Timelike Showers
The PYTHIA algorithm for timelike final-state showers is based on
the recent article [1], where a transverse-momentum-ordered
evolution scheme is introduced. This algorithm is influenced by
the previous mass-ordered algorithm in PYTHIA [2] and by the
dipole-emission formulation in Ariadne [3]. From the mass-ordered
algorithm it inherits a merging procedure for first-order
gluon-emission matrix elements in essentially all two-body decays
in the standard model and its minimal supersymmetric extension [4].
The normal user is not expected to call TimeShower
directly,
but only have it called from Pythia
. Some of the parameters
below, in particular TimeShower:alphaSvalue
, would be of
interest for a tuning exercise, however.
Main variables
The amount of QCD radiation in the shower is determined by
parameter name="TimeShower:alphaSvalue" default="0.1265"
min="0.06" max="0.25"
The alpha_strong value at scale M_Z^2. The default
value corresponds to the one tuned to LEP data (using a first-order
running), so should be taken rather seriously [5].
The actual value is then regulated by the running to the scale
pT^2, at which the shower evaluates alpha_strong
mode name="TimeShower:alphaSorder" default="1" min="0" max="2"
Order at which alpha_strong runs,
option value="0": zeroth order, i.e. alpha_strong is kept
fixed.
option value="1": first order, which is the normal value.
option value="2": second order. Since other parts of the code do
not go to second order there is no strong reason to use this option,
but there is also nothing wrong with it.
QED radiation is currently regulated by
StandardModel:alphaEMfix
,
since no QED running is implemented in the shower.
The rate of radiation if divergent in the pT -> 0 limit. Here,
however, perturbation theory is expected to break down. Therefore an
effective pT_min cutoff parameter is introduced, below which
no emissions are allowed. The cutoff may be different for QCD and QED
radiation off quarks, and is mainly a technical parameter for QED
radiation off leptons.
parameter name="TimeShower:pTmin" default="0.5" min="0.1" max="2.0"
Parton shower cut-off pT for QCD emissions.
parameter name="TimeShower:pTminChgQ" default="0.5" min="0.1" max="2.0"
Parton shower cut-off pT for photon coupling to coloured particle.
parameter name="TimeShower:pTminChgL" default="0.0005" min="0.0001"
max="2.0"
Parton shower cut-off pT for pure QED branchings.
Assumed smaller than (or equal to) pTminChgQ
.
Further variables
These should normally not be touched. Their only function is for
cross-checks.
There are three flags you can use to switch on or off selected
branchings in the shower:
flag name="TimeShower:QCDshower" default="on"
Allow a QCD shower, i.e. branchings q -> q g, g -> g g
and g -> q qbar; on/off = true/false.
flag name="TimeShower:QEDshowerByQ" default="on"
Allow quarks to radiate photons, i.e. branchings q -> q gamma;
on/off = true/false.
flag name="TimeShower:QEDshowerByL" default="on"
Allow leptons to radiate photons, i.e. branchings l -> l gamma;
on/off = true/false.
There are three further possibilities to simplify the shower:
flag name="TimeShower:MEcorrections" default="on"
Use of matrix element corrections; on/off = true/false.
flag name="TimeShower:phiPolAsym" default="on"
Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
mode name="TimeShower:nQuark" default="5" min="0" max="5"
Number of allowed quark flavours in g -> q qbar branchings
(phase space permitting). A change to 4 would exclude
g -> b bbar, etc.
References
- T. Sjostrand and P.Z. Skands, Eur. Phys. J. C39 (2005) 129
- M. Bengtsson and T. Sjostrand, Nucl. Phys. B289 (1987) 810
- G. Gustafson, Phys. Lett. B175 (1986) 453;
G. Gustafson and U. Pettersson, Nucl. Phys. B306 (1988) 746;
L. Lonnblad, Computer Physics Commun. 71 (1992) 15
- E. Norrbin and T. Sjostrand, Nucl. Phys. B603 (2001) 297
- G. Rudolph, private communication