flag
NewGaugeBoson:ffbar2gmZZprime
(default = off
)
Scattering f fbar →Z'^0.
Code 3001.
mode
Zprime:gmZmode
(default = 0
; minimum = 0
; maximum = 6
)
Choice of full gamma^*/Z^0/Z'^0 structure or not in
the above process. Note that, with the Z'^0 part switched
off, this process is reduced to what already exists among
electroweak processes,
so those options are here only for crosschecks.
option
0 : full gamma^*/Z^0/Z'^0 structure,
with interference included.
option
1 : only pure gamma^* contribution.
option
2 : only pure Z^0 contribution.
option
3 : only pure Z'^0 contribution.
option
4 : only the gamma^*/Z^0 contribution,
including interference.
option
5 : only the gamma^*/Z'^0 contribution,
including interference.
option
6 : only the Z^0/Z'^0 contribution,
including interference.
Note: irrespective of the option used, the particle produced
will always be assigned code 32 for Z'^0, and open decay channels
is purely dictated by what is set for the Z'^0.
The couplings of the Z'^0 to quarks and leptons can either be assumed universal, i.e. generation-independent, or not. In the former case eight numbers parametrize the vector and axial couplings of down-type quarks, up-type quarks, leptons and neutrinos, respectively. Depending on your assumed neutrino nature you may want to restrict your freedom in that sector, but no limitations are enforced by the program. The default corresponds to the same couplings as that of the Standard Model Z^0, with axial couplings a_f = +-1 and vector couplings v_f = a_f - 4 e_f sin^2(theta_W), with sin^2(theta_W) = 0.23. Without universality the same eight numbers have to be set separately also for the second and the third generation. The choice of fixed axial and vector couplings implies a resonance width that increases linearly with the Z'^0 mass.
By a suitable choice of the parameters, it is possible to simulate just about any imaginable Z'^0 scenario, with full interference effects in cross sections and decay angular distributions and generation-dependent couplings; the default values should mainly be viewed as placeholders. The conversion from the coupling conventions in a set of different Z'^0 models in the literature to those used in PYTHIA is described in [Cio08].
flag
Zprime:universality
(default = on
)
If on then you need only set the first-generation couplings
below, and these are automatically also used for the second and
third generation. If off, then couplings can be chosen separately
for each generation.
Here are the couplings always valid for the first generation, and normally also for the second and third by trivial analogy:
parm
Zprime:vd
(default = -0.693
)
vector coupling of d quarks.
parm
Zprime:ad
(default = -1.
)
axial coupling of d quarks.
parm
Zprime:vu
(default = 0.387
)
vector coupling of u quarks.
parm
Zprime:au
(default = 1.
)
axial coupling of u quarks.
parm
Zprime:ve
(default = -0.08
)
vector coupling of e leptons.
parm
Zprime:ae
(default = -1.
)
axial coupling of e leptons.
parm
Zprime:vnue
(default = 1.
)
vector coupling of nu_e neutrinos.
parm
Zprime:anue
(default = 1.
)
axial coupling of nu_e neutrinos.
Here are the further couplings that are specific for
a scenario with Zprime:universality
switched off:
parm
Zprime:vs
(default = -0.693
)
vector coupling of s quarks.
parm
Zprime:as
(default = -1.
)
axial coupling of s quarks.
parm
Zprime:vc
(default = 0.387
)
vector coupling of c quarks.
parm
Zprime:ac
(default = 1.
)
axial coupling of c quarks.
parm
Zprime:vmu
(default = -0.08
)
vector coupling of mu leptons.
parm
Zprime:amu
(default = -1.
)
axial coupling of mu leptons.
parm
Zprime:vnumu
(default = 1.
)
vector coupling of nu_mu neutrinos.
parm
Zprime:anumu
(default = 1.
)
axial coupling of nu_mu neutrinos.
parm
Zprime:vb
(default = -0.693
)
vector coupling of b quarks.
parm
Zprime:ab
(default = -1.
)
axial coupling of b quarks.
parm
Zprime:vt
(default = 0.387
)
vector coupling of t quarks.
parm
Zprime:at
(default = 1.
)
axial coupling of t quarks.
parm
Zprime:vtau
(default = -0.08
)
vector coupling of tau leptons.
parm
Zprime:atau
(default = -1.
)
axial coupling of tau leptons.
parm
Zprime:vnutau
(default = 1.
)
vector coupling of nu_tau neutrinos.
parm
Zprime:anutau
(default = 1.
)
axial coupling of nu_tau neutrinos.
The coupling to the decay channel Z'^0 → W^+ W^- is more model-dependent. By default it is therefore off, but can be switched on as follows.
parm
Zprime:coup2WW
(default = 0.
; minimum = 0.
)
the coupling Z'^0 → W^+ W^- is taken to be this number
times m_W^2 / m_Z'^2 times the Z^0 → W^+ W^-
coupling. Thus a unit value corresponds to the
Z^0 → W^+ W^- coupling, scaled down by a factor
m_W^2 / m_Z'^2, and gives a Z'^0 partial
width into this channel that again increases linearly. If you
cancel this behaviour, by letting Zprime:coup2WW
be
proportional to m_Z'^2 / m_W^2, you instead obtain a
partial width that goes like the fifth power of the Z'^0
mass. These two extremes correspond to the "extended gauge model"
and the "reference model", respectively, of [Alt89].
Note that this channel only includes the pure Z' part,
while f fbar → gamma^*/Z^*0 → W^+ W^- is available
as a separate electroweak process.
Furthermore, we have left some amount of
freedom in the choice of decay angular correlations in this
channel, but obviously alternative shapes could be imagined.
parm
Zprime:anglesWW
(default = 0.
; minimum = 0.
; maximum = 1.
)
in the decay chain Z'^0 → W^+ W^- →f_1 fbar_2 f_3 fbar_4
the decay angular distributions is taken to be a mixture of two
possible shapes. This parameter gives the fraction that is distributed
as in Higgs h^0 → W^+ W^- (longitudinal bosons),
with the remainder (by default all) is taken to be the same as for
Z^0 → W^+ W^- (a mixture of transverse and longitudinal
bosons).
A massive Z'^0 is also likely to decay into Higgs bosons and potentially into other now unknown particles. Such possibilities clearly are quite model-dependent, and have not been included for now.
Finally, to allow the exploration of more BSM physics scenarios, we include the possibility of the Z'^0 (and hence the gamma and Z^0) coupling to a fourth generation of fermions. This provides redundancy with and extensions beyond those processes implemented as fourth-generation processes. By default, the decay channels for the fourth-generation and not included. They are enabled using:
flag
Zprime:coup2gen4
(default = off
)
Z'^0 couples to 4th generation fermions.
Here are the further couplings that are specific for
a scenario with Zprime:universality
switched off:
parm
Zprime:vbPrime
(default = -0.693
)
vector coupling of b' quarks.
parm
Zprime:abPrime
(default = -1.
)
axial coupling of b' quarks.
parm
Zprime:vtPrime
(default = 0.387
)
vector coupling of t' quarks.
parm
Zprime:atPrime
(default = 1.
)
axial coupling of t' quarks.
parm
Zprime:vtauPrime
(default = -0.08
)
vector coupling of tau' leptons.
parm
Zprime:atauPrime
(default = -1.
)
axial coupling of tau' leptons.
parm
Zprime:vnutauPrime
(default = 1.
)
vector coupling of nu_tau' neutrinos.
parm
Zprime:anutauPrime
(default = 1.
)
axial coupling of nu_tau' neutrinos.
flag
NewGaugeBoson:ffbar2Wprime
(default = off
)
Scattering f fbar' → W'^+-.
Code 3021.
The couplings of the W'^+- are here assumed universal, i.e. the same for all generations. One may set vector and axial couplings freely, separately for the q qbar' and the l nu_l decay channels. The defaults correspond to the V - A structure and normalization of the Standard Model W^+-, but can be changed to simulate a wide selection of models. One limitation is that, for simplicity, the same Cabibbo--Kobayashi--Maskawa quark mixing matrix is assumed as for the standard W^+-. Depending on your assumed neutrino nature you may want to restrict your freedom in the lepton sector, but no limitations are enforced by the program.
parm
Wprime:vq
(default = 1.
)
vector coupling of quarks.
parm
Wprime:aq
(default = -1.
)
axial coupling of quarks.
parm
Wprime:vl
(default = 1.
)
vector coupling of leptons.
parm
Wprime:al
(default = -1.
)
axial coupling of leptons.
The coupling to the decay channel W'^+- → W^+- Z^0 is more model-dependent, like for Z'^0 → W^+ W^- described above. By default it is therefore off, but can be switched on as follows. Furthermore, we have left some amount of freedom in the choice of decay angular correlations in this channel, but obviously alternative shapes could be imagined.
parm
Wprime:coup2WZ
(default = 0.
; minimum = 0.
)
the coupling W'^0 → W^+- Z^0 is taken to be this number
times m_W^2 / m_W'^2 times the W^+- → W^+- Z^0
coupling. Thus a unit value corresponds to the
W^+- → W^+- Z^0 coupling, scaled down by a factor
m_W^2 / m_W'^2, and gives a W'^+- partial
width into this channel that increases linearly with the
W'^+- mass. If you cancel this behaviour, by letting
Wprime:coup2WZ
be proportional to m_W'^2 / m_W^2,
you instead obtain a partial width that goes like the fifth power
of the W'^+- mass. These two extremes correspond to the
"extended gauge model" and the "reference model", respectively,
of [Alt89].
parm
Wprime:anglesWZ
(default = 0.
; minimum = 0.
; maximum = 1.
)
in the decay chain W'^+- → W^+- Z^0 →f_1 fbar_2 f_3 fbar_4
the decay angular distributions is taken to be a mixture of two
possible shapes. This parameter gives the fraction that is distributed
as in Higgs H^+- → W^+- Z^0 (longitudinal bosons),
with the remainder (by default all) is taken to be the same as for
W^+- → W^+- Z^0 (a mixture of transverse and longitudinal
bosons).
A massive W'^+- is also likely to decay into Higgs bosons and potentially into other now unknown particles. Such possibilities clearly are quite model-dependent, and have not been included for now.
flag
NewGaugeBoson:ffbar2R0
(default = off
)
Scattering f_1 fbar_2 → R^0 → f_3 fbar_4, where
f_1 and fbar_2 are separated by +- one
generation and similarly for f_3 and fbar_4.
Thus possible final states are e.g. d sbar, u cbar
s bbar, c tbar, e- mu+ and
mu- tau+.
Code 3041.