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Charm Production in ep-Collisions

a r X i v :h e p -e x /0505078v 1 25 M a y 2005

CHARM PRODUCTION IN EP-COLLISIONS

J.Wagner,representing the ZEUS and H1Collaborations

DESY FH1,Notkestrasse 85D-22603Hamburg,Germany

Recent results on the production of open charm in electron proton scattering at HERA are reviewed.Results on the fragmentation of charm are presented and compared to the measure-ments at e +e ?colliders.Recent results on the charm contribution to the proton structure function F 2are shown.Furthermore,measurements of inclusive D ?meson,D ?+jet and D ?+muon production are presented.These results are compared with perturbative QCD calculations at next-to-leading order as well as with Monte Carlo predictions.

1Introduction

At HERA,electrons (or positrons)of energy 27.5GeV are collided with 920GeV protons,pro-viding a center-of-mass energy of

0.2

0.40.6

H1

ZEUS (prel.)

F 2

c c

–HERA F

cc

–10

10

10

10

10

10

10

x

a)

b)

Figure 1:a)Fragmentation fractions obtained at HERA and compared with e +e ?measurements.b)Charm

contribution to the structure function F 2,F c ˉ

c 2,compare

d with th

e NLO QCD ?t.

hold in di?erent limits of the scale μused for the calculation of the hard cross section.In addi-tion the data are compared with the Monte Carlo generators PYTHIA 5,which implements the DGLAP evolution equation 6and CASCADE 7which is based on the CCFM evolution equation 8.

2Fragmentation

In most measurements of charm production at HERA,D ?mesons are used to identify the presence of charm quarks,exploiting the well known mass di?erence method.The charm quark cross section is then determined from the measured D ?cross section by using the fragmentation fraction f (c →D ?),as measured by other experiments.These measurements are based on the assumption of universality of charm fragmentation.

In recent measurements the H1and ZEUS experiments have determined the fragmentation fractions f (c →D )for the various charmed hadrons (D +,D 0,D s ,D ?+and Λc (ZEUS))in DIS or photoproduction respectively 9,10.In ?gure 1a),the fragmentation fractions obtained in ep collisions at HERA are presented.The results are compared with measurements at e +e ?colliders and good agreement is observed indicating that fragmentation is universal.

In the H1measurement the shape of the di?erential D meson cross sections was found to be very similar for di?erent D meson species.Also the fragmentation ratios,the ratio of u to d quarks R u/d ,the strangeness suppression factor γs and the fraction of D mesons produced as vector mesons P v ,obtained by ZEUS and H1are in good agreement with those from the LEP experiments.The uncertainties in particular on the ZEUS measurement are competitive with those from LEP.

b)

c)

Figure2:a)D?+jet cross section as a function of E jet

T

.b+c)Normalised D?

of x obs

g

(b)and p T(D?μ)(c).The bˉb quark contamination is approximately10%.

3Proton Structure

Both H1and ZEUS have determined the charm contribution F cˉc2to the structure function F2of the proton11,12.F cˉc2is shown as a function of Bjorken x in several bins of Q2in?gure1b).In the past F cˉc2was always determined by extrapolation of the measured visible D?cross section into the full phase space.In a recent measurement the H1collaboration made use of the H1 silicon vertex detector to measure the fraction of charm events from the lifetime distribution in an inclusive event sample.In this measurement the size of the extrapolation is substantially reduced.The results of this measurement are indicated in?gure1b)as?lled squares in the last two Q2bins.Generally,good agreement between the ZEUS and H1data and the NLO QCD prediction is obtained.This means that the prediction of the charm contribution to F2from scaling violations is consistent with the F cˉc2measurement.

4Production Mechanism

The measurement of the D?production cross section as a function of Q2for1.5

If at least one jet is required in addition to the D?meson,the transverse energy of the jet,E jet T, provides an additional hard scale.In an analysis of the ZEUS collaboration,carried out in the

photoproduction regime,the D?+jet cross section as a function of E jet T was determined15 and compared to the massive and massless NLO calculations(?gure2a)).At high jet transverse energies the data are above the massive calculation,while the massless NLO calculation describes the data better,at least in this regime.

In a very recent and even more exclusive analysis in the photoproduction regime,performed by the H1collaboration,a muon is required in addition to the D?meson16.The charge and angle correlations between the D?and the muon are used to select double tagged events.Due to the double tagging,details of the production mechanism can be studied.The normalised D?+muon cross sections as a function of x obs g and p T(D?μ)are shown in?gure2b+c).The quantity x obs g is a good estimate of the relative momentum fraction of the gluon x g,whereas the correlation is worse for p T(D?μ)and the transverse momentum of the gluon k T.The data are compared to the NLO calculation and the LO calculation(indicated as FMNR in the?gure)as well as to the Monte Carlo generators PYTHIA and CASCADE.All calculations give a reasonable description of the shape of the x obs g distribution.In the case of the p T(D?μ)distribution,the pure LO calculation is too soft to describe the data,while the NLO calculation and the Monte Carlo generators describe the data well.

5Summary

Charm production at HERA has been extensively https://www.sodocs.net/doc/3914135230.html,parisons of HERA data with measurements at e+e?colliders support the assumption that the fragmentation of charm is in-dependent of the underlying hard physics process.The measurements of the charm contribution F cˉc2to the proton structure are consistent with the expectations from the scaling violations of F2.NLO QCD e?ects are essential to predict charm photoproduction as shown for D?+muon events.The general description is reasonable although some details of the D?cross sections are poorly described.

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