Electroweak Processes
This page contains processes involving Prompt-photon, gamma^*/Z^0
and W^+- production, plus a few with t-channel boson
exchange.
Prompt photon processes
This group collects the processes where one or two photons are
produced by the hard process. Additional sources of photons
include parton showers and hadron decays. A pT cut
is required to stay away from the unphysical low-pT region.
An eikonalized description, intended to be valid at all pT,
is included as part of the multiparton-interactions framework.
flag PromptPhoton:all
(default = off)
Common switch for the group of all prompt photon processes,
as listed separately in the following.
flag PromptPhoton:qg2qgamma
(default = off)
Scattering q g → q gamma.
Code 201.
flag PromptPhoton:qqbar2ggamma
(default = off)
Scattering q qbar → g gamma.
Code 202.
flag PromptPhoton:gg2ggamma
(default = off)
Scattering g g → g gamma.
Note: This is a box graph. The full quark-mass dependence
in the loop leads to very complicated expressions. The current
implementation is based on assuming five massless quarks (see below),
and thus is questionable at small (pT < m_b) or large
(pT > m_t) transverse momenta.
Code 203.
flag PromptPhoton:ffbar2gammagamma
(default = off)
Scattering q qbar → gamma gamma.
Code 204.
flag PromptPhoton:gg2gammagamma
(default = off)
Scattering g g → gamma gamma.
Note: This is a box graph. The full quark-mass dependence
in the loop leads to very complicated expressions. The current
implementation is based on assuming five massless quarks (see below),
and thus is questionable at small (pT < m_b) or large
(pT > m_t) transverse momenta.
Code 205.
mode PromptPhoton:nQuarkLoop
(default = 5; minimum = 3; maximum = 6)
Number of quark flavours included in the box graphs responsible for
g g → g gamma and g g→ gamma gamma processes.
Owing to the complexity if the massive expressions, quarks are treated
as massless. The default value should be applicable in the range of
transverse momenta above the b mass but below the t one.
Weak boson processes
Under this heading we group processes involving the production
of a single electroweak gauge boson, i.e. a gamma^*/Z^0
or a W^+-, or a pair of them, or one of them in
combination with a parton. Since the three sets are partly
conflicting, each is associated with its own group flag.
In addition, t-channel exchange of such a boson
between two fermions form a separate group.
There is one flag that can be used to influence the gamma^*/Z^0
structure in all the processes below where it is produced, unless
otherwise stated.
mode WeakZ0:gmZmode
(default = 0; minimum = 0; maximum = 2)
Choice of full gamma^*/Z^0 structure or not in relevant
processes.
option 0 : full gamma^*/Z^0 structure,
with interference included.
option 1 : only pure gamma^* contribution.
option 2 : only pure Z^0 contribution.
Note: irrespective of the option used, the particle produced
will always be assigned code 23 for Z^0, and open decay channels
is purely dictated by what is set for the Z^0.
Boson exchange
The two processes in this subgroup is included as part of the
multiparton-interactions framework.
flag WeakBosonExchange:all
(default = off)
Common switch for the group of gamma^*/Z^0
or W^+- exchange between two fermions.
flag WeakBosonExchange:ff2ff(t:gmZ)
(default = off)
Scattering f f' → f f' via gamma^*/Z^0
t-channel exchange, with full interference
between the gamma^* and Z^0.
Code 211.
flag WeakBosonExchange:ff2ff(t:W)
(default = off)
Scattering f_1 f_2 → f_3 f_4 via W^+-
t-channel exchange.
Code 212.
Single boson
flag WeakSingleBoson:all
(default = off)
Common switch for the group of a single gamma^*/Z^0
or W^+- production.
flag WeakSingleBoson:ffbar2gmZ
(default = off)
Scattering f fbar → gamma^*/Z^0, with full interference
between the gamma^* and Z^0.
Code 221.
flag WeakSingleBoson:ffbar2W
(default = off)
Scattering f fbar' → W^+-.
Code 222.
flag WeakSingleBoson:ffbar2ffbar(s:gm)
(default = off)
Scattering f fbar → gamma^* → f' fbar'. Subset of
process 221, but written as a 2 → 2 process, so that
pT can be used as ordering variable, e.g. in multiparton
interactions. Hardcoded for the final state being either of the
five quark flavours or three lepton ones. Not included in the
WeakSingleBoson:all set, but included in the
multiparton-interactions framework.
Code 223.
flag WeakSingleBoson:ffbar2ffbar(s:gmZ)
(default = off)
Scattering f fbar → gamma^*/Z^0 → f' fbar'. Equivalent
to process 221, but written as a 2 → 2 process, so that
pT could be used as cut or ordering variable. Final-state
flavour selection is based on the Z^0 allowed decay modes,
and the WeakZ0:gmZmode options are implemented.
Not included in the WeakSingleBoson:all set.
Code 224.
flag WeakSingleBoson:ffbar2ffbar(s:W)
(default = off)
Scattering f_1 fbar_2 → W+- → f_3 f_4. Almost equivalent
to process 222, but written as a 2 → 2 process, so that
pT could be used as cut or ordering variable. Final-state
flavour selection is based on the W allowed decay modes.
There are two simplifications relative to the implementation in
process 222. Firstly, it is not possible to set different decay modes
for the W^+ and the W^-; instead the allowed
W^+ ones will be used throughout, with charge conjugation
for the W^-. Secondly, quark mass corrections are neglected
in the decay angular distribution.
Not included in the WeakSingleBoson:all set.
Code 225.
Boson pair
Note that, in the decay of the two vector bosons produced by an
f fbar → V V process, the full four-fermion correlations
from the leading-order f fbar → V V → 4f matrix elements
are included [Gun86] (with some extensions by the authors).
The matrix elements are provided in the double-resonant approach, i.e.
excludes graph like
f fbar → V → f fbar → f fbar V → 4f.
flag WeakDoubleBoson:all
(default = off)
Common switch for the group of pair production of gamma^*/Z^0
and W^+-.
flag WeakDoubleBoson:ffbar2gmZgmZ
(default = off)
Scattering f fbar' → gamma^*/Z^0 gamma^*/Z^0.
Code 231.
flag WeakDoubleBoson:ffbar2ZW
(default = off)
Scattering f fbar' → Z^0 W^+-. Note that here the
gamma^* contribution is not (currently) included.
Code 232.
flag WeakDoubleBoson:ffbar2WW
(default = off)
Scattering f fbar → W^+ W^-.
Code 233.
Boson and parton
flag WeakBosonAndParton:all
(default = off)
Common switch for the group of production of a single electroweak
gauge boson, i.e. a gamma^*/Z^0 or a W^+-, in
association with a parton, i.e. a quark, gluon, photon or lepton.
These processes give first-order corrections to the ones in the
WeakSingleBoson class, and both sets cannot be used
simultaneously without unphysical double-counting. The current class
should only be used to study the high-pT tail of the
gauge-boson production processes (for LHC applications at least
pT > 20 GeV), while the ones in WeakSingleBoson
should be used for inclusive production.
flag WeakBosonAndParton:qqbar2gmZg
(default = off)
Scattering q qbar → gamma^*/Z^0 g.
Code 241.
flag WeakBosonAndParton:qg2gmZq
(default = off)
Scattering q g → gamma^*/Z^0 q .
Code 242.
flag WeakBosonAndParton:ffbar2gmZgm
(default = off)
Scattering f fbar → gamma^*/Z^0 gamma.
Code 243.
flag WeakBosonAndParton:fgm2gmZf
(default = off)
Scattering f gamma → gamma^*/Z^0 f.
Code 244.
flag WeakBosonAndParton:qqbar2Wg
(default = off)
Scattering q qbar → W^+- g.
Code 251.
flag WeakBosonAndParton:qg2Wq
(default = off)
Scattering q g → W^+- q.
Code 252.
flag WeakBosonAndParton:ffbar2Wgm
(default = off)
Scattering f fbar → W^+- gamma.
Code 253.
flag WeakBosonAndParton:fgm2Wf
(default = off)
Scattering f gamma → W^+- f.
Code 254.
Photon Collision Processes
A few electroweak two-photon production processes are available.
To use them, photon PDFs have to be defined for the incoming
beam particles. This is the case for the internal NNPDF 2.3 QCD+QED
sets, as well as some further ones available in the LHAPDF library,
but not for all PDFs.
flag PhotonCollision:all
(default = off)
Common switch for the group of six processes presented below.
flag PhotonCollision:gmgm2qqbar
(default = off)
Scatterings gamma gamma → q qbar, where q
is a light quark (u, d, s) .
Code 261.
flag PhotonCollision:gmgm2ccbar
(default = off)
Scattering gamma gamma → c cbar.
Code 262.
flag PhotonCollision:gmgm2bbbar
(default = off)
Scattering gamma gamma → b bbar.
Code 263.
flag PhotonCollision:gmgm2ee
(default = off)
Scattering gamma gamma → e+ e-.
Code 264.
flag PhotonCollision:gmgm2mumu
(default = off)
Scattering gamma gamma → mu+ mu-.
Code 265.
flag PhotonCollision:gmgm2tautau
(default = off)
Scattering gamma gamma → tau+ tau-.
Code 266.
Photon-Parton Processes
Photon-parton production processes. See comment related to PDFs above.
Also relevant for direct-resolved processes in photon-photon collisions.
Two instances of a given process are created when mixing processes where
the direct photon can come from either side. In this case the process
code in the parentheses is used when the direct photon is from side A.
flag PhotonParton:all
(default = off)
Common switch for the processes 271-274 (also 281-284 when relevant).
flag PhotonParton:ggm2qqbar
(default = off)
Scattering g gamma → q qbar, where q
is a light quark (u, d, s).
Code 271 (281).
flag PhotonParton:ggm2ccbar
(default = off)
Scattering g gamma → c cbar.
Code 272 (282).
flag PhotonParton:ggm2bbbar
(default = off)
Scattering g gamma → b bbar.
Code 273 (283).
flag PhotonParton:qgm2qg
(default = off)
Scattering q gamma → q g.
Code 274 (284).
flag PhotonParton:qgm2qgm
(default = off)
Scattering q gamma → q gamma.
Code 275 (285).