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<title>Electroweak Processes</title>
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<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 </code><strong> PromptPhoton:all </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 </code><strong> PromptPhoton:qg2qgamma </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g → q gamma</i>.
Code 201.
<p/><code>flag </code><strong> PromptPhoton:qqbar2ggamma </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar → g gamma</i>.
Code 202.
<p/><code>flag </code><strong> PromptPhoton:gg2ggamma </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 < m_b</i>) or large
(<i>pT > m_t</i>) transverse momenta.
Code 203.
<p/><code>flag </code><strong> PromptPhoton:ffbar2gammagamma </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar → gamma gamma</i>.
Code 204.
<p/><code>flag </code><strong> PromptPhoton:gg2gammagamma </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 < m_b</i>) or large
(<i>pT > m_t</i>) transverse momenta.
Code 205.
<p/><code>mode </code><strong> PromptPhoton:nQuarkLoop </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 </code><strong> WeakZ0:gmZmode </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 </code><strong> WeakBosonExchange:all </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 </code><strong> WeakBosonExchange:ff2ff(t:gmZ) </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 </code><strong> WeakBosonExchange:ff2ff(t:W) </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 </code><strong> WeakSingleBoson:all </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 </code><strong> WeakSingleBoson:ffbar2gmZ </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 </code><strong> WeakSingleBoson:ffbar2W </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar' → W^+-</i>.
Code 222.
<p/><code>flag </code><strong> WeakSingleBoson:ffbar2ffbar(s:gm) </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.
<p/><code>flag </code><strong> WeakSingleBoson:ffbar2ffbar(s:gmZ) </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar → gamma^*/Z^0 → f' fbar'</i>. Equivalent
to process 221, but written as a <i>2 → 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 </code><strong> WeakSingleBoson:ffbar2ffbar(s:W) </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f_1 fbar_2 → W+- → f_3 f_4</i>. Almost equivalent
to process 222, but written as a <i>2 → 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 → 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 </code><strong> WeakDoubleBoson:all </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 </code><strong> WeakDoubleBoson:ffbar2gmZgmZ </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar' → gamma^*/Z^0 gamma^*/Z^0</i>.
Code 231.
<p/><code>flag </code><strong> WeakDoubleBoson:ffbar2ZW </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 </code><strong> WeakDoubleBoson:ffbar2WW </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 </code><strong> WeakBosonAndParton:all </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 </code><strong> WeakBosonAndParton:qqbar2gmZg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar → gamma^*/Z^0 g</i>.
Code 241.
<p/><code>flag </code><strong> WeakBosonAndParton:qg2gmZq </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g → gamma^*/Z^0 q </i>.
Code 242.
<p/><code>flag </code><strong> WeakBosonAndParton:ffbar2gmZgm </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar → gamma^*/Z^0 gamma</i>.
Code 243.
<p/><code>flag </code><strong> WeakBosonAndParton:fgm2gmZf </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f gamma → gamma^*/Z^0 f</i>.
Code 244.
<p/><code>flag </code><strong> WeakBosonAndParton:qqbar2Wg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q qbar → W^+- g</i>.
Code 251.
<p/><code>flag </code><strong> WeakBosonAndParton:qg2Wq </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>q g → W^+- q</i>.
Code 252.
<p/><code>flag </code><strong> WeakBosonAndParton:ffbar2Wgm </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>f fbar → W^+- gamma</i>.
Code 253.
<p/><code>flag </code><strong> WeakBosonAndParton:fgm2Wf </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 </code><strong> PhotonCollision:all </strong>
(<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of six processes presented below.
<p/><code>flag </code><strong> PhotonCollision:gmgm2qqbar </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 </code><strong> PhotonCollision:gmgm2ccbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma → c cbar</i>.
Code 262.
<p/><code>flag </code><strong> PhotonCollision:gmgm2bbbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma → b bbar</i>.
Code 263.
<p/><code>flag </code><strong> PhotonCollision:gmgm2ee </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma → e+ e-</i>.
Code 264.
<p/><code>flag </code><strong> PhotonCollision:gmgm2mumu </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma → mu+ mu-</i>.
Code 265.
<p/><code>flag </code><strong> PhotonCollision:gmgm2tautau </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scattering <i>gamma gamma → tau+ tau-</i>.
Code 266.
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