Higgs Processes

This page contains Higgs production in the Standard Model. It should eventually be expanded at least also to cover the MSSM.

Standard-Model Higgs, basic processes

This section provides the standard set of processes that can be run together to provide a reasonably complete overview of possible production channels for a single Standard-Model Higgs.

flag  SMHiggs:all   (default = off)
Common switch for the group of Higgs production within the Standard Model.

flag  SMHiggs:ffbar2H   (default = off)
Scattering f fbar -> H^0, where f sums over available flavours except top. Related to the mass-dependent Higgs point coupling to fermions, so at hadron colliders the bottom contribution will dominate. Code 901.

flag  SMHiggs:gg2H   (default = off)
Scattering g g -> H^0 via loop contributions primarily from top. Code 902.

flag  SMHiggs:gmgm2H   (default = off)
Scattering gamma gamma -> H^0 via loop contributions primarily from top and W. Code 903.

flag  SMHiggs:ffbar2HZ   (default = off)
Scattering f fbar -> H^0 Z^0 via s-channel Z^0 exchange. Code 904.

flag  SMHiggs:ffbar2HW   (default = off)
Scattering f fbar -> H^0 W^+- via s-channel W^+- exchange. Code 905.

flag  SMHiggs:ff2Hff(t:ZZ)   (default = off)
Scattering f f' -> H^0 f f' via Z^0 Z^0 fusion. Code 906.

flag  SMHiggs:ff2Hff(t:WW)   (default = off)
Scattering f_1 f_2 -> H^0 f_3 f_4 via W^+ W^- fusion. Code 907.

flag  SMHiggs:gg2Httbar   (default = off)
Scattering g g -> H^0 t tbar via t tbar fusion (or, alternatively put, Higgs radiation off a top line). Warning: unfortunately this process is rather slow, owing to a lengthy cross-section expression and inefficient phase-space selection. Code 908.

flag  SMHiggs:qqbar2Httbar   (default = off)
Scattering q qbar -> H^0 t tbar via t tbar fusion (or, alternatively put, Higgs radiation off a top line). Warning: unfortunately this process is rather slow, owing to a lengthy cross-section expression and inefficient phase-space selection. Code 909.

Standard-Model Higgs, further processes

A number of further production processes has been implemented, that are specializations of some of the above ones to the high-pT region. The sets therefore could not be used simultaneously without unphysical doublecounting, as further explained below. They are not switched on by the SMHiggs:all flag, but have to be switched on for each separate process after due consideration.

The first three processes in this section are related to the Higgs point coupling to fermions, and so primarily are of interest for b quarks. It is here useful to begin by reminding that a process like b bbar -> H^0 implies that a b/bbar is taken from each incoming hadron, leaving behind its respective antiparticle. The initial-state showers will then add one g -> b bbar branching on either side, so that effectively the process becomes g g -> H0 b bbar. This would be the same basic process as the g g -> H^0 t tbar one used for top. The difference is that (a) no PDF's are defined for top and (b) the shower approach would not be good enough to provide sensible kinematics for the H^0 t tbar subsystem. By contrast, owing to the b being much lighter than the Higgs, multiple gluon emissions must be resummed for b, as is done by PDF's and showers, in order to obtain a sensible description of the total production rate, when the b quarks predominantly are produced at small pT values.

flag  SMHiggs:qg2Hq   (default = off)
Scattering b g -> H^0 b. This process gives first-order corrections to the f fbar -> H^0 one above, and should only be used to study the high-pT tail, while f fbar -> H^0 should be used for inclusive production. Only the dominant cc and b contributions are included, and generated separately for technical reasons. Code 911.

flag  SMHiggs:gg2Hbbbar   (default = off)
Scattering g g -> H^0 b bbar. This process is yet one order higher of the b bbar -> H^0 and b g -> H^0 b chain, where now two quarks should be required above some large pT threshold. Warning: unfortunately this process is rather slow, owing to a lengthy cross-section expression and inefficient phase-space selection. Code 912.

flag  SMHiggs:qqbar2Hbbbar   (default = off)
Scattering q qbar -> H^0 b bbar via an s-channel gluon, so closely related to the previous one, but typically less important owing to the smaller rate of (anti)quarks relative to gluons. Warning: unfortunately this process is rather slow, owing to a lengthy cross-section expression and inefficient phase-space selection. Code 913.

The second set of processes are predominantly first-order corrections to the g g -> H^0 process, again dominated by the top loop. We here only provide the kinematical expressions obtained in the limit that the top quark goes to infinity, but scaled to the finite-top-mass coupling in g g -> H^0. (Complete loop expressions are available e.g. in PYTHIA 6.4 but are very lengthy.) This provides a reasonably accurate description for "intermediate" pT values, but fails when the pT scale approaches the top mass.

flag  SMHiggs:gg2Hg(l:t)   (default = off)
Scattering g g -> H^0 g via loop contributions primarily from top. Code 914.

flag  SMHiggs:qg2Hq(l:t)   (default = off)
Scattering q g -> H^0 q via loop contributions primarily from top. Not to be confused with the SMHiggs:bg2Hb process above, with its direct fermion-to-Higgs coupling. Code 915.

flag  SMHiggs:qqbar2Hg(l:t)   (default = off)
Scattering q qbar -> H^0 g via an s-channel gluon and loop contributions primarily from top. Is strictly speaking a "new" process, not directly derived from g g -> H^0, and could therefore be included in the standard mix without doublecounting, but is numerically negligible. Code 916.

Parameters for Higgs production and decay

flag  ResonanceSMH:linearWidthWWZZ   (default = on)
The partial width of a Higgs particle to a pair of gauge bosons, W^+ W^- or Z^0 Z^0, depends cubically on the Higgs mass. When selecting the Higgs according to a Breit-Wigner, so that the actual mass mHat does not agree with the nominal m_Higgs one, an ambiguity arises which of the two to use [Sey95]. The default is to use a linear dependence on mHat, i.e. a width proportional to m_Higgs^2 * mHat, while off gives a mHat^3 dependence. This does not affect the width to fermions, which only depends on mHat.

mode  ResonanceSMH:parity   (default = 1; minimum = 0; maximum = 3)
possibility to modify angular decay correlations in Higgs decay to Z^0 Z^0 or W^+ W^- to four fermions. Currently it does not affect the partial width of the channels.
option 0 : isotropic decays.
option 1 : assuming the Higgs is a pure scalar (CP-even), as in the Standard Model.
option 2 : assuming the Higgs is a pure pseudoscalar (CP-odd).
option 3 : assuming the Higgs is a mixture of the two, including the CP-violating interference term. The parameter eta, see below, sets the strength of the CP-odd admixture, with the interference term being proportional to eta and the CP-odd one to eta^2.

parm  ResonanceSMH:etaParity   (default = 0.)
The eta value of CP-violation in the ResonanceSMH:parity = 2 option.