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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 -> 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 -> 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 -> 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 -> 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 -> 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 -> g gamma</i> and <i>g g-> 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' -> 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 -> 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 -> 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' -> 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 -> gamma^* -> f' fbar'</i>. Subset of 
process 221, but written as a <i>2 -> 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.
  

<h4>Boson pair</h4>

Note that, in the decay of the two vector bosons produced by an 
<i>f fbar -> V V</i> process, the full four-fermion correlations 
from the leading-order <i>f fbar -> V V -> 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 -> V -> f fbar -> f fbar V -> 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' -> 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' -> 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 -> 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 -> 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 -> 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 -> 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 ->  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 -> 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 -> 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 -> 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 -> 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 -> 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 -> 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 -> 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 -> 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 -> 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 -> tau+ tau-</i>.
Code 266.
  

</body>
</html>

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