DCA has sign - correct cut to use absolute value

[u/mrichter/AliRoot.git] / HERWIG / herwig61.txt

    A new version of the Monte Carlo program HERWIG (version 6.1) is now
    available, and can be obtained from the following web site:

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

    This will temporarily be mirrored at CERN for the next few weeks:

            http://home.cern.ch/~seymour/herwig/

    More complete information on HERWIG  can be found in the publication
    G. Marchesini, B.R. Webber,  G. Abbiendi, I.G. Knowles, M.H. Seymour
    and L. Stanco, Computer Phys. Commun. 67 (1992) 465  and also in the
    documentation for the previous version (5.9), which are available at
    the  same site, together  with other  useful files  and information.
    Here we merely give the new features relative to 5.9.

    If you use HERWIG, please refer to it something along the lines of:

    HERWIG 6.1, hep-ph/9912396; G. Marchesini, B.R. Webber, G. Abbiendi,
    I.G. Knowles, M.H. Seymour and L. Stanco,
    Computer Phys. Commun. 67 (1992) 465.


                  *** NEW FEATURES OF THIS VERSION ***

     *---------------------------------------------------------------*
     | The main  new  features  are:  supersymmetric processes (both |
     | R-parity conserving & violating) in hadron-hadron collisions; |
     | new e+e- to four jets process;  matrix element corrections to |
     | top decay and Drell-Yan processes;  new soft underlying event |
     | options; updates to default particle data tables; new LaTeX & |
     | html printout options.                                        |
     *---------------------------------------------------------------*

  * [N.B. Default values for input variables shown in square brackets.]

  * All R-parity  conserving SUSY two-to-two  processes in hadron-hadron
    collisions have been added. Their process numbers are:

    +-------+----------------------------------------------------------+
    | IPROC | Process                                                  |
    +-------+----------------------------------------------------------+
    | 3000  | 2 parton to 2 sparticles: the sum of 3010,3020 and 3030  |
    | 3010  | 2 parton to 2 spartons                                   |
    | 3020  | 2 parton to 2 gauginos                                   |
    | 3030  | 2 parton to 2 sleptons                                   |
    +-------+----------------------------------------------------------+

    Further details  of the inclusion of superpartners  and their decays
    are given below.

    Additional  processes  for the  SUSY  two  Higgs  doublet model  are
    currently under test and will be released shortly.

  * All  R-parity violating SUSY  two-to-two processes  via resonant
    sleptons and  squarks in hadron  collisions have been  added.  Their
    process numbers are:

    +-------+----------------------------------------------------------+
    | IPROC | Processes derived from the LQD term in the superpotential|
    +-------+----------------------------------------------------------+
    | 4000  | The sum of 4010,4020,4040 and 4050                       |
    | 4010  | Neutralino lepton production (all neutralinos)           |
    | 401i  | As 4010 but only the ith neutralino                      |
    | 4020  | Chargino lepton production (all charginos)               |
    | 402i  | As 4020 but only the ith chargino                        |
    | 4040  | Slepton W/Z   production                                 |
    | 4050  | Slepton Higgs production                                 |
    +-------+----------------------------------------------------------+
    | 4060  | Sum of 4070 and 4080                                     |
    | 4070  | quark-antiquark production      via LQD                  |
    | 4080  | lepton production               via LLE and LQD          |
    +=======+==========================================================+
    | IPROC | Processes derived from the UDD term in the superpotential|
    +-------+----------------------------------------------------------+
    | 4100  | The sum of 4110, 4120, 4130, 4140 and 4150               |
    | 4110  | Neutralino quark production (all neutralinos)            |
    | 411i  | As 4110 but only the ith neutralino                      |
    | 4120  | Chargino quark production (all charginos)                |
    | 412i  | As 4120 but only the ith chargino                        |
    | 4130  | Gluino quark production                                  |
    | 4140  | Squark W/Z   production                                  |
    | 4150  | Squark Higgs production                                  |
    +-------+----------------------------------------------------------+
    | 4160  | quark-quark production                                   |
    +-------+----------------------------------------------------------+

    In addition  the R-parity violating  decays of all  superpartners is
    included.

  * A new process describing electron-positron annihilation to four jets
    has been added. This has IPROC=600+IQ, where a non-zero value for IQ
    guarantees production of quark flavour IQ whilst IQ=0 corresponds to
    the natural flavour mix.  IPROC=650+IQ is as above but without those
    terms in the matrix element which orient the event w.r.t. the lepton
    beam direction. The matrix elements are based on those of Ellis Ross
    & Terrano with orientation terms from Catani & Seymour. The soft and
    collinear divergences are avoided  by imposing a minimum y-cut, Y4JT
    [.01], on the initial 4 partons. The interjet distance is calculated
    using either the Durham or JADE metrics.  This choice is governed by
    the logical variable DURHAM [.TRUE.]. Note that parameterizations of
    the volume of four-body phase  space are used: these are accurate up
    to a  few percent for y-cut  values less than 0.14.  Note, also that
    the phase space is for massless partons, as are the matrix elements,
    though a mass threshold cut is applied. Finally, the matrix elements
    for the q-qbar-g-g & q-qbar-q-qbar (same flavour quark) final states
    receive contributions from 2 colour flows each, the treatment of the
    interference terms being controlled by the array IOP4JT:

        q-qbar-g-g case:
        IOP4JT(1)=0 neglect, =1 extreme 2341; =2 extreme 3421 [0]

        q-qbar-q-qbar (identical quark flavour) case:
        IOP4JT(2)=0 neglect, =1 extreme 4123; =2 extreme 2143 [0]

    The scale EMSCA for the  parton showers is set equal to SQRT(s*ymin)
    where ymin is the least  distance, according to the selected metric,
    between any two partons.

  * Matrix element corrections to the simulation of top quark decays and
    Drell-Yan processes are now  available using the same general method
    as  already implemented  for e+e-  annihilation and  DIS.  If HARDME
    [.TRUE.] then  fill the missing phase-space (`dead  zone') using the
    exact  1st-order  M.E.   result  (`hard  corrections').   If  SOFTME
    [.TRUE.] then  correct emissions in the  already-populated region of
    phase space  using the  exact amplitude for  every emission  that is
    capable of being the hardest so far (`soft corrections').

    - For t -> bW decays the routine HWBTOP implements hard corrections.
      HWBRAN has been modified to  implement the soft corrections to top
      decays. Since the dead zone  includes part of the soft singularity
      a cutoff is required: only gluons with energy above GCUTME [2 GeV]
      (in the top rest frame) are corrected. Physical quantities are not
      strongly dependent on GCUTME in the range 1 to 5 GeV.  For details
      see:

      G. Corcella and M.H. Seymour, Phys. Lett. B442 (1998) 417.

    - For the  Drell-Yan process the routine  HWBDYP implements the hard
      corrections whilst  HWSBRN has been modified to implement the soft
      corrections to the initial state radiation. For details see:

      G. Corcella and M.H. Seymour, hep-ph/9908338.

  * The  parameters of  the model  used  for soft  interactions are  now
    available to  the user for modification.  The model is  based on the
    minimum-bias event generator of  the UA5 Collaboration, which starts
    from a parametrization of  the pbar p inelastic charged multiplicity
    distribution as  a negative binomial. The parameters  are as follows
    (default  parameter  values  are  the  UA5  ones  used  in  previous
    versions):

              +-------+---------------------------+---------+
              | Name  |      Description          | Default |
              +-------+---------------------------+---------+
              | PMBN1 | a in <n> = a*S^b+c        |  9.11   |
              | PMBN2 | b in <n> = a*S^b+c        |  0.115  |
              | PMBN3 | c in <n> = a*S^b+c        | -9.50   |
              |       |                           |         |
              | PMBK1 | a in 1/k = a*log_e(S)+b   |  0.029  |
              | PMBK2 | b in 1/k = a*log_e(S)+b   | -0.104  |
              |       |                           |         |
              | PMBM1 | a in (M-m_1-m_2-a)e^{-bM} |  0.4    |
              | PMBM2 | b in (M-m_1-m_2-a)e^{-bM} |  2.0    |
              |       |                           |         |
              | PMBP1 | p_t slope for d,u         |  5.2    |
              | PMBP2 | p_t slope for s,c         |  3.0    |
              | PMBP3 | p_t slope for qq          |  5.2    |
              +-------+---------------------------+---------+

    The  first  three  parametrize  the  mean  charged  multiplicity  at
    c.m.  energy  \sqrt{s}  as  indicated.  The  next  two  specify  the
    parameter   k  in   the  negative   binomial   charged  multiplicity
    distribution.   The  parameters   PMBM1  and   PMBM2   describe  the
    distribution of cluster  masses M in the soft  collision. These soft
    clusters  are  generated  with  a flat  rapidity  distribution  with
    gaussian  shoulders. The  transverse momentum  distribution  of soft
    clusters has the form

    P(p_t)\propto p_t\exp(-b\sqrt{p_t^2+M^2})

    where the slope  parameter b depends as indicated  on the flavour of
    the quark or diquark pair created when the cluster was produced.

    As an  option, for underlying events  the value of  \sqrt{s} used to
    choose the  multiplicity n may be  enhanced by a  parameter ENSOF to
    allow for an enhanced underlying activity in hard events. The actual
    charged  multiplicity is  then taken  to be  n plus  the sum  of the
    moduli of the charges of the colliding hadrons or clusters.

  * There have been a number of additions/changes to the default hadrons
    included via HWUDAT.  Here the identification of hadrons follows the
    PDG ('98 edition) table 13.2 with numbering according to section 31.

    New isoscalars states have been added to try to complete the 1^3D_3,
    1^1D_2 and 1^3D_1 multiplets:

           IDHW  RNAME        IDPDG      IDHW  RNAME        IDPDG
           ----  -----        -----      ----  -----        -----
            395  OMEGA_3        227       396  PHI_3          337
            397  ETA_2(L)     10225       398  ETA_2(H)     10335
            399  OMEGA(H)     30223

    Also the following states have been re-identified/replaced:

           IDHW  RNAME        IDPDG      IDHW  RNAME        IDPDG
           ----  -----        -----      ----  -----        -----
             57  FH_1         20333
            293  F0P0       9010221       294  FH_00        10221
             62  A_0(H)0      10111       290  A_00       9000111
             63  A_0(H)+      10211       291  A_0+       9000211
             64  A_0(H)-     -10211       292  A_0-      -9000211

    The f_1(1420) state completely  replaces the f_1(1520) in the 1^3P_0
    multiplet, taking over 57. The f_0(1370) (294) replaces the f_0(980)
    (293) in the 1^3P_0 multiplet; the  latter is retained as it appears
    in the decays of several other states.  The new a_0(1450) states (62
    -64) replace the three old a_0(980) states (290 - 292) in the 1^3P_0
    multiplet; the latter are kept, as they appear in f_1(1285) decays.

    By default production of the f_0(980) and a_0(980) states in cluster
    decays is vetoed.

    Also, the PDG numbers for the remnant particles have been changed to
    98 for REMG and 99 for REMN.

  * Since version 6.1 contains a large number of supersymmetry processes
    several new particles have been added.

    Extra scalar bosons for the two Higgs Doublet (SUSY) scenario:

           IDHW  RNAME        IDPDG      IDHW  RNAME        IDPDG
           ----  -----        -----      ----  -----        -----
            203  HIGGSL0         26       206  HIGGS+          37
            204  HIGGSH0         35       207  HIGGS-         -37
            205  HIGGSA0         36

    Note that the lighter neutral scalar (203) is given the non-standard
    PDG number 26, in order to distinguish it from the minimal SM Higgs,
    PDG number 25.

    Extra sfermions and gauginos for SUSY scenarios:

           IDHW  RNAME        IDPDG      IDHW  RNAME        IDPDG
           ----  -----        -----      ----  -----        -----
            401  SSDL       1000001       413  SSDR       2000001
              |  |          |               |  |          |
            406  SST1       1000006       418  SST2       2000006
            407  SSDLBR    -1000001       419  SSDRBR    -2000001
              |  |          |               |  |          |
            412  SST1BR    -1000006       424  SST2BR    -2000006

            425  SSEL-      1000011       437  SSER-      2000011
              |  |          |               |  |          |
            430  SSNUTL     1000016       442  SSNUTR     2000016
            431  SSEL+     -1000011       443  SSER+     -2000011
              |  |          |               |  |          |
            436  SSNUTLBR  -1000016       448  SSNUTRBR  -2000016

            449  GLUINO     1000021       454  CHGINO1+   1000024
            450  NTLINO1    1000022       455  CHGINO2+   1000037
            451  NTLINO2    1000023       456  CHGINO1   -1000024
            452  NTLINO3    1000025       457  CHGINO2   -1000037
            453  NTLINO4    1000035       458  GRAVTINO   1000039

    The implementation of SUSY is discussed more fully below.  Note that
    the default masses of the SUSY particles are zero and that they have
    no decay modes. Before a SUSY process can be simulated you must load
    the appropriate  masses and decay modes generated  using ISAWIG (see
    below) or an equivalent program.

    These new states don't interfere  with the user's ability to add new
    particles as previously described.

  * It is now possible to create particle property and event listings in
    any combination of 3 formats - standard ASCII, LaTeX or html.  These
    options are controlled by the new, logical variables PRNDEF [.TRUE.]
    PRNTEX [.FALSE.] and PRNWEB [.FALSE.].  The ASCII output is directed
    to stout (screen / log file) as in previous versions. When a listing
    of particle properties is requested (IPRINT.GE.2 or HWUDPR is called
    explicitly) then the following files are produced:

      If (PRNTEX):   HW_decays.tex
      If (PRNWEB):   HW_decays/index.html
                              /PART0000001.html etc.

    The HW_decays.tex  file is written  to the working  directory whilst
    the many **.html files  appear in the sub-directory HW_decays/ which
    must have been created previously.   Paper sizes and offsets for the
    LaTeX output  are stored at the  top of the block  data file HWUDAT:
    they may  need modifying to  suit a particular printer.   When event
    listings  are requested  (NEVHEP.LE.MAXPR.NE.0 or  HWUEPR  is called
    explicitly) the  following files are created in  the current working
    directory:

      If (PRNTEX):   HWEV_*******.tex       where *******=0000001 etc.
      If (PRNWEB):   HWEV_*******.html      is the event number

    Note the .html file automatically makes links to the index.html file
    of particle properties assumed to be in the HW_decays sub-directory.

    A new integer variable NPRFMT [1] has been introduced to control how
    many significant figures  are shown in each of  the 3 event outputs.
    Basically NPRFMT=1 gives short compact outputs whilst NPRFMT=2 gives
    long formats.

    Note  that all  the LaTeX  files  use the  package longtable.sty  to
    format the tables.  Also if  NPRFMT=2 or PRVTX=.TRUE. then the LaTeX
    files are designed to be printed in landscape mode.

  * There were  previously some  inconsistencies and ambiguities  in our
    conventions for the  mixing of flavour `octet' and `singlet' mesons.
    They are now:

           Multiplet   Octet         Singlet          Mixing Angle
           ---------   -----         -------          ------------
            1^1S_0     eta           eta'             ETAMIX=-23.
            1^3S_1     phi           omega            PHIMIX=+36.
            1^1P_1     h_1(1380)     h_1(1170)        H1MIX =ANGLE
            1^3P_0     MISSING       f_0(1370)        F0MIX =ANGLE
            1^3P_1     f_1(1420)     f_1(1285)        F1MIX =ANGLE
            1^3P_2     f'_2          f_2              F2MIX =+26.
            1^1D_2     eta_2(1645)   eta_2(1870)      ET2MIX=ANGLE
            1^3D_1     MISSING       omega(1600)      OMHMIX=ANGLE
            1^3D_3     phi_3         omega_3          PH3MIX=+28.

    After mixing the  quark content of the physical  states is given, in
    terms of the mixing angle, theta, by:

                  (ddbar+uubar)/sqrt(2)           ssbar
                  ---------------------           -----
         Octet:    cos(theta+theta_0)      -sin(theta+theta_0)
       Singlet:    sin(theta+theta_0)       cos(theta+theta_0)

    where  theta_0=ATAN(SQRT(2)).   Hence, using  the  default value  of
    ANGLE=ATAN(1/SQRT(2))*180/ACOS(-ONE)  for theta gives  ideal mixing,
    that  is,   the  `octet'   state  =  ssbar   and  the   `singlet'  =
    (ddbar+uubar)/sqrt(2).   This  choice is  important  to avoid  large
    isospin violations in the 1^3P_0  and 1^3D_1 multiplets in which the
    octet member is unknown.

  * A new treatment of the  colour interference terms in matrix elements
    has been introduced in this version. A non-planar, interference term
    is now shared between the planar terms corresponding to well defined
    colour flows in proportion to the size of the planar terms. Existing
    two-to-two QCD processes which have been affected are:

              Light Quarks                          Heavy Quarks
              ============                          ============
           Process            IHPRO              Process           IHPRO
           -------            -----              -------           -----
    q   +q    --> q   +q       1,2       Q   +g    --> Q   +g      10,11
    q   +qbar --> q   +qbar    5,6       Qbar+g    --> Qbar+g      21,22
    qbar+q    --> qbar+q      13,14      g   +Q    --> g   +Q      23,24
    qbar+qbar --> qbar+qbar   18,19      g   +Qbar --> g   +Qbar   25,26
                                         g   +g    --> Q   +Qbar   27,28

    The present and previous treatments of the interference term are the
    same for the other two-to-two QCD processes which remain unaffected.

    This new procedure has been adopted for all the SUSY QCD processes.

    For details see: K. Odagiri, JHEP 10 (1998) 006

  * A new process, direct gamma-gamma to charged particle pairs has been
    added. This has IPROC=16000+IQ: if IQ=1-6 then only quark flavour IQ
    is produced, if IQ=7,8 or 9 then only lepton flavour e, mu or tau is
    produced and if IQ=10 then only W pairs are produced: in these cases
    particle masses effects are included. Whilst if IQ=0 the natural mix
    of quark pairs are produced  using massless MEs but including a mass
    threshold cut. The range of allowed transverse momenta is controlled
    by PTMIN & PTMAX as usual.

  * A new package ISAWIG has been created to work with ISAJET to produce
    a file of the SUSY  particle masses, lifetimes and decay modes which
    can be read in by HERWIG.

    This package takes  the outputs of the ISAJET  SUGRA or general MSSM
    programs and produces a data file in a format that can be read in by
    the HWISSP subroutine described below.

    In addition to the decay modes included in the ISAJET package ISAWIG
    allows for  the possibility of  violating R-parity and  includes the
    calculation of all 2-body squark and slepton, and 3-body gaugino and
    gluino R-parity violating decay modes.

  * A new subroutine HWISSP has been  added to read the file of particle
    properties produced by the ISAWIG program. In principle the user can
    produce a similar file provided that the correct format is used. The
    format should be as follows.

    First the SUSY particle and top quark masses and lifetimes are given
    as, for example:

         65
         401 927.3980    0.74510E-13
         402 925.3307    0.74009E-13
         ....etc.

    That is,

         NSUSY=Number of SUSY+top particles
         IDHW, RMASS(IDHW) & RLTIM(IDHW)
         repeated NSUSY times.

    Next  each  particle's decay  modes  together  with their  branching
    ratios and matrix element codes are given as, for example:

         6
         401  0.18842796E-01     0   450     1     0     0     0
           |               |     |     |     |     |     |     |
         401  0.32755006E-02     0   457     2     0     0     0
         6
         402  0.94147678E-02     0   450     2     0     0     0
         ....etc.

    That is,

         Number of decay modes for a given particle (IDK)
         IDK(*), BRFRAC(*), NME(*) & IDKPRD(1-5,*)
         repeated for each mode.

         Repeated for each particle (NSUSY times).

    The order in which the decay products appear is significant: this is
    important inorder to obtain appropriate showering and hadronization.
    The correct ordering for each decay mode is indicated below.

    +----------+------------------------+------------------------------+
    | Decaying | Type of Mode           |  Order of Decay Products:    |
    | Particle |                        |   1st   |   2nd   |   3rd    |
    +----------+------------------------+---------+---------+----------+
    | Top      | 2 body to Higgs        | Higgs   | Bottom  |          |
    |          +------------------------+---------+---------+----------+
    |          | 3 body via Higgs/W     | quarks or leptons | Bottom   |
    |          |                        |    from W/Higgs   |          |
    +----------+------------------------+---------+---------+----------+
    | Gluino   | 2 body modes:          |         |         |          |
    |          | without gluon          |     any order     |          |
    |          | with    gluon          | gluon   | colour  |          |
    |          |                        |         | neutral |          |
    |          +------------------------+---------+---------+----------+
    |          | 3 body modes:          | colour  |      q or qbar     |
    |          | R-parity conserved     | neutral |                    |
    +----------+------------------------+---------+---------+----------+
    | Squark/  | 2 body modes:          |         |         |          |
    | Slepton  | Gaugino/Gluino         | Gaugino | quark   |          |
    |          | Quark/Lepton           | Gluino  | lepton  |          |
    |          +------------------------+---------+---------+----------+
    |          | 3 body modes:          |sparticle| particles from     |
    |          | Weak                   |         | W decay            |
    +----------+------------------------+---------+---------+----------+
    | Squark   | 2 body modes:          |         |         |          |
    |          | Lepton Number Violated | quark   | lepton  |          |
    |          | Baryon Number Violated | quark   | quark   |          |
    +----------+------------------------+---------+---------+----------+
    | Slepton  | 2 body modes:          |      q or qbar    |          |
    |          | Lepton Number Violated |         |         |          |
    +----------+------------------------+---------+---------+----------+
    | Higgs    | 2 body modes:          |         |         |          |
    |          | (s)quark-(s)qbar       |   (s)q or (s)qbar |          |
    |          | (s)lepton-(s)lepton    |   (s)l or (s)lbar |          |
    |          +------------------------+---------+---------+----------+
    |          | 3 body modes:          | colour  |      q or qbar     |
    |          |                        | neutral |      l or lbar     |
    +----------+------------------------+---------+---------+----------+
    | Gaugino  | 2 body modes:          |         |         |          |
    |          | squark-quark           |      q or sq      |          |
    |          | slepton-lepton         |      l or sl      |          |
    |          +------------------------+---------+---------+----------+
    |          | 3 body modes:          | colour  |      q or qbar     |
    |          | R-parity conserved     | neutral |      l or lbar     |
    +----------+------------------------+---------+---------+----------+
    | Gaugino/ | 3 body modes:          | particles in the order i,j,k |
    | Gluino   | R-parity violating     |                              |
    +----------+------------------------+---------+---------+----------+

    A new matrix element code has been added for these decays:

       NME = 300     3 body R-parity violating gaugino and gluino decays

    in addition,  an extra matrix element code has been reserved for use
    in a forthcoming version:

       NME = 200     3 body top quark via charged Higgs

    The indices i,j,k in  R-parity violating gaugino/gluino decays refer
    to the ordering  of the indices in the  R-parity violating couplings
    in the superpotential.  The convention is as in:

    H.Dreiner, P.Richardson and M.H.Seymour, hep-ph/9912407.

    Next a number of parameters derived from the SUSY Lagrangian must be
    given. These are: the ratio of Higgs VEVs, tan(beta), and the scalar
    Higgs mixing  angle, alpha; the  mixing parameters for  the Higgses,
    gauginos and the sleptons; the trilinear couplings; and the Higgsino
    mass parameter mu.

    Finally the  logical variable  RPARTY should be  set: if  FALSE then
    R-parity is violated, and the R-parity violating couplings must also
    be supplied, otherwise not.

    Details of the FORMAT statements  employed can be found by examining
    the subroutine HWISSP.

    The integer argument in the  call to HWISSP(N) gives the unit number
    to be read from. If the data is stored in a `fort.N' file no further
    action is required but  if the data is to be read  from a file named
    `fname.dat' then appropriate OPEN and CLOSE statements must be added
    by hand:

      OPEN(UNIT=N,FORM='FORMATTED',STATUS='UNKNOWN',FILE='fname.dat')
      CALL HWISSP(N)
      CLOSE(UNIT=N)

    A number of sets of SUSY parameter files, produced using ISAWIG, for
    the standard LHC SUGRA and GMSB points are available from the HERWIG
    home page: http://hepwww.rl.ac.uk/theory/seymour/herwig/

  * A large number of changes have been made to enable SUSY processes to
    be included in hadron-hadron collisions. The main changes are:

    - The subroutine HWDHQK has been  replaced by HWDHOB which does both
      heavy quark and SUSY particle decays.

    - The subroutines HWBCON HWCGSP & HWCFOR have been adapted to handle
      the colour connections found in normal SUSY decays.

    - The subroutine HWBRCN has been included to deal with the inter-jet
      colour connections arising in R-parity violating SUSY. Also HWCBVI
      HWCBVT and HWCBCT  have been added to handle  the hadronization of
      baryon number violating SUSY decays and processes. If the variable
      RPARTY=.TRUE. [default] then the old HWBCON colour connection code
      is used else the new HWBRCN

  * The option of separate treatments for `light' and b-quark containing
    clusters are now available.   The 3 variables, PSPLT (which controls
    the mass spectrum of the fragments in heavy cluster splitting) CLDIR
    (which controls whether perturbatively produced (anti-)quarks retain
    some knowledge of their direction  in cluster decays to hadrons) and
    CLSMR (which defines to what extent the hadron and constituent quark
    directions are aligned), have been made two dimensional.

          ARRAY(1) controls clusters that do NOT contain a b quark
          ARRAY(2) controls clusters that do     contain a b quark

    [ Default ARRAY(1)=ARRAY(2) equivalent to earlier versions. ]

  * A new variable EFFMIN [1E-3] has been introduced, it allows the user
    to set the minimum acceptable efficiency for event generation.

  * All hadron & lepton masses are now given to five significant figures
    whenever possible.

  * The treatment of the perturbative g --> qqbar vertex in the partonic
    showers  has been  improved. The  total rate  is unchanged,  but the
    angular distribution  now covers the  full range, rather  than being
    confined to the angular-ordered region as before.

  * The treatment of the intrinsic transverse momentum  of partons in an
    incoming  hadron has  been improved.   It is  now chosen  before the
    initial state  cascade is performed, and  is held fixed  even if the
    generated cascade  is rejected.  This removes  a correlation between
    the amount of  perturbative and non-perturbative transverse momentum
    generated that existed before.

  * Space-time  positioning of clusters  is now  smeared according  to a
    Gaussian distribution of width 1/(cluster mass).

  * For e+e- processes with ISR a check was added requiring TMNISR to be
    greater than the light quark threshold.

  * The treatment of the W resonance in top decays has been improved.

  * The common block file HERWIG61.INC has had many new variables added,
    these are listed at the top of the file.

  * Corrections for bugs have been made affecting the following:

    - eta-eta' mixing: the parameterization was nonstandard (see above).

    - 4/5 body phase space generation: was not flat - affected resonance
      decays only.

    - Drell-Yan: the overall normalization was too small by a factor 2/3
      also the t-channel contribution to q-qbar-> q-qbar was incorrectly
      normalized.

    - HWHV1J: the normalization of Z+jet  production rate was a factor 4
      too small; there was an incorrect correlation between the (signed)
      W and jet rapidities; the treatment of the W/Z Breit-Wigner lead a
      normalization error by a factor 3/pi.

    - HWHWPR: there was an overall normalization error of (M_ff'/M_w)^2,
      this only affected the line shape and normalization for the t-bbar
      final state for which M_ff' is large.

    - B_d/_s mixing: an incorrect formula was used.

    - VMIN2: the effective cut-off on the space-time distances travelled
      by light partons in a shower was incorrectly implemented. Also its
      default  value has  been  increased to  [0.1],  which affects  the
      colour reconnection probability.

    - A number of fixes to improve safety against overflowing the HEPEVT
      common block.

    - Fix to the underlying event to prevent errors with heavy quarks.

    - HWMODK/HWIODK: a number of corrections were made and the code made
      more robust.

    - HWURES: the minimum threshold for the decay of diquark-antidiquark
      clusters was incorrectly set.

    - The calculation of the top lifetime has been corrected and the QCD
      corrections  included - this only affects the  treatment of colour
      reconnection.

    - The space-time positioning of clusters sometimes led to them being
      produced outside the forward lightcone.  This has been rectified.

    As usual, if you wish to be removed from the HERWIG mailing list, or
    if you  know someone  who wants to  be added,  please let one  of us
    know.

    Mike Seymour,  Bryan Webber,  Ian Knowles,  Peter Richardson, Kosuke
    Odagiri, Stefano Moretti, Gennaro Corcella, Pino Marchesini

    CERN, Edinburgh, Oxford, RAL, Rochester, Milano, etc,
    16th December 1999.