problem: sensitive to energy deposits from
multiple interactions (m.i.)multiple interactions (m.i.)
3.3. Perform a shower shape analysis: single Perform a shower shape analysis: single photons are more compactphotons are more compact-> Fit signal/background ratio for any result bin -> Fit signal/background ratio for any result bin
(independent of MC background rate!)(independent of MC background rate!)
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 5
Inclusive Prompt Photons in ep Photoproduction (HERA)Inclusive Prompt Photons in ep Photoproduction (HERA)
H1 and ZEUS data consistent within errors MC: shape OK but normalisation low by ~40-50%
: direct exchanged makes up less than ½ of the cross section Multiple interactions spoil isolation cut -> reduced cross section
H1 and ZEUS data consistent within errors MC: shape OK but normalisation low by ~40-50%
: direct exchanged makes up less than ½ of the cross section Multiple interactions spoil isolation cut -> reduced cross section
Comparison with PYTHIA and HERWIG event generators
Comparison with PYTHIA and HERWIG event generators
142 < W < 266 GeV
Q2 < 1GeV2
ℒint = 105 pb-1
142 < W < 266 GeV
Q2 < 1GeV2
ℒint = 105 pb-1
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 6
Inclusive Prompt Photons in ep Photoproduction (HERA)Inclusive Prompt Photons in ep Photoproduction (HERA)
Reasonable description by NLO calculations on parton level After corrections for hadronisation and multiple interactions:
normalisation 30-40% below the data
Reasonable description by NLO calculations on parton level After corrections for hadronisation and multiple interactions:
normalisation 30-40% below the data
Comparison with NLO pQCD:
Fontanaz, Guillet, Heinrich (FGH)
Krawczyk, Zembruski(K&Z)
Comparison with NLO pQCD:
Fontanaz, Guillet, Heinrich (FGH)
Krawczyk, Zembruski(K&Z)
142 < W < 266 GeV
Q2 < 1GeV2
ℒint = 105 pb-1
142 < W < 266 GeV
Q2 < 1GeV2
ℒint = 105 pb-1
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 7
Inclusive Prompt Photons in Photon-Photon Collisions (LEP)Inclusive Prompt Photons in Photon-Photon Collisions (LEP)
PYTHIA: shape OK but normalisation low by ~50% NLO calculation on parton level gives good description of the data
Variation from photon p.d.f. small compared to experimental uncertainty
PYTHIA: shape OK but normalisation low by ~50% NLO calculation on parton level gives good description of the data
Variation from photon p.d.f. small compared to experimental uncertainty
Comparison with PYTHIA and NLO pQCD (Fontannaz et al)
Comparison with PYTHIA and NLO pQCD (Fontannaz et al)
<see>2 = (197 GeV)2
Q2 < 10 GeV2
ℒint = 649 pb-1
<see>2 = (197 GeV)2
Q2 < 10 GeV2
ℒint = 649 pb-1
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 8
Inclusive Prompt Photons in ep Deep Inelastic Scattering (HERA)Inclusive Prompt Photons in ep Deep Inelastic Scattering (HERA)
MC lower than data by factors 2 (PYTHIA) and 8 (HERWIG) Shape of Et well described, PYTHIA gives a poor description of
pseudorapidity
The MCs do not include wide-angle QED bremsstrahlung
MC lower than data by factors 2 (PYTHIA) and 8 (HERWIG) Shape of Et well described, PYTHIA gives a poor description of
pseudorapidity
The MCs do not include wide-angle QED bremsstrahlung
Comparison with PYTHIA and HERWIG event generators
Comparison with PYTHIA and HERWIG event generators
Q2>35GeV2
ℒint=121pb-1
Q2>35GeV2
ℒint=121pb-1
(x2.3)
(x7.9)
(x2.3)
(x7.9)
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 9
Jet + Prompt Photon: a handle to direct and resolved reactionsJet + Prompt Photon: a handle to direct and resolved reactions
x: Fractional part of the incoming photon energy, taking part in interaction Definitions are infrared safe for x -> 1 Jet Et does not enter the definition (but hadronic final state does via y )
x: Fractional part of the incoming photon energy, taking part in interaction Definitions are infrared safe for x -> 1 Jet Et does not enter the definition (but hadronic final state does via y )
pp
jetjet
prompt photonprompt photon
electronelectron
protonproton
p
e
q
eyEx 2
eyE2
eE
HERA
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 10
Jet + Prompt Photon in ep Deep Inelastic Scattering (HERA)Jet + Prompt Photon in ep Deep Inelastic Scattering (HERA)
NLO calculation on parton level: normalisation reasonable poor description at low Et and in the more forward (proton beam) direction
PYTHIA and HERWIG still by factors 2-4 too low (not shown)
NLO calculation on parton level: normalisation reasonable poor description at low Et and in the more forward (proton beam) direction
PYTHIA and HERWIG still by factors 2-4 too low (not shown)
Comparison with calculation to O(3s
1) (Kramer, Spiesberger)
Comparison with calculation to O(3s
1) (Kramer, Spiesberger)
Q2 > 35 GeV2
ℒint = 121 pb-1
Q2 > 35 GeV2
ℒint = 121 pb-1
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 11
Jet + Prompt Photon in ep Photoproduction (HERA)Jet + Prompt Photon in ep Photoproduction (HERA)
Reasonable description of the data by both NLO calculation on parton level
If a jet is required: slightly better description, NLO/LO correction more moderate
Corrections for hadronisation and multiple interactions, taken from PYTHIA, do not improve the description
Reasonable description of the data by both NLO calculation on parton level
If a jet is required: slightly better description, NLO/LO correction more moderate
Corrections for hadronisation and multiple interactions, taken from PYTHIA, do not improve the description
Comparison with NLO pQCD:
Fonatanaz, Guillet, Heinrich (FGH)
Krawczyk, Zembruski (K&Z)
Comparison with NLO pQCD:
Fonatanaz, Guillet, Heinrich (FGH)
Krawczyk, Zembruski (K&Z)
Q2 < 1GeV2
ℒint = 105 pb-1
Q2 < 1GeV2
ℒint = 105 pb-1
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 12
Jet + Prompt Photon in ep Photoproduction and Photon-Photon Collisions Jet + Prompt Photon in ep Photoproduction and Photon-Photon Collisions
Shape of the distributions different for HERA and LEP Both: NLO consistent with the data multiple interactions more important at low x (resolved photon)
Shape of the distributions different for HERA and LEP Both: NLO consistent with the data multiple interactions more important at low x (resolved photon)
Photon structure enters at lower x≲0.85 (resolved contribution)
Photon structure enters at lower x≲0.85 (resolved contribution)
ICHEP04 Th.Kluge: Prompt Photon Production at HERA 13
Prompt photons: study pQCD and p.d.f. of proton and photon
Alternative to jets: no hadronisation of the photon, good energy measurement small cross section, elaborate photon identification
Consistent findings for p,DIS (HERA) and (LEP)
– MC generators PYTHIA, HERWIG: always undershoot the data, shape in general OK
– NLO QCD calculations on parton level are in reasonable agreement with the data
– If multiple interactions and hadronisation are taken into account, the NLO calculations somewhat undershoot the data (H1)
Why are the predictions low? Higher orders? Non-perturbative effects?
Possible improvements of data analyses: extend phasespace, use more data (HERA II), refine 0 supression/subtraction
Prompt photons: study pQCD and p.d.f. of proton and photon
Alternative to jets: no hadronisation of the photon, good energy measurement small cross section, elaborate photon identification
Consistent findings for p,DIS (HERA) and (LEP)
– MC generators PYTHIA, HERWIG: always undershoot the data, shape in general OK
– NLO QCD calculations on parton level are in reasonable agreement with the data
– If multiple interactions and hadronisation are taken into account, the NLO calculations somewhat undershoot the data (H1)
Why are the predictions low? Higher orders? Non-perturbative effects?
Possible improvements of data analyses: extend phasespace, use more data (HERA II), refine 0 supression/subtraction
SummarySummary