<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 &rarr; q gamma</ei>. 
Code 201. 
</flag> 
 
<flag name="PromptPhoton:qqbar2ggamma" default="off"> 
Scattering <ei>q qbar &rarr; g gamma</ei>. 
Code 202. 
</flag> 
 
<flag name="PromptPhoton:gg2ggamma" default="off"> 
Scattering <ei>g g &rarr; 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 &lt; 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 &rarr; gamma gamma</ei>. 
Code 204. 
</flag> 
 
<flag name="PromptPhoton:gg2gammagamma" default="off"> 
Scattering <ei>g g &rarr; 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 &lt; 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 &rarr; g gamma</ei> and <ei>g g&rarr; 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/> 
Note that the <ei>Z^0</ei> and <ei>W^+-</ei> decay modes generated with 
the processes below only include those Standard-Model ones that are 
allowed for on-shell decays. Thus several other processes that receive 
contributions from off-shell resonances are listed separately, e.g. 
for <aloc href="TopProcesses">top</aloc> or 
<aloc href="FourthGenerationProcesses">fourth generation fermions</aloc>. 
 
<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 encode the standard Deeply Inelastic 
Scattering (DIS) of lepton-hadron collisions. In the latter context, 
it is useful to recall that the traditional <ei>Q^2</ei> facorization 
and renormalization scales need be set explicitly by 
<code>SigmaProcess:factorScale2 = 6</code> and 
<code>SigmaProcess:renormScale2 = 6</code>. The special phase space 
cut <code>PhaseSpace:Q2Min</code> may be more convenient than the 
traditional <ei>pT</ei> cut; to cover the phase space edges it may also 
be relevant to reduce <code>PhaseSpace:pTHatMinDiverge</code>. 
 
<p/> 
The processes are also included as part of the multiparton-interactions 
framework in hadron-hadron collisions. 
 
<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' &rarr; 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 &rarr; 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 &rarr; 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' &rarr; W^+-</ei>. 
Code 222. 
</flag> 
 
<flag name="WeakSingleBoson:ffbar2ffbar(s:gm)" default="off"> 
Scattering <ei>f fbar &rarr; gamma^* &rarr; f' fbar'</ei>. Subset of 
process 221, but written as a <ei>2 &rarr; 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 &rarr; gamma^*/Z^0 &rarr; f' fbar'</ei>. Equivalent 
to process 221, but written as a <ei>2 &rarr; 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 &rarr; W+- &rarr; f_3 f_4</ei>. Almost equivalent 
to process 222, but written as a <ei>2 &rarr; 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 &rarr; V V</ei> process, the full four-fermion correlations 
from the leading-order <ei>f fbar &rarr; V V &rarr; 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 &rarr; V &rarr; f fbar &rarr; f fbar V &rarr; 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' &rarr; gamma^*/Z^0 gamma^*/Z^0</ei>. 
Code 231. 
</flag> 
 
<flag name="WeakDoubleBoson:ffbar2ZW" default="off"> 
Scattering <ei>f fbar' &rarr; 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 &rarr; 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 &rarr; gamma^*/Z^0 g</ei>. 
Code 241. 
</flag> 
 
<flag name="WeakBosonAndParton:qg2gmZq" default="off"> 
Scattering <ei>q g &rarr; gamma^*/Z^0 q </ei>. 
Code 242. 
</flag> 
 
<flag name="WeakBosonAndParton:ffbar2gmZgm" default="off"> 
Scattering <ei>f fbar &rarr; gamma^*/Z^0 gamma</ei>. 
Code 243. 
</flag> 
 
<flag name="WeakBosonAndParton:fgm2gmZf" default="off"> 
Scattering <ei>f gamma &rarr;  gamma^*/Z^0 f</ei>. 
Code 244. 
</flag> 
 
<flag name="WeakBosonAndParton:qqbar2Wg" default="off"> 
Scattering <ei>q qbar &rarr; W^+- g</ei>. 
Code 251. 
</flag> 
 
<flag name="WeakBosonAndParton:qg2Wq" default="off"> 
Scattering <ei>q g &rarr; W^+- q</ei>. 
Code 252. 
</flag> 
 
<flag name="WeakBosonAndParton:ffbar2Wgm" default="off"> 
Scattering <ei>f fbar &rarr; W^+- gamma</ei>. 
Code 253. 
</flag> 
 
<flag name="WeakBosonAndParton:fgm2Wf" default="off"> 
Scattering <ei>f gamma &rarr; 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. 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 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 &rarr; 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 &rarr; c cbar</ei>. 
Code 262. 
</flag> 
 
<flag name="PhotonCollision:gmgm2bbbar" default="off"> 
Scattering <ei>gamma gamma &rarr; b bbar</ei>. 
Code 263. 
</flag> 
 
<flag name="PhotonCollision:gmgm2ee" default="off"> 
Scattering <ei>gamma gamma &rarr; e+ e-</ei>. 
Code 264. 
</flag> 
 
<flag name="PhotonCollision:gmgm2mumu" default="off"> 
Scattering <ei>gamma gamma &rarr; mu+ mu-</ei>. 
Code 265. 
</flag> 
 
<flag name="PhotonCollision:gmgm2tautau" default="off"> 
Scattering <ei>gamma gamma &rarr; tau+ tau-</ei>. 
Code 266. 
</flag> 
 
<h3> Photon-Parton Processes</h3> 
 
<p> 
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. 
</p> 
 
<flag name="PhotonParton:all" default="off"> 
Common switch for the processes 271-274 (also 281-284 when relevant). 
</flag> 
 
<flag name="PhotonParton:ggm2qqbar" default="off"> 
Scattering <ei>g gamma &rarr; q qbar</ei>, where <ei>q</ei> 
is a light quark (<ei>u, d, s</ei>). 
Code 271 (281). 
</flag> 
 
<flag name="PhotonParton:ggm2ccbar" default="off"> 
Scattering <ei>g gamma &rarr; c cbar</ei>. 
Code 272 (282). 
</flag> 
 
<flag name="PhotonParton:ggm2bbbar" default="off"> 
Scattering <ei>g gamma &rarr; b bbar</ei>. 
Code 273 (283). 
</flag> 
 
<flag name="PhotonParton:qgm2qg" default="off"> 
Scattering <ei>q gamma &rarr; q g</ei>. 
Code 274 (284). 
</flag> 
 
<flag name="PhotonParton:qgm2qgm" default="off"> 
Scattering <ei>q gamma &rarr; q gamma</ei>. 
Code 275 (285). 
</flag> 
 
</chapter> 
 
<!-- Copyright (C) 2020 Torbjorn Sjostrand --> 
