Single production

`MSEL` = 11, 12, 13, 14, 15, (21)
`ISUB` =

1 | |

2 | |

15 | |

16 | |

19 | |

20 | |

30 | |

31 | |

35 | |

36 | |

(141) | |

(142) |

This group consists of processes, i.e. production of a single resonance, and processes, where the resonance is recoiling against a jet or a photon. The processes 141 and 142, which also are listed here, are described further elsewhere.

With initial-state showers turned on, the processes also generate additional jets; in order to avoid double-counting, the corresponding processes should therefore not be turned on simultaneously. The basic rule is to use the processes for inclusive generation of , i.e. where the bulk of the events studied have . With the introduction of explicit matrix-element-inspired corrections to the parton shower [Miu99], also the high- tail is well described in this approach, thus offering an overall good description of the full spectrum of gauge bosons [Bál01].

If one is interested in the high- tail only, however, the generation efficiency will be low. It is here better to start from the matrix elements and add showers to these. However, the matrix elements are divergent for , and should not be used down to the low- region, or one may get unphysical cross sections. As soon as the generated cross section corresponds to a non-negligible fraction of the total one, say 10%-20%, Sudakov effects are likely to be affecting the shape of the spectrum to a corresponding extent, and results should not be trusted.

The problems of double-counting and Sudakov effects apply not only
to
production in hadron colliders, but also to a process
like
, which clearly is part of the
initial-state radiation corrections to
obtained for
`MSTP(11) = 1`. As is the case for production in association
with jets, the process should therefore only be used for the
high- region.

The of subprocess 1 includes the full interference structure
; via `MSTP(43)` you can select to produce only
, only , or the full
. The same holds true
for the of subprocess 141; via `MSTP(44)` any combination
of , and can be selected. Thus, subprocess
141 with `MSTP(44) = 4` is essentially equivalent to subprocess
1 with `MSTP(43) = 3`; however, process 141 also includes the
possibility of a decay into Higgs bosons. Also processes
15, 19, 30 and 35 contain the full mixture of
, with
`MSTP(43)` available to change this. Note that the
decay products can have an invariant mass as small
as the program cutoff. This can be changed using `CKIN`.

Note that process 1, with only allowed, and studied in the region well below the mass, is what is conventionally called Drell-Yan. This latter process therefore does not appear under a separate heading, but can be obtained by a suitable setting of switches and parameters.

A process like requires some comment. When the boson decays, photons can be radiated off the decay products. The full interference between photon radiation off the incoming fermions, the intermediate boson, and the decay products is not included in the treatment. If such effects are important, a full matrix element calculation is preferred. Some caution must therefore be exercised; see also section for related comments.

For the processes, the Breit-Wigner includes an
-dependent width, which should provide an improved
description of line shapes. In fact, from a line-shape point of view,
process 1 should provide a more accurate simulation of
annihilation events than the dedicated
generation scheme of
`PYEEVT` (see section ). Another difference is
that `PYEEVT` only allows the generation of
,
while process 1 additionally contains
and
. The parton-shower and fragmentation
descriptions are the same, but the process 1 implementation only
contains a partial interface to the first- and second-order
matrix-element options available in `PYEEVT`, see `MSTP(48)`.

All processes in this group have been included with the correct angular distribution in the subsequent decays. In process 1 also fermion mass effects have been included in the angular distributions, while this is not the case for the other ones. Normally mass effects are not large anyway.

As noted earlier, some approximations can be used to simulate higher-order
processes.
The process
can be simulated in two
different ways. One is to make use of the `sea' distribution
inside , i.e. have splittings
.
This can be obtained, together with ordinary production, by
using subprocess 1, with `MSTP(11) = 1` and `MSTP(12) = 1`. Then
the contribution of the type above is 5.0 pb for a 500 GeV
collider, compared with the correct 6.2 pb [Hag91]. Alternatively
one may use process 35, with `MSTP(11) = 1` and `MSTP(12) = 0`,
relying on the splitting
.
This process has a singularity in the forward direction, regularized by
the electron mass and also sensitive to the virtuality of the photon.
It is therefore among the few where the incoming masses have been
included in the matrix element expression. Nevertheless, it may be
advisable to set small lower cut-offs, e.g.
`CKIN(3) = CKIN(5) = 0.01`,
if one should experience problems (e.g. at higher energies).

Process 36,
may have corresponding
problems; except that in
the forward scattering amplitude for
is killed (radiation zero), which means
that the differential cross section is vanishing for
.
It is therefore feasible to use the default `CKIN(3)` and
`CKIN(5)` values in
, and one also comes closer to the
correct cross section.

The process
, formerly available as process
131, has been removed from the current version, since the implementation
turned out to be slow and unstable. However, process 1 with incoming
flavours set to be
(by
`KFIN(1,5) = KFIN(1,-5) = KFIN(2,5) = KFIN(2,-5) = 1` and everything
else `= 0`) provides an alternative description, where the
additional
are generated by
branchings
in the initial-state showers. (Away from the low- region,
process 30 with `KFIN` values as above except that also incoming
gluons are allowed, offers yet another description. Here it is in terms
of
, with only one further
branching
constructed by the shower.) At first glance, the shower approach would
seem less reliable than the full matrix element. The relative
lightness of the quark will generate large logs of the type
, however, that ought to be resummed [Car00].
This is implicit in the parton-density approach of incoming quarks
but absent from the lowest-order
matrix elements.
Therefore actually the shower approach may be the more accurate of the
two in the region of intermediate transverse momenta.