Heavy Standard Model Higgs

`ISUB` =

5 | |

8 | |

71 | (longitudinal) |

72 | (longitudinal) |

73 | (longitudinal) |

76 | (longitudinal) |

77 | (longitudinal) |

Processes 5 and 8 are the simple versions of what is now available in 123 and 124 with the full kinematics. For low Higgs masses processes 5 and 8 overestimate the correct cross sections and should not be used, whereas good agreement between the and descriptions is observed when heavy Higgs production is studied.

The subprocesses 5 and 8, , which contribute to the processes , show a bad high-energy behaviour. Here denotes a longitudinal intermediate gauge boson, or . This can be cured only by the inclusion of all graphs, as is done in subprocesses 71, 72, 73, 76 and 77. In particular, subprocesses 5 and 8 give rise to a fictitious high-mass tail of the Higgs. If this tail is thrown away, however, the agreement between the -channel graphs only (subprocesses 5 and 8) and the full set of graphs (subprocesses 71 etc.) is very good: for a Higgs of nominal mass 300 (800) GeV, a cut at 600 (1200) GeV retains 95% (84%) of the total cross section, and differs from the exact calculation, cut at the same values, by only 2% (11%) (numbers for SSC energies). With this prescription there is therefore no need to use subprocesses 71 etc. rather than subprocesses 5 and 8.

For subprocess 77, there is an option, see `MSTP(45)`, to select
the charge combination of the scattering 's: like-sign,
opposite-sign (relevant for Higgs), or both.

Process 77 contains a divergence for due to -exchange contributions. This leads to an infinite total cross section, which is entirely fictitious, since the simple parton-distribution function approach to the longitudinal flux is not appropriate in this limit. For this process, it is therefore necessary to make use of a cut, e.g. .

For subprocesses 71, 72, 76 and 77, an option is included (see
`MSTP(46)`) whereby you can select only the -channel
Higgs graph; this will then be essentially equivalent to running
subprocess 5 or 8 with the proper decay channels (i.e.
or
) set via `MDME`. The difference is that the
Breit-Wigner distributions in subprocesses 5 and 8 contain a mass-dependent
width, whereas the width in subprocesses 71-77 is calculated at
the nominal Higgs mass; also, higher-order corrections to the widths
are treated more accurately in subprocesses 5 and 8. Further,
processes 71-77 assume the incoming
to be on the mass shell,
with associated kinematics factors, while processes 5 and 8 have
correctly space-like. All this leads to differences in the
cross sections by up to a factor of 1.5.

In the absence of a Higgs, the sector of longitudinal and
scattering will become strongly interacting at energies above 1 TeV.
The models proposed by Dobado, Herrero and Terron [Dob91] to
describe this kind of physics have been included as alternative matrix
elements for subprocesses 71, 72, 73, 76 and 77, selectable by
`MSTP(46)`. From the point of view of the general classification
scheme for subprocesses, this model should appropriately be
included as separate subprocesses with numbers above 100, but the
current solution allows a more efficient reuse of existing code.
By a proper choice of parameters, it is also here possible to
simulate the production of a techni- (see section
).

Currently, the scattering of transverse gauge bosons has not been included, neither that of mixed transverse-longitudinal scatterings. These are expected to be less important at high energies, and do not contain an resonance peak, but need not be entirely negligible in magnitude. As a rule of thumb, processes 71-77 should not be used for invariant masses below 500 GeV.

The decay products of the longitudinal gauge bosons are correctly distributed in angle.