3 <title>Electroweak Processes</title>
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9 <h2>Electroweak Processes</h2>
11 This page contains processes involving Prompt-photon, <i>gamma^*/Z^0</i>
12 and <i>W^+-</i> production, plus a few with <i>t</i>-channel boson
15 <h3>Prompt photon processes</h3>
17 This group collects the processes where one or two photons are
18 produced by the hard process. Additional sources of photons
19 include parton showers and hadron decays. A <i>pT</i> cut
20 is required to stay away from the unphysical low-<i>pT</i> region.
21 An eikonalized description, intended to be valid at all <i>pT</i>,
22 is included as part of the multiparton-interactions framework.
24 <p/><code>flag </code><strong> PromptPhoton:all </strong>
25 (<code>default = <strong>off</strong></code>)<br/>
26 Common switch for the group of all prompt photon processes,
27 as listed separately in the following.
30 <p/><code>flag </code><strong> PromptPhoton:qg2qgamma </strong>
31 (<code>default = <strong>off</strong></code>)<br/>
32 Scattering <i>q g -> q gamma</i>.
36 <p/><code>flag </code><strong> PromptPhoton:qqbar2ggamma </strong>
37 (<code>default = <strong>off</strong></code>)<br/>
38 Scattering <i>q qbar -> g gamma</i>.
42 <p/><code>flag </code><strong> PromptPhoton:gg2ggamma </strong>
43 (<code>default = <strong>off</strong></code>)<br/>
44 Scattering <i>g g -> g gamma</i>.
45 <br/><b>Note:</b> This is a box graph. The full quark-mass dependence
46 in the loop leads to very complicated expressions. The current
47 implementation is based on assuming five massless quarks (see below),
48 and thus is questionable at small (<i>pT < m_b</i>) or large
49 (<i>pT > m_t</i>) transverse momenta.
53 <p/><code>flag </code><strong> PromptPhoton:ffbar2gammagamma </strong>
54 (<code>default = <strong>off</strong></code>)<br/>
55 Scattering <i>q qbar -> gamma gamma</i>.
59 <p/><code>flag </code><strong> PromptPhoton:gg2gammagamma </strong>
60 (<code>default = <strong>off</strong></code>)<br/>
61 Scattering <i>g g -> gamma gamma</i>.
62 <br/><b>Note:</b> This is a box graph. The full quark-mass dependence
63 in the loop leads to very complicated expressions. The current
64 implementation is based on assuming five massless quarks (see below),
65 and thus is questionable at small (<i>pT < m_b</i>) or large
66 (<i>pT > m_t</i>) transverse momenta.
70 <p/><code>mode </code><strong> PromptPhoton:nQuarkLoop </strong>
71 (<code>default = <strong>5</strong></code>; <code>minimum = 3</code>; <code>maximum = 6</code>)<br/>
72 Number of quark flavours included in the box graphs resposible for
73 <i>g g -> g gamma</i> and <i>g g-> gamma gamma</i> processes.
74 Owing to the complexity if the massive expressions, quarks are treated
75 as massless. The default value should be applicable in the range of
76 transverse momenta above the <i>b</i> mass but below the <i>t</i> one.
79 <h3>Weak boson processes</h3>
81 Under this heading we group processes involving the production
82 of a single electroweak gauge boson, i.e. a <i>gamma^*/Z^0</i>
83 or a <i>W^+-</i>, or a pair of them, or one of them in
84 combination with a parton. Since the three sets are partly
85 conflicting, each is associated with its own group flag.
86 In addition, <i>t</i>-channel exchange of such a boson
87 between two fermions form a separate group.
90 There is one flag that can be used to influence the <i>gamma^*/Z^0</i>
91 structure in all the processes below where it is produced, unless
93 <p/><code>mode </code><strong> WeakZ0:gmZmode </strong>
94 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
95 Choice of full <i>gamma^*/Z^0</i> structure or not in relevant
97 <br/><code>option </code><strong> 0</strong> : full <i>gamma^*/Z^0</i> structure,
98 with interference included.
99 <br/><code>option </code><strong> 1</strong> : only pure <i>gamma^*</i> contribution.
100 <br/><code>option </code><strong> 2</strong> : only pure <i>Z^0</i> contribution.
101 <br/><b>Note</b>: irrespective of the option used, the particle produced
102 will always be assigned code 23 for <i>Z^0</i>, and open decay channels
103 is purely dictated by what is set for the <i>Z^0</i>.
106 <h4>Boson exchange</h4>
108 The two processes in this subgroup is included as part of the
109 multiparton-interactions framework.
111 <p/><code>flag </code><strong> WeakBosonExchange:all </strong>
112 (<code>default = <strong>off</strong></code>)<br/>
113 Common switch for the group of <i>gamma^*/Z^0</i>
114 or <i>W^+-</i> exchange between two fermions.
117 <p/><code>flag </code><strong> WeakBosonExchange:ff2ff(t:gmZ) </strong>
118 (<code>default = <strong>off</strong></code>)<br/>
119 Scattering <i>f f' -> f f'</i> via </i>gamma^*/Z^0</i>
120 <i>t</i>-channel exchange, with full interference
121 between the <i>gamma^*</i> and <i>Z^0</i>.
125 <p/><code>flag </code><strong> WeakBosonExchange:ff2ff(t:W) </strong>
126 (<code>default = <strong>off</strong></code>)<br/>
127 Scattering <i>f_1 f_2 -> f_3 f_4</i> via </i>W^+-</i>
128 <i>t</i>-channel exchange.
132 <h4>Single boson</h4>
134 <p/><code>flag </code><strong> WeakSingleBoson:all </strong>
135 (<code>default = <strong>off</strong></code>)<br/>
136 Common switch for the group of a single <i>gamma^*/Z^0</i>
137 or <i>W^+-</i> production.
140 <p/><code>flag </code><strong> WeakSingleBoson:ffbar2gmZ </strong>
141 (<code>default = <strong>off</strong></code>)<br/>
142 Scattering <i>f fbar -> gamma^*/Z^0</i>, with full interference
143 between the <i>gamma^*</i> and <i>Z^0</i>.
147 <p/><code>flag </code><strong> WeakSingleBoson:ffbar2W </strong>
148 (<code>default = <strong>off</strong></code>)<br/>
149 Scattering <i>f fbar' -> W^+-</i>.
153 <p/><code>flag </code><strong> WeakSingleBoson:ffbar2ffbar(s:gm) </strong>
154 (<code>default = <strong>off</strong></code>)<br/>
155 Scattering <i>f fbar -> gamma^* -> f' fbar'</i>. Subset of
156 process 221, but written as a <i>2 -> 2</i> process, so that
157 <i>pT</i> can be used as ordering variable, e.g. in multiparton
158 interactions. Hardcoded for the final state being either of the
159 five quark flavours or three lepton ones. Not included in the
160 <code>WeakSingleBoson:all</code> set, but included in the
161 multiparton-interactions framework.
167 Note that, in the decay of the two vector bosons produced by an
168 <i>f fbar -> V V</i> process, the full four-fermion correlations
169 from the leading-order <i>f fbar -> V V -> 4f</i> matrix elements
170 are included [<a href="Bibliography.html" target="page">Gun86</a>] (with some extensions by the authors).
171 The matrix elements are provided in the double-resonant approach, i.e.
172 excludes graph like <i>f fbar -> V -> f fbar -> f fbar V -> 4f</i>.
174 <p/><code>flag </code><strong> WeakDoubleBoson:all </strong>
175 (<code>default = <strong>off</strong></code>)<br/>
176 Common switch for the group of pair production of <i>gamma^*/Z^0</i>
180 <p/><code>flag </code><strong> WeakDoubleBoson:ffbar2gmZgmZ </strong>
181 (<code>default = <strong>off</strong></code>)<br/>
182 Scattering <i>f fbar' -> gamma^*/Z^0 gamma^*/Z^0</i>.
186 <p/><code>flag </code><strong> WeakDoubleBoson:ffbar2ZW </strong>
187 (<code>default = <strong>off</strong></code>)<br/>
188 Scattering <i>f fbar' -> Z^0 W^+-</i>. Note that here the
189 <i>gamma^*</i> contribution is not (currently) included.
193 <p/><code>flag </code><strong> WeakDoubleBoson:ffbar2WW </strong>
194 (<code>default = <strong>off</strong></code>)<br/>
195 Scattering <i>f fbar -> W^+ W^-</i>.
199 <h4>Boson and parton</h4>
201 <p/><code>flag </code><strong> WeakBosonAndParton:all </strong>
202 (<code>default = <strong>off</strong></code>)<br/>
203 Common switch for the group of production of a single electroweak
204 gauge boson, i.e. a <i>gamma^*/Z^0</i> or a <i>W^+-</i>, in
205 association with a parton, i.e. a quark, gluon, photon or lepton.
206 These processes give first-order corrections to the ones in the
207 <code>WeakSingleBoson</code> class, and both sets cannot be used
208 simultaneously without unphysical doublecounting. The current class
209 should only be used to study the high-<i>pT</i> tail of the
210 gauge-boson production processes (for LHC applications at least
211 <i>pT</i> > 20 GeV), while the ones in <code>WeakSingleBoson</code>
212 should be used for inclusive production.
215 <p/><code>flag </code><strong> WeakBosonAndParton:qqbar2gmZg </strong>
216 (<code>default = <strong>off</strong></code>)<br/>
217 Scattering <i>q qbar -> gamma^*/Z^0 g</i>.
221 <p/><code>flag </code><strong> WeakBosonAndParton:qg2gmZq </strong>
222 (<code>default = <strong>off</strong></code>)<br/>
223 Scattering <i>q g -> gamma^*/Z^0 q </i>.
227 <p/><code>flag </code><strong> WeakBosonAndParton:ffbar2gmZgm </strong>
228 (<code>default = <strong>off</strong></code>)<br/>
229 Scattering <i>f fbar -> gamma^*/Z^0 gamma</i>.
233 <p/><code>flag </code><strong> WeakBosonAndParton:fgm2gmZf </strong>
234 (<code>default = <strong>off</strong></code>)<br/>
235 Scattering <i>f gamma -> gamma^*/Z^0 f</i>.
239 <p/><code>flag </code><strong> WeakBosonAndParton:qqbar2Wg </strong>
240 (<code>default = <strong>off</strong></code>)<br/>
241 Scattering <i>q qbar -> W^+- g</i>.
245 <p/><code>flag </code><strong> WeakBosonAndParton:qg2Wq </strong>
246 (<code>default = <strong>off</strong></code>)<br/>
247 Scattering <i>q g -> W^+- q</i>.
251 <p/><code>flag </code><strong> WeakBosonAndParton:ffbar2Wgm </strong>
252 (<code>default = <strong>off</strong></code>)<br/>
253 Scattering <i>f fbar -> W^+- gamma</i>.
257 <p/><code>flag </code><strong> WeakBosonAndParton:fgm2Wf </strong>
258 (<code>default = <strong>off</strong></code>)<br/>
259 Scattering <i>f gamma -> W^+- f</i>.
263 <h3> Photon Collision Processes</h3>
265 A few electroweak two-photon production processes are available.
266 To use them, photon PDFs have to be defined for the incoming
267 beam particles. For proton beams an appropriate set would be
268 MRST QED 2004 [<a href="Bibliography.html" target="page">Mar05</a>], available in the LHAPDF library.
270 <p/><code>flag </code><strong> PhotonCollision:all </strong>
271 (<code>default = <strong>off</strong></code>)<br/>
272 Common switch for the group of six processes presented below.
275 <p/><code>flag </code><strong> PhotonCollision:gmgm2qqbar </strong>
276 (<code>default = <strong>off</strong></code>)<br/>
277 Scatterings <i>gamma gamma -> q qbar</i>, where <i>q</i>
278 is a light quark (<i>u, d, s</i>) .
282 <p/><code>flag </code><strong> PhotonCollision:gmgm2ccbar </strong>
283 (<code>default = <strong>off</strong></code>)<br/>
284 Scattering <i>gamma gamma -> c cbar</i>.
288 <p/><code>flag </code><strong> PhotonCollision:gmgm2bbbar </strong>
289 (<code>default = <strong>off</strong></code>)<br/>
290 Scattering <i>gamma gamma -> b bbar</i>.
294 <p/><code>flag </code><strong> PhotonCollision:gmgm2ee </strong>
295 (<code>default = <strong>off</strong></code>)<br/>
296 Scattering <i>gamma gamma -> e+ e-</i>.
300 <p/><code>flag </code><strong> PhotonCollision:gmgm2mumu </strong>
301 (<code>default = <strong>off</strong></code>)<br/>
302 Scattering <i>gamma gamma -> mu+ mu-</i>.
306 <p/><code>flag </code><strong> PhotonCollision:gmgm2tautau </strong>
307 (<code>default = <strong>off</strong></code>)<br/>
308 Scattering <i>gamma gamma -> tau+ tau-</i>.
315 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->