1 <chapter name="Electroweak Processes">
3 <h2>Electroweak Processes</h2>
5 This page contains processes involving Prompt-photon, <ei>gamma^*/Z^0</ei>
6 and <ei>W^+-</ei> production, plus a few with <ei>t</ei>-channel boson
9 <h3>Prompt photon processes</h3>
11 This group collects the processes where one or two photons are
12 produced by the hard process. Additional sources of photons
13 include parton showers and hadron decays. A <ei>pT</ei> cut
14 is required to stay away from the unphysical low-<ei>pT</ei> region.
15 An eikonalized description, intended to be valid at all <ei>pT</ei>,
16 is included as part of the multiple-interactions framework.
18 <flag name="PromptPhoton:all" default="off">
19 Common switch for the group of all prompt photon processes,
20 as listed separately in the following.
23 <flag name="PromptPhoton:qg2qgamma" default="off">
24 Scattering <ei>q g -> q gamma</ei>.
28 <flag name="PromptPhoton:qqbar2ggamma" default="off">
29 Scattering <ei>q qbar -> g gamma</ei>.
33 <flag name="PromptPhoton:gg2ggamma" default="off">
34 Scattering <ei>g g -> g gamma</ei>.
35 <note>Note:</note> This is a box graph. The full quark-mass dependence
36 in the loop leads to very complicated expressions. The current
37 implementation is based on assuming five massless quarks (see below),
38 and thus is questionable at small (<ei>pT < m_b</ei>) or large
39 (<ei>pT > m_t</ei>) transverse momenta.
43 <flag name="PromptPhoton:ffbar2gammagamma" default="off">
44 Scattering <ei>q qbar -> gamma gamma</ei>.
48 <flag name="PromptPhoton:gg2gammagamma" default="off">
49 Scattering <ei>g g -> gamma gamma</ei>.
50 <note>Note:</note> This is a box graph. The full quark-mass dependence
51 in the loop leads to very complicated expressions. The current
52 implementation is based on assuming five massless quarks (see below),
53 and thus is questionable at small (<ei>pT < m_b</ei>) or large
54 (<ei>pT > m_t</ei>) transverse momenta.
58 <modeopen name="PromptPhoton:nQuarkLoop" default="5" min="3" max="6">
59 Number of quark flavours included in the box graphs resposible for
60 <ei>g g -> g gamma</ei> and <ei>g g-> gamma gamma</ei> processes.
61 Owing to the complexity if the massive expressions, quarks are treated
62 as massless. The default value should be applicable in the range of
63 transverse momenta above the <ei>b</ei> mass but below the <ei>t</ei> one.
66 <h3>Weak boson processes</h3>
68 Under this heading we group processes involving the production
69 of a single electroweak gauge boson, i.e. a <ei>gamma^*/Z^0</ei>
70 or a <ei>W^+-</ei>, or a pair of them, or one of them in
71 combination with a parton. Since the three sets are partly
72 conflicting, each is associated with its own group flag.
73 In addition, <ei>t</ei>-channel exchange of such a boson
74 between two fermions form a separate group.
77 There is one flag that can be used to influence the <ei>gamma^*/Z^0</ei>
78 structure in all the processes below where it is produced, unless
80 <modepick name="WeakZ0:gmZmode" default="0" min="0" max="2">
81 Choice of full <ei>gamma^*/Z^0</ei> structure or not in relevant
83 <option value="0">full <ei>gamma^*/Z^0</ei> structure,
84 with interference included.</option>
85 <option value="1">only pure <ei>gamma^*</ei> contribution.</option>
86 <option value="2">only pure <ei>Z^0</ei> contribution.</option>
87 <note>Note</note>: irrespective of the option used, the particle produced
88 will always be assigned code 23 for <ei>Z^0</ei>, and open decay channels
89 is purely dictated by what is set for the <ei>Z^0</ei>.
92 <h4>Boson exchange</h4>
94 The two processes in this subgroup is included as part of the
95 multiple-interactions framework.
97 <flag name="WeakBosonExchange:all" default="off">
98 Common switch for the group of <ei>gamma^*/Z^0</ei>
99 or <ei>W^+-</ei> exchange between two fermions.
102 <flag name="WeakBosonExchange:ff2ff(t:gmZ)" default="off">
103 Scattering <ei>f f' -> f f'</ei> via </ei>gamma^*/Z^0</ei>
104 <ei>t</ei>-channel exchange, with full interference
105 between the <ei>gamma^*</ei> and <ei>Z^0</ei>.
109 <flag name="WeakBosonExchange:ff2ff(t:W)" default="off">
110 Scattering <ei>f_1 f_2 -> f_3 f_4</ei> via </ei>W^+-</ei>
111 <ei>t</ei>-channel exchange.
115 <h4>Single boson</h4>
117 <flag name="WeakSingleBoson:all" default="off">
118 Common switch for the group of a single <ei>gamma^*/Z^0</ei>
119 or <ei>W^+-</ei> production.
122 <flag name="WeakSingleBoson:ffbar2gmZ" default="off">
123 Scattering <ei>f fbar -> gamma^*/Z^0</ei>, with full interference
124 between the <ei>gamma^*</ei> and <ei>Z^0</ei>.
128 <flag name="WeakSingleBoson:ffbar2W" default="off">
129 Scattering <ei>f fbar' -> W^+-</ei>.
133 <flag name="WeakSingleBoson:ffbar2ffbar(s:gm)" default="off">
134 Scattering <ei>f fbar -> gamma^* -> f' fbar'</ei>. Subset of
135 process 221, but written as a <ei>2 -> 2</ei> process, so that
136 <ei>pT</ei> can be used as ordering variable, e.g. in multiple
137 interactions. Hardcoded for the final state being either of the
138 five quark flavours or three lepton ones. Not included in the
139 <code>WeakSingleBoson:all</code> set, but included in the
140 multiple-interactions framework.
146 <flag name="WeakDoubleBoson:all" default="off">
147 Common switch for the group of pair production of <ei>gamma^*/Z^0</ei>
151 <flag name="WeakDoubleBoson:ffbar2gmZgmZ" default="off">
152 Scattering <ei>f fbar' -> gamma^*/Z^0 gamma^*/Z^0</ei>.
156 <flag name="WeakDoubleBoson:ffbar2ZW" default="off">
157 Scattering <ei>f fbar' -> Z^0 W^+-</ei>. Note that here the
158 <ei>gamma^*</ei> contribution is not (currently) included.
162 <flag name="WeakDoubleBoson:ffbar2WW" default="off">
163 Scattering <ei>f fbar -> W^+ W^-</ei>.
167 <h4>Boson and parton</h4>
169 <flag name="WeakBosonAndParton:all" default="off">
170 Common switch for the group of production of a single electroweak
171 gauge boson, i.e. a <ei>gamma^*/Z^0</ei> or a <ei>W^+-</ei>, in
172 association with a parton, i.e. a quark, gluon, photon or lepton.
173 These processes give first-order corrections to the ones in the
174 <code>WeakSingleBoson</code> class, and both sets cannot be used
175 simultaneously without unphysical doublecounting. The current class
176 should only be used to study the high-<ei>pT</ei> tail of the
177 gauge-boson production processes (for LHC applications at least
178 <ei>pT</ei> > 20 GeV), while the ones in <code>WeakSingleBoson</code>
179 should be used for inclusive production.
182 <flag name="WeakBosonAndParton:qqbar2gmZg" default="off">
183 Scattering <ei>q qbar -> gamma^*/Z^0 g</ei>.
187 <flag name="WeakBosonAndParton:qg2gmZq" default="off">
188 Scattering <ei>q g -> gamma^*/Z^0 q </ei>.
192 <flag name="WeakBosonAndParton:ffbar2gmZgm" default="off">
193 Scattering <ei>f fbar -> gamma^*/Z^0 gamma</ei>.
197 <flag name="WeakBosonAndParton:fgm2gmZf" default="off">
198 Scattering <ei>f gamma -> gamma^*/Z^0 f</ei>.
202 <flag name="WeakBosonAndParton:qqbar2Wg" default="off">
203 Scattering <ei>q qbar -> W^+- g</ei>.
207 <flag name="WeakBosonAndParton:qg2Wq" default="off">
208 Scattering <ei>q g -> W^+- q</ei>.
212 <flag name="WeakBosonAndParton:ffbar2Wgm" default="off">
213 Scattering <ei>f fbar -> W^+- gamma</ei>.
217 <flag name="WeakBosonAndParton:fgm2Wf" default="off">
218 Scattering <ei>f gamma -> W^+- f</ei>.
222 <h3> Photon Collision Processes</h3>
224 A few electroweak two-photon production processes are available.
225 To use them, photon PDFs have to be defined for the incoming
226 beam particles. For proton beams an appropriate set would be
227 MRST QED 2004 <ref>Mar05</ref>, available in the LHAPDF library.
229 <flag name="PhotonCollision:all" default="off">
230 Common switch for the group of six processes presented below.
233 <flag name="PhotonCollision:gmgm2qqbar" default="off">
234 Scatterings <ei>gamma gamma -> q qbar</ei>, where <ei>q</ei>
235 is a light quark (<ei>u, d, s</ei>) .
239 <flag name="PhotonCollision:gmgm2ccbar" default="off">
240 Scattering <ei>gamma gamma -> c cbar</ei>.
244 <flag name="PhotonCollision:gmgm2bbbar" default="off">
245 Scattering <ei>gamma gamma -> b bbar</ei>.
249 <flag name="PhotonCollision:gmgm2ee" default="off">
250 Scattering <ei>gamma gamma -> e+ e-</ei>.
254 <flag name="PhotonCollision:gmgm2mumu" default="off">
255 Scattering <ei>gamma gamma -> mu+ mu-</ei>.
259 <flag name="PhotonCollision:gmgm2tautau" default="off">
260 Scattering <ei>gamma gamma -> tau+ tau-</ei>.
266 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->