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Wolfram Zeuner/DESY
on behalf of
Open Heavy Flavour Production at HERA
• Introduction Selected Results from
• Charm - Photoproduction and DIS• Charm Fragmentation• Beauty - Photoproduction and DIS
• Summary
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Open Heavy Flavour Production at HERA
• Study c and b production• Hard scale from mQ tests of pQCD• Access to gluon in proton and photon• Study non-perturbative effects as fragmentation
LO (s)
Direct BGF Resolved photon
Flavour excitationsIn higher order only the sumof all contributions is well defined
+ ...
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Kinematic Variables
framerestpinEE
tconstituenpoffractionmomentum
transfermomentumGeVenergycms
e/
)4(318/300
2
p
p
p
222pe
2p
PkPq
y
qP2
2Qx
)k(kqQE4E)P(ks
• ep collisions: Ee=27.5 GeV, Ep=820/920 GeV• HERA produces boson proton interactions
Luminosities e-p: 15 pb-1
e+p: 120 pb-1
Kinematic regions• Photoproduction (p) : Q20• Deep Inelastic Scattering (DIS): 1Q2 5·104 GeV2
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Calculations and ModelsPerturbative QCD 1. Fixed order NLO QCD (FO) calculations Heavy Quark produced only in hard scattering DIS: B. W. Harris, J. Smith, Phys. Rev D57, 2806 (1998) - HVQDIS MC p: Frixione et al.,Nucl. Phys. B454 (1997) 3
2. Resummed calculations in NLO (RS) Heavy Quark is an active flavour inside the proton and photon B. Kniehl et al. Z. Phys C76 (1997) 690; J. Binnewies et al., Z. Phys C76 (1997) 677; Phys. Rev. D58 (1998) 14014 M. Cacciari et al., Phys. Rev. D55 (1997) 2736; ibid 7134
3. Variable flavour numbers (VFNS), flavours are active depending on scale M.A.G. Aivaizis et al. Phys. Rev. D50 3102 (1994) (ACOT)
LO Monte Carlo Calculations – (BGF)• AROMA (direct only), HERWIG and PYTHIA (direct+resolved) DGLAP evolution – (Q2 ordering)• CASCADE based on CCFM evolution – (angular ordering)
Typical scalesp: DIS: 22)( TqT pmqm 224 Qmq
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• semi leptonic electrons - via dE/dx•
•
•
•
Open Charm Production
• Charm tagging through D*± mesons
s
ss
K
KDD
0
31350 ± 240
• Other decays and mesons are used
using M method
01
02
01
0
)2536(
)2460(),2420(
ss
s
KDD
DDD
DKD
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• Problems to describe shape• Too low at high • Sensitive to -PDF • Best with GS-G-HO, but u=c
Charm cross sections - D* and Ds in pZEUS 96+97
RS QCD (Kniehl et al.)p: CTEQ4M, mc=1.5 GeV, R= mT= F/2
FO QCD (FMNR) p: MRSG, : GRV-G HOThin: mc=1.5 GeV, R= mT= F/2Thick: mc=1.2 GeV, R= 0.5 mT
f(cD*)=0.235 (LEP)
• Data more precise than prediction• Normalisation, low and par. dependent
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Charm Production in DISDouble differential D* production cross section
• QCD predictions: bands (mc,c) : (1.3 GeV,0.35) - (1.5 GeV, 0.01)• HVQDIS - NLO DGLAP prediction too low for low pt and positive • CASCADE - LO CCFM MC describes data better
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Charm Production in DIS - ),( 22 QxF cc
)Qx,(F)1(1xQ
2dxdQ
)Qx,(d 2cc2
24
2
2
22
y
Assumptions• NLO FO calculation (BGF)• Neglect bound charm (2.5-4.5%)• Neglect • BR(c D*) = 0.222 - 0.233 from e+e-
Extrapolate to full phase space in pT and Fragmentation effect on theextrapolation is small
ccLF
• All measurements agree• Data described by NLO QCD• Strong scaling violations• Steep rise toward low x• Ratio grows towards low x ( >25% for Q2 6.5 GeV2)
22 FF cc
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Open Charm Production - The Gluon Density Distribution
Reconstruct gluon momentum fractionxg from e and D*
Extract xgg(xg,Q2) by unfoldingdifferential D* cross sectionsusing FO NLO calculations
Gluon parton density fromD* in p and DIS agree withQCD fit to inclusive DIS cross section
xg
x gg(x
g,Q2 )
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Charm Production in DIS - D*
Unexpected observation )()( XDepeXDepe
• The ratio (e-p)/(e+p) rises towards high Q2 and high x• Both cross sections are compatible with NLO prediction within errors
More data needed
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Charm Fragmentation - Universal ?
OPAL 0.3)%0.41.6( ZEUS)%14.018.024.1()(
CLEO 0.3)%1.9( ZEUS)%41.058.000.2()(
CLEO 0.3)%1.8( ZEUS)%06.018.046.1()(
08.006.01
40.148.0
0*2
33.027.0
01
sDcf
Dcf
Dcf
-
03.005.0
ee 0.040.43
ZEUS07.041.0
XDXD
ep
sep
-ee03.026.0ZEUS05.027.0
s
• Strangeness production from Ds (K+ K-) vs. D*D0 (K ) Strangeness suppression factor s of LUND model
• Fragmentation fractions of excited D and Ds mesons0
10
201 )2536(and)2460(),2420( ss KDDDDD
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Charm Fragmentation - Universal ?
• Production of vector mesons (D0) vs pseudoscalar mesons (D*)
assume (D*±) = (D*0)
PV= 0.546±0.045±0.028 ZEUS 0.57 ±0.05 OPAL 0.595 ±0.045 ALEPH
0
0
DDD
VPSVPV
D0 not from D*±
D0 from D*±
• No difference in charm fragmentation found between ep and e+e- collisions• All experimental results favour universality of charm fragmentation
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Open Beauty Production
B Tagging: • Semi-leptonic decays with a pt
rel method - H1+ZEUSJet Axis orThrust Axis
Lepton
ptrel
• Impact parameter method - H1 using lifetime information - Intercept of muons from semi-leptonic decays in the transverse plane
JetJet
<0>0
High quark mass pQCD, but cross section low b/ c 10-2
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H1 - Muon Analysis
Combined likelihood fit to ptrel and
Combine with earlier ptrel measurement
pb25170)( ' XXbbepvis
Beauty in Photoproduction
B fraction 27±3 % - from fit to MC
Predictions:• NLO (FMNR) 54 ±9 pb• AROMA (LO no resolved) 38 pb• CASCADE (LO CCFM-splitting) 66 pb
All predictions significantly below data
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Beauty in Photoproduction
pb4.69.24 2.43.7
eb
Xedijetepe
Fitted b fraction: 14.7±3.8% - HERWIG predicts 16.2%Cross section by MC (LO) on the low sideHERWIG: 8pb, Pythia: 18 pb, Cascade 20 pb
ZEUS electron analysis• Electrons identified by dE/dx of tracks
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Beauty in PhotoproductionZEUS Muon Analysis
• Pythia with b excitation describes data reasonably well• NLO ? – more data on tape to be analysed
From fit:fb~30%
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Beauty in DISFirst observation of b production in DIS by H1; analysis like in p
Sample: 168 eventsCombined fit in pt
rel and fb= 43±8%
pb10839)( ' XXbbepvis Predictions: NLO QCD: = (11±2) pb LO MC - AROMA: = 9 pb
CASCADE: = 15 pb
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Summary - Open Beauty Production at HERA
Measured cross sectionexceeds NLO calculationin p and DIS
• LO -Monte Carlo predictions are always on the low side• Quark excitation improves agreement in central region• CCFM improves agreement in central region• Forward region too low in all predictions
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SUMMARY• Large mass of c and b defines hard scale for pQCD • Charm - total cross sections predictable
• p cross section higher than predicted by LO-MCs and NLO calc. at high • DIS cross section higher than predicted at high , low x and low Q2
• Beauty Cross section measured in p and DIS• clearly underestimated by NLO predictions - surprising
• NNLO Calculations and MCs are needed• Investigation of interactions with p-remnant (DIS) or -remnant (factorisation) • Need to investigate non-perturbative effects
What comes next ?• HERA has been upgraded for high luminosity - 1fb-1 until 2006 • Detectors have been upgraded - higher sensitivity and efficiency
Precision measurements, QCD tests and detailed investigations of the gluon in the proton, surprises not excluded
• Charm fragmentation - no difference seen between e+e- and ep