%================================================================
% LaTeX file with preferred layout for the contributed papers to
% the ICHEP Conference 98 in Vancouver
% process with:  latex hep98.tex
%                dvips -D600 hep98
%================================================================
\documentclass[12pt]{article}
\usepackage{epsfig}
\usepackage{amsmath}
\usepackage{hhline}
\usepackage{amssymb}
\usepackage{times}
\usepackage{pstricks}

\renewcommand{\topfraction}{1.0}
\renewcommand{\bottomfraction}{1.0}
\renewcommand{\textfraction}{0.0}
\newlength{\dinwidth}
\newlength{\dinmargin}
\setlength{\dinwidth}{21.0cm}
\textheight23.5cm \textwidth16.0cm
\setlength{\dinmargin}{\dinwidth}
\setlength{\unitlength}{1mm}
\addtolength{\dinmargin}{-\textwidth}
\setlength{\dinmargin}{0.5\dinmargin}
\oddsidemargin -1.0in
\addtolength{\oddsidemargin}{\dinmargin}
\setlength{\evensidemargin}{\oddsidemargin}
\setlength{\marginparwidth}{0.9\dinmargin}
\marginparsep 8pt \marginparpush 5pt
\topmargin -42pt
\headheight 12pt
\headsep 30pt \footskip 24pt
\parskip 3mm plus 2mm minus 2mm

%===============================title page=============================

% Some useful tex commands
%
\newcommand{\GeV}{\rm GeV}
\newcommand{\TeV}{\rm TeV}
\newcommand{\pb}{\rm pb}
\newcommand{\cm}{\rm cm}
\newcommand{\hdick}{\noalign{\hrule height1.4pt}}

\newcommand{\ra}{\rightarrow}
\newcommand{\ccb}{c\bar{c}}
\newcommand{\bbb}{b\bar{b}}
%
\begin{document}

\pagestyle{empty}
\begin{titlepage}

\noindent
%{\bf Version of \today} \\
%Please send comments to O.~Behnke, J.~Kroseberg, F.~Sefkow 
%and the referee, T.~Naumann 
%\\[5mm] 
Submitted to the 30th International Conference on 
High-Energy Physics ICHEP2000, \\ 
Osaka, Japan, July 2000

\vspace*{3cm}

\begin{center}
  \Large
  {\bf 
Measurement of the Beauty Production Cross Section \\ 
at HERA Using Lifetime Information}

  \vspace*{1cm}
    {\Large H1 Collaboration} 
\end{center}

\begin{abstract}

\noindent
Beauty production has been observed in $ep$ collisions at a 
centre-of-mass energy of 300~GeV. 
%For the first time at HERA 
A central silicon vertex detector is used to identify $b$-hadrons 
by their decay length. 
The cross section is extracted from the impact parameter distribution
of muons in dijet events.
The result, based on this observable alone, is 
$\sigma_{vis}(ep\ra\bbb X\ra \mu X) =[159\;\pm\;30\;(stat.)\; \pm 29\;(syst.)\;]$ pb
for the kinematic range
$Q^2<1$ GeV$^2$, 
$0.1<y<0.8\,$, 
$p_T(\mu)>2$ GeV
and $35^\circ<\theta(\mu)<130^\circ$.
%Using an independent method, 
It confirms the earlier H1 measurement 
based on the transverse momentum of muons relative to jets. 
%The sensitivity of the measurement is further 
Using both observables and combining with the earlier result,   
the cross section is determined to be  $\sigma_{vis} = (170 \pm 25)$ pb. 
It again exceeds NLO QCD predictions.   
\end{abstract}


\vfill
\begin{flushleft}
  {\bf Abstract: 979,982 } \\
  {\bf Parallel session: 7} \\
  {\bf Plenary talk: 3} 
\end{flushleft}

\end{titlepage}

\pagestyle{plain}

\section{Introduction}

At HERA, heavy long-lived quarks are studied to
test QCD, to probe proton structure and more general 
the structure of the interaction in 
$ep$ collisions~\cite{h1charm,zeuscharm,h1beauty}.  
The masses of the charm, and even more so of the beauty quark 
introduce a scale that renders QCD perturbation theory 
to be applicable to the calculation of the  
production process~\cite{hqnlo,frixi}.
However, it has been observed in hadron hadron collisions~\cite{hadronb}, 
and recently also in two photon interactions~\cite{ggb},
that measurements of $b$ production cross sections 
tend to overshoot theoretical expectations in next-to-leading order.
The first measurement of open $b$ production at HERA~\cite{h1beauty}
revealed a photoproduction cross section 
almost a factor of two above theoretical prediction, 
albeit with limited experimental accuracy. 

The previous H1 result~\cite{h1beauty} has relied on the
signature of semileptonic decays of $b$~hadrons in jets
and is based on the high transverse momentum $p_T^{rel}$ with respect to the 
jet direction, which the lepton acquires due to the higher $b$ mass. 
The long lifetimes of $b$ hadrons, which can be measured 
with micro-vertex detectors, provide an independent signature.
Beauty production at HERA has a small cross 
section, which is two and three orders of magnitude below the charm 
production and total cross sections respectively, 
and which falls steeply 
with increasing transverse momentum.
Therefore most $b$ hadrons
are produced with small boost in the laboratory frame. 
Here, for he first time, is presented a measurement 
of the $b$ production cross section, 
which uses
the H1 central silicon tracker (CST)~\cite{cst}. 

The analysis is performed with a sample of dijet photoproduction 
events with an identified muon, collected 
with the H1 detector~\cite{h1det} in 1997 when the 
CST was fully commissioned. 
For each muon candidate, the impact parameter $\delta$ is calculated
in the plane transverse to the beam axis.
Its magnitude is given by the distance 
of closest approach of the track
to the primary event vertex.
Its sign is positive if the intercept of the track with the jet axis 
is downstream of the primary vertex, and negative 
otherwise. Decays of long-lived particles are signalled by positive 
impact parameters, whereas the finite track resolution yields 
a symmetric distribution. 
The $b$~cross section is extracted by decomposing the impact 
parameter distribution.

The paper starts with a brief description of the CST and its performance. 
The event selection and the impact parameter analysis are described
subsequently. 
Finally, the cross section is extracted and discussed with respect to
the previous H1 measurement and to theoretical expectations.  


\section{The H1 Central Silicon Tracker}

The H1 detector has been 
described elsewhere~\cite{h1det}. 
The CST consists of two cylindrical
layers of silicon strip detectors, 
surrounding the beam pipe  
at radii of $R=57.5$ mm and $R=97 $ mm,
respectively, from the beam axis. 
With an effective length of 358 mm the CST covers 
a large part
of the $ep$ interaction region
and has a polar angle acceptance of $30^0 <\theta< 150^0$
for the outer layer.
Double sided silicon detectors with readout strip pitches 
of 50 $\mu$m and 88 $\mu$m provide
resolutions of 12 $\mu$m in $r\phi$ and 25 $\mu$m in $z$.
Average hit efficiencies reach values of 97 (92)\% in 
$r\phi$ ($z$).
%CST hits are associated to driftchamber tracks by 
%choosing the hits, which minimize the $\chi^2$ of a
%combined track fit. 
% 

The achieved $dca$\footnote{The $dca$ is the
transverse distance of the track to the
center of the H1 detector.} 
resolution of tracks with
CST hits in both layers can be parameterized as
$\sigma_{dca} \approx 40\;\mu\mbox{m} 
\oplus 100 \;\mu\mbox{m} /p_T [\mbox{GeV}]$.
%
The first term represents the intrinsic
resolution and the second the contribution
from multiple scattering in the beam pipe.
%
The intrinsic resolution is about 20\% worse than
expected from the Monte Carlo simulation, which is 
caused by imperfections in the 
calibrations of the drift chamber and the CST.
%
In the analysis this is taken into account by an
appropriate additional smearing of the Monte Carlo simulation.

The impact parameter method requires the precise knowledge
of the $ep$-interaction point.
%
The transverse profile of the interaction region at HERA
has a Gaussian width of about $150 \; \mu\mbox{m}$  in the horizontal
and of about $30\; \mu\mbox{m}$ in the vertical direction.
%
The average $x$ and $y$ coordinates are determined
for each run  
by collecting information from many events.
%

\section{Event Selection}

The events were triggered by a coincidence of signals 
from the muon system, the central drift chamber and the 
multi-wire proportional chambers. 
The data set corresponds to an integrated luminosity of 14.7 pb$^{-1}$.  

Photoproduction events are selected by requiring that there be no 
electromagnetic cluster with an energy above 8 GeV in the backward calorimeter.
The accepted range of negative four-momentum
transfer squared is thus restricted to $Q^2<1$ GeV$^2$.  
An inelasticity  cut 0.1 $<y<$ 0.8, 
where $y$ is calculated using the Jacquet-Blondel method~\cite{JB}, 
further reduces remaining background from deep inelastic scattering. 
%
Jets are reconstructed using the inclusive $k_t$ algorithm~\cite{kt} 
with parameter $R=1$ in the energy weighted recombination scheme. 
Calorimeter clusters and charged particle tracks are combined,
avoiding double counting. 
The selection requires at least 2 jets with transverse energy $E_T>5$ GeV,
of which at least one contains a muon candidate.
% 
Muons are identified in the barrel part of the instrumented iron,
corresponding to polar angles $35^o < \theta (\mu)<130^o$,
and are required to have transverse momentum $p_T>2$ GeV. 
At least two hits in the CST must be associated with the 
muon candidate track, measured in the central drift chambers.  
The combined track fit probability must exceed 10\%.  
%  
The final sample consists of 1403 events with 1415 muon candidates. 

Monte Carlo event samples for the processes 
$ep\ra\bbb X$ and $ep\ra\ccb X$
%corresponding to integrated luminosities of 
%1.2 fb$^{-1}$ and 0.8 fb$^{-1}$, respectively, 
have been generated using the AROMA program~\cite{aroma}
which is based on leading order QCD and parton showers.  
The selection efficiency (10\%) has been obtained from these Monte Carlo 
simulations. 
The trigger efficiency (79\%) was determined from data, 
using an independent, calorimeter-based trigger. 

% 
The background from fake muons, i.e. hadrons misidentified as muons, 
is determined using tracks from a tagged photoproduction 
event sample, which fulfill the same selection criteria as 
the signal sample, except that no muon identification is required.  


\section{Impact Parameter Analysis}
%
For each event the knowledge of the 
$ep$ collision point is significantly improved 
by applying a primary vertex fit to selected tracks
with CST information.
%
The muon track candidate under consideration is excluded from this fit.
%
An average muon impact pa\-ra\-me\-ter resolution
of 80 $\mu$m has been achieved with comparable contributions
from the muon track reconstruction and the primary vertex determination. 
%

Figure~\ref{fig:delta} shows the observed impact parameter
distribution in the data together with histograms 
indicating the contributions from 
$b$ production and from the backgrounds.
%
%--- delta fit
%
\begin{figure}[t]
\begin{center}
\unitlength5mm
\begin{picture}(24,20)
\put(0., 0.){\epsfig{figure=pap_delta.eps,height=12cm}}
%
\end{picture}
\caption{\label{fig:delta}
Impact parameter distribution and decomposition from the likelihood fit.}
\end{center}
\end{figure}         
%
The decomposition is obtained
from a likelihood fit to the $\delta$ spectrum, following
the method of~\cite{barlow}.
%
The fit uses the shapes of the $\delta$ distributions of 
beauty and charm events from the AROMA Monte Carlo simulation 
and of fake muon events from real data.
%
The relative weights of all three components are adjusted
such that the likelihood 
% to describe the data spectrum by the sum of the components 
is maximized.
%
The fit yields a sample composition of  
$f_b = (26 \pm 5)\%$  (beauty), 
$f_c = (24 \pm 12)\%$ (charm) 
and $f_{fake} = (50 \pm 10)\%$ (fake muons).
%$f_b = 26.4 \pm 5.0\%$  (beauty), 
%$f_c = 23.8 \pm 12.4\%$ (charm) 
%and $f_{fake} = 49.8 \pm 9.8\%$ (misidentified hadrons).
%
The correlation coefficients are $\rho_{b,c} = -0.7$, 
$\rho_{b,fake} = 0.4$ and $\rho_{c,fake} =-0.9$. 
%
The statistical errors of the fit include 
a non negligible contribution
from the limited statistics of the samples representing
the different components. 
%

As a cross-check the fractions 
can be compared with predictions
from the Monte Carlo or measurements in the real data.
%  
The charm result translates into a total $\ccb$ cross section of 
840 nb. Within errors this is in agreement with the H1 measurement of 
$D^*$ photoproduction~\cite{h1charm}.
%However the statistical errors for the charm component  
%are too large to draw firm conclusions.
The fraction of fake muons from the fit is in good
agreement with an estimate
of (56$\pm$6)\%
from momentum and polar angle dependent misidentification
probabilities~\cite{h1beauty}.

The results from the impact parameter analysis can be compared
with an independent measurement using the 
transverse momentum $p_T^{rel}$ of the muon candidate relative 
to the jet axis.
Figure~\ref{fig:ptrel} shows the observed $p_T^{rel}$  
distribution of the data.
%
%--- ptrel fit
%
\begin{figure}[t]
\begin{center}
\unitlength5mm
\begin{picture}(24,20)
\put(0., 0.){\epsfig{figure=pap_ptrel.eps,height=12cm}}
%
\end{picture}
\caption{\label{fig:ptrel}
$p_T^{rel}$ distribution and decomposition from the likelihood fit
with fixed normalization for the fake muon contribution.}
\end{center}
\end{figure}    
%
The estimated signal and backgrounds, also shown in 
Figure~\ref{fig:ptrel}, are taken from a likelihood fit, 
using the same method as for the $\delta$ fit.
As the $p_T^{rel}$ distributions of charm and fake muon events cannot be
well distinguished, the fake muon contribution
is fixed at $f_{fake} = 56\%$.
%
The fit yields a $b$ fraction of $f_b = 27 \pm 3\%$.
% and $f_c = 17 \pm 3\%$.
% and a correlation coefficient of
% $\rho_{b,c} =  -0.6$.
%
%The fit yields a relative sample composition of  $f_b = 26.8 \pm 2.9\%$  
%and $f_c = 17.3 \pm 3.2\%$ and a correlation coefficient of
%$\rho_{b,c} =  -0.60$.
%
The obtained $p_T^{rel}$ fit results for the sample composition
are in good agreement with the values from the $\delta$ fit.
The $p_T^{rel}$ method
% using the fixed fake background normalization, 
exhibits smaller statistical errors,
due to a better shape discrimination of the beauty events in $p_T^{rel}$,
%as compared to the $\delta$ method.
%however the $p_T^{rel}$ method alone suffers from the fact, that
but the fake muon component must here be fixed in order to obtain meaningful  
fit results.

To further elucidate the consistency of the two observables
$\delta$ and $p_T^{rel}$ one can enrich the $b$ component
in the events by restricting the range of one variable and then studying
the distribution of the other quantity.
%
Figure~\ref{fig:enrich} shows the observed $\delta$ spectrum obtained
after a cut $p_T^{rel} >2\; $GeV.
The different contributions, 
shown in Figure~\ref{fig:enrich}a, 
are the absolute predictions from the above $\delta$ fit 
for the limited $p_T^{rel}$ region. 
%
The observed impact parameter spectrum and the fit prediction,
with a dominating beauty component, agree within the errors.
%
Figure~\ref{fig:enrich}b shows the $p_T^{rel}$ spectrum after a
cut on $\delta > 500\;\mu$m.
%
The spectrum 
%is softer than expected for an almost pure $b$-sample, but 
also agrees within the statistical
errors with the fit prediction.

%
%--- delta for ptrel>5, ptrel for delta>0.05 
%
\begin{figure}[t]
\begin{center}
\unitlength5mm
\begin{picture}(24,15)
\put(-4., 0.){\epsfig{figure=pap_delta_highptrel.eps,height=9cm}}
\put(12., 0.){\epsfig{figure=pap_ptrel_highdelta.eps,height=9cm}}
%
%
\end{picture}
\caption{\label{fig:enrich}
Impact parameter distribution for muon candidates with $p_T^{rel}>$ 2 GeV (a)
and 
$p_T^{rel}$ distribution for muon candidates with $\delta > 500\;\mu$m (b),
with estimated contributions.}
%as predicted from the impact parameter fit for 
%the full sample.}
\end{center}
\end{figure}    
%
The observed distributions in the regions of high $b$ purity add 
further confidence that the data are well understood. 
The two observables $\delta$ and $p_T^{rel}$ 
complement each other in the discrimination 
of the beauty component in the data 
against the different background sources. 
The separation power
can be combined in a likelihood fit of the two-dimensional distribution 
of these variables. 
The fit 
yields
$f_b = (27 \pm 3)\,\%$,
$f_c = (27 \pm 7)\,\%$  and 
$f_{fake} = (46 \pm 7)\,\%$
% 
%$f_b = (26.6 \pm 2.7)\,\%$,
%$f_c = (27.3 \pm 6.9)\,\%$, and 
%$f_{fake} = (46.2 \pm 7.3)\,\%$, 
%again consistent with the estimation using misidentification probabilities. 
%
with correlation coefficients of $\rho_{b,c} = -0.04$, 
$\rho_{b,fake} = -0.2$ and $\rho_{c,fake} =-0.9$.
%
The result for the fake muon background, and for the charm fraction,  
are again consistent with the
estimates and measurements quoted above.

 
\section{Beauty Cross Section}

The cross section for the production of central muons from $b$ decays
for the kinematic range
$Q^2<1$ GeV$^2$, 
$0.1<y<0.8\,$, 
$p_T(\mu)>2$ GeV
and $35^\circ<\theta(\mu)<130^\circ$,
as determined by the fit to the impact parameter distribution alone 
yields 
\[ \sigma_{vis}(ep\ra\bbb X\ra\mu X) =
[159\;\pm\;30\;(stat.)\; \pm 29\;(syst.)\;]\;{\rm pb}\ . \]
The cross section was previously measured 
by H1 with the $p_T^{rel}$ method, 
using data taken in 1996 and a different event selection,
to be~\cite{h1beauty} 
\[ \sigma_{vis}(ep\ra\bbb X\ra\mu X) =
[176\;\pm\;16\;(stat.)\;^{+27}_{-17}\; (syst.)\; ] \;{\rm pb} \]
in the same range.  
The published result is thus confirmed,
using an independent signature and data set.

The fit of the two-dimensional ($\delta$, $p_T^{rel}$) distribution 
of the new data yields
\[ \sigma_{vis}(ep\ra\bbb X\ra\mu X) =
[160\;\pm\;16\;(stat.)\; \pm 29\;(syst.)\;]\;{\rm pb}\ . \]

The systematic error from variations of 
the impact parameter analysis and from the modeling of the 
tracking resolution is 
10\%.   
%
Uncertainties due to the Monte Carlo modeling of the final state 
affect mostly the selection efficiency and only weakly the impact parameter
distribution. 
%Using the RAPGAP Monte Carlo generator~\cite{rapgap}, which includes 
%a contribution of 7\% from resolved photoproduction 
%in the visible range, 
%results in a change of the cross section of 5\%,
%which increases to 10\% if 
%the resolved contribution is artificially enhanced to 35\%.
%Performing the jet reconstruction with the CDF cone algorithm~\cite{cdfc}
%yields 30\% more events, but no significant change in the cross section. 
%Following these studies, 
Following studies with the RAPGAP Monte Carlo generator~\cite{rapgap}, 
which includes contributions from resolved photoproduction, 
a systematic error of 
10\% 
is assigned due to uncertainties in the modeling of the hadronic final state. 
Performing the jet reconstruction with the CDF cone algorithm~\cite{cdfc}
yields 30\% more events, but no significant change in the cross section. 
%
Further contributions to the systematic error arise 
from the statistical error of the trigger efficiency,
%(5\%), 
%from the simulation of the muon reconstruction
%(6\%),
%of the jet chamber and the CST efficiency
%(3\% and 2\%, respectively),
%from a 3\% uncertainty in the hadronic energy scale 
%(7\%),
%and from the uncertainty in the luminosity measurement
%(1.5\%).
from the detector simulation, and from the luminosity measurement.
Added in quadrature, the total systematic error amounts to 18\%. 
 
The measurement is consistent with the previous H1 result~\cite{h1beauty}
within the statistical uncertainty. 
Both can be combined, taking correlated systematic uncertainties into account. 
The result is 
\[ \sigma_{vis}(ep\ra\bbb X\ra\mu X) 
= (170 \pm 25) \; {\rm pb}\; . \]

The NLO QCD prediction, using a calculation by Frixione et al.~\cite{frixi}
and a Peterson type fragmentation ansatz~\cite{peterson}
is (104 $\pm$ 17) pb~\cite{h1beauty}
where the error reflects the uncertainties due to variations of the 
renormalization or factorization scale, and to the fragmentation.   
%This expectation undershoots 
%also the new and the combined result. 
That NLO QCD undershoots the measurement is hereby confirmed. 
Such a discrepancy is now established in both $ep$ and 
$\bar{p}p$ interactions~\cite{hadronb}.
 

\section{Conclusion}

Open beauty production at HERA has been measured, using for the first time  
lifetime information.
The cross section has been extracted from 
an analysis of the muon impact parameter spectrum. 
The result confirms the first H1 measurement, based on kinematic observables. 
Both measurements are found to 
be above NLO QCD expectations.


\section*{Acknowledgments}

We are grateful to the HERA machine group whose outstanding
efforts have made and continue to make this experiment possible. 
We thank
the engineers and technicians for their work in constructing and now
maintaining the H1 detector, our funding agencies for 
financial support, the
DESY technical staff for continual assistance, 
and the DESY directorate for the
hospitality which they extend to the non-DESY 
members of the collaboration.


\begin{thebibliography}{99}

\bibitem{h1charm}
% H1 D*gp
H1 Coll., S. Aid {\it et al.},
Nucl.\,Phys.\ B472 (1996) 32, \\
% H1 F2c 
H1 Coll., C. Adloff {\it et al.}, Z. Phys.\ C 72 (1996) 593,
% H1 D*glue
% C. Adloff {\it et al.}, 
Nucl.\ Phys.\ B545 (1999) 21.

\bibitem{zeuscharm} 
% Zeus D* gp
ZEUS Coll., J. Breitweg {\it et al.}, 
Eur.\, Phys.\, J. C6 (1999) 67,
% Zeus F2c
Eur.\, Phys.\, J.  C12 (2000) 1.

\bibitem{h1beauty} 
% H1 openb
H1 Coll., C. Adloff {\it et al.},  Phys.\ Lett.\ B467 (1999) 156.

%\bibitem{zeusbeauty}
%% ZEUS b Tampere
%ZEUS Coll.,  
%{\em Measuremeent of Beauty Photoproduction in Events with Two
%Jets and a Lepton at HERA},
%contributed paper to the 
%International Europhysics Conference High Energy Physics, 
%Tampere, Finland, 1999.

\bibitem{hqnlo} 
% HQ prod in NLO QCD
R.K.\,Ellis and P.\,Nason, Nucl.\,Phys.\ B312 (1989) 551; \\
J.\,Smith and W.L.\,Van Neerven, Nucl.\,Phys.\ B374 (1992) 36; \\
S. Frixione, M.L. Mangano, P. Nason, and G. Ridolfi,
Nucl.\,Phys.\ B412 (1994) 225; \\
%Phys.\ Lett.\ B348 (1995) 633; \\
B.W. Harris and J. Smith, 
Nucl.\,Phys.\ B452 (1995) 109;      
Phys.\,Lett.\ B353 (1995) 535;
Phys.\,Rev.\ D57 (1998) 2806.

\bibitem{frixi} 
% fmnr 
S. Frixione, M.L. Mangano, P. Nason, and G. Ridolfi,
Phys.\ Lett.\ B348 (1995) 633.

\bibitem{hadronb} 
% CDF, D0 open b
CDF Coll., F. Abe {\it et al.}, 
Phys.\ Rev.\ Let. 71 (1993) 2396, Phys.\ Rev.\ D 53 (1996) 1051; \\
D0 Coll., S. Abachi {\it et al.}, 
Phys.\ Rev.\ Let 74 (1995) 3548, Phys.\ Let.\ B 370 (1996) 239.

\bibitem{ggb} 
% L3 gg->bb
L3 Coll, {\em Measuremnent of Charm and Beauty Production in 
$\gamma\gamma$ Collisons at LEP}, contributed paper to the 
International Europhysics Conference High Energy Physics, 
Tampere, Finland, 1999.    

\bibitem{cst} 
% CST paper
D.~Pitzl {\it et al}, 
{\em The H1 Silicon Vertex Detector},
preprint ETHZ-IPP-PR-2000-01, hep-ex/0002044, 2000. 

\bibitem{h1det}
% H1 detector
I. Abt {\it et al.} (H1 Collaboration)
Nucl.\,Instr.\,Meth.\  A386 (1997) 310 and 348.

\bibitem{JB} 
% Jacquet, Blondel
F.~Jacquet, A.~Blondel, 
in: Proc.\ {\em Study of an ep Facility for Europe} (Ed.\ U. Amaldi), 
DESY 79/48, p 391 (1979).

\bibitem{kt} 
% incl kt
S.~Catani, Yu.~Dokshitzer, M.H.Seymour and B.R.~Webber,
Nucl.\ Phys.\  B 406 (1993) 187.

\bibitem{aroma} 
% aroma
G. Ingelman, J. Rathsman and G.A. Schuler,
Comput.\,Phys.\,Commun.\ 101 (1997) 135.

\bibitem{barlow} 
% R.Barlow fit
R.~Barlow and C.~Beeston,
%Fitting using finite Monte Carlo Samples. 
Comp.\ Phys.\ Comm.\ 77 (1993) 219. 

\bibitem{rapgap} 
%rapgap
H.Jung, 
Comput.\ Phys.\ Comm.\ 86 (1995) 147. 

\bibitem{cdfc} 
% cdfcone (review)
%S.D.~Ellis, D.E.~Soper, 
%Phys.\ Rev.\  D48 (1993) 3160.
J.E.~Huth {\it et al.}, 
{\em Towards a standardization of jet definitions}, 
presented at Summer Study on High Energy Physics, Research
Directions for the Decade, Snowmass, CO, June 25 - July 13, 1990.

\bibitem{peterson}
% Peterson
C. Peterson, D. Schlatter, I. Schmitt, and P.M. Zerwas, 
Phys.\,Rev.\ D 27 (1983) 105.

\end{thebibliography}

\end{document}