1. 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

2. + ... 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

3. Kinematic Variables 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) : Q20
Deep Inelastic Scattering (DIS): 1Q2  5·104 GeV2

4. 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. D (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 scales
p: DIS:

5. 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

6. Charm cross sections - D* and Ds in p
ZEUS 96+97
RS QCD (Kniehl et al.)
p: CTEQ4M, mc=1.5 GeV, R= mT= F/2
FO QCD (FMNR)
p: MRSG, : GRV-G HO
Thin: mc=1.5 GeV, R= mT= F/2
Thick: mc=1.2 GeV, R= 0.5 mT
f(cD*)=0.235 (LEP)
Data more precise than prediction
Normalisation, low and par. dependent
Problems to describe shape
Too low at high 
Sensitive to -PDF
Best with GS-G-HO, but u=c

7. 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

8. Charm Production in DIS -
Assumptions
NLO FO calculation (BGF)
Neglect bound charm ( %)
Neglect
BR(c D*) = from e+e-
Extrapolate to full phase space
in pT and h
Fragmentation effect on the
extrapolation is small
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)

9. 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

10. Charm Production in DIS - D*
Unexpected observation
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

11. 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

12. Charm Fragmentation - Universal ?
Production of vector mesons (D0) vs pseudoscalar mesons (D*)
assume (D*±) = (D*0)
PV= 0.546±0.045± ZEUS
0.57 ± OPAL
0.595 ± ALEPH
D0 from D*±
D0 not from D*±
No difference in charm fragmentation found between ep and e+e- collisions
All experimental results favour universality of charm fragmentation

13. Open Beauty Production
High quark mass  pQCD, but cross section low b/ c 10-2
B Tagging:
Semi-leptonic decays with a ptrel method - H1+ZEUS
Jet Axis or
Thrust Axis
Lepton
ptrel
Jet 
<0
>0
Impact parameter method - H1
using lifetime information -
Intercept of muons from semi-leptonic
decays in the transverse plane

14. Beauty in Photoproduction
H1 - Muon Analysis
Combined likelihood fit to ptrel and 
Combine with earlier ptrel measurement
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

15. Beauty in Photoproduction
ZEUS electron analysis
Electrons identified by dE/dx of tracks
Fitted b fraction: 14.7±3.8% - HERWIG predicts 16.2%
Cross section by MC (LO) on the low side
HERWIG: 8pb, Pythia: 18 pb, Cascade 20 pb

16. Beauty in Photoproduction
ZEUS Muon Analysis
From fit:
fb~30%
Pythia with b excitation describes data reasonably well
NLO ? – more data on tape to be analysed

17. 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= 43±8%
Predictions: NLO QCD:  = (11±2) pb
LO MC - AROMA:  = 9 pb
CASCADE:  = 15 pb

18. 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

19. 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 e+e- 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