1 <chapter name="SUSY Processes">
5 Here is collected processes involving supersymmetric particle
6 production, with the exception of the (extended) Higgs sector.
7 Since the number of separate but closely related processes is so big,
8 there will not be switches for each separate process but only for a
9 reasonable set of subgroups. However, the general
10 switches <code>SUSY:idA</code> and <code>SUSY:idB</code> may be used in
11 conjunction with any of these groups to provide some additional
12 flexibility to concentrate on processes involving only specific (s)particle
13 final states, see below.
17 Most of the SUSY implementation in PYTHIA 8 has been written by
18 N. Desai, in combination with an MCnet studentship at CERN in
19 2010, with further work having been carried out during 2011.
20 The implementation is documented in <ref>Des11</ref>. Please give due
21 credit to external contributions to PYTHIA 8, such as this one, by
22 including the original work in your list of references when using this
26 <p>Since the implementation of SUSY processes was only recently
27 completed <ref>Des11</ref>, case-by-case validations against other codes
28 are still recommended. In all cases, a set of default validations have
29 already been carried out by the authors. These validations compared to
30 the PYTHIA 6 SUSY implementation, using an sps1a spectrum. Most of the
31 SLHA2-specific extensions have not been explicitly validated,
32 however, with the exception of the R-parity violating single-sparticle
33 production cross sections.
36 <note>Important Note on SLHA:</note>
37 In order to simulate SUSY processes it is required to read in the
38 couplings and masses relevant for the scenario to be studied. This
39 is done with the help of the SUSY Les Houches Accord (SLHA), including
40 the SLHA2 extensions and generalizations. (Internally, the SLHA2
41 conventions are used. SLHA1 spectra are automatically translated into
42 SLHA2 notation during initialization.) The
43 reading of a relevant SLHA file <b>must</b> be set up, as described
44 on <aloc href="SUSYLesHouchesAccord">the SLHA page</aloc>.
45 Attempting to generate SUSY processes without a properly initialized
46 SLHA spectrum is strongly discouraged and may lead to unexpected
47 results. Always check for warnings and errors reported by the SLHA
48 reader during the initialization stage.
50 <h3>SUSY Processes</h3>
52 <note>Note 1:</note> Decays of SUSY particles are described
53 separately <a href="#decays">below</a>.
55 <note>Note 2:</note> One special possibility is that the gluino or
56 some squark(s) are sufficiently long-lived to hadronize. See
57 <aloc href="RHadrons">the R-hadrons page</aloc> for further details.
59 <note>Note 3:</note> lepton- and photon-initial states are not yet available.
60 Only quark/gluon-initiated <ei>2 -> 2</ei> and <ei>2 -> 1</ei> (RPV)
61 processes have been implemented. Likewise, direct slepton production
62 has not yet been implemented (i.e., <ei>2 -> 2</ei> processes
63 involving sleptons in the final state). Sleptons will of course still be
64 produced through cascade decays of heavier (s)particles.
66 <note>Note 4:</note> cross sections will be correctly folded with open
67 branching fractions of cascade decays, but at present any difference between
68 particle and antiparticle decay tables is not taken into account. This
69 possibility will be included in a future update.
71 <flag name="SUSY:all" default="off">
72 Common switch for production of supersymmetric particles, i.e.
73 particles with R-parity -1.
76 <modeopen name="SUSY:idA" default="0" min="0">
77 Option to limit the sum over possible outgoing states in SUSY
78 <ei>2 -> 2</ei> processes to ones including a specific particle
79 identity code. The default corresponds to summing over all possible
80 indices. A non-zero value of <code>SUSY:idA</code> selects only processes
81 that contain the state corresponding to that particular particle identity
82 code in the fundamental <ei>2 -> 2</ei> scattering process (symmetrized
83 over particle/antiparticle). It is the user's responsibility to ensure
84 that (a subset of) the processes be to simulated actually include this
85 particle at the <ei>2 -> 2</ei> level; thus, asking for the lightest
86 neutralino (code 1000021) to be present in a squark-squark production
87 process will give no match.
90 <modeopen name="SUSY:idB" default="0" min="0">
91 As for <code>SUSY:idA</code>, but requires an additional particle
92 with PDG code <code>SUSY:idB</code> to be present in the <ei>2 -> 2</ei>
93 process. Thus, using <code>SUSY:idA</code> and <code>SUSY:idB</code>
94 a specific subprocess can be selected. Again only the absolute sign is
95 used, i.e. the summation over particle and antiparticle is retained.
98 <modepick name="SUSY:sin2thetaWMode" default="2" min="1" max="3">
99 The value of <ei>sin2(thetaW)</ei> should be taken from
100 <option value="1">SM value, defined at <ei>M_Z</ei>, taken from
101 PYTHIA's <code>StandardModel:sin2thetaW</code> parameter.</option>
102 <option value="2">SUSY value, defined at <ei>M_SUSY</ei>, derived from the
103 running gauge couplings in <code>BLOCK GAUGE</code> in the SLHA
104 file. Note: if no such block is present in the input file, this option
105 will default back to option 1 above, i.e., the SM value.</option>
106 <option value="3">Pole value, defined by <ei>1 - M_W^2/M_Z^2</ei>,
107 using the pole masses stored in the SLHA <code>BLOCK MASS</code>, or,
108 alternatively, PYTHIA's internal pole masses if no such block is
112 <h4>Gluino Pair Production</h4>
114 <flag name="SUSY:gg2gluinogluino" default="off">
115 Pair production of gluinos by gluon-gluon initial states.
118 <flag name="SUSY:qqbar2gluinogluino" default="off">
119 Pair production of gluinos by quark-antiquark annihilation and
120 <ei>t</ei>-channel squark exchange. So far, these cross sections assume the
121 squark CKM is aligned with the quark CKM and that all quantities are
122 real, so effects of non-minimal flavour violation and/or CP violation
123 are not yet included.
126 <h4>Associated Squark-Gluino Production</h4>
128 <flag name="SUSY:qg2squarkgluino" default="off">
129 Associated production of a squark with a gluino. Only implemented for the
130 flavor-diagonal case. So far, these cross sections assume the
131 squark CKM is aligned with the quark CKM and that all quantities are
132 real, so effects of non-minimal flavour violation and/or CP violation
133 are not yet included.
136 <h4>Squark Pair Production</h4>
138 <flag name="SUSY:gg2squarkantisquark" default="off">
139 Pair production of a scalar quark together with a scalar antiquark by
140 gluon annhilation via <ei>s</ei>-channel gluon exhange, <ei>t</ei>- and
141 <ei>u</ei>-channel squark exchange, and the direct 4-point coupling.
142 The cross section expression follows <ref>Boz07</ref>.
143 Validation of the FLV and CPV cases has not yet been completed.
146 <flag name="SUSY:qqbar2squarkantisquark" default="off">
147 Pair production of a scalar quark together with a scalar antiquark
148 by quark-antiquark annihilation.
149 For same-isospin <ei>~q~q*</ei> production (i.e., <ei>~u~u*</ei>,
150 <ei>~u~c*</ei>, ...), the <ei>s</ei>-channel gluon, photon, and
151 <ei>Z</ei> and <ei>t</ei>-channel gluino contributions have so far
152 been implemented (i.e., the <ei>t</ei>-channel neutralino contributions
153 are neglected). For opposite-isospin <ei>~q~q*</ei> production
154 (<ei>~u~d*</ei>, <ei>~u~s*</ei>, ...), the <ei>s</ei>-channel <ei>W</ei>
155 and <ei>t</ei>-channel gluino contributions have been implemented
156 (i.e., the <ei>t</ei>-channel neutralino contributions are neglected).
157 The cross section expressions follow <ref>Boz07</ref>.
158 Validation of the FLV and CPV cases has not yet been completed.
159 (Note to PYTHIA 6 users:
160 in older PYTHIA 6 versions, a bug caused the <ei>~t1~t2*</ei> cross to be
161 overcounted by a factor of 2. Starting from version 6.4.24, that
162 generator now agrees with the implementation here.)
165 <flag name="SUSY:qqbar2squarkantisquark:onlyQCD" default="off">
166 When switched <code>on</code> this flag switches off all but the
167 <ei>s</ei>-channel gluon contribution in the
168 calculation of same-isospin squark-antisquark production cross
169 sections. Intended for reference only. For the most
170 accurate physics simulation, leave this flag in the <code>off</code>
174 <flag name="SUSY:qq2squarksquark" default="off">
175 Pair production of scalar quarks (squark-squark and its charge
176 conjugate process; for squark-antisquark production see above)
177 by <ei>t</ei>- and <ei>u</ei>-channel gluino, neutralino, and
178 chargino exchange. The cross section expressions follow <ref>Boz07</ref>.
179 Validation of the FLV and CPV cases has not yet been completed.
180 (Note to PYTHIA 6 users: PYTHIA 6 only included the gluino exchange
181 contribution, which typically dominates due to the size of the strong
182 coupling; for counterchecks,
183 the flag <code>SUSY:qq2squarksquark:onlyQCD</code>
184 below can be switched on to eliminate the chargino and neutralino
188 <flag name="SUSY:qq2squarksquark:onlyQCD" default="off">
189 When switched <code>on</code> this flag causes the <ei>t</ei>- or
190 <ei>u</ei>-channel neutralino and chargino contributions to be
191 ignored in the calculation of squark pair production cross sections.
192 Intended for reference only. For the most accurate physics simulation,
193 leave this flag in the <code>off</code> position.
196 <h4>Neutralino and Chargino Pair Production</h4>
198 <flag name="SUSY:qqbar2chi0chi0" default="off">
199 Pair production of neutralinos by quark-antiquark annihilation. With
200 four neutralino species this gives ten separate processes, codes
201 1201 - 1210. The cross section expressions follow <ref>Boz07</ref>.
202 Validation of the FLV and CPV cases has not yet been completed.
205 <flag name="SUSY:qqbar2chi+-chi0" default="off">
206 Associated chargino-neutralino production by quark-antiquark
207 annihilation. With four neutralino species, two chargino ones, and
208 maintaining charge conjugate proceeses separate, this gives 16
209 separate processes, codes 1221 - 1236. The cross section expressions
210 follow <ref>Boz07</ref>.
211 Validation of the FLV and CPV cases has not yet been completed.
214 <flag name="SUSY:qqbar2chi+chi-" default="off">
215 Pair production of charginos by quark-antiquark annihilation. With
216 two chargino species and maintaining mutually charge conjugate
217 processes separate, this gives four separate processes, codes
218 1241 - 1244. The cross section expressions follow <ref>Boz07</ref>.
219 Validation of the FLV and CPV cases has not yet been completed.
222 <h4>Associated Neutralino/Chargino + Squark/Gluino Production</h4>
224 <flag name="SUSY:qg2chi0squark" default="off">
225 Pair production of neutralinos from quark-gluon initial states.
226 The cross section expressions follow <ref>Boz07</ref>.
227 Validation of the FLV and CPV cases has not yet been completed.
230 <flag name="SUSY:qg2chi+-squark" default="off">
231 Associated chargino-squark production from quark-gluon initial states.
232 annihilation. The cross section expressions
233 follow <ref>Boz07</ref>.
234 Validation of the FLV and CPV cases has not yet been completed.
237 <flag name="SUSY:qqbar2chi0gluino" default="off">
238 Associated neutralino-gluino production by quark-antiquark
239 annihilation. Status: not implemented yet.
242 <flag name="SUSY:qqbar2chi+-gluino" default="off">
243 Associated chargino-gluino production by quark-antiquark
244 annihilation. Status: not implemented yet.
247 <h4>Slepton Production</h4>
249 No 2->2 slepton pair production or associated slepton production
250 cross sections have been implemented yet.
252 <h4>R-parity violating squark production</h4>
254 <flag name="SUSY:qq2antisquark" default="off">
255 Resonant squark production via R-parity violating UDD couplings. The
256 couplings must be input using the SLHA2 structure.
259 <a name="decays"></a>
260 <h3>Decays of SUSY Particles</h3>
262 Based on the parameters read in from the SLHA, PYTHIA 8 will normally
263 compute the decay modes of SUSY particles automatically, using the
264 <code>SusyResonanceDecays</code> class(es). Essentially all tree-level
265 2-body decays in the MSSM
266 have been implemented this way, excepting so far only those involving
267 Higgs bosons (either in the in- or out-state) or gravitinos.
268 Available channels so far include:
270 <li>~q → q + ~chi</li>
271 <li>~q → ~q + W/Z</li>
272 <li>~q → q + q (RPV UDD)</li>
273 <li>~q → l + q (RPV LQD)</li>
274 <li>~g → ~q + q</li>
275 <li>~chi → ~chi + Z/W</li>
276 <li>~chi → ~q + q</li>
277 <li>~chi → ~l/~nu + l/nu</li>
278 <li>~chi0 → q + q + q (RPV UDD)</li>
279 <li>~l/~nu → l/nu + ~chi</li>
280 <li>~l/~nu → ~l/~nu + W/Z</li>
282 All channels are still undergoing validation, so this
283 implementation should be considered preliminary.
284 Still missing but to be included in a forthcoming update
285 are: 3-body decays of charginos (via RPV), and 2-body decays of squarks and
286 gauginos with Higgs as one of the decay products.
290 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->