Wolfram ZeunerDESY

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Open Heavy Flavour Production at HERA
Wolfram Zeuner/DESY on behalf of

• 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
...
In higher order only the sum of all contributions
is well defined
Flavour excitations
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Kinematic Variables

• ep collisions Ee27.5 GeV, Ep820/920 GeV
• HERA produces boson proton interactions

Luminosities e-p ? 15 pb-1 ep ?120 pb-1

• Kinematic regions
• Photoproduction (?p) Q2?0
• Deep Inelastic Scattering (DIS) 1?Q2 ? 5104
  GeV2

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Calculations and Models
Perturbative 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
  (directresolved)
• DGLAP evolution (Q2 ordering)
• CASCADE based on CCFM evolution (angular
  ordering)

Typical scales ?p
DIS
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Open Charm Production

• Charm tagging through D mesons

31350 240
using ?M method

• Other decays and mesons are used

• semi leptonic electrons - via dE/dx

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Charm cross sections - D and Ds in ?p

• FO QCD (FMNR)
• p MRSG, ? GRV-G HO
• Thin mc1.5 GeV, ?R mT ?F/2
• Thick mc1.2 GeV, ?R 0.5 mT
• f(c?D)0.235 (LEP)
• Data more precise than prediction
• Normalisation, low and par. dependent

RS QCD (Kniehl et al.) p CTEQ4M, mc1.5 GeV, ?R
mT ?F/2

• Problems to describe shape
• Too low at high ?
• Sensitive to ?-PDF
• Best with GS-G-HO, but uc

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Charm Production in DIS
Double 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 -

• All measurements agree
• Data described by NLO QCD
• Strong scaling violations
• Steep rise toward low x
• Ratio grows towards
• low x ( gt25 for Q2 ? 6.5 GeV2)

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Open Charm Production - The Gluon Density
Distribution
Reconstruct gluon momentum fraction xg from e and
D Extract xgg(xg,Q2) by unfolding differential
D cross sections using FO NLO calculations
xgg(xg,Q2)
xg
Gluon parton density from D in ?p and DIS agree
with QCD fit to inclusive DIS cross section
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Charm Production in DIS - D

• The ratio ?(e-p)/?(ep) 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 ?

• Fragmentation fractions of excited D and Ds
  mesons

• Strangeness production from
• Ds ? ?? ? (K K-) ? vs. D?D0 ? ?(K ?) ?
• ? Strangeness suppression factor ?s of LUND
  model

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Charm Fragmentation - Universal ?

• Production of vector mesons (D0) vs pseudoscalar
  mesons (D)

assume ?(D) ?(D0)
PV 0.5460.0450.028 ZEUS 0.57 0.05
OPAL 0.595 0.045
ALEPH
D0 from D
D0 not from D

• No difference in charm fragmentation found
  between ep and ee- collisions
• All experimental results favour universality of
  charm fragmentation

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Open Beauty Production
High quark mass ? pQCD, but cross section low ?b/
?c ?10-2

• B Tagging
• Semi-leptonic decays with a ptrel method -
  H1ZEUS

• Impact parameter method - H1
• using lifetime information -
• Intercept of muons from semi-leptonic
• decays in the transverse plane

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Beauty in Photoproduction
H1 - Muon Analysis
Combined likelihood fit to ptrel and ? Combine
with earlier ptrel measurement
B fraction 273 - 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

• ZEUS electron analysis
• Electrons identified by dE/dx of tracks

Fitted b fraction 14.73.8 - HERWIG predicts
16.2 Cross section by MC (LO) on the low
side HERWIG 8pb, Pythia 18 pb, Cascade 20 pb
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Beauty in Photoproduction
ZEUS Muon Analysis
From fit fb30

• Pythia with b excitation describes data
  reasonably well
• NLO ? more data on tape to be analysed

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Beauty in DIS
First observation of b production in DIS by H1
analysis like in ?p
Sample 168 events Combined fit in ptrel and
? fb 438
Predictions NLO QCD ? (112)
pb LO MC - AROMA ?
9 pb CASCADE ? 15 pb
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Summary - Open Beauty Production at HERA
Measured cross section exceeds NLO calculation in
?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

• Charm fragmentation - no difference seen between
  ee- and ep

• 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