A new version of the Monte Carlo program HERWIG (version 5.9) is
     now available, and can be obtained from the following web site:

             http://hepwww.rl.ac.uk/theory/seymour/herwig/

                  *** NEW FEATURES OF THIS VERSION ***

  * The common block file HERWIG59.INC has been significantly rearranged
    and tidied up.

  * Many new hadrons have been added.  All S & P wave mesons are present
    including the 1^P_0 & 3^P_1 states and many new, excited B^**, B_c &
    quarkonium states. Also all D wave kaons and some `light' I=3 states
    [pi_2, rho(1700) & rho_3].  All the baryons (singlet/octet/decuplet)
    containing up to one heavy (c,b) quark are included.

       --- Consequently the default parameters require retuning ---

  * New 8-character particle names have been introduced and  the revised
    7 digit PDG numbering scheme,  as advocated in the LEP2 report,  has
    been adopted.

  * The layout of HWUDAT  has been altered to make it easier to identify
    and modify particle propeties. Three new arrays have been introduced
    RLTIM, RSPIN & IFLAV. These are: the particle's lifetime (ps), spin,
    and a code which specifies the flavour content of each hadron - used
    (in HWURES) to create sets of iso-flavour hadrons for cluster decay.
    Using the standard numbering of quark flavours the convention is:

       mesons:          n_q n_qbar    Eg. pi^+:  21, pi^-:      12
       baryons: +/-n_q1 n_q2 n_q3     Eg. Xi^0: 332, Xi^0bar: -332  etc.
               (-ve for antibaryons; digits in decreasing order)

    Light, neutral mesons are identified as: 11 if I=1: pi^0,rho^0,...
                                             33 if I=0: eta, eta'.. etc.

    Some parts of the program have been automated so that it is possible
    for the user to add new particles by specifying their properties via
    the arrays in /HWPROP/ & /HWUNAM/ and increasing NRES appropriately:
    this should be done before a call to HWUINC.

    As an example following lines add an isoscalar, spin pi state 'STAN'
    and a (very light) stable toponium state 'BEER' with the decay mode:
    STAN ---> BEER+BEER+BEER.

      NRES=NRES+1
      RNAME(NRES)='STAN    '
      IDPDG(NRES)=666
      IFLAV(NRES)=11
      ICHRG(NRES)=0.
      RMASS(NRES)=0.5
      RLTIM(NRES)=1.000D-10
      RSPIN(NRES)=3.142
      NRES=NRES+1
      RNAME(NRES)='BEER    '
      IDPDG(NRES)=66
      IFLAV(NRES)=66
      ICHRG(NRES)=0.
      RMASS(NRES)=0.1
      RLTIM(NRES)=1.000D+30
      RSPIN(NRES)=0.0
      CALL HWMODK(666,1.D0,0,66,66,66,0,0)

  * The mixing angles of all the light, I=0 mesons can now be set using:

     ETAMIX:  eta       <-> eta'        F0MIX:  f_0(1300) <-> f_0(980)
     PHIMIX:  omega     <-> phi,        F1MIX:  f_1(1285) <-> f_1(1510)
     H1MIX:   h_1(1170) <-> h_1(1380)   F2MIX:  f_2       <-> f_2'

  * Using the logical arrays VTOCDK & VTORDK the production of specified
    particles can be stopped in both cluster decays and via the decay of
    other unstable resonances.

  * A priori weights for the relative production rates in cluster decays
    of mesons and baryons differing only via their S & L quantum numbers
    can be supplied using SNGWT & DECWT for singlet (i.e. Lambda-like) &
    decuplet baryons and REPWT for mesons. The old VECWT now corresponds
    to REPWT(0,1,0) and TENWT to REPWT(0,2,0).

  * The default masses of the c and b quarks have been lowered to 1.55 &
    4.95 repectively: this corresponds to the mass of the lightest meson
    minus the u/d quark mass. This increases the number of heavy mesons,
    and hence total multiplicities,  and slightly softens their momentum
    spectrum.  The rate of photoproduced charm states increases and B-pi
    momentum correlations become smoother.

  * The resonance decay tables supplied in the program have been largely
    revised. Measured/expected modes with branching fraction at or above
    1 per mille are given, including 4 & 5 body decays. To print the new
    tables call HWUDPR.

  * The arrays FBTM, FTOP & FHVY which stored the branching fractions of
    the bottom, top & heavier quarks' `partonic' decays are now nolonger
    used.  Such decays are  specified in the same way as all other decay
    modes: this permits different decays to be given to individual heavy
    hadrons.  Partonic decays of charm hadrons and quarkonium states are
    also now supported.  The products' order in a partonic decay mode is
    significant. For example if the decay is: Q --> W+q --> (f+fbar')+q,
    occuring inside a Q-sbar hadron the required ordering is:

             Q+sbar --->(f+fbar')+(q+sbar)
                     or (q+fbar')+(f+sbar)  `colour rearranged'

    In both cases the (V-A)^2 ME^2 is proportional to: p_0.p_2 * p_1*p_3

  * The structure of the program has been altered so that secondary hard
    subrocess and subsequent fragmentation associated with each partonic
    heavy hadron decay appear separately. Thus pre-hadronization t quark
    decays are treated individually  as are any subsequent bottom hadron
    partonic decays.

  * Additionally decays of heavy hadrons to exclusive non-partonic final
    states are supported. No check against double counting from partonic
    modes is included. However this isn't expected to be a major problem
    for the semi-leptonic and 2-body hadronic modes supplied.

  * An array NME has been introduced to enable a possible matrix element
    to be specified for each decay mode.

    NME = 0   Isotropic decay
          100 Free particle (V-A)*(V-A): p_0.p_2 * p_1.p_3
          101 Bound quark (V-A)*(V-A):   p_0.p_2 * p_1*[p_3 - xs*p_0]
                        xs = m_Q/M_0 - spectator quark momentum fraction
          130 Ore & Powell ortho-positronium ME^2: onium --> gg+g/gamma.

    The list of matrix elements presently supported is modest, users are
    urged to contact an author to have other MEs implimentated.

  * The decay tables can be written to/read from a file by using HWIODK,
    adopting the format advocated in the LEP2 report. In addition to the
    PDG numbering of particles the HERWIG numbers or character names can
    be used. This permits easy alteration of the decay tables. In HWUINC
    a call is made to HWUDKS which sets up HERWIGs internal pointers and
    performs some basic checks of the decay tables. Each decay mode must
    conserve charge and be kinematically allowed  and not contain vetoed
    decay products. The sum of a particles branching ratios is set to 1.
    Also  a warning is printed  if an antiparticle does not have all the
    charge conjugate decays modes of the particle.

  * HWMODK enables changes to the decay tables to be made by alterating/
    adding single decay modes including on an event by event basis. This
    can be done before HWUINC, in which case when altering the BR and/or
    ME code of an existing mode a warning is given of a duplicate second
    mode which supercedes the first. BRs set below 10^-6 are eliminated,
    whilst if one mode is within 10^-6 of 1 all other modes are removed.
    Note  that some forethought is required if the BRs of 2 modes of the
    same particle are changed since  the operation of rescaling to 1 the
    BR sum causes a non-commutativity in the order of the calls.

  * Production vertex information is now made available, using VHEP, for
    all partons, clusters and final state particles: set PRVTX=.TRUE. to
    print them. The vertices of partons and clusters are given wrt local
    coordinates associated with their individual hard sub-process.

  * All partonic and resonace rest frame lifetimes are generated with an
    exponential distribution: exp(-t/<tau>)/<tau>. The average lifetime,
    <tau>, is given in terms of the particles mass, width and virtuality
    by:
                              hbar.sqrt(q^2)
        <tau>(q^2) =   -----------------------------
                     \/(q^2-M^2)^2 + (Gamma.q^2/M)^2

                   = hbar/Gamma        for an on-shell       particle
                   ~ hbar.q/(q^2-M^2)      a  highly virtual particle

    For partons an effective width = sqrt(VMIN2), to act as a cut-off on
    lifetimes, is introduced.

  * The space-time picture for cluster formation and splitting is partly
    ad hoc and partly string inspired - no physics depends upon it.

  * All particles with lifetimes greater than PLTCUT are set stable.

  * If PIPSMR=.TRUE. the primary interaction point's spatial position is
    is smeared according to the triple Gaussian in HWRPIP: this position
    is assigned to the CMF track.

  * If MAXDKL=.TRUE. then each putative decay is tested in HWDXLM to see
    that it occurs within a specified volume (cylinder/sphere for IOPDKL
    =1/2): if not it is set stable.

  * If MIXING=.TRUE. then  B^0_d,s mesons are allowed to oscillate: XMIX
    and YMIX contain Delta-M/Gamma and Delta-gamma/2*Gamma respectively.
    A new particle, ISTHEP=200,  is introduced giving the flavour of the
    neutral B meson at production in addition to the `decaying' track.

  * A multiple intra & inter-jet colour rearrangement model is available
    for  CLRECO=.TRUE.  The q-qbar pairings in two non-adjacent clusters
    are interchanged with probability PRECO if the distances between the
    production vertices of both q-qbar pairs when added in quadrature is
    reduced. EXAG can be used to artificially scale the lifetimes of any
    weak bosons.

  * A number of bugs have been corrected: in HWEPRO for weighted events;
    in  HWSBRN  affecting the reconstruction of the photon beam remnant;
    and in HWHEPG stopping event generation. Plus minor modifications to
    HWBGEN;  in the use of HWHIGM by HWHIGJ; and small changes in HWHDIS
    & HWHEGG.

  * A significant bug in HWDHQK, affecting top quark decays, was present
    in version 5.8 ONLY.  The scale of the top decay had been set to the
    b-quark mass,  stopping gluon radiation  from the b  and restricting
    that from the W decay products to have transverse momentum less than
    the b mass. The scales are now correctly set for top decays.

  * Improved efficiency of photon generation in HWEGAM.

  * New hard sub-process have been added:

    - Compton scattering, gamma + q --> gamma + q, IPROC=5300.

    - Two-to-two  parton  scattering  via  exchange  of a colour singlet
      IPROC=2400 Mueller-Tang pomeron: the fixed alpha_s and omega_0 are
      given by ASFIXD and OMEGA0 respectively.
      IPROC=2450 photon exchange, for like flavour qqbar pairs including
      the t-channel component of the interference with q-qbar -> q-qbar.

    - Drell-Yan has been extended to the production of all fermion pairs
      IPROC=1399; 1300 gives all quark flavours 1300+IQ a specific quark
      flavour, 1350 all leptons (including neutrinos) 1350+IL a specific
      lepton flavour.   The s-channel component of the interference with
      like flavour q-qbar scattering is included here.

    - Z+jet production is included as IPROC=2150 (HWHW1J becomes HWHV1J)

  * Running coupling now used for prompt J/PSI production in DIS.

  * The phase-space limits for the momentum fraction of incoming photons
    in the Weizsacker-Williams approximation is now set by the variables
    YWWMIN  &  YWWMAX,  allowing  different  ranges  for  the tagged and
    untagged photons in two-photon DIS.

  * Interfaced to the  Schuler-Sjostrand  parton distribution functions,
    version 2.  These appear as  PDFLIB  sets with author group 'SaSph',
    but are actually implemented via a call to their  SASGAM  code.  The
    value in MODPDF specifices the set (1-4 for 1D [recommended set],1M,
    2D,2M), whether the Bethe-Heitler process is used for heavy flavours
    (add 10), whether the P^2-dependence is included (add 20), and which
    of their P^2 models is used (add 100 times their IP2 parameter).

  * New variables ANOMSC(1 or 2,IBEAM) record the evolution scale and Pt
    at which an anomalous (gamma* --> q+qbar) splitting was generated in
    the backward evolution of beam IBEAM. set 0 if no such splitting was
    generated. This is implemented in HWBGEN and HWSBRN.

  * In preparation for multiple interactions, several routines have been
    added or modified.  New are:  HWHREM for identifying and cleaning up
    the beam remnants;  HWHSCT to administer the extra scatters.   Minor
    modifications to:  HWBGEN & HWSBRN, don't report energy conservation
    errors when ISLENT = -1;  HWSSPC, improved approximation for remnant
    mass at high energies;  and HWUPCM, improved safety against negative
    square roots.

  * Photon Initial State Radiation in  e+e- annihilation events allowed.
    TMNISR sets the minimum s-hat/s value,   ZMXISR sets the (arbitrary)
    separation between unresolved and resolved emission;  using ZMXISR=0
    switches off photon ISR.

  * Numerical integral in HWBDED now done analytically removing the need
    to reintegrate for each new energy; in principle allowing use in  5-
    jet WW events, but this is not yet implemented.

  * New phase-space variable WHMIN added.  This sets the minimum allowed
    hadronic mass and affects photoproduction reactions  (gamma-hadron &
    gamma-gamma) and DIS.  In lepton-hadron DIS it is largely irrelevant
    since there is already a cut on Bjorken y which at fixed s is almost
    the same but for lepton-gamma DIS it makes a big difference.

  * A new treatment of running Higgs width and non-resonant diagrams, as
    suggested in M.H. Seymour, Phys. Lett. B354 (1995) 409.  Selected by
    setting IOPHIG=2 or 3 (default);  previous options 2 and 3 have been
    withdrawn. Note that including the non-resonant diagrams changes the
    meaning of what is generated: IOPHIG = 0  or  1, gives the s-channel
    diagram,  an unphysical choice of part of the amplitude;  IOPHIG = 2
    or 3,  gives the I=0 & J=0 part of the excess over the cross section
    expected for a zero mass Higgs boson,  a physical choice  of part of
    the cross section. The inclusion of non-resonant diagrams causes the
    cross section to increase below and decrease above resonance.

  * New treatment of the splitting in two of clusters  containing hadron
    (or photon) remnants.  Previous versions gave the 2 fragments a mass
    spectrum typical of soft processes: dn/dm**2 = Gaussian.  In the new
    version  the child  containing the remnant  is treated as before but
    the other cluster, containing a perturbative parton, is treated as a
    normal clusters: dn/dm = m**psplt. IOPREM controls this behaviour: 0
    = old version, 1 = new (default).

  * Direct gamma+gamma* -> q+qbar is included in the hard correction for
    lepton-gamma DIS; plus minor bug fixed in HWBDIS.

  * The dummy routine  IUCOMP  has been removed, this avoids errors when
    the program is linked to CERNLIB.

  * It  has  been  noticed  that differences in the way quark masses are
    treated  in  different  processes  can cause inconsistencies between
    different  ways of generating the same process.  The most noticeable
    example is in direct photoproduction, where one can use process 9130
    or  5000.  See the note at the end of Section 5 of the documentation
    for more information on the strategies used in different processes.

  Version  5.1  of HERWIG was described in detail in the main reference,
  Comput. Phys. Commun. 67 (1992) 465.  The new features of each version
  released since may be found in the documentation file HERWIG59.DOC.

               Ian Knowles     Mike Seymour     Bryan Webber

                                   CERN

                              July 22nd 1996