source: HiSusy/trunk/Pythia8/pythia8170/xmldoc/ElectroweakProcesses.xml @ 1

<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 &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 -> 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 &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 resposible 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>

<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 doublecounting. 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>

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