<chapter name="Electroweak Processes">
<h2>Electroweak Processes</h2>
This page contains processes involving Prompt-photon, <ei>gamma^*/Z^0</ei>
and <ei>W^+-</ei> production, plus a few with <ei>t</ei>-channel boson
exchange.
<h3>Prompt photon processes</h3>
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 <ei>pT</ei> cut
is required to stay away from the unphysical low-<ei>pT</ei> region.
An eikonalized description, intended to be valid at all <ei>pT</ei>,
is included as part of the multiparton-interactions framework.
<flag name="PromptPhoton:all" default="off">
Common switch for the group of all prompt photon processes,
as listed separately in the following.
</flag>
<flag name="PromptPhoton:qg2qgamma" default="off">
Scattering <ei>q g → q gamma</ei>.
Code 201.
</flag>
<flag name="PromptPhoton:qqbar2ggamma" default="off">
Scattering <ei>q qbar → g gamma</ei>.
Code 202.
</flag>
<flag name="PromptPhoton:gg2ggamma" default="off">
Scattering <ei>g g → g gamma</ei>.
<note>Note:</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 (<ei>pT < m_b</ei>) or large
(<ei>pT > m_t</ei>) transverse momenta.
Code 203.
</flag>
<flag name="PromptPhoton:ffbar2gammagamma" default="off">
Scattering <ei>q qbar → gamma gamma</ei>.
Code 204.
</flag>
<flag name="PromptPhoton:gg2gammagamma" default="off">
Scattering <ei>g g → gamma gamma</ei>.
<note>Note:</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 (<ei>pT < m_b</ei>) or large
(<ei>pT > m_t</ei>) transverse momenta.
Code 205.
</flag>
<modeopen name="PromptPhoton:nQuarkLoop" default="5" min="3" max="6">
Number of quark flavours included in the box graphs responsible for
<ei>g g → g gamma</ei> and <ei>g g→ gamma gamma</ei> 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 <ei>b</ei> mass but below the <ei>t</ei> one.
</modeopen>
<h3>Weak boson processes</h3>
Under this heading we group processes involving the production
of a single electroweak gauge boson, i.e. a <ei>gamma^*/Z^0</ei>
or a <ei>W^+-</ei>, 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, <ei>t</ei>-channel exchange of such a boson
between two fermions form a separate group.
<p/>
There is one flag that can be used to influence the <ei>gamma^*/Z^0</ei>
structure in all the processes below where it is produced, unless
otherwise stated.
<modepick name="WeakZ0:gmZmode" default="0" min="0" max="2">
Choice of full <ei>gamma^*/Z^0</ei> structure or not in relevant
processes.
<option value="0">full <ei>gamma^*/Z^0</ei> structure,
with interference included.</option>
<option value="1">only pure <ei>gamma^*</ei> contribution.</option>
<option value="2">only pure <ei>Z^0</ei> contribution.</option>
<note>Note</note>: irrespective of the option used, the particle produced
will always be assigned code 23 for <ei>Z^0</ei>, and open decay channels
is purely dictated by what is set for the <ei>Z^0</ei>.
</modepick>
<h4>Boson exchange</h4>
The two processes in this subgroup is included as part of the
multiparton-interactions framework.
<flag name="WeakBosonExchange:all" default="off">
Common switch for the group of <ei>gamma^*/Z^0</ei>
or <ei>W^+-</ei> exchange between two fermions.
</flag>
<flag name="WeakBosonExchange:ff2ff(t:gmZ)" default="off">
Scattering <ei>f f' → f f'</ei> via </ei>gamma^*/Z^0</ei>
<ei>t</ei>-channel exchange, with full interference
between the <ei>gamma^*</ei> and <ei>Z^0</ei>.
Code 211.
</flag>
<flag name="WeakBosonExchange:ff2ff(t:W)" default="off">
Scattering <ei>f_1 f_2 → f_3 f_4</ei> via </ei>W^+-</ei>
<ei>t</ei>-channel exchange.
Code 212.
</flag>
<h4>Single boson</h4>
<flag name="WeakSingleBoson:all" default="off">
Common switch for the group of a single <ei>gamma^*/Z^0</ei>
or <ei>W^+-</ei> production.
</flag>
<flag name="WeakSingleBoson:ffbar2gmZ" default="off">
Scattering <ei>f fbar → gamma^*/Z^0</ei>, with full interference
between the <ei>gamma^*</ei> and <ei>Z^0</ei>.
Code 221.
</flag>
<flag name="WeakSingleBoson:ffbar2W" default="off">
Scattering <ei>f fbar' → W^+-</ei>.
Code 222.
</flag>
<flag name="WeakSingleBoson:ffbar2ffbar(s:gm)" default="off">
Scattering <ei>f fbar → gamma^* → f' fbar'</ei>. Subset of
process 221, but written as a <ei>2 → 2</ei> process, so that
<ei>pT</ei> 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
<code>WeakSingleBoson:all</code> set, but included in the
multiparton-interactions framework.
Code 223.
</flag>
<flag name="WeakSingleBoson:ffbar2ffbar(s:gmZ)" default="off">
Scattering <ei>f fbar → gamma^*/Z^0 → f' fbar'</ei>. Equivalent
to process 221, but written as a <ei>2 → 2</ei> process, so that
<ei>pT</ei> could be used as cut or ordering variable. Final-state
flavour selection is based on the <ei>Z^0</ei> allowed decay modes,
and the <code>WeakZ0:gmZmode</code> options are implemented.
Not included in the <code>WeakSingleBoson:all</code> set.
Code 224.
</flag>
<flag name="WeakSingleBoson:ffbar2ffbar(s:W)" default="off">
Scattering <ei>f_1 fbar_2 → W+- → f_3 f_4</ei>. Almost equivalent
to process 222, but written as a <ei>2 → 2</ei> process, so that
<ei>pT</ei> could be used as cut or ordering variable. Final-state
flavour selection is based on the <ei>W</ei> 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 <ei>W^+</ei> and the <ei>W^-</ei>; instead the allowed
<ei>W^+</ei> ones will be used throughout, with charge conjugation
for the <ei>W^-</ei>. Secondly, quark mass corrections are neglected
in the decay angular distribution.
Not included in the <code>WeakSingleBoson:all</code> set.
Code 225.
</flag>
<h4>Boson pair</h4>
Note that, in the decay of the two vector bosons produced by an
<ei>f fbar → V V</ei> process, the full four-fermion correlations
from the leading-order <ei>f fbar → V V → 4f</ei> matrix elements
are included <ref>Gun86</ref> (with some extensions by the authors).
The matrix elements are provided in the double-resonant approach, i.e.
excludes graph like
<ei>f fbar → V → f fbar → f fbar V → 4f</ei>.
<flag name="WeakDoubleBoson:all" default="off">
Common switch for the group of pair production of <ei>gamma^*/Z^0</ei>
and <ei>W^+-</ei>.
</flag>
<flag name="WeakDoubleBoson:ffbar2gmZgmZ" default="off">
Scattering <ei>f fbar' → gamma^*/Z^0 gamma^*/Z^0</ei>.
Code 231.
</flag>
<flag name="WeakDoubleBoson:ffbar2ZW" default="off">
Scattering <ei>f fbar' → Z^0 W^+-</ei>. Note that here the
<ei>gamma^*</ei> contribution is not (currently) included.
Code 232.
</flag>
<flag name="WeakDoubleBoson:ffbar2WW" default="off">
Scattering <ei>f fbar → W^+ W^-</ei>.
Code 233.
</flag>
<h4>Boson and parton</h4>
<flag name="WeakBosonAndParton:all" default="off">
Common switch for the group of production of a single electroweak
gauge boson, i.e. a <ei>gamma^*/Z^0</ei> or a <ei>W^+-</ei>, in
association with a parton, i.e. a quark, gluon, photon or lepton.
These processes give first-order corrections to the ones in the
<code>WeakSingleBoson</code> class, and both sets cannot be used
simultaneously without unphysical double-counting. The current class
should only be used to study the high-<ei>pT</ei> tail of the
gauge-boson production processes (for LHC applications at least
<ei>pT</ei> > 20 GeV), while the ones in <code>WeakSingleBoson</code>
should be used for inclusive production.
</flag>
<flag name="WeakBosonAndParton:qqbar2gmZg" default="off">
Scattering <ei>q qbar → gamma^*/Z^0 g</ei>.
Code 241.
</flag>
<flag name="WeakBosonAndParton:qg2gmZq" default="off">
Scattering <ei>q g → gamma^*/Z^0 q </ei>.
Code 242.
</flag>
<flag name="WeakBosonAndParton:ffbar2gmZgm" default="off">
Scattering <ei>f fbar → gamma^*/Z^0 gamma</ei>.
Code 243.
</flag>
<flag name="WeakBosonAndParton:fgm2gmZf" default="off">
Scattering <ei>f gamma → gamma^*/Z^0 f</ei>.
Code 244.
</flag>
<flag name="WeakBosonAndParton:qqbar2Wg" default="off">
Scattering <ei>q qbar → W^+- g</ei>.
Code 251.
</flag>
<flag name="WeakBosonAndParton:qg2Wq" default="off">
Scattering <ei>q g → W^+- q</ei>.
Code 252.
</flag>
<flag name="WeakBosonAndParton:ffbar2Wgm" default="off">
Scattering <ei>f fbar → W^+- gamma</ei>.
Code 253.
</flag>
<flag name="WeakBosonAndParton:fgm2Wf" default="off">
Scattering <ei>f gamma → W^+- f</ei>.
Code 254.
</flag>
<h3> Photon Collision Processes</h3>
A few electroweak two-photon production processes are available.
To use them, photon PDFs have to be defined for the incoming
beam particles. For proton beams an appropriate set would be
MRST QED 2004 <ref>Mar05</ref>, available in the LHAPDF library.
<flag name="PhotonCollision:all" default="off">
Common switch for the group of six processes presented below.
</flag>
<flag name="PhotonCollision:gmgm2qqbar" default="off">
Scatterings <ei>gamma gamma → q qbar</ei>, where <ei>q</ei>
is a light quark (<ei>u, d, s</ei>) .
Code 261.
</flag>
<flag name="PhotonCollision:gmgm2ccbar" default="off">
Scattering <ei>gamma gamma → c cbar</ei>.
Code 262.
</flag>
<flag name="PhotonCollision:gmgm2bbbar" default="off">
Scattering <ei>gamma gamma → b bbar</ei>.
Code 263.
</flag>
<flag name="PhotonCollision:gmgm2ee" default="off">
Scattering <ei>gamma gamma → e+ e-</ei>.
Code 264.
</flag>
<flag name="PhotonCollision:gmgm2mumu" default="off">
Scattering <ei>gamma gamma → mu+ mu-</ei>.
Code 265.
</flag>
<flag name="PhotonCollision:gmgm2tautau" default="off">
Scattering <ei>gamma gamma → tau+ tau-</ei>.
Code 266.
</flag>
</chapter>
<!-- Copyright (C) 2014 Torbjorn Sjostrand -->
Marcin Chrzaszcz