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. The Coulomb singularity for elastic scatterings,
if simulated, is handled separately.)
These constraints apply in the rest frame of the hard 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 could be applied.
Here we have tried to remain with the useful minimum, however. More
generic possibilities could be handled by the
user hooks facility.
Cuts in all processes
parm
PhaseSpace:mHatMin
(default = 4.
; minimum = 0.
)
The minimum invariant mass.
parm
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
PhaseSpace:pTHatMin
(default = 0.
; minimum = 0.
)
The minimum invariant pT.
parm
PhaseSpace:pTHatMax
(default = -1.
)
The maximum invariant pT.
A value below pTHatMin
means there is no upper limit.
parm
PhaseSpace:pTHatMinDiverge
(default = 1.
; minimum = 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
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
PhaseSpace:minWidthBreitWigners
(default = 0.01
; minimum = 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.
Cuts in 2 -> 3 processes
Currently no further cuts have been introduced for 2 -> 3
processes than those already available for 2 -> 2 ones.
A 2 -> 3 process in principle would allow to define a
separate pT range for each of the three particles (with some
correlations), but for now all three are restricted exactly the
same way by pTHatMin
and pTHatMax
.
As above, Breit-Wigner mass ranges can be restricted.
Documentation
flag
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
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.