Phase Space Cuts
PhaseSpace
is base class for all hard-process phase-space
generators, either generic 2 -> 1 or 2 -> 2 ones,
or specialized ones like for elastic and diffractive scattering.
In it, it is possible to constrain the kinematics of most processes.
(Exceptions are "soft physics", i.e. minimum bias, elastic and
diffractive processes.) These constraints apply in the rest frame
of the hars subprocess, and topologies normally would be changed
e.g. by subsequent showering activity. The cross section of a process
is adjusted to only correspond to the allowed phase space.
The more particles in the final state, the more cuts can be applied.
Cuts in all processes
parm name="PhaseSpace:mHatMin" default="4." min="0."
The minimum invariant mass.
parm name="PhaseSpace:mHatMax" default="-1."
The maximum invariant mass.
A value below mHatMin
means there is no upper limit.
Cuts in 2 -> 1 processes
When a resonance id
is produced, the
mMin(id)
and mMax(id)
methods restrict the allowed mass range
of this resonance. Therefore the allowed range is chosen to be the
overlap of this range and the mHatMin
to
mHatMax
range above. Most resonances by default have no
upper mass limit, so effects mainly concern the lower limit.
Should there be no overlap between the two ranges then the process
will be switched off.
Cuts in 2 -> 2 processes
parm name="PhaseSpace:pTHatMin" default="0." min="0."
The minimum invariant pT.
parm name="PhaseSpace:pTHatMax" default="-1."
The maximum invariant pT.
A value below pTHatMin
means there is no upper limit.
parm name="PhaseSpace:pTHatMinDiverge" default="1." min="0.5"
Extra pT cut to avoid the divergences of some processes
in the limit pT -> 0. Specifically, if either or both
produced particles have a mass below pTHatMinDiverge
then pT is limited from below by the larger of
pTHatMin
and pTHatMinDiverge
.
flag name="PhaseSpace:useBreitWigners" default="on"
Allows masses to be selected according to Breit-Wigner shapes in
2 -> 2 processes, whenever particles have been declared
with a nonvanishing width above the threshold below. In those cases
also the limits below will be used for the mass selection. For
2 -> 1 processes the Breit-Wigner shape is part of the
cross section itself, and therefore always included.
parm name="PhaseSpace:minWidthBreitWigners" default="0.01" min="1e-6"
The minimum width a resonance must have for the mass to be dynamically
selected according to a Breit-Wigner shape, within the limits set below.
Only applies when useBreitWigners
is on; else the nominal
mass value is always used.
For a particle with a Breit-Wigner shape selected, according to the
rules above and to the rules of the particle species itself, the
mMin(id)
and mMax(id)
methods restrict the allowed mass range
of the particle, just like for the 2 -> 1 processes.
Documentation
flag name="PhaseSpace:showSearch" default="off"
Possibility to print information on the search for phase-space
coefficients that (in a multichannel approach) provides an analytical
upper envelope of the differential cross section, and the
corresponding upper estimate of the cross section. Of interest
for crosschecks by expert users only.
flag name="PhaseSpace:showViolation" default="off"
Possibility to print information whenever the assumed maximum
differential cross section of a process is violated, i.e. when
the initial maximization procedure did not find the true maximum.
Also, should negative cross sections occur, print whenever a more
negative value is encountered.