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%       tables
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%     Use \small capitals for acronyms
%                  Define command for quoting variable names etc
%                    (to be 100% consistent, any variables which include
%                     brackets should also be in maths mode)
\def\v{\begingroup\obeyspaces\u}
% (unusual definition is so that the argument\
%  is not expanded until after the \obeyspaces}
\def\u#1{\verb!#1!\endgroup}
\def\IQ{\v{IQ}}
\def\ISQ{\v{ISQ}}
\def\IL{\v{IL}}
\def\ID{\v{ID}}
\def\IP{\v{IPROC}}
\def\PD{{\small PDG}}
\def\AL{{\small ALPGEN}}
\def\HD{{\small HDECAY}}
\def\HP{{\small HERWIG++}}
\def\HW{{\small HERWIG}}
\def\PY{{\small PYTHIA}}
\def\IS{{\small ISAJET}}
\def\IW{{\small ISAWIG}}
\def\JS{{\small JETSET}}
\def\MS{{\small MSSM}}
\def\MN{{\small MC@NLO}}
\def\SM{{\small SM}}
\def\SY{{\small SUSY}}
\def\ee{$e^+e^-$}
\def\l{\ell}
\def\qbar{\bar q}   
\def\SMH{H^0_{\mbox{\scriptsize SM}}}
\begin{document}
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\noindent
Cavendish-HEP-02/17\hfill
CERN-TH/2002-270\\
DAMTP-2002-124\hfill
%DCPT/02/16\\
IPPP/02/58\\
KEK--TH-850\hfill
MC-TH-2002-7\\
MPI-PhT 2002-55\hfill
October 2002

\begin{center}

{\Large \bf HERWIG 6.5 Release Note}\\[4mm]

{G. Corcella\\[0.4mm]
\it Max-Planck-Institut f\"ur Physik, Werner-Heisenberg-Institut,
Munich\\[0.4mm]
E-mail: \tt{corcella@mppmu.mpg.de}}\\[4mm]

{I.G.\ Knowles\\[0.4mm]
\it Department of Physics and Astronomy, University of Edinburgh\\[0.4mm]
E-mail: \tt{iknowles@supanet.com}}\\[4mm]

{G.\ Marchesini\\[0.4mm]
\it Dipartimento di Fisica, Universit\`a di Milano-Bicocca, and I.N.F.N.,
Sezione di Milano\\[0.4mm]
E-mail: \tt{Giuseppe.Marchesini@mi.infn.it}}\\[4mm]

{S.\ Moretti\\[0.4mm]
\it Theory Division, CERN, and Institute for Particle Physics Phenomenology,
University of Durham\\[0.4mm]
E-mail: \tt{stefano.moretti@cern.ch}}\\[4mm]

{K.\ Odagiri\\[0.4mm]
\it Theory Division, KEK\\[0.4mm]
E-mail: \tt{odagirik@post.kek.jp}}\\[4mm]

{P.\ Richardson\\[0.4mm]
\it Department of Applied Mathematics and Theoretical Physics and\\
Cavendish Laboratory, University of Cambridge\\[0.4mm]
E-mail: \tt{richardn@hep.phy.cam.ac.uk}}\\[4mm]

{M.H.\ Seymour\\[0.4mm]
\it Department of Physics and Astronomy,
University of Manchester\\[0.4mm]
E-mail: \tt{M.Seymour@rl.ac.uk}}\\[4mm]

{B.R.\ Webber\\[0.4mm]
\it Cavendish Laboratory, University of Cambridge\\[0.4mm]
E-mail: \tt{webber@hep.phy.cam.ac.uk}}\\[4mm]

\end{center}

\vspace*{\fill}

\begin{abstract}{\small\noindent
    A new release of the Monte Carlo program \HW\ (version 6.5) is now
    available. The main  new  features  are: support for the Les Houches
    interface to matrix element generators;
    additional \SM\ and \MS\ Higgs processes in lepton collisions;
    additional matrix elements for the spin correlation algorithm;
    a new version of the \IW\ interface; interface to the \MN\ program
    for gauge boson pair production in hadron collisions.
    This is planned to be the last major release of Fortran \HW.
    Future developments will be implemented in a new C++ event generator,
    \HP.}

\end{abstract}

\vspace*{\fill}
\newpage
\tableofcontents
\setcounter{page}{1}

\section{Introduction}

The last major public version (6.2) of \HW\ was reported in detail
in \cite{AllHW}.  The new features of version 6.3 and 6.4 are described
in refs.~\cite{Corcella:2001pi} and \cite{Corcella:2001wc}, respectively.
In this note we describe the main modifications and new features
included in the latest public version, 6.5.

Please refer to \cite{AllHW} and to the present paper if
using version 6.5 of the program.
When running \MS\ processes starting from version 6.1, please add 
reference to \cite{Moretti:2002eu}.

The program, together  with other useful files and information,
can be obtained from the following web site:
\small\begin{quote}\tt
            http://hepwww.rl.ac.uk/theory/seymour/herwig/
\end{quote}\normalsize
    This will be mirrored at CERN:
\small\begin{quote}\tt
            http://home.cern.ch/seymour/herwig/
\end{quote}\normalsize

It is anticipated that version 6.5, with minor modifications in the
series 6.5xx, will be the {\em last Fortran version} of \HW.  Substantial
physics improvements will be included in the new C++ event
generator \HP\ \cite{HW++}, to be released in 2003.

\section{Les Houches interface}\label{sec:lesh}
    We now  include  support  for the  interface  between  parton  level
    generators and \HW\ using the Les Houches  Accord as  described in
    \cite{Boos:2001cv}. In  general we have  tried to code  the interface in
    such a way that from the user point of  view it behaves in  the same
    way as that already included in \PY\ \cite{Sjostrand:2000wi}.


    The interface  operates in the following  way. If the \IP\ code is
    set  negative  then \HW\  assumes  that the user wants an external
    hard process  using the Les Houches  accord.

    If this  option is  used the  initialization will  call the  routine
    \v{UPINIT} to  initialize  the external hard  process; this  name is the
    same  as that used by \PY. This routine  should set the values of
    the run parameters in the Les Houches run common block.

    After  the  initialization  during the  event  generation  phase the 
    routine  \v{UPEVNT} (again this name is the same as that used by \PY)
    is  called.  This  routine  should  fill the  event  common block as
    described in \cite{Boos:2001cv}.

    Dummy  copies of both these  routines are  supplied with  \HW\ and 
    should be deleted and  replaced if you are using this option. Due to
    the internal  structure of  \HW\ two  new parameters are needed to
    control  the  interface, in  addition  to those  in the  Les Houches
    common block. The logical input variable\footnote{Default values for
    input variables are shown in square brackets.} \v{LHSOFT} [\v{.TRUE}]
    controls the  generation of  the soft underlying event; the default
    is to generate a soft underlying event and so \v{LHSOFT} must be set
    \v{.FALSE.} for lepton-lepton  processes.  The second  variable
    \v{LHGLSF} [\v{.FALSE.}] controls the  treatment of colour
    self-connected  gluons, which  may occur  with some methods of colour
    decomposition. The  default is to kill events with self-connected
    gluons  whereas if \v{LHGLSF} = \v{.TRUE.} an  information-only 
    warning is issued instead.

    We have not included full support for the interface. In  particular
\begin{itemize}
\item It is assumed that all events have two particles of status \v{IDUP}=--1;
\item We do not support  the status codes --2 and --9;
\item The treatment of the code \v{IDUP}=3 is the same as \v{IDUP}=2,
      i.e.\ all intermediate resonances have their masses preserved.
\end{itemize}
    These restrictions are the same as those 
    imposed by \PY. If you have any  problems with the  interface or 
    need the options we have not yet supported please let us know.

    It  should  also be noted  that while  the interface has been tested
    with  Standard  Model  processes, for example  all the  processes in
    \AL\ \cite{Mangano:2002ea}, it is  less well tested
    for \SY\ processes.

\section{New SM and MSSM Higgs processes}
\SM\ and \MS\ Higgs production in association with fermion pairs
in lepton-lepton collisions is now available.
These processes were introduced in \cite{Djouadi:1991tk} and their
phenomenological relevance was discussed in \cite{Djouadi:gp}.
The relevant \IP\ codes are given in table~\ref{tab1}. 

Two new subroutines, \v{HWHIGE} and \v{HWH2HE}, had to be included. 
Fermion masses are retained in the final state
according  to the \HW\ defaults. The same values appear in
the Yukawa couplings. Notice that in the case of charged Higgs boson
production the Cabibbo-Kobayashi-Maskawa mixing matrix has been assumed 
to be diagonal. Furthermore, due to the rather different phase space 
distribution of the final state products, all processes can only be produced 
separately, not collectively. Initial- and final-state radiation (both QED 
and QCD) and beamsstrahlung are included via the usual \HW\ algorithms. 
Finally notice that the use of the \IP\ series 1000 and 1100 for $\l^+\l^-$
processes required some internal modification to \HW, which was 
originally designed to generate leptonic processes only for $\IP< 1000$.
Now we assume an $\l^+\l^-$ process whenever $\IP< 1300$.
These modifications have no implications for the traditional user, but
may affect more knowledgeable ones who have edited previous versions
of the main \HW\ code.
\newpage
\begin{table}[t]
\begin{center}
\begin{tabular}{|c|l|}
\hline
\IP & Process\\
\hline
1000+\ID & $\l^+\l^-\to t\,\bar t\,\SMH$ (\ID\ as in \IP=300+\ID)\\
1110+\IQ & $\l^+\l^-\to q\,\qbar\, h^0$  (\IQ\ as in \IP=100+\IQ)\\
1116+\IL & $\l^+\l^-\to \l^+\l^- h^0$ (\IL=1,2,3 for $e,\mu,\tau$)\\
1120+\IQ & $\l^+\l^-\to q\,\qbar\, H^0$ (\IQ\ as in \IP=100+\IQ)\\
1126+\IL & $\l^+\l^-\to \l^+\l^- H^0$ (\IL=1,2,3 for $e,\mu,\tau$)\\
1130+\IQ & $\l^+\l^-\to q\,\qbar\, A^0$ (\IQ\ as in \IP=100+\IQ)\\
1136+\IL & $\l^+\l^-\to \l^+\l^- A^0$ (\IL=1,2,3 for $e,\mu,\tau$)\\
1140    &  $\l^+\l^-\to d\,\bar u\, H^+ +$ c.c.\\
1141    &  $\l^+\l^-\to s\,\bar c\, H^+ +$ c.c.\\
1142    &  $\l^+\l^-\to b\,\bar t\, H^+ +$ c.c.\\
1143    &  $\l^+\l^-\to e\,\bar\nu_e H^+ +$ c.c.\\
1144    &  $\l^+\l^-\to \mu\,\bar\nu_\mu H^+ +$ c.c.\\
1145    &  $\l^+\l^-\to \tau\,\bar\nu_\tau H^+ +$ c.c.\\
\hline
\end{tabular}
\caption{New \SM\ and \MS\ Higgs processes.}\label{tab1}
\end{center}
\end{table}

\section{Spin correlations in  R-parity violating decays}
    When we included spin correlations in \HW6.4 \cite{Richardson:2001df}
    we did not  include
    either R-parity violating  decays or decays producing  gravitinos in
    the  algorithm. This  led to \HW\  stopping when  such decays were 
    included. This of  course could be  stopped by  switching  the  spin
    correlations  off, i.e.\  \v{SYSPIN}=\v{.FALSE.}.
    We have  now  included the 
    relevant  matrix  elements for R-parity  violating  decays and  hard 
    processes and decays producing gravitinos. At the same  time we have
    made changes so that at both the initialization and event generation
    stages many of the terminal warnings  which were caused  by the code
    not having the correct matrix elements are now information-only
    warnings. If you still  get terminal  error messages from any of the
    spin correlation  routines please let us know.


\section{Other interfaces}
\subsection{ISAWIG}
    To  coincide with  the release of  \HW\ 6.5 we have  produced a new
    version  of the \IW\ interface to  \IS\ for the  calculation of
    \MS\ spectra and  decay rates.  Due to stability  problems with the 
    Oxford  web-server  we have moved the web-page to  Cambridge. The new
    address is 
\small\begin{quote}
    \v{http://www.hep.phy.cam.ac.uk/}$\sim$\v{richardn/HERWIG/ISAWIG/}
\end{quote}\normalsize
    At  the moment  the old page will redirect  users here but we cannot
    be sure how long this will continue.

    Since the original version of \IW\ there have been a number of new
    versions of \IS, often with changes in the  sizes of  the common
    blocks from which we extract the information we need. This has
    necessitated periodical updating of the code to run with the most
    recent version  of  \IS, with the result that the code could
    no longer be run with older versions. However, the Snowmass points and
    slopes \cite{Allanach:2002nj} are  defined  with  \IS\ version
    7.58 and so we  can no  longer
    continue  in  this way and  still be able to generate  these  points.
    Therefore, starting  with  the  new  \IW\ version  1.2, we are using C
    preprocessing so that users can define the version of \IS\ they
    are using at  compile time.  When compiling the main \IW\ code and 
    the modified {\small SUGRUN} and {\small SSRUN}
    programs the following compiler options should 
    be specified:
\begin{itemize}
\item \v{-DISAJET758}  to use \IS7.58, 
\item \v{-DISAJET763}  to use \IS7.63.
\end{itemize}
    The default at the moment is to compile code to run with \IS7.64.
    In  the future  this will change  so that the most recent version of 
    \IS\ becomes the default but we will continue to support the older
    versions.

\subsection{HDECAY}
    There has also been a new release of the \HD\ package \cite{Djouadi:1997yw},
    version 3.
    In order to support this  version, and the  previous version 2.0, we 
    are also using C  preprocessing  to control  the version  of  \HD\
    used. In order to achieve  this the \HD\  interface code has  been
    merged with the main \IW\ program. The following options should be
    used when compiling \IW\ if you are using \HD:
\begin{itemize}
\item \v{-DHDECAY2} if you are using version 2 of \HD
\item \v{-DHDECAY3} if you are using version 3 of \HD.
\end{itemize}
    As  before  the default  is to  compile a  dummy routine. If you are
    using \HD\ you must  link with a version of the \HD\  code which 
    has the main \HD\ program removed.

\subsection{MC@NLO}
  The program \MN\ \cite{Frixione:2002ik} generates events with
\begin{itemize}
\item  exclusive rates and distributions accurate to next-to-leading order
    when expanded in $\alpha_s$;
\item multiple  soft/collinear  parton emission generated by \HW\ parton
    showering;
\item hadronization according to the \HW\ cluster model.
\end{itemize}
    At present only the processes of gauge boson pair production in hadron
    ($h=p$ or $\bar p$) collisions are implemented in \MN, making use
    of the \HW\ process codes in table~\ref{tab2}.

\begin{table}[h!]
\begin{center}
\begin{tabular}{|c|l|}
\hline
\v{IPROC} & Process\\
\hline
    2850    &   $h\, h \to W^+ W^- X$\\
    2860    &   $h\, h \to Z^0 Z^0 X$\\
    2870    &   $h\, h \to W^+ Z^0 X$\\
    2880    &   $h\, h \to W^- Z^0 X$\\
\hline
\end{tabular}\caption{\MN\ diboson processes.}\label{tab2}
\end{center}
\end{table}

To use these codes, one must delete the dummy \HW\ subroutine \v{HWHVVJ}
and replace it by the one in the \MN\ package.  This will then read and
process an input file of parton configurations prepared by \MN.
For further details see the \MN\ web page:
\small\begin{quote}\tt    
            http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/
\end{quote}\normalsize

Implementation of other processes, such as heavy quark production, in \MN\ is
in progress \cite{Frixione:inprep}.  These future development will make use
of the Les Houches interface discussed in sect.~\ref{sec:lesh}, rather than
special process codes.

\section{Miscellaneous changes and corrections}
\begin{itemize}
\item
    A new logical input variable, \v{PRESPL} [\v{.TRUE.}],  has  been
    introduced to control whether  the  longitudinal  momentum 
    (\v{PRESPL} = \v{.TRUE.}),  or  rapidity (\v{PRESPL} = \v{.FALSE.}),
    of the hard process centre-of-mass is preserved in
    hadron collisions after initial-state parton showering.   At present
    the only function of this variable is to allow users to study the
    effects of momentum reshuffling,  which is necessary after showering
    to compensate for jet masses. In future, it is anticipated that setting
    \v{PRESPL}=\v{.FALSE.} will simplify the treatment of other processes
    in \MN.

\item A bug has been  fixed  in the  backward  evolution in the  logical
      structure structure of an \v{IF} statement controlling the branching.
      This was  preventing the forced  branching  to valence partons and
      caused many  of the \v{HWSBRN}=104  warnings.  Users  should  find the
      number of such warnings is significantly reduced.

\item When polarization  effects in lepton  collisions  were added a bug 
      was introduced in squark pair production which has now been fixed.

\item A bug in the matrix element for $q\,\qbar\to g\,\SMH$ (\IP=2300
      etc.) has been corrected.

\item The effects of off-shell $WW$ and $ZZ$ decays of the SM Higgs boson
      are now included in the generated weights of $2\to 3$ processes.
\end{itemize}

\section*{Acknowledgements}
Since this is expected to be the last (Fortran) \HW\ release note, we would
like to take this opportunity to thank again all those colleagues and users
who have contributed to the development of the program, whether by providing
code, suggesting improvements or reporting problems.  Thanks also to those
who worked so hard to establish the Les Houches accord, which should make
it possible to expand the application of Fortran \HW\ to new processes
without (much) further intervention by the authors.  Meanwhile, many of us
will be transferring our main efforts to \HP, in preparation for the new
era of object-oriented event generation.
 
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%\bibitem{Frixione:2002bd}
%S.~Frixione and B.~R.~Webber,
``The \MN\ event generator,''
;
%%CITATION = ;%%

\bibitem{Frixione:inprep}
S.~Frixione, P.~Nason and B.~R.~Webber,
``Matching NLO QCD computations and parton showers in
heavy quark production'', in preparation.

\end{thebibliography}
\end{document}

