3 <title>New-Gauge-Boson Processes</title>
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9 <h2>New-Gauge-Boson Processes</h2>
11 This page contains the production of new <i>Z'^0</i> and
12 <i>W'^+-</i> gauge bosons, e.g. within the context of a new
13 <i>U(1)</i> or <i>SU(2)</i> gauge group, and also a
14 (rather speculative) horizontal gauge boson <i>R^0</i>.
15 Left-right-symmetry scenarios also contain new gauge bosons,
17 <a href="LeftRightSymmetryProcesses.html" target="page">separately</a>.
21 This group only contains one subprocess, with the full
22 <i>gamma^*/Z^0/Z'^0</i> interference structure for couplings
23 to fermion pairs. It is possible to pick only a subset, e.g, only
24 the pure <i>Z'^0</i> piece. No higher-order processes are
25 available explicitly, but the ISR showers contain automatic
26 matching to the <i>Z'^0</i> + 1 jet matrix elements, as for
27 the corresponding <i>gamma^*/Z^0</i> process.
29 <p/><code>flag </code><strong> NewGaugeBoson:ffbar2gmZZprime </strong>
30 (<code>default = <strong>off</strong></code>)<br/>
31 Scattering <i>f fbar ->Z'^0</i>.
35 <p/><code>mode </code><strong> Zprime:gmZmode </strong>
36 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>; <code>maximum = 6</code>)<br/>
37 Choice of full <i>gamma^*/Z^0/Z'^0</i> structure or not in
38 the above process. Note that, with the <i>Z'^0</i> part switched
39 off, this process is reduced to what already exists among
40 <a href="ElectroweakProcesses.html" target="page">electroweak processes</a>,
41 so those options are here only for crosschecks.
42 <br/><code>option </code><strong> 0</strong> : full <i>gamma^*/Z^0/Z'^0</i> structure,
43 with interference included.
44 <br/><code>option </code><strong> 1</strong> : only pure <i>gamma^*</i> contribution.
45 <br/><code>option </code><strong> 2</strong> : only pure <i>Z^0</i> contribution.
46 <br/><code>option </code><strong> 3</strong> : only pure <i>Z'^0</i> contribution.
47 <br/><code>option </code><strong> 4</strong> : only the <i>gamma^*/Z^0</i> contribution,
48 including interference.
49 <br/><code>option </code><strong> 5</strong> : only the <i>gamma^*/Z'^0</i> contribution,
50 including interference.
51 <br/><code>option </code><strong> 6</strong> : only the <i>Z^0/Z'^0</i> contribution,
52 including interference.
53 <br/><b>Note</b>: irrespective of the option used, the particle produced
54 will always be assigned code 32 for <i>Z'^0</i>, and open decay channels
55 is purely dictated by what is set for the <i>Z'^0</i>.
59 The couplings of the <i>Z'^0</i> to quarks and leptons can
60 either be assumed universal, i.e. generation-independent, or not.
61 In the former case eight numbers parametrize the vector and axial
62 couplings of down-type quarks, up-type quarks, leptons and neutrinos,
63 respectively. Depending on your assumed neutrino nature you may
64 want to restrict your freedom in that sector, but no limitations
65 are enforced by the program. The default corresponds to the same
66 couplings as that of the Standard Model <i>Z^0</i>, with axial
67 couplings <i>a_f = +-1</i> and vector couplings
68 <i>v_f = a_f - 4 e_f sin^2(theta_W)</i>, with
69 <i>sin^2(theta_W) = 0.23</i>. Without universality
70 the same eight numbers have to be set separately also for the
71 second and the third generation. The choice of fixed axial and
72 vector couplings implies a resonance width that increases linearly
73 with the <i>Z'^0</i> mass.
76 By a suitable choice of the parameters, it is possible to simulate
77 just about any imaginable <i>Z'^0</i> scenario, with full
78 interference effects in cross sections and decay angular
79 distributions and generation-dependent couplings; the default values
80 should mainly be viewed as placeholders. The conversion
81 from the coupling conventions in a set of different <i>Z'^0</i>
82 models in the literature to those used in PYTHIA is described by
83 <a href="http://www.hep.uiuc.edu/home/catutza/nota12.ps">C.
86 <p/><code>flag </code><strong> Zprime:universality </strong>
87 (<code>default = <strong>on</strong></code>)<br/>
88 If on then you need only set the first-generation couplings
89 below, and these are automatically also used for the second and
90 third generation. If off, then couplings can be chosen separately
95 Here are the couplings always valid for the first generation,
96 and normally also for the second and third by trivial analogy:
98 <p/><code>parm </code><strong> Zprime:vd </strong>
99 (<code>default = <strong>-0.693</strong></code>)<br/>
100 vector coupling of <i>d</i> quarks.
103 <p/><code>parm </code><strong> Zprime:ad </strong>
104 (<code>default = <strong>-1.</strong></code>)<br/>
105 axial coupling of <i>d</i> quarks.
108 <p/><code>parm </code><strong> Zprime:vu </strong>
109 (<code>default = <strong>0.387</strong></code>)<br/>
110 vector coupling of <i>u</i> quarks.
113 <p/><code>parm </code><strong> Zprime:au </strong>
114 (<code>default = <strong>1.</strong></code>)<br/>
115 axial coupling of <i>u</i> quarks.
118 <p/><code>parm </code><strong> Zprime:ve </strong>
119 (<code>default = <strong>-0.08</strong></code>)<br/>
120 vector coupling of <i>e</i> leptons.
123 <p/><code>parm </code><strong> Zprime:ae </strong>
124 (<code>default = <strong>-1.</strong></code>)<br/>
125 axial coupling of <i>e</i> leptons.
128 <p/><code>parm </code><strong> Zprime:vnue </strong>
129 (<code>default = <strong>1.</strong></code>)<br/>
130 vector coupling of <i>nu_e</i> neutrinos.
133 <p/><code>parm </code><strong> Zprime:anue </strong>
134 (<code>default = <strong>1.</strong></code>)<br/>
135 axial coupling of <i>nu_e</i> neutrinos.
139 Here are the further couplings that are specific for
140 a scenario with <code>Zprime:universality</code> swiched off:
142 <p/><code>parm </code><strong> Zprime:vs </strong>
143 (<code>default = <strong>-0.693</strong></code>)<br/>
144 vector coupling of <i>s</i> quarks.
147 <p/><code>parm </code><strong> Zprime:as </strong>
148 (<code>default = <strong>-1.</strong></code>)<br/>
149 axial coupling of <i>s</i> quarks.
152 <p/><code>parm </code><strong> Zprime:vc </strong>
153 (<code>default = <strong>0.387</strong></code>)<br/>
154 vector coupling of <i>c</i> quarks.
157 <p/><code>parm </code><strong> Zprime:ac </strong>
158 (<code>default = <strong>1.</strong></code>)<br/>
159 axial coupling of <i>c</i> quarks.
162 <p/><code>parm </code><strong> Zprime:vmu </strong>
163 (<code>default = <strong>-0.08</strong></code>)<br/>
164 vector coupling of <i>mu</i> leptons.
167 <p/><code>parm </code><strong> Zprime:amu </strong>
168 (<code>default = <strong>-1.</strong></code>)<br/>
169 axial coupling of <i>mu</i> leptons.
172 <p/><code>parm </code><strong> Zprime:vnumu </strong>
173 (<code>default = <strong>1.</strong></code>)<br/>
174 vector coupling of <i>nu_mu</i> neutrinos.
177 <p/><code>parm </code><strong> Zprime:anumu </strong>
178 (<code>default = <strong>1.</strong></code>)<br/>
179 axial coupling of <i>nu_mu</i> neutrinos.
182 <p/><code>parm </code><strong> Zprime:vb </strong>
183 (<code>default = <strong>-0.693</strong></code>)<br/>
184 vector coupling of <i>b</i> quarks.
187 <p/><code>parm </code><strong> Zprime:ab </strong>
188 (<code>default = <strong>-1.</strong></code>)<br/>
189 axial coupling of <i>b</i> quarks.
192 <p/><code>parm </code><strong> Zprime:vt </strong>
193 (<code>default = <strong>0.387</strong></code>)<br/>
194 vector coupling of <i>t</i> quarks.
197 <p/><code>parm </code><strong> Zprime:at </strong>
198 (<code>default = <strong>1.</strong></code>)<br/>
199 axial coupling of <i>t</i> quarks.
202 <p/><code>parm </code><strong> Zprime:vtau </strong>
203 (<code>default = <strong>-0.08</strong></code>)<br/>
204 vector coupling of <i>tau</i> leptons.
207 <p/><code>parm </code><strong> Zprime:atau </strong>
208 (<code>default = <strong>-1.</strong></code>)<br/>
209 axial coupling of <i>tau</i> leptons.
212 <p/><code>parm </code><strong> Zprime:vnutau </strong>
213 (<code>default = <strong>1.</strong></code>)<br/>
214 vector coupling of <i>nu_tau</i> neutrinos.
217 <p/><code>parm </code><strong> Zprime:anutau </strong>
218 (<code>default = <strong>1.</strong></code>)<br/>
219 axial coupling of <i>nu_tau</i> neutrinos.
223 The coupling to the decay channel <i>Z'^0 -> W^+ W^-</i> is
224 more model-dependent. By default it is therefore off, but can be
225 switched on as follows. Furthermore, we have left some amount of
226 freedom in the choice of decay angular correlations in this
227 channel, but obviously alternative shapes could be imagined.
229 <p/><code>parm </code><strong> Zprime:coup2WW </strong>
230 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
231 the coupling <i>Z'^0 -> W^+ W^-</i> is taken to be this number
232 times <i>m_W^2 / m_Z'^2</i> times the <i>Z^0 -> W^+ W^-</i>
233 coupling. Thus a unit value corresponds to the
234 <i>Z^0 -> W^+ W^-</i> coupling, scaled down by a factor
235 <i>m_W^2 / m_Z'^2</i>, and gives a <i>Z'^0</i> partial
236 width into this channel that again increases linearly. If you
237 cancel this behaviour, by letting <code>Zprime:coup2WW</code> be
238 proportional to <i>m_Z'^2 / m_W^2</i>, you instead obtain a
239 partial width that goes like the fifth power of the <i>Z'^0</i>
240 mass. These two extremes correspond to the "extended gauge model"
241 and the "reference model", respectively, of [<a href="Bibliography.html" target="page">Alt89</a>].
242 Note that this channel only includes the pure <i>Z'</i> part,
243 while <i>f fbar -> gamma^*/Z^*0 -> W^+ W^-</i> is available
244 as a separate electroweak process.
247 <p/><code>parm </code><strong> Zprime:anglesWW </strong>
248 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
249 in the decay chain <i>Z'^0 -> W^+ W^- ->f_1 fbar_2 f_3 fbar_4</i>
250 the decay angular distributions is taken to be a mixture of two
251 possible shapes. This parameter gives the fraction that is distributed
252 as in Higgs <i>h^0 -> W^+ W^-</i> (longitudinal bosons),
253 with the remainder (by default all) is taken to be the same as for
254 <i>Z^0 -> W^+ W^-</i> (a mixture of transverse and longitudinal
259 A massive <i>Z'^0</i> is also likely to decay into Higgses
260 and potentially into other now unknown particles. Such possibilities
261 clearly are quite model-dependent, and have not been included
264 <h3><i>W'^+-</i></h3>
266 The <i>W'^+-</i> implementation is less ambitious than the
267 <i>Z'^0</i>. Specifically, while indirect detection of a
268 <i>Z'^0</i> through its interference contribution is
269 a possible discovery channel in lepton colliders, there is no
270 equally compelling case for <i>W^+-/W'^+-</i> interference
271 effects being of importance for discovery, and such interference
272 has therefore not been implemented for now. Related to this, a
273 <i>Z'^0</i> could appear on its own in a new <i>U(1)</i> group,
274 while <i>W'^+-</i> would have to sit in a <i>SU(2)</i> group
275 and thus have a <i>Z'^0</i> partner that is likely to be found
276 first. Only one process is implemented but, like for the
277 <i>W^+-</i>, the ISR showers contain automatic matching to the
278 <i>W'^+-</i> + 1 jet matrix elements.
280 <p/><code>flag </code><strong> NewGaugeBoson:ffbar2Wprime </strong>
281 (<code>default = <strong>off</strong></code>)<br/>
282 Scattering <i>f fbar' -> W'^+-</i>.
287 The couplings of the <i>W'^+-</i> are here assumed universal,
288 i.e. the same for all generations. One may set vector and axial
289 couplings freely, separately for the <i>q qbar'</i> and the
290 <i>l nu_l</i> decay channels. The defaults correspond to the
291 <i>V - A</i> structure and normalization of the Standard Model
292 <i>W^+-</i>, but can be changed to simulate a wide selection
293 of models. One limitation is that, for simplicity, the same
294 Cabibbo--Kobayashi--Maskawa quark mixing matrix is assumed as for
295 the standard <i>W^+-</i>. Depending on your assumed neutrino
296 nature you may want to restrict your freedom in the lepton sector,
297 but no limitations are enforced by the program.
299 <p/><code>parm </code><strong> Wprime:vq </strong>
300 (<code>default = <strong>1.</strong></code>)<br/>
301 vector coupling of quarks.
304 <p/><code>parm </code><strong> Wprime:aq </strong>
305 (<code>default = <strong>-1.</strong></code>)<br/>
306 axial coupling of quarks.
309 <p/><code>parm </code><strong> Wprime:vl </strong>
310 (<code>default = <strong>1.</strong></code>)<br/>
311 vector coupling of leptons.
314 <p/><code>parm </code><strong> Wprime:al </strong>
315 (<code>default = <strong>-1.</strong></code>)<br/>
316 axial coupling of leptons.
320 The coupling to the decay channel <i>W'^+- -> W^+- Z^0</i> is
321 more model-dependent, like for <i>Z'^0 -> W^+ W^-</i> described
322 above. By default it is therefore off, but can be
323 switched on as follows. Furthermore, we have left some amount of
324 freedom in the choice of decay angular correlations in this
325 channel, but obviously alternative shapes could be imagined.
327 <p/><code>parm </code><strong> Wprime:coup2WZ </strong>
328 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>)<br/>
329 the coupling <i>W'^0 -> W^+- Z^0</i> is taken to be this number
330 times <i>m_W^2 / m_W'^2</i> times the <i>W^+- -> W^+- Z^0</i>
331 coupling. Thus a unit value corresponds to the
332 <i>W^+- -> W^+- Z^0</i> coupling, scaled down by a factor
333 <i>m_W^2 / m_W'^2</i>, and gives a <i>W'^+-</i> partial
334 width into this channel that increases linearly with the
335 <i>W'^+-</i> mass. If you cancel this behaviour, by letting
336 <code>Wprime:coup2WZ</code> be proportional to <i>m_W'^2 / m_W^2</i>,
337 you instead obtain a partial width that goes like the fifth power
338 of the <i>W'^+-</i> mass. These two extremes correspond to the
339 "extended gauge model" and the "reference model", respectively,
340 of [<a href="Bibliography.html" target="page">Alt89</a>].
343 <p/><code>parm </code><strong> Wprime:anglesWZ </strong>
344 (<code>default = <strong>0.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
345 in the decay chain <i>W'^+- -> W^+- Z^0 ->f_1 fbar_2 f_3 fbar_4</i>
346 the decay angular distributions is taken to be a mixture of two
347 possible shapes. This parameter gives the fraction that is distributed
348 as in Higgs <i>H^+- -> W^+- Z^0</i> (longitudinal bosons),
349 with the remainder (by default all) is taken to be the same as for
350 <i>W^+- -> W^+- Z^0</i> (a mixture of transverse and longitudinal
355 A massive <i>W'^+-</i> is also likely to decay into Higgses
356 and potentially into other now unknown particles. Such possibilities
357 clearly are quite model-dependent, and have not been included
362 The <i>R^0</i> boson (particle code 41) represents one possible
363 scenario for a horizontal gauge boson, i.e. a gauge boson
364 that couples between the generations, inducing processes like
365 <i>s dbar -> R^0 -> mu^- e^+</i>. Experimental limits on
366 flavour-changing neutral currents forces such a boson to be fairly
367 heavy. In spite of being neutral the antiparticle is distinct from
368 the particle: one carries a net positive generation number and
369 the other a negative one. This particular model has no new
370 parameters beyond the <i>R^0</i> mass. Decays are assumed isotropic.
371 For further details see [<a href="Bibliography.html" target="page">Ben85</a>].
373 <p/><code>flag </code><strong> NewGaugeBoson:ffbar2R0 </strong>
374 (<code>default = <strong>off</strong></code>)<br/>
375 Scattering <i>f_1 fbar_2 -> R^0 -> f_3 fbar_4</i>, where
376 <i>f_1</i> and <i>fbar_2</i> are separated by <i>+-</i> one
377 generation and similarly for <i>f_3</i> and <i>fbar_4</i>.
378 Thus possible final states are e.g. <i>d sbar</i>, <i>u cbar</i>
379 <i>s bbar</i>, <i>c tbar</i>, <i>e- mu+</i> and
387 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->