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

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

+ ...

Kinematic Variables

framerestpinEE

tconstituenpoffractionmomentum

transfermomentum

GeVenergycms

e/

)4(

318/3002

p

p

p

222pe

2p

Pk

Pqy

qP2

2Qx

)k(kqQ

E4E)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

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

• 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

D

KD

• 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 HO

Thin: 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

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

Charm Production in DIS - ),( 22 QxF cc

)Qx,(F)1(1xQ

2

dxdQ

)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

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 )

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

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

XD

XD

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

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

DD

D

VPS

VPV

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

Open Beauty Production

B Tagging: • Semi-leptonic decays with a pt

rel method - H1+ZEUS

Jet 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

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

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

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%

Beauty in DIS

First 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

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

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