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1 | // MiniStringFragmentation.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 | // MiniStringFragmentation class | |
8 | ||
9 | #include "MiniStringFragmentation.h" | |
10 | ||
11 | namespace Pythia8 { | |
12 | ||
13 | //========================================================================== | |
14 | ||
15 | // The MiniStringFragmentation 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 | // Since diffractive by definition is > 1 particle, try hard. | |
23 | const int MiniStringFragmentation::NTRYDIFFRACTIVE = 200; | |
24 | ||
25 | // After one-body fragmentation failed, try two-body once more. | |
26 | const int MiniStringFragmentation::NTRYLASTRESORT = 100; | |
27 | ||
28 | // Loop try to combine available endquarks to valid hadron. | |
29 | const int MiniStringFragmentation::NTRYFLAV = 10; | |
30 | ||
31 | //-------------------------------------------------------------------------- | |
32 | ||
33 | // Initialize and save pointers. | |
34 | ||
35 | void MiniStringFragmentation::init(Info* infoPtrIn, Settings& settings, | |
36 | ParticleData* particleDataPtrIn, Rndm* rndmPtrIn, | |
37 | StringFlav* flavSelPtrIn, StringPT* pTSelPtrIn, StringZ* zSelPtrIn) { | |
38 | ||
39 | // Save pointers. | |
40 | infoPtr = infoPtrIn; | |
41 | particleDataPtr = particleDataPtrIn; | |
42 | rndmPtr = rndmPtrIn; | |
43 | flavSelPtr = flavSelPtrIn; | |
44 | pTSelPtr = pTSelPtrIn; | |
45 | zSelPtr = zSelPtrIn; | |
46 | ||
47 | // Initialize the MiniStringFragmentation class proper. | |
48 | nTryMass = settings.mode("MiniStringFragmentation:nTry"); | |
49 | ||
50 | // Initialize the b parameter of the z spectrum, used when joining jets. | |
51 | bLund = zSelPtr->bAreaLund(); | |
52 | ||
53 | } | |
54 | ||
55 | //-------------------------------------------------------------------------- | |
56 | ||
57 | // Do the fragmentation: driver routine. | |
58 | ||
59 | bool MiniStringFragmentation::fragment(int iSub, ColConfig& colConfig, | |
60 | Event& event, bool isDiff) { | |
61 | ||
62 | // Read in info on system to be treated. | |
63 | iParton = colConfig[iSub].iParton; | |
64 | flav1 = FlavContainer( event[ iParton.front() ].id() ); | |
65 | flav2 = FlavContainer( event[ iParton.back() ].id() ); | |
66 | pSum = colConfig[iSub].pSum; | |
67 | mSum = colConfig[iSub].mass; | |
68 | m2Sum = mSum*mSum; | |
69 | isClosed = colConfig[iSub].isClosed; | |
70 | ||
71 | // Do not want diffractive systems to easily collapse to one particle. | |
72 | int nTryFirst = (isDiff) ? NTRYDIFFRACTIVE : nTryMass; | |
73 | ||
74 | // First try to produce two particles from the system. | |
75 | if (ministring2two( nTryFirst, event)) return true; | |
76 | ||
77 | // If this fails, then form one hadron and shuffle momentum. | |
78 | if (ministring2one( iSub, colConfig, event)) return true; | |
79 | ||
80 | // If also this fails, then try harder to produce two particles. | |
81 | if (ministring2two( NTRYLASTRESORT, event)) return true; | |
82 | ||
83 | // Else complete failure. | |
84 | infoPtr->errorMsg("Error in MiniStringFragmentation::fragment: " | |
85 | "no 1- or 2-body state found above mass threshold"); | |
86 | return false; | |
87 | ||
88 | } | |
89 | ||
90 | //-------------------------------------------------------------------------- | |
91 | ||
92 | // Attempt to produce two particles from the ministring. | |
93 | ||
94 | bool MiniStringFragmentation::ministring2two( int nTry, Event& event) { | |
95 | ||
96 | // Properties of the produced hadrons. | |
97 | int idHad1 = 0; | |
98 | int idHad2 = 0; | |
99 | double mHad1 = 0.; | |
100 | double mHad2 = 0.; | |
101 | double mHadSum = 0.; | |
102 | ||
103 | // Allow a few attempts to find a particle pair with low enough masses. | |
104 | for (int iTry = 0; iTry < nTry; ++iTry) { | |
105 | ||
106 | // For closed gluon loop need to pick an initial flavour. | |
107 | if (isClosed) do { | |
108 | int idStart = flavSelPtr->pickLightQ(); | |
109 | FlavContainer flavStart(idStart, 1); | |
110 | flavStart = flavSelPtr->pick( flavStart); | |
111 | flav1 = flavSelPtr->pick( flavStart); | |
112 | flav2.anti(flav1); | |
113 | } while (flav1.id == 0 || flav1.nPop > 0); | |
114 | ||
115 | // Create a new q qbar flavour to form two hadrons. | |
116 | // Start from a diquark, if any. | |
117 | do { | |
118 | FlavContainer flav3 = | |
119 | (flav1.isDiquark() || (!flav2.isDiquark() && rndmPtr->flat() < 0.5) ) | |
120 | ? flavSelPtr->pick( flav1) : flavSelPtr->pick( flav2).anti(); | |
121 | idHad1 = flavSelPtr->combine( flav1, flav3); | |
122 | idHad2 = flavSelPtr->combine( flav2, flav3.anti()); | |
123 | } while (idHad1 == 0 || idHad2 == 0); | |
124 | ||
125 | // Check whether the mass sum fits inside the available phase space. | |
126 | mHad1 = particleDataPtr->mass(idHad1); | |
127 | mHad2 = particleDataPtr->mass(idHad2); | |
128 | mHadSum = mHad1 + mHad2; | |
129 | if (mHadSum < mSum) break; | |
130 | } | |
131 | if (mHadSum >= mSum) return false; | |
132 | ||
133 | // Define an effective two-parton string, by splitting intermediate | |
134 | // gluon momenta in proportion to their closeness to either endpoint. | |
135 | Vec4 pSum1 = event[ iParton.front() ].p(); | |
136 | Vec4 pSum2 = event[ iParton.back() ].p(); | |
137 | if (iParton.size() > 2) { | |
138 | Vec4 pEnd1 = pSum1; | |
139 | Vec4 pEnd2 = pSum2; | |
140 | Vec4 pEndSum = pEnd1 + pEnd2; | |
141 | for (int i = 1; i < int(iParton.size()) - 1 ; ++i) { | |
142 | Vec4 pNow = event[ iParton[i] ].p(); | |
143 | double ratio = (pEnd2 * pNow) / (pEndSum * pNow); | |
144 | pSum1 += ratio * pNow; | |
145 | pSum2 += (1. - ratio) * pNow; | |
146 | } | |
147 | } | |
148 | ||
149 | // Set up a string region based on the two effective endpoints. | |
150 | StringRegion region; | |
151 | region.setUp( pSum1, pSum2); | |
152 | ||
153 | // Generate an isotropic decay in the ministring rest frame, | |
154 | // suppressed at large pT by a fragmentation pT Gaussian. | |
155 | double pAbs2 = 0.25 * ( pow2(m2Sum - mHad1*mHad1 - mHad2*mHad2) | |
156 | - pow2(2. * mHad1 * mHad2) ) / m2Sum; | |
157 | double pT2 = 0.; | |
158 | do { | |
159 | double cosTheta = rndmPtr->flat(); | |
160 | pT2 = (1. - pow2(cosTheta)) * pAbs2; | |
161 | } while (pTSelPtr->suppressPT2(pT2) < rndmPtr->flat() ); | |
162 | ||
163 | // Construct the forward-backward asymmetry of the two particles. | |
164 | double mT21 = mHad1*mHad1 + pT2; | |
165 | double mT22 = mHad2*mHad2 + pT2; | |
166 | double lambda = sqrtpos( pow2(m2Sum - mT21 - mT22) - 4. * mT21 * mT22 ); | |
167 | double probReverse = 1. / (1. + exp( min( 50., bLund * lambda) ) ); | |
168 | ||
169 | // Construct kinematics, as viewed in the transverse rest frame. | |
170 | double xpz1 = 0.5 * lambda/ m2Sum; | |
171 | if (probReverse > rndmPtr->flat()) xpz1 = -xpz1; | |
172 | double xmDiff = (mT21 - mT22) / m2Sum; | |
173 | double xe1 = 0.5 * (1. + xmDiff); | |
174 | double xe2 = 0.5 * (1. - xmDiff ); | |
175 | ||
176 | // Distribute pT isotropically in angle. | |
177 | double phi = 2. * M_PI * rndmPtr->flat(); | |
178 | double pT = sqrt(pT2); | |
179 | double px = pT * cos(phi); | |
180 | double py = pT * sin(phi); | |
181 | ||
182 | // Translate this into kinematics in the string frame. | |
183 | Vec4 pHad1 = region.pHad( xe1 + xpz1, xe1 - xpz1, px, py); | |
184 | Vec4 pHad2 = region.pHad( xe2 - xpz1, xe2 + xpz1, -px, -py); | |
185 | ||
186 | // Add produced particles to the event record. | |
187 | int iFirst = event.append( idHad1, 82, iParton.front(), iParton.back(), | |
188 | 0, 0, 0, 0, pHad1, mHad1); | |
189 | int iLast = event.append( idHad2, 82, iParton.front(), iParton.back(), | |
190 | 0, 0, 0, 0, pHad2, mHad2); | |
191 | ||
192 | // Set decay vertex when this is displaced. | |
193 | if (event[iParton.front()].hasVertex()) { | |
194 | Vec4 vDec = event[iParton.front()].vDec(); | |
195 | event[iFirst].vProd( vDec ); | |
196 | event[iLast].vProd( vDec ); | |
197 | } | |
198 | ||
199 | // Set lifetime of hadrons. | |
200 | event[iFirst].tau( event[iFirst].tau0() * rndmPtr->exp() ); | |
201 | event[iLast].tau( event[iLast].tau0() * rndmPtr->exp() ); | |
202 | ||
203 | // Mark original partons as hadronized and set their daughter range. | |
204 | for (int i = 0; i < int(iParton.size()); ++i) { | |
205 | event[ iParton[i] ].statusNeg(); | |
206 | event[ iParton[i] ].daughters(iFirst, iLast); | |
207 | } | |
208 | ||
209 | // Successfully done. | |
210 | return true; | |
211 | ||
212 | } | |
213 | ||
214 | //-------------------------------------------------------------------------- | |
215 | ||
216 | // Attempt to produce one particle from a ministring. | |
217 | // Current algorithm: find the system with largest invariant mass | |
218 | // relative to the existing one, and boost that system appropriately. | |
219 | // Try more sophisticated alternatives later?? (Z0 mass shifted??) | |
220 | // Also, if problems, attempt several times to obtain closer mass match?? | |
221 | ||
222 | bool MiniStringFragmentation::ministring2one( int iSub, | |
223 | ColConfig& colConfig, Event& event) { | |
224 | ||
225 | // Cannot handle qq + qbarqbar system. | |
226 | if (abs(flav1.id) > 100 && abs(flav2.id) > 100) return false; | |
227 | ||
228 | // For closed gluon loop need to pick an initial flavour. | |
229 | if (isClosed) do { | |
230 | int idStart = flavSelPtr->pickLightQ(); | |
231 | FlavContainer flavStart(idStart, 1); | |
232 | flav1 = flavSelPtr->pick( flavStart); | |
233 | flav2 = flav1.anti(); | |
234 | } while (abs(flav1.id) > 100); | |
235 | ||
236 | // Select hadron flavour from available quark flavours. | |
237 | int idHad = 0; | |
238 | for (int iTryFlav = 0; iTryFlav < NTRYFLAV; ++iTryFlav) { | |
239 | idHad = flavSelPtr->combine( flav1, flav2); | |
240 | if (idHad != 0) break; | |
241 | } | |
242 | if (idHad == 0) return false; | |
243 | ||
244 | // Find mass. | |
245 | double mHad = particleDataPtr->mass(idHad); | |
246 | ||
247 | // Find the untreated parton system which combines to the largest | |
248 | // squared mass above mimimum required. | |
249 | int iMax = -1; | |
250 | double deltaM2 = mHad*mHad - mSum*mSum; | |
251 | double delta2Max = 0.; | |
252 | for (int iRec = iSub + 1; iRec < colConfig.size(); ++iRec) { | |
253 | double delta2Rec = 2. * (pSum * colConfig[iRec].pSum) - deltaM2 | |
254 | - 2. * mHad * colConfig[iRec].mass; | |
255 | if (delta2Rec > delta2Max) { iMax = iRec; delta2Max = delta2Rec;} | |
256 | } | |
257 | if (iMax == -1) return false; | |
258 | ||
259 | // Construct kinematics of the hadron and recoiling system. | |
260 | Vec4& pRec = colConfig[iMax].pSum; | |
261 | double mRec = colConfig[iMax].mass; | |
262 | double vecProd = pSum * pRec; | |
263 | double coefOld = mSum*mSum + vecProd; | |
264 | double coefNew = mHad*mHad + vecProd; | |
265 | double coefRec = mRec*mRec + vecProd; | |
266 | double coefSum = coefOld + coefNew; | |
267 | double sHat = coefOld + coefRec; | |
268 | double root = sqrtpos( (pow2(coefSum) - 4. * sHat * mHad*mHad) | |
269 | / (pow2(vecProd) - pow2(mSum * mRec)) ); | |
270 | double k2 = 0.5 * (coefOld * root - coefSum) / sHat; | |
271 | double k1 = (coefRec * k2 + 0.5 * deltaM2) / coefOld; | |
272 | Vec4 pHad = (1. + k1) * pSum - k2 * pRec; | |
273 | Vec4 pRecNew = (1. + k2) * pRec - k1 * pSum; | |
274 | ||
275 | // Add the produced particle to the event record. | |
276 | int iHad = event.append( idHad, 81, iParton.front(), iParton.back(), | |
277 | 0, 0, 0, 0, pHad, mHad); | |
278 | ||
279 | // Set decay vertex when this is displaced. | |
280 | if (event[iParton.front()].hasVertex()) { | |
281 | Vec4 vDec = event[iParton.front()].vDec(); | |
282 | event[iHad].vProd( vDec ); | |
283 | } | |
284 | ||
285 | // Set lifetime of hadron. | |
286 | event[iHad].tau( event[iHad].tau0() * rndmPtr->exp() ); | |
287 | ||
288 | // Mark original partons as hadronized and set their daughter range. | |
289 | for (int i = 0; i < int(iParton.size()); ++i) { | |
290 | event[ iParton[i] ].statusNeg(); | |
291 | event[ iParton[i] ].daughters(iHad, iHad); | |
292 | } | |
293 | ||
294 | // Copy down recoiling system, with boosted momentum. Update current partons. | |
295 | RotBstMatrix M; | |
296 | M.bst(pRec, pRecNew); | |
297 | for (int i = 0; i < colConfig[iMax].size(); ++i) { | |
298 | int iOld = colConfig[iMax].iParton[i]; | |
299 | // Do not touch negative iOld = beginning of new junction leg. | |
300 | if (iOld >= 0) { | |
301 | int iNew = event.copy(iOld, 72); | |
302 | event[iNew].rotbst(M); | |
303 | colConfig[iMax].iParton[i] = iNew; | |
304 | } | |
305 | } | |
306 | colConfig[iMax].pSum = pRecNew; | |
307 | colConfig[iMax].isCollected = true; | |
308 | ||
309 | // Successfully done. | |
310 | return true; | |
311 | ||
312 | } | |
313 | ||
314 | //========================================================================== | |
315 | ||
316 | } // end namespace Pythia8 |