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<html>
<head>
<title>Electroweak Processes</title>
<link rel="stylesheet" type="text/css" href="pythia.css"/>
<link rel="shortcut icon" href="pythia32.gif"/>
</head>
<body>
 
<h2>Electroweak Processes</h2> 
 
This page contains processes involving Prompt-photon, <i>gamma^*/Z^0</i> 
and <i>W^+-</i> production, plus a few with <i>t</i>-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 <i>pT</i> cut 
is required to stay away from the unphysical low-<i>pT</i> region. 
An eikonalized description, intended to be valid at all <i>pT</i>, 
is included as part of the multiparton-interactions framework. 
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of all prompt photon processes, 
as listed separately in the following. 
   
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:qg2qgamma &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g &rarr; q gamma</i>. 
Code 201. 
   
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:qqbar2ggamma &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar &rarr; g gamma</i>. 
Code 202. 
   
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:gg2ggamma &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>g g &rarr; g gamma</i>. 
<br/><b>Note:</b> 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 (<i>pT &lt; m_b</i>) or large 
(<i>pT > m_t</i>) transverse momenta. 
Code 203. 
   
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:ffbar2gammagamma &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar &rarr; gamma gamma</i>. 
Code 204. 
   
 
<p/><code>flag&nbsp; </code><strong> PromptPhoton:gg2gammagamma &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>g g &rarr; gamma gamma</i>. 
<br/><b>Note:</b> 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 (<i>pT &lt; m_b</i>) or large 
(<i>pT > m_t</i>) transverse momenta. 
Code 205. 
   
 
<p/><code>mode&nbsp; </code><strong> PromptPhoton:nQuarkLoop &nbsp;</strong> 
 (<code>default = <strong>5</strong></code>; <code>minimum = 3</code>; <code>maximum = 6</code>)<br/>
Number of quark flavours included in the box graphs responsible for 
<i>g g &rarr; g gamma</i> and <i>g g&rarr; gamma gamma</i> 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 <i>b</i> mass but below the <i>t</i> one. 
   
 
<h3>Weak boson processes</h3> 
 
Under this heading we group processes involving the production 
of a single electroweak gauge boson, i.e. a <i>gamma^*/Z^0</i> 
or a <i>W^+-</i>, 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, <i>t</i>-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 <i>gamma^*/Z^0</i> 
structure in all the processes below where it is produced, unless 
otherwise stated. 
<p/><code>mode&nbsp; </code><strong> WeakZ0:gmZmode &nbsp;</strong> 
 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
Choice of full <i>gamma^*/Z^0</i> structure or not in relevant 
processes. 
<br/><code>option </code><strong> 0</strong> : full <i>gamma^*/Z^0</i> structure, 
with interference included.   
<br/><code>option </code><strong> 1</strong> : only pure <i>gamma^*</i> contribution.   
<br/><code>option </code><strong> 2</strong> : only pure <i>Z^0</i> contribution.   
<br/><b>Note</b>: irrespective of the option used, the particle produced 
will always be assigned code 23 for <i>Z^0</i>, and open decay channels 
is purely dictated by what is set for the <i>Z^0</i>. 
   
 
<h4>Boson exchange</h4> 
 
The two processes in this subgroup is included as part of the 
multiparton-interactions framework. 
 
<p/><code>flag&nbsp; </code><strong> WeakBosonExchange:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of <i>gamma^*/Z^0</i> 
or <i>W^+-</i> exchange between two fermions. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakBosonExchange:ff2ff(t:gmZ) &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f f' &rarr; f f'</i> via </i>gamma^*/Z^0</i> 
<i>t</i>-channel exchange, with full interference 
between the <i>gamma^*</i> and <i>Z^0</i>. 
Code 211. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakBosonExchange:ff2ff(t:W) &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f_1 f_2 &rarr; f_3 f_4</i> via </i>W^+-</i> 
<i>t</i>-channel exchange. 
Code 212. 
   
 
<h4>Single boson</h4> 
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of a single <i>gamma^*/Z^0</i> 
or <i>W^+-</i> production. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:ffbar2gmZ &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; gamma^*/Z^0</i>, with full interference 
between the <i>gamma^*</i> and <i>Z^0</i>. 
Code 221. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:ffbar2W &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar' &rarr; W^+-</i>. 
Code 222. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:ffbar2ffbar(s:gm) &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; gamma^* &rarr; f' fbar'</i>. Subset of 
process 221, but written as a <i>2 &rarr; 2</i> process, so that 
<i>pT</i> 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. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:ffbar2ffbar(s:gmZ) &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; gamma^*/Z^0 &rarr; f' fbar'</i>. Equivalent 
to process 221, but written as a <i>2 &rarr; 2</i> process, so that 
<i>pT</i> could be used as cut or ordering variable. Final-state 
flavour selection is based on the <i>Z^0</i> allowed decay modes, 
and the <code>WeakZ0:gmZmode</code> options are implemented. 
Not included in the <code>WeakSingleBoson:all</code> set. 
Code 224. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakSingleBoson:ffbar2ffbar(s:W) &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f_1 fbar_2 &rarr; W+- &rarr; f_3 f_4</i>. Almost equivalent 
to process 222, but written as a <i>2 &rarr; 2</i> process, so that 
<i>pT</i> could be used as cut or ordering variable. Final-state 
flavour selection is based on the <i>W</i> 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 <i>W^+</i> and the <i>W^-</i>; instead the allowed 
<i>W^+</i> ones will be used throughout, with charge conjugation 
for the <i>W^-</i>. Secondly, quark mass corrections are neglected 
in the decay angular distribution. 
Not included in the <code>WeakSingleBoson:all</code> set. 
Code 225. 
   
 
<h4>Boson pair</h4> 
 
Note that, in the decay of the two vector bosons produced by an 
<i>f fbar &rarr; V V</i> process, the full four-fermion correlations 
from the leading-order <i>f fbar &rarr; V V &rarr; 4f</i> matrix elements 
are included [<a href="Bibliography.html" target="page">Gun86</a>] (with some extensions by the authors). 
The matrix elements are provided in the double-resonant approach, i.e. 
excludes graph like 
<i>f fbar &rarr; V &rarr; f fbar &rarr; f fbar V &rarr; 4f</i>. 
 
<p/><code>flag&nbsp; </code><strong> WeakDoubleBoson:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of pair production of <i>gamma^*/Z^0</i> 
and <i>W^+-</i>. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakDoubleBoson:ffbar2gmZgmZ &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar' &rarr; gamma^*/Z^0 gamma^*/Z^0</i>. 
Code 231. 
   
 
<p/><code>flag&nbsp; </code><strong> WeakDoubleBoson:ffbar2ZW &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar' &rarr; Z^0 W^+-</i>. Note that here the 
<i>gamma^*</i> contribution is not (currently) included. 
Code 232. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakDoubleBoson:ffbar2WW &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; W^+ W^-</i>. 
Code 233. 
   
 
<h4>Boson and parton</h4> 
 
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of production of a single electroweak 
gauge boson, i.e. a <i>gamma^*/Z^0</i> or a <i>W^+-</i>, 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-<i>pT</i> tail of the 
gauge-boson production processes (for LHC applications at least 
<i>pT</i> > 20 GeV), while the ones in <code>WeakSingleBoson</code> 
should be used for inclusive production. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:qqbar2gmZg &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar &rarr; gamma^*/Z^0 g</i>. 
Code 241. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:qg2gmZq &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g &rarr; gamma^*/Z^0 q </i>. 
Code 242. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:ffbar2gmZgm &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; gamma^*/Z^0 gamma</i>. 
Code 243. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:fgm2gmZf &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f gamma &rarr;  gamma^*/Z^0 f</i>. 
Code 244. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:qqbar2Wg &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar &rarr; W^+- g</i>. 
Code 251. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:qg2Wq &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g &rarr; W^+- q</i>. 
Code 252. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:ffbar2Wgm &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar &rarr; W^+- gamma</i>. 
Code 253. 
   
  
<p/><code>flag&nbsp; </code><strong> WeakBosonAndParton:fgm2Wf &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f gamma &rarr; W^+- f</i>. 
Code 254. 
   
 
<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 [<a href="Bibliography.html" target="page">Mar05</a>], available in the LHAPDF library. 
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:all &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of six processes presented below. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2qqbar &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>gamma gamma &rarr; q qbar</i>, where <i>q</i> 
is a light quark (<i>u, d, s</i>) . 
Code 261. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2ccbar &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma &rarr; c cbar</i>. 
Code 262. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2bbbar &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma &rarr; b bbar</i>. 
Code 263. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2ee &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma &rarr; e+ e-</i>. 
Code 264. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2mumu &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma &rarr; mu+ mu-</i>. 
Code 265. 
   
 
<p/><code>flag&nbsp; </code><strong> PhotonCollision:gmgm2tautau &nbsp;</strong> 
 (<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma &rarr; tau+ tau-</i>. 
Code 266. 
   
 
</body>
</html>
 
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