Top Processes

flag  Top:all   (default = off)
Common switch for the group of top production.

flag  Top:gg2ttbar   (default = off)
Scatterings g g → t tbar. Code 601.

flag  Top:qqbar2ttbar   (default = off)
Scatterings q qbar → t tbar by gluon exchange. Code 602.

flag  Top:qq2tq(t:W)   (default = off)
Scatterings q q' → t q'' by t-channel exchange of a W^+- boson. Code 603.

flag  Top:ffbar2ttbar(s:gmZ)   (default = off)
Scatterings f fbar → t tbar by s-channel exchange of a gamma^*/Z^0 boson. Code 604.

flag  Top:ffbar2tqbar(s:W)   (default = off)
Scatterings f fbar' → t q'' by s-channel exchange of a W^+- boson. Code 605.

flag  Top:gmgm2ttbar   (default = off)
Scatterings gamma gamma → t tbar. Code 606.

flag  Top:ggm2ttbar   (default = off)
Scatterings g gamma → t tbar. Code 607 when g gamma → t tbar and 617 when gamma g → t tbar.

By default top always decays to a W and a down-type quark. It is possible to switch on the t → H+ b decay mode. Note that its partial width is calculated using the tan(beta) value stored in HiggsHchg:tanBeta, so that it can be used without having to read in a SUSY parameter file. For the H+ to decay also Higgs:useBSM = on is necessary.

Threshold enhancements

Relevant code is implemented as a new class TopThreshold in SigmaQCD.h/.cc, which is accessed by the internal gg → ttbar and qqbar → ttbar classes in the same files. The implemented scenarios and free parameters within them are as follows.

mode  TopThreshold:model   (default = 0; minimum = 0; maximum = 4)
The choice of threshold behaviour for the g g → t tbar and q qbar → t tbar processes.
option 0 : no modifications to threshold, i.e. pure Born (leading order) matrix elements.
option 1 : the simple Coulomb enhancement, which only works above the threshold, E = mHat(t + tbar) - m(t) - m(tbar) > 0.
option 2 : the Green's function correction in the threshold region, for the positive part only, E > 0. At larger E it transitions to the simple Coulomb enhancement, see width below. The reason is that the Green's function is only valid in the threshold region, and diverges above it.
option 3 : the below-threshold part of the Green's function, E < 0 is mirrored into the positive region, E' = -E > 0. This negative part is damped-out for small E, see width below. Note that subset of events generated with options 2 and 3 can be combined to a complete scenario.
option 4 : the Green's function in the whole threshold region, both E > 0 and E < 0, with transition to Coulomb above and damped-out below (see width below). The procedure is to first pick m(t) and m(tbar) by Breit-Wigners, and then pick an mHat distributed all the way down to m(t) + m(tbar) - 2 * width. If then E = mHat - m(t) - m(tbar) > 0 everything works as in option 2. If not, then a new m'(t) < m(t) and a new m'(tbar) < m(tbar) are picked according to Breit-Wigners under the requirement that E' = mHat - m'(t) - m'(tbar) > 0. Finally the event is accepted or rejected according to the naive cross section reweighted to the Green's function value. This handling of the E < 0 part is more realistic than the previous options, and the best bet, is but not perfect.

parm  TopThreshold:width   (default = 10.; minimum = 5.; maximum = 20.)
the Green's function, when used, is assumed valid in the threshold region [-width, +width]. Above threshold, in the region [width, 2 * width], it linearly transitions to the Coulomb expression. Below threshold, in the region [-2 * width, -width], it is linearly damped to zero.

mode  TopThreshold:alphasOrder   (default = 2; minimum = 0; maximum = 2)
the order of the running of the alpha_strong, used for the top threshold factors (and nowhere else).
option 0 : no running.
option 1 : first-order running.
option 2 : second-order running.

parm  TopThreshold:alphasValue   (default = 0.118; minimum = 0.10; maximum = 0.25)
the alpha_strong value at scale M_Z^2, that then runs according to the order defined above.

parm  TopThreshold:ggSingletFrac   (default = 0.28571; minimum = 0.; maximum = 1.)
in the g g → t tbar process, colour factors gives 2/7 singlet and the rest octet, but dynamics might modify this.

parm  TopThreshold:qqSingletFrac   (default = 0.; minimum = 0.; maximum = 1.)
in the q qbar → t tbar process colour arguments gives all octet and no singlet, but again modifications are possible.

Note 1: Model 4, for E < 0 redefines the t and tbar mass values in order to achieve a new E' > 0. In order still to have access to the original quantities, before the new masses were selected, the following quantities are saved and retrievable:
pythia.info.toponiumE the original threshold energy E, which may have either sign;
pythia.info.toponiumm3, pythia.info.toponiumm4 the two original t and tbar masses, which are larger than the m' ones found in the event record.

Note 2: When generating only the below-threshold region, which is nonzero only in a limited energy/mass range, the initialization may miss to sample this range and conclude that the cross section vanishes everywhere. Therefore we want to set a nonvanishing smallest value during initialization, but not during event generation. To this end a new method pythia.info.getInInit() returns true in pythia.init() and false in pythia.next().

Note 3: Four main program explore the usage of this code:
main368.cc plots the shape of the pure singlet and octet enhancements;
main369.cc and main370.cc histogram a few quantities for many different scenarios, to allow direct comparisons, with code for parallelization using either OpenMP or PythiaParallel, respectively;
main371.cc histograms a wider set of quantities, but only for one model at a time, which can be chosen among a smaller selection.