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Stupid bug fix in new superlight mode (from Zurich airport)
[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8170 / src / FragmentationFlavZpT.cxx
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63ba5337 1// FragmentationFlavZpT.cc is a part of the PYTHIA event generator.
2// Copyright (C) 2012 Torbjorn Sjostrand.
3// PYTHIA is licenced under the GNU GPL version 2, see COPYING for details.
4// Please respect the MCnet Guidelines, see GUIDELINES for details.
5
6// Function definitions (not found in the header) for the
7// StringFlav, StringZ and StringPT classes.
8
9#include "FragmentationFlavZpT.h"
10
11namespace Pythia8 {
12
13//==========================================================================
14
15// The StringFlav class.
16
17//--------------------------------------------------------------------------
18
19// Constants: could be changed here if desired, but normally should not.
20// These are of technical nature, as described for each.
21
22// Offset for different meson multiplet id values.
23const int StringFlav::mesonMultipletCode[6]
24 = { 1, 3, 10003, 10001, 20003, 5};
25
26// Clebsch-Gordan coefficients for baryon octet and decuplet are
27// fixed once and for all, so only weighted sum needs to be edited.
28// Order: ud0 + u, ud0 + s, uu1 + u, uu1 + d, ud1 + u, ud1 + s.
29const double StringFlav::baryonCGOct[6]
30 = { 0.75, 0.5, 0., 0.1667, 0.0833, 0.1667};
31const double StringFlav::baryonCGDec[6]
32 = { 0., 0., 1., 0.3333, 0.6667, 0.3333};
33
34//--------------------------------------------------------------------------
35
36// Initialize data members of the flavour generation.
37
38void StringFlav::init(Settings& settings, Rndm* rndmPtrIn) {
39
40 // Save pointer.
41 rndmPtr = rndmPtrIn;
42
43 // Basic parameters for generation of new flavour.
44 probQQtoQ = settings.parm("StringFlav:probQQtoQ");
45 probStoUD = settings.parm("StringFlav:probStoUD");
46 probSQtoQQ = settings.parm("StringFlav:probSQtoQQ");
47 probQQ1toQQ0 = settings.parm("StringFlav:probQQ1toQQ0");
48
49 // Parameters derived from above.
50 probQandQQ = 1. + probQQtoQ;
51 probQandS = 2. + probStoUD;
52 probQandSinQQ = 2. + probSQtoQQ * probStoUD;
53 probQQ1corr = 3. * probQQ1toQQ0;
54 probQQ1corrInv = 1. / probQQ1corr;
55 probQQ1norm = probQQ1corr / (1. + probQQ1corr);
56
57 // Parameters for normal meson production.
58 for (int i = 0; i < 4; ++i) mesonRate[i][0] = 1.;
59 mesonRate[0][1] = settings.parm("StringFlav:mesonUDvector");
60 mesonRate[1][1] = settings.parm("StringFlav:mesonSvector");
61 mesonRate[2][1] = settings.parm("StringFlav:mesonCvector");
62 mesonRate[3][1] = settings.parm("StringFlav:mesonBvector");
63
64 // Parameters for L=1 excited-meson production.
65 mesonRate[0][2] = settings.parm("StringFlav:mesonUDL1S0J1");
66 mesonRate[1][2] = settings.parm("StringFlav:mesonSL1S0J1");
67 mesonRate[2][2] = settings.parm("StringFlav:mesonCL1S0J1");
68 mesonRate[3][2] = settings.parm("StringFlav:mesonBL1S0J1");
69 mesonRate[0][3] = settings.parm("StringFlav:mesonUDL1S1J0");
70 mesonRate[1][3] = settings.parm("StringFlav:mesonSL1S1J0");
71 mesonRate[2][3] = settings.parm("StringFlav:mesonCL1S1J0");
72 mesonRate[3][3] = settings.parm("StringFlav:mesonBL1S1J0");
73 mesonRate[0][4] = settings.parm("StringFlav:mesonUDL1S1J1");
74 mesonRate[1][4] = settings.parm("StringFlav:mesonSL1S1J1");
75 mesonRate[2][4] = settings.parm("StringFlav:mesonCL1S1J1");
76 mesonRate[3][4] = settings.parm("StringFlav:mesonBL1S1J1");
77 mesonRate[0][5] = settings.parm("StringFlav:mesonUDL1S1J2");
78 mesonRate[1][5] = settings.parm("StringFlav:mesonSL1S1J2");
79 mesonRate[2][5] = settings.parm("StringFlav:mesonCL1S1J2");
80 mesonRate[3][5] = settings.parm("StringFlav:mesonBL1S1J2");
81
82 // Store sum over multiplets for Monte Carlo generation.
83 for (int i = 0; i < 4; ++i) mesonRateSum[i]
84 = mesonRate[i][0] + mesonRate[i][1] + mesonRate[i][2]
85 + mesonRate[i][3] + mesonRate[i][4] + mesonRate[i][5];
86
87 // Parameters for uubar - ddbar - ssbar meson mixing.
88 for (int spin = 0; spin < 6; ++spin) {
89 double theta;
90 if (spin == 0) theta = settings.parm("StringFlav:thetaPS");
91 else if (spin == 1) theta = settings.parm("StringFlav:thetaV");
92 else if (spin == 2) theta = settings.parm("StringFlav:thetaL1S0J1");
93 else if (spin == 3) theta = settings.parm("StringFlav:thetaL1S1J0");
94 else if (spin == 4) theta = settings.parm("StringFlav:thetaL1S1J1");
95 else theta = settings.parm("StringFlav:thetaL1S1J2");
96 double alpha = (spin == 0) ? 90. - (theta + 54.7) : theta + 54.7;
97 alpha *= M_PI / 180.;
98 // Fill in (flavour, spin)-dependent probability of producing
99 // the lightest or the lightest two mesons of the nonet.
100 mesonMix1[0][spin] = 0.5;
101 mesonMix2[0][spin] = 0.5 * (1. + pow2(sin(alpha)));
102 mesonMix1[1][spin] = 0.;
103 mesonMix2[1][spin] = pow2(cos(alpha));
104 }
105
106 // Additional suppression of eta and etaPrime.
107 etaSup = settings.parm("StringFlav:etaSup");
108 etaPrimeSup = settings.parm("StringFlav:etaPrimeSup");
109
110 // Sum of baryon octet and decuplet weights.
111 decupletSup = settings.parm("StringFlav:decupletSup");
112 for (int i = 0; i < 6; ++i) baryonCGSum[i]
113 = baryonCGOct[i] + decupletSup * baryonCGDec[i];
114
115 // Maximum SU(6) weight for ud0, ud1, uu1 types.
116 baryonCGMax[0] = max( baryonCGSum[0], baryonCGSum[1]);
117 baryonCGMax[1] = baryonCGMax[0];
118 baryonCGMax[2] = max( baryonCGSum[2], baryonCGSum[3]);
119 baryonCGMax[3] = baryonCGMax[2];
120 baryonCGMax[4] = max( baryonCGSum[4], baryonCGSum[5]);
121 baryonCGMax[5] = baryonCGMax[4];
122
123 // Popcorn baryon parameters.
124 popcornRate = settings.parm("StringFlav:popcornRate");
125 popcornSpair = settings.parm("StringFlav:popcornSpair");
126 popcornSmeson = settings.parm("StringFlav:popcornSmeson");
127
128 // Suppression of leading (= first-rank) baryons.
129 suppressLeadingB = settings.flag("StringFlav:suppressLeadingB");
130 lightLeadingBSup = settings.parm("StringFlav:lightLeadingBSup");
131 heavyLeadingBSup = settings.parm("StringFlav:heavyLeadingBSup");
132
133 // Begin calculation of derived parameters for baryon production.
134
135 // Enumerate distinguishable diquark types (in diquark first is popcorn q).
136 enum Diquark {ud0, ud1, uu1, us0, su0, us1, su1, ss1};
137
138 // Maximum SU(6) weight by diquark type.
139 double barCGMax[8];
140 barCGMax[ud0] = baryonCGMax[0];
141 barCGMax[ud1] = baryonCGMax[4];
142 barCGMax[uu1] = baryonCGMax[2];
143 barCGMax[us0] = baryonCGMax[0];
144 barCGMax[su0] = baryonCGMax[0];
145 barCGMax[us1] = baryonCGMax[4];
146 barCGMax[su1] = baryonCGMax[4];
147 barCGMax[ss1] = baryonCGMax[2];
148
149 // Diquark SU(6) survival = Sum_quark (quark tunnel weight) * SU(6).
150 double dMB[8];
151 dMB[ud0] = 2. * baryonCGSum[0] + probStoUD * baryonCGSum[1];
152 dMB[ud1] = 2. * baryonCGSum[4] + probStoUD * baryonCGSum[5];
153 dMB[uu1] = baryonCGSum[2] + (1. + probStoUD) * baryonCGSum[3];
154 dMB[us0] = (1. + probStoUD) * baryonCGSum[0] + baryonCGSum[1];
155 dMB[su0] = dMB[us0];
156 dMB[us1] = (1. + probStoUD) * baryonCGSum[4] + baryonCGSum[5];
157 dMB[su1] = dMB[us1];
158 dMB[ss1] = probStoUD * baryonCGSum[2] + 2. * baryonCGSum[3];
159 for (int i = 1; i < 8; ++i) dMB[i] = dMB[i] / dMB[0];
160
161 // Tunneling factors for diquark production; only half a pair = sqrt.
162 double probStoUDroot = sqrt(probStoUD);
163 double probSQtoQQroot = sqrt(probSQtoQQ);
164 double probQQ1toQQ0root = sqrt(probQQ1toQQ0);
165 double qBB[8];
166 qBB[ud1] = probQQ1toQQ0root;
167 qBB[uu1] = probQQ1toQQ0root;
168 qBB[us0] = probSQtoQQroot;
169 qBB[su0] = probStoUDroot * probSQtoQQroot;
170 qBB[us1] = probQQ1toQQ0root * qBB[us0];
171 qBB[su1] = probQQ1toQQ0root * qBB[su0];
172 qBB[ss1] = probStoUDroot * pow2(probSQtoQQroot) * probQQ1toQQ0root;
173
174 // spin * (vertex factor) * (half-tunneling factor above).
175 double qBM[8];
176 qBM[ud1] = 3. * qBB[ud1];
177 qBM[uu1] = 6. * qBB[uu1];
178 qBM[us0] = probStoUD * qBB[us0];
179 qBM[su0] = qBB[su0];
180 qBM[us1] = probStoUD * 3. * qBB[us1];
181 qBM[su1] = 3. * qBB[su1];
182 qBM[ss1] = probStoUD * 6. * qBB[ss1];
183
184 // Combine above two into total diquark weight for q -> B Bbar.
185 for (int i = 1; i < 8; ++i) qBB[i] = qBB[i] * qBM[i];
186
187 // Suppression from having strange popcorn meson.
188 qBM[us0] *= popcornSmeson;
189 qBM[us1] *= popcornSmeson;
190 qBM[ss1] *= popcornSmeson;
191
192 // Suppression for a heavy quark of a diquark to fit into a baryon
193 // on the other side of popcorn meson: (0) s/u for q -> B M;
194 // (1) s/u for rank 0 diquark su -> M B; (2) ditto for s -> c/b.
195 double uNorm = 1. + qBM[ud1] + qBM[uu1] + qBM[us0] + qBM[us1];
196 scbBM[0] = (2. * (qBM[su0] + qBM[su1]) + qBM[ss1]) / uNorm;
197 scbBM[1] = scbBM[0] * popcornSpair * qBM[su0] / qBM[us0];
198 scbBM[2] = (1. + qBM[ud1]) * (2. + qBM[us0]) / uNorm;
199
200 // Include maximum of Clebsch-Gordan coefficients.
201 for (int i = 1; i < 8; ++i) dMB[i] *= qBM[i];
202 for (int i = 1; i < 8; ++i) qBM[i] *= barCGMax[i] / barCGMax[0];
203 for (int i = 1; i < 8; ++i) qBB[i] *= barCGMax[i] / barCGMax[0];
204
205 // Popcorn fraction for normal diquark production.
206 double qNorm = uNorm * popcornRate / 3.;
207 double sNorm = scbBM[0] * popcornSpair;
208 popFrac = qNorm * (1. + qBM[ud1] + qBM[uu1] + qBM[us0] + qBM[us1]
209 + sNorm * (qBM[su0] + qBM[su1] + 0.5 * qBM[ss1])) / (1. + qBB[ud1]
210 + qBB[uu1] + 2. * (qBB[us0] + qBB[us1]) + 0.5 * qBB[ss1]);
211
212 // Popcorn fraction for rank 0 diquarks, depending on number of s quarks.
213 popS[0] = qNorm * qBM[ud1] / qBB[ud1];
214 popS[1] = qNorm * 0.5 * (qBM[us1] / qBB[us1]
215 + sNorm * qBM[su1] / qBB[su1]);
216 popS[2] = qNorm * sNorm * qBM[ss1] / qBB[ss1];
217
218 // Recombine diquark weights to flavour and spin ratios. Second index:
219 // 0 = s/u popcorn quark ratio.
220 // 1, 2 = s/u ratio for vertex quark if popcorn quark is u/d or s.
221 // 3 = q/q' vertex quark ratio if popcorn quark is light and = q.
222 // 4, 5, 6 = (spin 1)/(spin 0) ratio for su, us and ud.
223
224 // Case 0: q -> B B.
225 dWT[0][0] = (2. * (qBB[su0] + qBB[su1]) + qBB[ss1])
226 / (1. + qBB[ud1] + qBB[uu1] + qBB[us0] + qBB[us1]);
227 dWT[0][1] = 2. * (qBB[us0] + qBB[us1]) / (1. + qBB[ud1] + qBB[uu1]);
228 dWT[0][2] = qBB[ss1] / (qBB[su0] + qBB[su1]);
229 dWT[0][3] = qBB[uu1] / (1. + qBB[ud1] + qBB[uu1]);
230 dWT[0][4] = qBB[su1] / qBB[su0];
231 dWT[0][5] = qBB[us1] / qBB[us0];
232 dWT[0][6] = qBB[ud1];
233
234 // Case 1: q -> B M B.
235 dWT[1][0] = (2. * (qBM[su0] + qBM[su1]) + qBM[ss1])
236 / (1. + qBM[ud1] + qBM[uu1] + qBM[us0] + qBM[us1]);
237 dWT[1][1] = 2. * (qBM[us0] + qBM[us1]) / (1. + qBM[ud1] + qBM[uu1]);
238 dWT[1][2] = qBM[ss1] / (qBM[su0] + qBM[su1]);
239 dWT[1][3] = qBM[uu1] / (1. + qBM[ud1] + qBM[uu1]);
240 dWT[1][4] = qBM[su1] / qBM[su0];
241 dWT[1][5] = qBM[us1] / qBM[us0];
242 dWT[1][6] = qBM[ud1];
243
244 // Case 2: qq -> M B; diquark inside chain.
245 dWT[2][0] = (2. * (dMB[su0] + dMB[su1]) + dMB[ss1])
246 / (1. + dMB[ud1] + dMB[uu1] + dMB[us0] + dMB[us1]);
247 dWT[2][1] = 2. * (dMB[us0] + dMB[us1]) / (1. + dMB[ud1] + dMB[uu1]);
248 dWT[2][2] = dMB[ss1] / (dMB[su0] + dMB[su1]);
249 dWT[2][3] = dMB[uu1] / (1. + dMB[ud1] + dMB[uu1]);
250 dWT[2][4] = dMB[su1] / dMB[su0];
251 dWT[2][5] = dMB[us1] / dMB[us0];
252 dWT[2][6] = dMB[ud1];
253
254}
255
256//--------------------------------------------------------------------------
257
258// Pick a new flavour (including diquarks) given an incoming one.
259
260FlavContainer StringFlav::pick(FlavContainer& flavOld) {
261
262 // Initial values for new flavour.
263 FlavContainer flavNew;
264 flavNew.rank = flavOld.rank + 1;
265
266 // For original diquark assign popcorn quark and whether popcorn meson.
267 int idOld = abs(flavOld.id);
268 if (flavOld.rank == 0 && idOld > 1000) assignPopQ(flavOld);
269
270 // Diquark exists, to be forced into baryon now.
271 bool doOldBaryon = (idOld > 1000 && flavOld.nPop == 0);
272 // Diquark exists, but do meson now.
273 bool doPopcornMeson = flavOld.nPop > 0;
274 // Newly created diquark gives baryon now, antibaryon later.
275 bool doNewBaryon = false;
276
277 // Choose whether to generate a new meson or a new baryon.
278 if (!doOldBaryon && !doPopcornMeson && probQandQQ * rndmPtr->flat() > 1.) {
279 doNewBaryon = true;
280 if ((1. + popFrac) * rndmPtr->flat() > 1.) flavNew.nPop = 1;
281 }
282
283 // Optional suppression of first-rank baryon.
284 if (flavOld.rank == 0 && doNewBaryon && suppressLeadingB) {
285 double leadingBSup = (idOld < 4) ? lightLeadingBSup : heavyLeadingBSup;
286 if (rndmPtr->flat() > leadingBSup) {
287 doNewBaryon = false;
288 flavNew.nPop = 0;
289 }
290 }
291
292 // Single quark for new meson or for baryon where diquark already exists.
293 if (!doPopcornMeson && !doNewBaryon) {
294 flavNew.id = pickLightQ();
295 if ( (flavOld.id > 0 && flavOld.id < 9) || flavOld.id < -1000 )
296 flavNew.id = -flavNew.id;
297
298 // Done for simple-quark case.
299 return flavNew;
300 }
301
302 // Case: 0 = q -> B B, 1 = q -> B M B, 2 = qq -> M B.
303 int iCase = flavNew.nPop;
304 if (flavOld.nPop == 1) iCase = 2;
305
306 // Flavour of popcorn quark (= q shared between B and Bbar).
307 if (doNewBaryon) {
308 double sPopWT = dWT[iCase][0];
309 if (iCase == 1) sPopWT *= scbBM[0] * popcornSpair;
310 double rndmFlav = (2. + sPopWT) * rndmPtr->flat();
311 flavNew.idPop = 1;
312 if (rndmFlav > 1.) flavNew.idPop = 2;
313 if (rndmFlav > 2.) flavNew.idPop = 3;
314 } else flavNew.idPop = flavOld.idPop;
315
316 // Flavour of vertex quark.
317 double sVtxWT = dWT[iCase][1];
318 if (flavNew.idPop >= 3) sVtxWT = dWT[iCase][2];
319 if (flavNew.idPop > 3) sVtxWT *= 0.5 * (1. + 1./dWT[iCase][4]);
320 double rndmFlav = (2. + sVtxWT) * rndmPtr->flat();
321 flavNew.idVtx = 1;
322 if (rndmFlav > 1.) flavNew.idVtx = 2;
323 if (rndmFlav > 2.) flavNew.idVtx = 3;
324
325 // Special case for light flavours, possibly identical.
326 if (flavNew.idPop < 3 && flavNew.idVtx < 3) {
327 flavNew.idVtx = flavNew.idPop;
328 if (rndmPtr->flat() > dWT[iCase][3]) flavNew.idVtx = 3 - flavNew.idPop;
329 }
330
331 // Pick 2 * spin + 1.
332 int spin = 3;
333 if (flavNew.idVtx != flavNew.idPop) {
334 double spinWT = dWT[iCase][6];
335 if (flavNew.idVtx == 3) spinWT = dWT[iCase][5];
336 if (flavNew.idPop >= 3) spinWT = dWT[iCase][4];
337 if ((1. + spinWT) * rndmPtr->flat() < 1.) spin = 1;
338 }
339
340 // Form outgoing diquark. Done.
341 flavNew.id = 1000 * max(flavNew.idVtx, flavNew.idPop)
342 + 100 * min(flavNew.idVtx, flavNew.idPop) + spin;
343 if ( (flavOld.id < 0 && flavOld.id > -9) || flavOld.id > 1000 )
344 flavNew.id = -flavNew.id;
345 return flavNew;
346
347}
348
349//--------------------------------------------------------------------------
350
351// Combine two flavours (including diquarks) to produce a hadron.
352// The weighting of the combination may fail, giving output 0.
353
354int StringFlav::combine(FlavContainer& flav1, FlavContainer& flav2) {
355
356 // Recognize largest and smallest flavour.
357 int id1Abs = abs(flav1.id);
358 int id2Abs = abs(flav2.id);
359 int idMax = max(id1Abs, id2Abs);
360 int idMin = min(id1Abs, id2Abs);
361
362 // Construct a meson.
363 if (idMax < 9 || idMin > 1000) {
364
365 // Popcorn meson: use only vertex quarks. Fail if none.
366 if (idMin > 1000) {
367 id1Abs = flav1.idVtx;
368 id2Abs = flav2.idVtx;
369 idMax = max(id1Abs, id2Abs);
370 idMin = min(id1Abs, id2Abs);
371 if (idMin == 0) return 0;
372 }
373
374 // Pick spin state and preliminary code.
375 int flav = (idMax < 3) ? 0 : idMax - 2;
376 double rndmSpin = mesonRateSum[flav] * rndmPtr->flat();
377 int spin = -1;
378 do rndmSpin -= mesonRate[flav][++spin];
379 while (rndmSpin > 0.);
380 int idMeson = 100 * idMax + 10 * idMin + mesonMultipletCode[spin];
381
382 // For nondiagonal mesons distinguish particle/antiparticle.
383 if (idMax != idMin) {
384 int sign = (idMax%2 == 0) ? 1 : -1;
385 if ( (idMax == id1Abs && flav1.id < 0)
386 || (idMax == id2Abs && flav2.id < 0) ) sign = -sign;
387 idMeson *= sign;
388
389 // For light diagonal mesons include uubar - ddbar - ssbar mixing.
390 } else if (flav < 2) {
391 double rMix = rndmPtr->flat();
392 if (rMix < mesonMix1[flav][spin]) idMeson = 110;
393 else if (rMix < mesonMix2[flav][spin]) idMeson = 220;
394 else idMeson = 330;
395 idMeson += mesonMultipletCode[spin];
396
397 // Additional suppression of eta and eta' may give failure.
398 if (idMeson == 221 && etaSup < rndmPtr->flat()) return 0;
399 if (idMeson == 331 && etaPrimeSup < rndmPtr->flat()) return 0;
400 }
401
402 // Finished for mesons.
403 return idMeson;
404 }
405
406 // SU(6) factors for baryon production may give failure.
407 int idQQ1 = idMax / 1000;
408 int idQQ2 = (idMax / 100) % 10;
409 int spinQQ = idMax % 10;
410 int spinFlav = spinQQ - 1;
411 if (spinFlav == 2 && idQQ1 != idQQ2) spinFlav = 4;
412 if (idMin != idQQ1 && idMin != idQQ2) spinFlav++;
413 if (baryonCGSum[spinFlav] < rndmPtr->flat() * baryonCGMax[spinFlav])
414 return 0;
415
416 // Order quarks to form baryon. Pick spin.
417 int idOrd1 = max( idMin, max( idQQ1, idQQ2) );
418 int idOrd3 = min( idMin, min( idQQ1, idQQ2) );
419 int idOrd2 = idMin + idQQ1 + idQQ2 - idOrd1 - idOrd3;
420 int spinBar = (baryonCGSum[spinFlav] * rndmPtr->flat()
421 < baryonCGOct[spinFlav]) ? 2 : 4;
422
423 // Distinguish Lambda- and Sigma-like.
424 bool LambdaLike = false;
425 if (spinBar == 2 && idOrd1 > idOrd2 && idOrd2 > idOrd3) {
426 LambdaLike = (spinQQ == 1);
427 if (idOrd1 != idMin && spinQQ == 1) LambdaLike = (rndmPtr->flat() < 0.25);
428 else if (idOrd1 != idMin) LambdaLike = (rndmPtr->flat() < 0.75);
429 }
430
431 // Form baryon code and return with sign.
432 int idBaryon = (LambdaLike)
433 ? 1000 * idOrd1 + 100 * idOrd3 + 10 * idOrd2 + spinBar
434 : 1000 * idOrd1 + 100 * idOrd2 + 10 * idOrd3 + spinBar;
435 return (flav1.id > 0) ? idBaryon : -idBaryon;
436
437}
438
439//--------------------------------------------------------------------------
440
441// Assign popcorn quark inside an original (= rank 0) diquark.
442
443void StringFlav::assignPopQ(FlavContainer& flav) {
444
445 // Safety check that intended to do something.
446 int idAbs = abs(flav.id);
447 if (flav.rank > 0 || idAbs < 1000) return;
448
449 // Make choice of popcorn quark.
450 int id1 = (idAbs/1000)%10;
451 int id2 = (idAbs/100)%10;
452 double pop2WT = 1.;
453 if (id1 == 3) pop2WT = scbBM[1];
454 else if (id1 > 3) pop2WT = scbBM[2];
455 if (id2 == 3) pop2WT /= scbBM[1];
456 else if (id2 > 3) pop2WT /= scbBM[2];
457 // Agrees with Patrik code, but opposite to intention??
458 flav.idPop = ((1. + pop2WT) * rndmPtr->flat() > 1.) ? id2 : id1;
459 flav.idVtx = id1 + id2 - flav.idPop;
460
461 // Also determine if to produce popcorn meson.
462 flav.nPop = 0;
463 double popWT = popS[0];
464 if (id1 == 3) popWT = popS[1];
465 if (id2 == 3) popWT = popS[2];
466 if (idAbs%10 == 1) popWT *= sqrt(probQQ1toQQ0);
467 if ((1. + popWT) * rndmPtr->flat() > 1.) flav.nPop = 1;
468
469}
470
471//--------------------------------------------------------------------------
472
473// Combine two quarks to produce a diquark.
474// Normally according to production composition, but nonvanishing idHad
475// means diquark from known hadron content, so use SU(6) wave fucntion.
476
477int StringFlav::makeDiquark(int id1, int id2, int idHad) {
478
479 // Initial values.
480 int idMin = min( abs(id1), abs(id2));
481 int idMax = max( abs(id1), abs(id2));
482 int spin = 1;
483
484 // Select spin of diquark formed from two valence quarks in proton.
485 // (More hadron cases??)
486 if (abs(idHad) == 2212) {
487 if (idMin == 1 && idMax == 2 && rndmPtr->flat() < 0.75) spin = 0;
488
489 // Else select spin of diquark according to production composition.
490 } else {
491 if (idMin != idMax && rndmPtr->flat() > probQQ1norm) spin = 0;
492 }
493
494 // Combined diquark code.
495 int idNewAbs = 1000 * idMax + 100 * idMin + 2 * spin + 1;
496 return (id1 > 0) ? idNewAbs : -idNewAbs;
497
498}
499
500//==========================================================================
501
502// The StringZ class.
503
504//--------------------------------------------------------------------------
505
506// Constants: could be changed here if desired, but normally should not.
507// These are of technical nature, as described for each.
508
509// When a or c are close to special cases, default to these.
510const double StringZ::CFROMUNITY = 0.01;
511const double StringZ::AFROMZERO = 0.02;
512const double StringZ::AFROMC = 0.01;
513
514// Do not take exponent of too large or small number.
515const double StringZ::EXPMAX = 50.;
516
517//--------------------------------------------------------------------------
518
519// Initialize data members of the string z selection.
520
521void StringZ::init(Settings& settings, ParticleData& particleData,
522 Rndm* rndmPtrIn) {
523
524 // Save pointer.
525 rndmPtr = rndmPtrIn;
526
527 // c and b quark masses.
528 mc2 = pow2( particleData.m0(4));
529 mb2 = pow2( particleData.m0(5));
530
531 // Paramaters of Lund/Bowler symmetric fragmentation function.
532 aLund = settings.parm("StringZ:aLund");
533 bLund = settings.parm("StringZ:bLund");
534 aExtraDiquark = settings.parm("StringZ:aExtraDiquark");
535 rFactC = settings.parm("StringZ:rFactC");
536 rFactB = settings.parm("StringZ:rFactB");
537 rFactH = settings.parm("StringZ:rFactH");
538
539 // Flags and parameters of Peterson/SLAC fragmentation function.
540 usePetersonC = settings.flag("StringZ:usePetersonC");
541 usePetersonB = settings.flag("StringZ:usePetersonB");
542 usePetersonH = settings.flag("StringZ:usePetersonH");
543 epsilonC = settings.parm("StringZ:epsilonC");
544 epsilonB = settings.parm("StringZ:epsilonB");
545 epsilonH = settings.parm("StringZ:epsilonH");
546
547 // Parameters for joining procedure.
548 stopM = settings.parm("StringFragmentation:stopMass");
549 stopNF = settings.parm("StringFragmentation:stopNewFlav");
550 stopS = settings.parm("StringFragmentation:stopSmear");
551
552}
553
554//--------------------------------------------------------------------------
555
556// Generate the fraction z that the next hadron will take,
557// using either Lund/Bowler or, for heavy, Peterson/SLAC functions.
558// Note: for a heavy new coloured particle we assume pT negligible.
559
560double StringZ::zFrag( int idOld, int idNew, double mT2) {
561
562 // Find if old or new flavours correspond to diquarks.
563 int idOldAbs = abs(idOld);
564 int idNewAbs = abs(idNew);
565 bool isOldDiquark = (idOldAbs > 1000 && idOldAbs < 10000);
566 bool isNewDiquark = (idNewAbs > 1000 && idNewAbs < 10000);
567
568 // Find heaviest quark in fragmenting parton/diquark.
569 int idFrag = idOldAbs;
570 if (isOldDiquark) idFrag = max( idOldAbs / 1000, (idOldAbs / 100) % 10);
571
572 // Use Peterson where explicitly requested for heavy flavours.
573 if (idFrag == 4 && usePetersonC) return zPeterson( epsilonC);
574 if (idFrag == 5 && usePetersonB) return zPeterson( epsilonB);
575 if (idFrag > 5 && usePetersonH) {
576 double epsilon = epsilonH * mb2 / mT2;
577 return zPeterson( epsilon);
578 }
579
580 // Shape parameters of Lund symmetric fragmentation function.
581 double aShape = aLund;
582 if (isOldDiquark) aShape += aExtraDiquark;
583 double bShape = bLund * mT2;
584 double cShape = 1.;
585 if (isOldDiquark) cShape -= aExtraDiquark;
586 if (isNewDiquark) cShape += aExtraDiquark;
587 if (idFrag == 4) cShape += rFactC * bLund * mc2;
588 if (idFrag == 5) cShape += rFactB * bLund * mb2;
589 if (idFrag > 5) cShape += rFactH * bLund * mT2;
590 return zLund( aShape, bShape, cShape);
591
592}
593
594//--------------------------------------------------------------------------
595
596// Generate a random z according to the Lund/Bowler symmetric
597// fragmentation function f(z) = (1 -z)^a * exp(-b/z) / z^c.
598// Normalized so that f(z_max) = 1 it can also be written as
599// f(z) = exp( a * ln( (1 - z) / (1 - z_max) ) + b * (1/z_max - 1/z)
600// + c * ln(z_max/z) ).
601
602double StringZ::zLund( double a, double b, double c) {
603
604 // Special cases for c = 1, a = 0 and a = c.
605 bool cIsUnity = (abs( c - 1.) < CFROMUNITY);
606 bool aIsZero = (a < AFROMZERO);
607 bool aIsC = (abs(a - c) < AFROMC);
608
609 // Determine position of maximum.
610 double zMax;
611 if (aIsZero) zMax = (c > b) ? b / c : 1.;
612 else if (aIsC) zMax = b / (b + c);
613 else { zMax = 0.5 * (b + c - sqrt( pow2(b - c) + 4. * a * b)) / (c - a);
614 if (zMax > 0.9999 && b > 100.) zMax = min(zMax, 1. - a / b); }
615
616 // Subdivide z range if distribution very peaked near either endpoint.
617 bool peakedNearZero = (zMax < 0.1);
618 bool peakedNearUnity = (zMax > 0.85 && b > 1.);
619
620 // Find integral of trial function everywhere bigger than f.
621 // (Dummy start values.)
622 double fIntLow = 1.;
623 double fIntHigh = 1.;
624 double fInt = 2.;
625 double zDiv = 0.5;
626 double zDivC = 0.5;
627 // When z_max is small use that f(z)
628 // < 1 for z < z_div = 2.75 * z_max,
629 // < (z_div/z)^c for z > z_div (=> logarithm for c = 1, else power).
630 if (peakedNearZero) {
631 zDiv = 2.75 * zMax;
632 fIntLow = zDiv;
633 if (cIsUnity) fIntHigh = -zDiv * log(zDiv);
634 else { zDivC = pow( zDiv, 1. - c);
635 fIntHigh = zDiv * (1. - 1./zDivC) / (c - 1.);}
636 fInt = fIntLow + fIntHigh;
637 // When z_max large use that f(z)
638 // < exp( b * (z - z_div) ) for z < z_div with z_div messy expression,
639 // < 1 for z > z_div.
640 // To simplify expressions the integral is extended to z = -infinity.
641 } else if (peakedNearUnity) {
642 double rcb = sqrt(4. + pow2(c / b));
643 zDiv = rcb - 1./zMax - (c / b) * log( zMax * 0.5 * (rcb + c / b) );
644 if (!aIsZero) zDiv += (a/b) * log(1. - zMax);
645 zDiv = min( zMax, max(0., zDiv));
646 fIntLow = 1. / b;
647 fIntHigh = 1. - zDiv;
648 fInt = fIntLow + fIntHigh;
649 }
650
651 // Choice of z, preweighted for peaks at low or high z. (Dummy start values.)
652 double z = 0.5;
653 double fPrel = 1.;
654 double fVal = 1.;
655 do {
656 // Choice of z flat good enough for distribution peaked in the middle;
657 // if not this z can be reused as a random number in general.
658 z = rndmPtr->flat();
659 fPrel = 1.;
660 // When z_max small use flat below z_div and 1/z^c above z_div.
661 if (peakedNearZero) {
662 if (fInt * rndmPtr->flat() < fIntLow) z = zDiv * z;
663 else if (cIsUnity) {z = pow( zDiv, z); fPrel = zDiv / z;}
664 else { z = pow( zDivC + (1. - zDivC) * z, 1. / (1. - c) );
665 fPrel = pow( zDiv / z, c); }
666 // When z_max large use exp( b * (z -z_div) ) below z_div
667 // and flat above it.
668 } else if (peakedNearUnity) {
669 if (fInt * rndmPtr->flat() < fIntLow) {
670 z = zDiv + log(z) / b;
671 fPrel = exp( b * (z - zDiv) );
672 } else z = zDiv + (1. - zDiv) * z;
673 }
674
675 // Evaluate actual f(z) (if in physical range) and correct.
676 if (z > 0 && z < 1) {
677 double fExp = b * (1. / zMax - 1. / z)+ c * log(zMax / z);
678 if (!aIsZero) fExp += a * log( (1. - z) / (1. - zMax) );
679 fVal = exp( max( -EXPMAX, min( EXPMAX, fExp) ) ) ;
680 } else fVal = 0.;
681 } while (fVal < rndmPtr->flat() * fPrel);
682
683 // Done.
684 return z;
685
686}
687
688//--------------------------------------------------------------------------
689
690// Generate a random z according to the Peterson/SLAC formula
691// f(z) = 1 / ( z * (1 - 1/z - epsilon/(1-z))^2 )
692// = z * (1-z)^2 / ((1-z)^2 + epsilon * z)^2.
693
694double StringZ::zPeterson( double epsilon) {
695
696 double z, fVal;
697
698 // For large epsilon pick z flat and reject,
699 // knowing that 4 * epsilon * f(z) < 1 everywhere.
700 if (epsilon > 0.01) {
701 do {
702 z = rndmPtr->flat();
703 fVal = 4. * epsilon * z * pow2(1. - z)
704 / pow2( pow2(1. - z) + epsilon * z);
705 } while (fVal < rndmPtr->flat());
706 return z;
707 }
708
709 // Else split range, using that 4 * epsilon * f(z)
710 // < 4 * epsilon / (1 - z)^2 for 0 < z < 1 - 2 * sqrt(epsilon)
711 // < 1 for 1 - 2 * sqrt(epsilon) < z < 1
712 double epsRoot = sqrt(epsilon);
713 double epsComb = 0.5 / epsRoot - 1.;
714 double fIntLow = 4. * epsilon * epsComb;
715 double fInt = fIntLow + 2. * epsRoot;
716 do {
717 if (rndmPtr->flat() * fInt < fIntLow) {
718 z = 1. - 1. / (1. + rndmPtr->flat() * epsComb);
719 fVal = z * pow2( pow2(1. - z) / (pow2(1. - z) + epsilon * z) );
720 } else {
721 z = 1. - 2. * epsRoot * rndmPtr->flat();
722 fVal = 4. * epsilon * z * pow2(1. - z)
723 / pow2( pow2(1. - z) + epsilon * z);
724 }
725 } while (fVal < rndmPtr->flat());
726 return z;
727
728}
729
730//==========================================================================
731
732// The StringPT class.
733
734//--------------------------------------------------------------------------
735
736// Constants: could be changed here if desired, but normally should not.
737// These are of technical nature, as described for each.
738
739// To avoid division by zero one must have sigma > 0.
740const double StringPT::SIGMAMIN = 0.2;
741
742//--------------------------------------------------------------------------
743
744// Initialize data members of the string pT selection.
745
746void StringPT::init(Settings& settings, ParticleData& , Rndm* rndmPtrIn) {
747
748 // Save pointer.
749 rndmPtr = rndmPtrIn;
750
751 // Parameters of the pT width and enhancement.
752 double sigma = settings.parm("StringPT:sigma");
753 sigmaQ = sigma / sqrt(2.);
754 enhancedFraction = settings.parm("StringPT:enhancedFraction");
755 enhancedWidth = settings.parm("StringPT:enhancedWidth");
756
757 // Parameter for pT suppression in MiniStringFragmentation.
758 sigma2Had = 2. * pow2( max( SIGMAMIN, sigma) );
759
760}
761
762//--------------------------------------------------------------------------
763
764// Generate Gaussian pT such that <p_x^2> = <p_x^2> = sigma^2 = width^2/2,
765// but with small fraction multiplied up to a broader spectrum.
766
767pair<double, double> StringPT::pxy() {
768
769 double sigma = sigmaQ;
770 if (rndmPtr->flat() < enhancedFraction) sigma *= enhancedWidth;
771 pair<double, double> gauss2 = rndmPtr->gauss2();
772 return pair<double, double>(sigma * gauss2.first, sigma * gauss2.second);
773
774}
775
776//==========================================================================
777
778} // end namespace Pythia8