
%AUTHOR_STYLES_AND_DEFINITIONS%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%Please reduce your own definitions and macros to an absolute
%minimum since otherwise the editor will find it rather
%strenuous to compile all individual contributions to a
%single book file
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%\begin{document}
%
\title*{Measurement of Open $b$ Production at HERA}
\toctitle{Measurement of Open $b$ Production at HERA}
\titlerunning{Open $b$ Production}
% allows abbreviation of title, if the full title is too long
% to fit in the running head
%
\author{Daniel Pitzl}
\authorrunning{Daniel Pitzl}
% if there are more than two authors,
% please abbreviate author list for running head
%
%
\institute{DESY, Notkestrasse 85, D-22609 Hamburg, Germany}

\maketitle              % typesets the title of the contribution

\begin{abstract}
H1 and ZEUS have reported the first measurements of open $b$ 
photoproduction at HERA. Both experiments use semi-leptonic decays to
enrich a two-jet sample with heavy quarks.
The beauty fraction is extracted from a fit of the measured $p_{T}^{rel}$
distribution with contributions from $b$, $c$ and light quarks 
according to a leading order QCD simulation.
The measured cross section for $b$ quarks is found to be larger
than the  predictions from next-to-leading order perturbative QCD
calculations.
\end{abstract}
%
\section{Introduction}
%
The production of heavy quarks at HERA is an important testing ground
for perturbative QCD calculations, where the heavy quark mass provides
a hard scale.
The basic mechanism, in leading order, is photon-gluon fusion, where
a photon emitted from the electron and a gluon from the proton
generate a heavy quark-antiquark pair.
The dominant contribution to the cross section is due to the exchange
of an almost real photon, whose negative squared four-momentum
$Q^{2} = -q^{2}$ is small.
In this photoproduction regime, a second production mechanism becomes
important, where the photon fluctuates into a hadronic state
which is resolved into its partonic composition in the hard collision.
In next-to-leading order QCD, however, only the sum of direct and
resolved processes is unambiguously defined~\cite{p-frixione}.
\par
Heavy quark production at HERA is dominated by charm, with a total
cross section of about 1\,\umu b.
The prediction for open $b$ production is 3.8\,nb from the leading
order AROMA~\cite{p-aroma} Monte Carlo event generator, which uses
massive matrix elements and considers direct processes only.
Predictions from next-to-leading order
QCD calculations~\cite{p-ellis-nason,p-smith-neerven,p-frixione94}
are between 4.7\,nb and 10\,nb, depending on the mass of the $b$ quark
and the parton distribution functions used.
\par
During the years 1994 to 1997 HERA provided
positron-proton collisions at a center of mass energy
$\sqrt{s} = 300$\,GeV.
In photoproduction the photon-proton center of mass energy is given by
$W_{\gamma p} \approx \sqrt{ys}$, where the proton mass has been
neglected, and the inelasticity $y$ is given by
$y = q \cdot P / l \cdot P$, with $P$ and $l$ for the four momenta of 
the incoming proton and positron, respectively.
\par
In the analyses of both H1 and ZEUS heavy quarks are identified by their
semileptonic decays, which suppresses the light quark background.
The flight directions of the quarks are approximated by reconstructing
two jets in the final state. The contributions from $b$ and $c$ quarks
are distinguished statistically by exploiting the effects of the
higher $b$ quark mass on the kinematics of the final state.
%
\section{H1 muon analysis}
%
The first measurement of the open $b$ photoproduction cross section at 
HERA was reported by the H1 collaboration~\cite{b98}.
The final analysis~\cite{b99} uses a data sample corresponding to
an integrated luminosity of 6.6\,pb$^{-1}$ collected in 1996.
The virtuality of the exchanged photon was restricted to the range
$Q^2 < 1$\,GeV$^2$ by requiring that no scattered positron candidate
was found in the electromagnetic liquid argon or scintillating-fibre  
calorimeters.
The variable $y$ was calculated from a combination of calorimetric
and tracking information
using the Jacquet-Blondel method~\cite{yJB}
and was required to be in the range $0.1 <  y < 0.8$,
corresponding to 95\,GeV$<W_{\gamma p}<269$\,GeV.
The event selection required at least two jets in the pseudorapidity 
range $|\eta| < 2.5$ with transverse energies $E_{T}>6$\,GeV.
The jets were identified using a cone algorithm with a radius
$r = \sqrt{(\Delta\eta)^2 + (\Delta\phi)^2}<1$, where $\eta$ is the
pseudorapidity and $\phi$ is the azimuthal angle of the track or cluster.
\par
At least one muon candidate with a transverse momentum $p_t>2$\,GeV/c 
had to be identified in the instrumented iron system in the 
polar angle range $35^o < \theta < 130^o$. The muon candidate had to be 
found within one of the jets.
This sample contains muons from semi-leptonic decays of $b$ and $c$ 
hadrons and background from light quark decays and from hadrons 
faking a muon signature in the detector.
A detailed and extensive Monte Carlo simulation was performed
to determine  the probability $\mathcal{P}_h^{\mu}(p,\theta)$ that a
hadron $h = \pi, K$, or $p$ produces a muon signature.
These probabilities vary with momentum and polar angle but do not exceed
0.6\% for pions, 2\% for kaons and 0.2\% for protons.
They were tested  using reconstructed decays of $K^0_S$ and $\phi$ 
mesons as sources of charged pions and kaons in the data. 
Figure~\ref{fakefig} shows the distribution of pions from
$K^0_S$ decays that produce a muon signature.
It is well described, in shape and magnitude, by the muon fake 
probability for pions as obtained from the simulation.
From the $\phi$ decays, eight kaons produced a muon signature, 
compared to an expectation of 7.8.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl1.eps}
\end{center}
\caption[]{Pions from K$^0_S$ decays producing a muon signature in the data
(symbols with error bars) and the expectation from the muon fake 
probability $\mathcal{P}_{\pi}^{\mu}(p,\theta)$ (solid histogram).}
\label{fakefig}
\end{figure}
%
\par
The decay of a $b$ hadron releases an energy of around 3\,GeV
and can involve decay particles with transverse momenta up to 
2.3\,GeV/c relative to the $b$ hadron direction.
The $b$ hadron direction was experimentally approximated by the thrust
axis of the jet, defined as the axis which maximizes
$$ T = \max\frac{\sum_{i\ne\mu} |p_i^L|}{\sum_{i\ne\mu} |p_i|},$$
where the sum is over all particles in the jet except the muon.
The transverse momentum of the muon relative to the thrust axis
$p_{T,rel}^{\mu}$ was used to statistically separate $b$, $c$ and 
light quark contributions in the sample.
The $p_{T,rel}^{\mu}$ distribution for the 1996 H1 data is shown in
figure~\ref{pitzl2}. It has a maximum at about 0.7\,GeV/c and a tail
out to 4\,GeV/c.
\par
The $p_{T,rel}^{\mu}$ distribution of hadrons faking a muon signature 
was determined in shape and absolute normalization from a data
sample of similar two-jet events,
selected without the requirement of a muon candidate.
The muon-fake probabilities $\mathcal{P}_h^{\mu}(p,\theta)$ were then
applied to all particles in the jets, with a pion-kaon-proton ratio
as given by the JETSET fragmentation model.
The predicted background amounted to 25.9\%.
The expected shapes of the $p_{T,rel}^{\mu}$ distributions for $b$ and
$c$ decays were taken from the AROMA Monte Carlo and the normalizations
were determined from a fit to the data as $(50.8 \pm 4.9)$\% for $b$
and $(22.4 \pm 5.0)$\% for $c$, where the errors are statistical only.
The sum of the three contributions is in good agreement with the
measured distribution.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl2.eps}
\end{center}
\caption[]{The $p_{T,rel}^{\mu}$ distribution as measured by H1
(points with statistical error bars).
The solid line is the sum of the fixed hadron background
and the beauty and charm contributions from AROMA
with normalizations from a fit to the data.}
\label{pitzl2}
\end{figure}
%
\par
From the number of muons attributed to $b$ events the cross section
for open $b$ production in the range $Q^2<1$\,GeV$^2$, $0.2 < y < 0.8$,
$p_{T}^{\mu} > 2$\,GeV/c and $35^o < \theta^{\mu} < 130^{o}$
was determined as
$$ \sigma_{vis}( ep \rightarrow b\bar{b}X \rightarrow \mu Y)
= \left( 0.176 \pm 0.016\ (stat.) \ ^{+0.026}_{-0.017}\ (syst.) 
\,\right) \ \mathrm{nb.}$$
This cross section contains an extrapolation to the full phase space for the
two jets, where the AROMA Monte Carlo gives an acceptance of 42.9\%
for the requirement of two jets with $E_{T}>6$\,GeV.
The systematic error contains contributions due to uncertainties in
the muon reconstruction efficiency, the hadronic energy scale in the
calorimeter, and the luminosity determination.
The systematic error due to the light quark background was determined
by varying the fractions for pion, kaon and proton production and by
using different data samples.
The analysis was also performed using the HERWIG~\cite{p-herwig}
and RAPGAP~\cite{p-rapgap} leading order Monte Carlo Generators,
which also contain resolved photon contributions.
The effects of fragmentation was studied by varying the parameter
$\epsilon$ of the Peterson fragmentation function.
The variation of the cross section with different simulations amounts
to $\pm7.1$\% and is included in the systematic error.
The cross section predicted by AROMA in leading order and for direct 
photon processes only is 0.038\,nb, where a $b$-quark mass of
4.75\,GeV/c$^{2}$ and the MRS-G~\cite{p-mrsg} parton densities
for the proton were used.
\par
The measured cross section was compared to a next-to-leading order
QCD calculation by the authors of~\cite{p-fmnr}.
Their program generates a final state at the parton level.
In order to apply the cuts on the muon that define the visible cross section,
the fragmentation and weak decay processes had to be added~\cite{b99}.
The $b$-quark mass was taken as 4.75\,GeV/c$^{2}$ and the
MRS-G~\cite{p-mrsg} and GRV-HO~\cite{p-grv-ho} parton densities
for the proton and photon, respectively, were used.
The cross section from the NLO QCD calculation
in the kinematic range defined above was
$(0.104 \pm 0.017)$\,nb, where the uncertainty is due to the 
fragmentation process and a variation of the renormalization and 
factorization scales by a factor of two from the default value
$\mu_{R} = \mu_{F} = \sqrt{m_{b}^{2} + p_{t,b}^{2}}$.
The NLO QCD prediction for the open $b$ visible photoproduction cross
section is 2.5 standard deviations below the measured value, where
statistical, systematic and theoretical uncertainties were added
in quadrature.
\par
Nevertheless, the NLO QCD calculation~\cite{p-fmnr} was used to
extrapolate to the full kinematic phase space and to derive the
total photoproduction cross section for $b$-quarks.
The total branching ratio for a muon to be produced in the decay
chain of $b$ hadrons was applied.
The Weizs\"acker-Williams approximation was used to determine the
equivalent flux of photons emitted by the positron.
The average photon-proton center of mass energy is
$\left< W_{\gamma p} \right> \approx 180$\,GeV.
The measured cross section is
$$\sigma(\gamma p \rightarrow b\bar{b} X) = 
( 111 \pm 10 ^{+16}_{-11} \pm 17 ) \ \mathrm{nb,}$$
where the first and second error are the experimental statistical and
systematic error and the third error is due to the 
extrapolation procedure and the uncertainty in the semileptonic
branching ratio of $B$ hadrons.
The H1 measurement is shown in figure~\ref{pitzl3} together with
the NLO QCD prediction and a measurement by EMC~\cite{p-emc}
at fixed target energy.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl3.eps}
\end{center}
\caption[]{The total photoproduction cross section
$\sigma(\gamma p \rightarrow b\bar{b}X)$.
The inner error bar is statistical while the outer error bar includes
all systematic errors as explained in the text.
The horizontal bar indicates the range of the measurement.
The solid curve shows the NLO QCD calculation~\cite{p-fmnr}
and the shaded band indicates a change in the factorization scale by
a factor of two. The dashed line shows the NLO prediction when the
MRST~\cite{p-mrst} instead of the MRS-G~\cite{p-mrsg} parton densities
for the proton are used.}
\label{pitzl3}
\end{figure}
%
\section{ZEUS electron analysis}
%
The ZEUS collaboration has reported a measurement of the
$b$ production cross section using decays into electrons~\cite{zeus-b}.
The data were collected during 1996 and 1997 when HERA was running
with positrons.
The integrated luminosity amounts to 36.9\,pb$^{-1}$.
The kinematic range in the analysis was restricted to the region
$Q^2 < 1$\,GeV$^2$ and $0.2 < y < 0.8$.
\par
The $k_{T}$ clustering algorithm~\cite{p-kt} was used to reconstruct jets
and at least two jets with $E_{T}^{jet1} > 7$\,GeV and $E_{T}^{jet2} > 6$\,GeV
in the pseudorapidity range $|\eta^{jet}| < 2.4$ were required.
\par
Electrons were identified by using information from the central drift
chamber and the uranium-scintillator calorimeter.
A transverse momentum $p_{T}^{e^-} >1.6$\,GeV/c was required and
the pseudorapidity was restricted to the range $|\eta^{e^-}| < 1.1$.
The specific energy loss $dE/dx$ in the drift chamber is about
1.4\,mip for electrons due to the relativistic rise, while
hadrons are minimum ionizing.
Tracks were linked to calorimeter clusters, where a cluster is called
electron enriched if it has more than 90\% of its energy deposited in 
the electromagnetic section. Conversely, a cluster is called hadron
enriched if it has more than 60\% of its energy deposited in the
hadronic section. The hadron enriched sample contains essentially no
electrons and can be used as a background sample.
\par
Figure~\ref{pitzl5} shows the $dE/dx$ spectra of the electron and
hadron enriched samples.
The shaded region $dE/dx < 1.1$\,mips is used for normalization.
The lower part of figure~\ref{pitzl5} shows the subtraction of the
two samples with a clear electron signal.
A background of electrons from photon conversions in the material
in front of the drift chamber is determined from the data and
the detector simulation and is also shown in figure~\ref{pitzl5}.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl5.eps}
\end{center}
\caption[]{(top) The dE/dx distributions for the electron enriched
and the hadron enriched samples, normalized in the region
dE/dx $< 1.1$\.mips.
(bottom) The subtraction shows the electron signal.
The background from photon conversions is indicated by the 
dashed-dotted line.}
\label{pitzl5}
\end{figure}
%
\par
The differential cross section $d\sigma/dp_{T,rel}$, where
$p_{T,rel}$ is the transverse momentum of the electron relative to
the closest jet, has been measured for the process
$e^{+}\ p \rightarrow \mathrm{dijet} \ e^{-} \ X$
and is shown in figure~\ref{pitzl7}.
The ZEUS data points show the statistical error as the inner
error bar and the statistical and systematic errors
added in quadrature as the outer bars.
The uncertainty due to the ZEUS calorimeter energy scale is
shown separately as the shaded band.
Figure~\ref{pitzl7} also shows the prediction from the
HERWIG~\cite{p-herwig} leading order Monte Carlo program
for beauty production (17\%), for charm and light quarks (83\%)
and their sum. The overall rate was scaled by a factor of 3.7 in order
to match the measured cross section.
The HERWIG program was used with the default heavy quark masses
($m_c = 1.55$\,GeV/c$^2$ and $m_b = 4.95$\,GeV/c$^2$)
and with the CTEQ-4D~\cite{p-cteq} and GRV-LO~\cite{p-grv}
structure functions for the proton and photon, respectively.
Allowing the contributions for beauty and charm plus light flavours to 
vary, a fraction of $(20 \pm 6 \ (stat.) \ ^{+12}_{-7} \ (syst.)\ )$\%
for beauty production was determined.
Using this value, the beauty production cross section,
measured in the kinematic range
 $Q^2 < 1$\,GeV$^2$, $0.2 < y < 0.8$, $E_{T}^{jet1} > 7$\,GeV,
$E_{T}^{jet2} > 6$\,GeV, $|\eta^{jet}| < 2.4$, $p_{T}^{e^-} >1.6$\,GeV/c,
and $|\eta^{e^-}|<1.1$, was determined as
$$ \sigma_{b\bar{b}}^{vis}
(e^{+}\ p \rightarrow \mathrm{dijet} \ e^{-} \ X) = 
\left( 39 \pm 11 \ (stat.) \ ^{+23}_{-16} \ (syst.) \ \right)
\mathrm{pb, ZEUS\ preliminary}$$
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl7.eps}
\end{center}
\caption[]{The differential cross section in $p_{T,rel}$ for dijet 
events with an electron. The ZEUS data points are compared to the 
HERWIG Monte Carlo, which was scaled by a factor 3.7 to match the data.
A beauty fraction of 17\% is the HERWIG prediction.}
\label{pitzl7}
\end{figure}
%
The measured $b\bar{b}$ cross section is compared to leading order
QCD Monte Carlo predictions in figure~\ref{pitzl8} as a function
of the $b$-quark mass assumed in the simulation.
The HERWIG cross section decreases with mass and shows only a
small dependence on the choice of the proton and photon structure
function.
The PYTHIA~\cite{p-pythia} program contains massless matrix elements
and was used with the GRV94-LO and GRV-LO~\cite{p-grv} parton
distributions functions for the proton and photon, respectively.
It predicts a higher $b\bar{b}$ cross section than HERWIG. 
Due to the large errors, the present measurement cannot rule out
either prediction.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl8.eps}
\end{center}
\caption[]{The measured cross section for $b\bar{b}$ production
with two jets  and an electron in the final state,
in the kinematic range specified in the text, compared to the HERWIG
Monte Carlo with massive matrix elements and different
proton and photon parton densities and to the PYTHIA Monte Carlo
with massless matrix elements.}
\label{pitzl8}
\end{figure}
%
\par
Further insight in the production mechanism for heavy quarks can be
gained by measuring the differential cross section in the variable
$$x_{\gamma}^{obs} =
\frac{ \sum_{2 jets} E_{T}^{jet}\ e^{-\eta^{jet}} }{2yE_e},$$
where $E_e$ is the energy of the incoming positron.
It is the fraction of the photon's momentum
contributing to the production of the two jets.
Direct processes involving pointlike photons are observed at
$x_{\gamma}^{obs} \approx 1$,
whereas resolved processes involving the partonic structure of the
photon lead to smaller values.
\par
Figure~\ref{pitzl6} shows the measured cross section
$d\sigma/dx_{\gamma}^{obs}$ for events with two jets and an electron
in the final state. It has a maximum at high $x_{\gamma}^{obs} $
and a significant tail at low $x_{\gamma}^{obs} $ which cannot
be explained by direct processes and resolution effects alone.
The HERWIG Monte Carlo predicts a 40\% contribution from resolved photon
processes, which, when added to the directed photon processes,
provide a good description of the shape of the measured cross section.
The overall normalization was scaled by a factor 3.7 to match the data.
Fitting the ratio of resolved to direct photon processes yields a
resolved contribution of $(35 \pm 6\ (stat.)\,)$\% which is consistent
with the HERWIG prediction and with the value measured in charm
production~\cite{zeus-dstar}.
A similar result is obtained when the measured cross section is 
compared to the PYTHIA Monte Carlo generator, which predicts
a resolved contribution of 48\%.
%
\begin{figure}[hbt]
\begin{center}
\includegraphics[width=0.6\textwidth]{pitzl6.eps}
\end{center}
\caption[]{The differential cross section in $x_{\gamma}^{obs}$ for dijet 
events with an electron.
The ZEUS data points are shown with statistical errors and with
statistical and systematic errors added in quadrature.
The shaded band shows the uncertainty due the calorimeter energy
scale separately.
The HERWIG prediction for direct and resolved contributions
was scaled by a factor 3.7 to match the data.}
\label{pitzl6}
\end{figure}
%
\section{Conclusions}
%
The first measurements of open $b$ photoproduction have been 
reported by H1 and ZEUS. Both experiments find cross sections that are
about a factor of 4 larger than the expectations from leading order
Monte Carlo Generators with massive matrix elements.
The cross sections are measured in a visible range
that involves the kinematics of the decay lepton.
A comparison to next-to-leading order QCD calculations at the parton
level requires an extrapolation to the full kinematic range or the
addition of fragmentation and weak decays to the calculations.
Both approaches add further systematic errors.
The second approach has been followed by H1, which finds a cross
section significantly larger than the NLO expectation.
%A complete NLO Monte Carlo generator is highly desirable.
\par
The $b$ purity is 50\% for the H1 muon analysis and 20\% for the
ZEUS electron analysis. A higher purity can be obtained by using 
lifetime information from existing or future silicon vertex detectors.
It would allow a study of the resolved photon contribution to $b$
photoproduction.
\par
At higher $Q^2$ the present data samples should allow the first
observation of open $b$ production in deep inelastic scattering.
A significant measurement of the structure function $F_2^b(x,Q^2)$,
however, requires the increased luminosity expected from the HERA
upgrade.
%
\begin{thebibliography}{8.}
\addcontentsline{toc}{section}{References}
%
\bibitem{p-frixione} S.\,Frixione, M.L.Mangano, P.\,Nason, G.\,Ridolfi
in \emph{Heavy Flavours II},
ed. by A.J.\,Buras and M.\,Lindner, World Scientific, 1998
%
\bibitem{p-aroma} G.\,Ingelman, J.\,Rathsman, G.A.\,Schuler,
Comp. Phys. Comm. 101 (1997) 135
%
\bibitem{p-ellis-nason} R.K.\,Ellis, P.\,Nason,
Nucl. Phys. B312 (1989) 551
%
\bibitem{p-smith-neerven} J.\,Smith, W.L.\,van Neerven,
Nucl. Phys. B374 (1992) 36
%
\bibitem{p-frixione94} S.\,Frixione et al., Phys. Lett. B348 (1994) 633
%
\bibitem{b98} U.\,Langenegger, to appear in
\emph{QCD and High Energy Hadronic Interactions.
Proceedings, 33rd Rencontres de Moriond}, Les Arcs, France,
March 1998
%By ÊJean Tran Thanh Van (ed.). Ê Ed. Frontieres, 1997. 673 p. 
\newline
G.\,Tsipolitis,
in \emph{Deep Inelastic Scattering and QCD.
Proceedings, 6th International Workshop, DIS'98}, Brussels, Belgium,
April 4--8, 1998,
ed. by G.\,Coremans and R.\,Roosen, World Scientific, 1998
\newline
H1 Collaboration,
contributed paper no. 575 to the 29th International Conference
on High Energy Physics, ICHEP'98, Vancouver, Canada, July 23-29, 1998
%By ÊAlan Astbury (ed.) , David Axen (ed.) and Jacob Robinson (ed.). 
%World Scientific, 1999, Vol. 2
%
\bibitem{b99} H1 Collaboration, C.\,Adloff et al.,
Phys. Lett. B467 (1999) 156
%DESY 99-126
%
\bibitem{yJB} F.\,Jacquet, A.\,Blondel in
\emph{Proceddings of the Study of an ep facility for Europe},
ed. by U.\,Amaldi, DESY 79-48 (1979) 391
%
\bibitem{p-mrsg} A.D.\,Martin, R.G.\,Roberts, W.J.\,Stirling,
Phys. Lett. B354 (1995) 155
%
\bibitem{p-rapgap} H.\,Jung,
Comp. Phys. Comm. 86 (1995) 147
%
\bibitem{p-fmnr} S.\,Frixione, M.L.Mangano, P.\,Nason, G.\,Ridolfi,
Phys. Lett. B348 (1995) 348
%
\bibitem{p-grv-ho} M.\,Gl\"uck, E.\,Reya, A.\,Vogt,
Phys. Rev. D46 (1992) 1973
%
\bibitem{p-emc} EMC Collaboration, J.-J.\, Aubert et al.,
Phys. Lett. B106 (1981) 419
%
\bibitem{p-mrst} A.D.\,Martin, R.G.\,Roberts, W.J.\,Stirling, 
R.S.\,Thorne,
Eur. Phys. J. C4 (1998) 463
%
\bibitem{zeus-b}M. Hayes,
in \emph{34th Rencontres de Moriond on QCD and Hadronic Interactions},
March 20-27, 1999, Les Arcs, France, and hep-ex/9905033
\newline
M.\,Wing,
in \emph{7th International Workshop on Deep Inelastic Scattering and 
QCD},
April 19-23, 1999 , Zeuthen, Germany, and hep-ex/9905051
\newline
ZEUS Collaboration, contributed paper no. 498 to the International
Europhysics Conference on High Energy Physics, Tampere, Finland,
July 1999
%
\bibitem{p-kt} S.\,Catani, Y.L.\,Dokshitzer, B.R.\,Webber,
Phys. Lett. B285 (1992) 291

\bibitem{p-herwig} G.\,Marchesini et al., 
Comp. Phys. Comm. 67 (1992) 465

\bibitem{p-cteq}  H.L.\,Lai et al., Phys. Rev. D55 (1997) 1280

\bibitem{p-grv} M.\,Gl\"uck, E.\,Reya, A.\,Vogt,
Phys. Rev. D45 (1992) 3986

\bibitem{p-pythia} H.-U.\,Bengtsson, T.\,Sj\"ostrand,
Comp. Phys. Comm. 46 (1987) 43

\bibitem{zeus-dstar} ZEUS Collaboration, J.\,Breitweg et al.,
Eur. Phys. J. C6 (1999) 67

\end{thebibliography}

%\end{document}

