1 <chapter name="New-Gauge-Boson Processes">
3 <h2>New-Gauge-Boson Processes</h2>
5 This 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>.
9 Left-right-symmetry scenarios also contain new gauge bosons,
11 <aloc href"LeftRightSymmetryProcesses">separately</aloc>.
13 <h3><ei>Z'^0</ei></h3>
15 This group only contains one subprocess, with the full
16 <ei>gamma^*/Z^0/Z'^0</ei> interference structure for couplings
17 to fermion pairs. It is possible to pick only a subset, e.g, only
18 the pure <ei>Z'^0</ei> piece. No higher-order processes are
19 available explicitly, but the ISR showers contain automatic
20 matching to the <ei>Z'^0</ei> + 1 jet matrix elements, as for
21 the corresponding <ei>gamma^*/Z^0</ei> process.
23 <flag name="NewGaugeBoson:ffbar2gmZZprime" default="off">
24 Scattering <ei>f fbar ->Z'^0</ei>.
28 <modepick name="Zprime:gmZmode" default="0" min="0" max="6">
29 Choice of full <ei>gamma^*/Z^0/Z'^0</ei> structure or not in
30 the above process. Note that, with the <ei>Z'^0</ei> part switched
31 off, this process is reduced to what already exists among
32 <aloc href="ElectroweakProcesses">electroweak processes</aloc>,
33 so those options are here only for crosschecks.
34 <option value="0">full <ei>gamma^*/Z^0/Z'^0</ei> structure,
35 with 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,
40 including interference.</option>
41 <option value="5">only the <ei>gamma^*/Z'^0</ei> contribution,
42 including interference.</option>
43 <option value="6">only the <ei>Z^0/Z'^0</ei> contribution,
44 including interference.</option>
45 <note>Note</note>: irrespective of the option used, the particle produced
46 will always be assigned code 32 for <ei>Z'^0</ei>, and open decay channels
47 is purely dictated by what is set for the <ei>Z'^0</ei>.
51 The couplings of the <ei>Z'^0</ei> to quarks and leptons can
52 either be assumed universal, i.e. generation-independent, or not.
53 In the former case eight numbers parametrize the vector and axial
54 couplings of down-type quarks, up-type quarks, leptons and neutrinos,
55 respectively. Depending on your assumed neutrino nature you may
56 want to restrict your freedom in that sector, but no limitations
57 are enforced by the program. The default corresponds to the same
58 couplings as that of the Standard Model <ei>Z^0</ei>, with axial
59 couplings <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
62 the same eight numbers have to be set separately also for the
63 second and the third generation. The choice of fixed axial and
64 vector couplings implies a resonance width that increases linearly
65 with the <ei>Z'^0</ei> mass.
68 By a suitable choice of the parameters, it is possible to simulate
69 just about any imaginable <ei>Z'^0</ei> scenario, with full
70 interference effects in cross sections and decay angular
71 distributions and generation-dependent couplings; the default values
72 should mainly be viewed as placeholders. The conversion
73 from the coupling conventions in a set of different <ei>Z'^0</ei>
74 models in the literature to those used in PYTHIA is described by
75 <a href="http://www.hep.uiuc.edu/home/catutza/nota12.ps">C.
78 <flag name="Zprime:universality" default="on">
79 If on then you need only set the first-generation couplings
80 below, and these are automatically also used for the second and
81 third generation. If off, then couplings can be chosen separately
86 Here are the couplings always valid for the first generation,
87 and normally also for the second and third by trivial analogy:
89 <parm name="Zprime:vd" default="-0.693">
90 vector coupling of <ei>d</ei> quarks.
93 <parm name="Zprime:ad" default="-1.">
94 axial coupling of <ei>d</ei> quarks.
97 <parm name="Zprime:vu" default="0.387">
98 vector coupling of <ei>u</ei> quarks.
101 <parm name="Zprime:au" default="1.">
102 axial coupling of <ei>u</ei> quarks.
105 <parm name="Zprime:ve" default="-0.08">
106 vector coupling of <ei>e</ei> leptons.
109 <parm name="Zprime:ae" default="-1.">
110 axial coupling of <ei>e</ei> leptons.
113 <parm name="Zprime:vnue" default="1.">
114 vector coupling of <ei>nu_e</ei> neutrinos.
117 <parm name="Zprime:anue" default="1.">
118 axial coupling of <ei>nu_e</ei> neutrinos.
122 Here are the further couplings that are specific for
123 a scenario with <code>Zprime:universality</code> swiched off:
125 <parm name="Zprime:vs" default="-0.693">
126 vector coupling of <ei>s</ei> quarks.
129 <parm name="Zprime:as" default="-1.">
130 axial coupling of <ei>s</ei> quarks.
133 <parm name="Zprime:vc" default="0.387">
134 vector coupling of <ei>c</ei> quarks.
137 <parm name="Zprime:ac" default="1.">
138 axial coupling of <ei>c</ei> quarks.
141 <parm name="Zprime:vmu" default="-0.08">
142 vector coupling of <ei>mu</ei> leptons.
145 <parm name="Zprime:amu" default="-1.">
146 axial coupling of <ei>mu</ei> leptons.
149 <parm name="Zprime:vnumu" default="1.">
150 vector coupling of <ei>nu_mu</ei> neutrinos.
153 <parm name="Zprime:anumu" default="1.">
154 axial coupling of <ei>nu_mu</ei> neutrinos.
157 <parm name="Zprime:vb" default="-0.693">
158 vector coupling of <ei>b</ei> quarks.
161 <parm name="Zprime:ab" default="-1.">
162 axial coupling of <ei>b</ei> quarks.
165 <parm name="Zprime:vt" default="0.387">
166 vector coupling of <ei>t</ei> quarks.
169 <parm name="Zprime:at" default="1.">
170 axial coupling of <ei>t</ei> quarks.
173 <parm name="Zprime:vtau" default="-0.08">
174 vector coupling of <ei>tau</ei> leptons.
177 <parm name="Zprime:atau" default="-1.">
178 axial coupling of <ei>tau</ei> leptons.
181 <parm name="Zprime:vnutau" default="1.">
182 vector coupling of <ei>nu_tau</ei> neutrinos.
185 <parm name="Zprime:anutau" default="1.">
186 axial coupling of <ei>nu_tau</ei> neutrinos.
190 The coupling to the decay channel <ei>Z'^0 -> W^+ W^-</ei> is
191 more model-dependent. By default it is therefore off, but can be
192 switched on as follows. Furthermore, we have left some amount of
193 freedom in the choice of decay angular correlations in this
194 channel, but obviously alternative shapes could be imagined.
196 <parm name="Zprime:coup2WW" default="0." min="0.">
197 the coupling <ei>Z'^0 -> W^+ W^-</ei> is taken to be this number
198 times <ei>m_W^2 / m_Z'^2</ei> times the <ei>Z^0 -> W^+ W^-</ei>
199 coupling. 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
202 width into this channel that again increases linearly. If you
203 cancel this behaviour, by letting <code>Zprime:coup2WW</code> be
204 proportional to <ei>m_Z'^2 / m_W^2</ei>, you instead obtain a
205 partial width that goes like the fifth power of the <ei>Z'^0</ei>
206 mass. These two extremes correspond to the "extended gauge model"
207 and the "reference model", respectively, of <ref>Alt89</ref>.
208 Note that this channel only includes the pure <ei>Z'</ei> part,
209 while <ei>f fbar -> gamma^*/Z^*0 -> W^+ W^-</ei> is available
210 as a separate electroweak process.
213 <parm name="Zprime:anglesWW" default="0." min="0." max="1.">
214 in the decay chain <ei>Z'^0 -> W^+ W^- ->f_1 fbar_2 f_3 fbar_4</ei>
215 the decay angular distributions is taken to be a mixture of two
216 possible shapes. This parameter gives the fraction that is distributed
217 as in Higgs <ei>h^0 -> W^+ W^-</ei> (longitudinal bosons),
218 with 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
224 A massive <ei>Z'^0</ei> is also likely to decay into Higgses
225 and potentially into other now unknown particles. Such possibilities
226 clearly are quite model-dependent, and have not been included
229 <h3><ei>W'^+-</ei></h3>
231 The <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
234 a possible discovery channel in lepton colliders, there is no
235 equally compelling case for <ei>W^+-/W'^+-</ei> interference
236 effects being of importance for discovery, and such interference
237 has 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,
239 while <ei>W'^+-</ei> would have to sit in a <ei>SU(2)</ei> group
240 and thus have a <ei>Z'^0</ei> partner that is likely to be found
241 first. 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.
245 <flag name="NewGaugeBoson:ffbar2Wprime" default="off">
246 Scattering <ei>f fbar' -> W'^+-</ei>.
251 The couplings of the <ei>W'^+-</ei> are here assumed universal,
252 i.e. the same for all generations. One may set vector and axial
253 couplings 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
257 of models. One limitation is that, for simplicity, the same
258 Cabibbo--Kobayashi--Maskawa quark mixing matrix is assumed as for
259 the standard <ei>W^+-</ei>. Depending on your assumed neutrino
260 nature you may want to restrict your freedom in the lepton sector,
261 but no limitations are enforced by the program.
263 <parm name="Wprime:vq" default="1.">
264 vector coupling of quarks.
267 <parm name="Wprime:aq" default="-1.">
268 axial coupling of quarks.
271 <parm name="Wprime:vl" default="1.">
272 vector coupling of leptons.
275 <parm name="Wprime:al" default="-1.">
276 axial coupling of leptons.
280 The coupling to the decay channel <ei>W'^+- -> W^+- Z^0</ei> is
281 more model-dependent, like for <ei>Z'^0 -> W^+ W^-</ei> described
282 above. By default it is therefore off, but can be
283 switched on as follows. Furthermore, we have left some amount of
284 freedom in the choice of decay angular correlations in this
285 channel, but obviously alternative shapes could be imagined.
287 <parm name="Wprime:coup2WZ" default="0." min="0.">
288 the coupling <ei>W'^0 -> W^+- Z^0</ei> is taken to be this number
289 times <ei>m_W^2 / m_W'^2</ei> times the <ei>W^+- -> W^+- Z^0</ei>
290 coupling. 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
293 width 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>,
296 you instead obtain a partial width that goes like the fifth power
297 of the <ei>W'^+-</ei> mass. These two extremes correspond to the
298 "extended gauge model" and the "reference model", respectively,
302 <parm name="Wprime:anglesWZ" default="0." min="0." max="1.">
303 in the decay chain <ei>W'^+- -> W^+- Z^0 ->f_1 fbar_2 f_3 fbar_4</ei>
304 the decay angular distributions is taken to be a mixture of two
305 possible shapes. This parameter gives the fraction that is distributed
306 as in Higgs <ei>H^+- -> W^+- Z^0</ei> (longitudinal bosons),
307 with 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
313 A massive <ei>W'^+-</ei> is also likely to decay into Higgses
314 and potentially into other now unknown particles. Such possibilities
315 clearly are quite model-dependent, and have not been included
318 <h3><ei>R^0</ei></h3>
320 The <ei>R^0</ei> boson (particle code 41) represents one possible
321 scenario for a horizontal gauge boson, i.e. a gauge boson
322 that couples between the generations, inducing processes like
323 <ei>s dbar -> R^0 -> mu^- e^+</ei>. Experimental limits on
324 flavour-changing neutral currents forces such a boson to be fairly
325 heavy. In spite of being neutral the antiparticle is distinct from
326 the particle: one carries a net positive generation number and
327 the other a negative one. This particular model has no new
328 parameters beyond the <ei>R^0</ei> mass. Decays are assumed isotropic.
329 For further details see <ref>Ben85</ref>.
331 <flag name="NewGaugeBoson:ffbar2R0" default="off">
332 Scattering <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
334 generation and similarly for <ei>f_3</ei> and <ei>fbar_4</ei>.
335 Thus 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
343 <!-- Copyright (C) 2008 Torbjorn Sjostrand -->