<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/> 
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' &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. 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 &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> 
 
</chapter> 
 
<!-- Copyright (C) 2014 Torbjorn Sjostrand --> 
 
