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.