<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 </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. </body> </html> <!-- Copyright (C) 2014 Torbjorn Sjostrand -->
