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[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8130 / xmldoc / NewGaugeBosonProcesses.xml
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5ad4eb21 1<chapter name="New-Gauge-Boson Processes">
2
3<h2>New-Gauge-Boson Processes</h2>
4
5This page contains the production of new <ei>Z'^0</ei> and
6<ei>W'^+-</ei> gauge bosons, e.g. within the context of a new
7<ei>U(1)</ei> or <ei>SU(2)</ei> gauge group, and also a
8(rather speculative) horizontal gauge boson <ei>R^0</ei>.
9Left-right-symmetry scenarios also contain new gauge bosons,
10but are described
11<aloc href"LeftRightSymmetryProcesses">separately</aloc>.
12
13<h3><ei>Z'^0</ei></h3>
14
15This group only contains one subprocess, with the full
16<ei>gamma^*/Z^0/Z'^0</ei> interference structure for couplings
17to fermion pairs. It is possible to pick only a subset, e.g, only
18the pure <ei>Z'^0</ei> piece. No higher-order processes are
19available explicitly, but the ISR showers contain automatic
20matching to the <ei>Z'^0</ei> + 1 jet matrix elements, as for
21the corresponding <ei>gamma^*/Z^0</ei> process.
22
23<flag name="NewGaugeBoson:ffbar2gmZZprime" default="off">
24Scattering <ei>f fbar ->Z'^0</ei>.
25Code 3001.
26</flag>
27
28<modepick name="Zprime:gmZmode" default="0" min="0" max="6">
29Choice of full <ei>gamma^*/Z^0/Z'^0</ei> structure or not in
30the above process. Note that, with the <ei>Z'^0</ei> part switched
31off, this process is reduced to what already exists among
32<aloc href="ElectroweakProcesses">electroweak processes</aloc>,
33so those options are here only for crosschecks.
34<option value="0">full <ei>gamma^*/Z^0/Z'^0</ei> structure,
35with interference included.</option>
36<option value="1">only pure <ei>gamma^*</ei> contribution.</option>
37<option value="2">only pure <ei>Z^0</ei> contribution.</option>
38<option value="3">only pure <ei>Z'^0</ei> contribution.</option>
39<option value="4">only the <ei>gamma^*/Z^0</ei> contribution,
40including interference.</option>
41<option value="5">only the <ei>gamma^*/Z'^0</ei> contribution,
42including interference.</option>
43<option value="6">only the <ei>Z^0/Z'^0</ei> contribution,
44including interference.</option>
45<note>Note</note>: irrespective of the option used, the particle produced
46will always be assigned code 32 for <ei>Z'^0</ei>, and open decay channels
47is purely dictated by what is set for the <ei>Z'^0</ei>.
48</modepick>
49
50<p/>
51The couplings of the <ei>Z'^0</ei> to quarks and leptons can
52either be assumed universal, i.e. generation-independent, or not.
53In the former case eight numbers parametrize the vector and axial
54couplings of down-type quarks, up-type quarks, leptons and neutrinos,
55respectively. Depending on your assumed neutrino nature you may
56want to restrict your freedom in that sector, but no limitations
57are enforced by the program. The default corresponds to the same
58couplings as that of the Standard Model <ei>Z^0</ei>, with axial
59couplings <ei>a_f = +-1</ei> and vector couplings
60<ei>v_f = a_f - 4 e_f sin^2(theta_W)</ei>, with
61<ei>sin^2(theta_W) = 0.23</ei>. Without universality
62the same eight numbers have to be set separately also for the
63second and the third generation. The choice of fixed axial and
64vector couplings implies a resonance width that increases linearly
65with the <ei>Z'^0</ei> mass.
66
67<p/>
68By a suitable choice of the parameters, it is possible to simulate
69just about any imaginable <ei>Z'^0</ei> scenario, with full
70interference effects in cross sections and decay angular
71distributions and generation-dependent couplings; the default values
72should mainly be viewed as placeholders. The conversion
73from the coupling conventions in a set of different <ei>Z'^0</ei>
74models in the literature to those used in PYTHIA is described by
75<a href="http://www.hep.uiuc.edu/home/catutza/nota12.ps">C.
76Ciobanu et al.</a>
77
78<flag name="Zprime:universality" default="on">
79If on then you need only set the first-generation couplings
80below, and these are automatically also used for the second and
81third generation. If off, then couplings can be chosen separately
82for each generation.
83</flag>
84
85<p/>
86Here are the couplings always valid for the first generation,
87and normally also for the second and third by trivial analogy:
88
89<parm name="Zprime:vd" default="-0.693">
90vector coupling of <ei>d</ei> quarks.
91</parm>
92
93<parm name="Zprime:ad" default="-1.">
94axial coupling of <ei>d</ei> quarks.
95</parm>
96
97<parm name="Zprime:vu" default="0.387">
98vector coupling of <ei>u</ei> quarks.
99</parm>
100
101<parm name="Zprime:au" default="1.">
102axial coupling of <ei>u</ei> quarks.
103</parm>
104
105<parm name="Zprime:ve" default="-0.08">
106vector coupling of <ei>e</ei> leptons.
107</parm>
108
109<parm name="Zprime:ae" default="-1.">
110axial coupling of <ei>e</ei> leptons.
111</parm>
112
113<parm name="Zprime:vnue" default="1.">
114vector coupling of <ei>nu_e</ei> neutrinos.
115</parm>
116
117<parm name="Zprime:anue" default="1.">
118axial coupling of <ei>nu_e</ei> neutrinos.
119</parm>
120
121<p/>
122Here are the further couplings that are specific for
123a scenario with <code>Zprime:universality</code> swiched off:
124
125<parm name="Zprime:vs" default="-0.693">
126vector coupling of <ei>s</ei> quarks.
127</parm>
128
129<parm name="Zprime:as" default="-1.">
130axial coupling of <ei>s</ei> quarks.
131</parm>
132
133<parm name="Zprime:vc" default="0.387">
134vector coupling of <ei>c</ei> quarks.
135</parm>
136
137<parm name="Zprime:ac" default="1.">
138axial coupling of <ei>c</ei> quarks.
139</parm>
140
141<parm name="Zprime:vmu" default="-0.08">
142vector coupling of <ei>mu</ei> leptons.
143</parm>
144
145<parm name="Zprime:amu" default="-1.">
146axial coupling of <ei>mu</ei> leptons.
147</parm>
148
149<parm name="Zprime:vnumu" default="1.">
150vector coupling of <ei>nu_mu</ei> neutrinos.
151</parm>
152
153<parm name="Zprime:anumu" default="1.">
154axial coupling of <ei>nu_mu</ei> neutrinos.
155</parm>
156
157<parm name="Zprime:vb" default="-0.693">
158vector coupling of <ei>b</ei> quarks.
159</parm>
160
161<parm name="Zprime:ab" default="-1.">
162axial coupling of <ei>b</ei> quarks.
163</parm>
164
165<parm name="Zprime:vt" default="0.387">
166vector coupling of <ei>t</ei> quarks.
167</parm>
168
169<parm name="Zprime:at" default="1.">
170axial coupling of <ei>t</ei> quarks.
171</parm>
172
173<parm name="Zprime:vtau" default="-0.08">
174vector coupling of <ei>tau</ei> leptons.
175</parm>
176
177<parm name="Zprime:atau" default="-1.">
178axial coupling of <ei>tau</ei> leptons.
179</parm>
180
181<parm name="Zprime:vnutau" default="1.">
182vector coupling of <ei>nu_tau</ei> neutrinos.
183</parm>
184
185<parm name="Zprime:anutau" default="1.">
186axial coupling of <ei>nu_tau</ei> neutrinos.
187</parm>
188
189<p/>
190The coupling to the decay channel <ei>Z'^0 -> W^+ W^-</ei> is
191more model-dependent. By default it is therefore off, but can be
192switched on as follows. Furthermore, we have left some amount of
193freedom in the choice of decay angular correlations in this
194channel, but obviously alternative shapes could be imagined.
195
196<parm name="Zprime:coup2WW" default="0." min="0.">
197the coupling <ei>Z'^0 -> W^+ W^-</ei> is taken to be this number
198times <ei>m_W^2 / m_Z'^2</ei> times the <ei>Z^0 -> W^+ W^-</ei>
199coupling. Thus a unit value corresponds to the
200<ei>Z^0 -> W^+ W^-</ei> coupling, scaled down by a factor
201<ei>m_W^2 / m_Z'^2</ei>, and gives a <ei>Z'^0</ei> partial
202width into this channel that again increases linearly. If you
203cancel this behaviour, by letting <code>Zprime:coup2WW</code> be
204proportional to <ei>m_Z'^2 / m_W^2</ei>, you instead obtain a
205partial width that goes like the fifth power of the <ei>Z'^0</ei>
206mass. These two extremes correspond to the "extended gauge model"
207and the "reference model", respectively, of <ref>Alt89</ref>.
208Note that this channel only includes the pure <ei>Z'</ei> part,
209while <ei>f fbar -> gamma^*/Z^*0 -> W^+ W^-</ei> is available
210as a separate electroweak process.
211</parm>
212
213<parm name="Zprime:anglesWW" default="0." min="0." max="1.">
214in the decay chain <ei>Z'^0 -> W^+ W^- ->f_1 fbar_2 f_3 fbar_4</ei>
215the decay angular distributions is taken to be a mixture of two
216possible shapes. This parameter gives the fraction that is distributed
217as in Higgs <ei>h^0 -> W^+ W^-</ei> (longitudinal bosons),
218with the remainder (by default all) is taken to be the same as for
219<ei>Z^0 -> W^+ W^-</ei> (a mixture of transverse and longitudinal
220bosons).
221</parm>
222
223<p/>
224A massive <ei>Z'^0</ei> is also likely to decay into Higgses
225and potentially into other now unknown particles. Such possibilities
226clearly are quite model-dependent, and have not been included
227for now.
228
229<h3><ei>W'^+-</ei></h3>
230
231The <ei>W'^+-</ei> implementation is less ambitious than the
232<ei>Z'^0</ei>. Specifically, while indirect detection of a
233<ei>Z'^0</ei> through its interference contribution is
234a possible discovery channel in lepton colliders, there is no
235equally compelling case for <ei>W^+-/W'^+-</ei> interference
236effects being of importance for discovery, and such interference
237has therefore not been implemented for now. Related to this, a
238<ei>Z'^0</ei> could appear on its own in a new <ei>U(1)</ei> group,
239while <ei>W'^+-</ei> would have to sit in a <ei>SU(2)</ei> group
240and thus have a <ei>Z'^0</ei> partner that is likely to be found
241first. Only one process is implemented but, like for the
242<ei>W^+-</ei>, the ISR showers contain automatic matching to the
243<ei>W'^+-</ei> + 1 jet matrix elements.
244
245<flag name="NewGaugeBoson:ffbar2Wprime" default="off">
246Scattering <ei>f fbar' -> W'^+-</ei>.
247Code 3021.
248</flag>
249
250<p/>
251The couplings of the <ei>W'^+-</ei> are here assumed universal,
252i.e. the same for all generations. One may set vector and axial
253couplings freely, separately for the <ei>q qbar'</ei> and the
254<ei>l nu_l</ei> decay channels. The defaults correspond to the
255<ei>V - A</ei> structure and normalization of the Standard Model
256<ei>W^+-</ei>, but can be changed to simulate a wide selection
257of models. One limitation is that, for simplicity, the same
258Cabibbo--Kobayashi--Maskawa quark mixing matrix is assumed as for
259the standard <ei>W^+-</ei>. Depending on your assumed neutrino
260nature you may want to restrict your freedom in the lepton sector,
261but no limitations are enforced by the program.
262
263<parm name="Wprime:vq" default="1.">
264vector coupling of quarks.
265</parm>
266
267<parm name="Wprime:aq" default="-1.">
268axial coupling of quarks.
269</parm>
270
271<parm name="Wprime:vl" default="1.">
272vector coupling of leptons.
273</parm>
274
275<parm name="Wprime:al" default="-1.">
276axial coupling of leptons.
277</parm>
278
279<p/>
280The coupling to the decay channel <ei>W'^+- -> W^+- Z^0</ei> is
281more model-dependent, like for <ei>Z'^0 -> W^+ W^-</ei> described
282above. By default it is therefore off, but can be
283switched on as follows. Furthermore, we have left some amount of
284freedom in the choice of decay angular correlations in this
285channel, but obviously alternative shapes could be imagined.
286
287<parm name="Wprime:coup2WZ" default="0." min="0.">
288the coupling <ei>W'^0 -> W^+- Z^0</ei> is taken to be this number
289times <ei>m_W^2 / m_W'^2</ei> times the <ei>W^+- -> W^+- Z^0</ei>
290coupling. Thus a unit value corresponds to the
291<ei>W^+- -> W^+- Z^0</ei> coupling, scaled down by a factor
292<ei>m_W^2 / m_W'^2</ei>, and gives a <ei>W'^+-</ei> partial
293width into this channel that increases linearly with the
294<ei>W'^+-</ei> mass. If you cancel this behaviour, by letting
295<code>Wprime:coup2WZ</code> be proportional to <ei>m_W'^2 / m_W^2</ei>,
296you instead obtain a partial width that goes like the fifth power
297of the <ei>W'^+-</ei> mass. These two extremes correspond to the
298"extended gauge model" and the "reference model", respectively,
299of <ref>Alt89</ref>.
300</parm>
301
302<parm name="Wprime:anglesWZ" default="0." min="0." max="1.">
303in the decay chain <ei>W'^+- -> W^+- Z^0 ->f_1 fbar_2 f_3 fbar_4</ei>
304the decay angular distributions is taken to be a mixture of two
305possible shapes. This parameter gives the fraction that is distributed
306as in Higgs <ei>H^+- -> W^+- Z^0</ei> (longitudinal bosons),
307with the remainder (by default all) is taken to be the same as for
308<ei>W^+- -> W^+- Z^0</ei> (a mixture of transverse and longitudinal
309bosons).
310</parm>
311
312<p/>
313A massive <ei>W'^+-</ei> is also likely to decay into Higgses
314and potentially into other now unknown particles. Such possibilities
315clearly are quite model-dependent, and have not been included
316for now.
317
318<h3><ei>R^0</ei></h3>
319
320The <ei>R^0</ei> boson (particle code 41) represents one possible
321scenario for a horizontal gauge boson, i.e. a gauge boson
322that couples between the generations, inducing processes like
323<ei>s dbar -> R^0 -> mu^- e^+</ei>. Experimental limits on
324flavour-changing neutral currents forces such a boson to be fairly
325heavy. In spite of being neutral the antiparticle is distinct from
326the particle: one carries a net positive generation number and
327the other a negative one. This particular model has no new
328parameters beyond the <ei>R^0</ei> mass. Decays are assumed isotropic.
329For further details see <ref>Ben85</ref>.
330
331<flag name="NewGaugeBoson:ffbar2R0" default="off">
332Scattering <ei>f_1 fbar_2 -> R^0 -> f_3 fbar_4</ei>, where
333<ei>f_1</ei> and <ei>fbar_2</ei> are separated by <ei>+-</ei> one
334generation and similarly for <ei>f_3</ei> and <ei>fbar_4</ei>.
335Thus possible final states are e.g. <ei>d sbar</ei>, <ei>u cbar</ei>
336<ei>s bbar</ei>, <ei>c tbar</ei>, <ei>e- mu+</ei> and
337<ei>mu- tau+</ei>.
338Code 3041.
339</flag>
340
341</chapter>
342
343<!-- Copyright (C) 2008 Torbjorn Sjostrand -->
344