1 // StringFragmentation.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.
6 // Function definitions (not found in the header) for the StringEnd and
7 // StringFragmentation classes.
9 #include "StringFragmentation.h"
13 //==========================================================================
15 // The StringEnd class.
17 //--------------------------------------------------------------------------
19 // Constants: could be changed here if desired, but normally should not.
21 // Avoid unphysical solutions to equation system.
22 const double StringEnd::TINY = 1e-6;
24 // Assume two (eX, eY) regions are related if pT2 differs by less.
25 const double StringEnd::PT2SAME = 0.01;
27 //--------------------------------------------------------------------------
29 // Set up initial endpoint values from input.
31 void StringEnd::setUp(bool fromPosIn, int iEndIn, int idOldIn, int iMaxIn,
32 double pxIn, double pyIn, double GammaIn, double xPosIn, double xNegIn) {
34 // Simple transcription from input.
38 flavOld = FlavContainer(idOldIn);
42 iPosOld = (fromPos) ? 0 : iMax;
43 iNegOld = (fromPos) ? iMax : 0;
49 //--------------------------------------------------------------------------
51 // Fragment off one hadron from the string system, in flavour and pT.
53 void StringEnd::newHadron() {
55 // Pick new flavour and form a new hadron.
57 flavNew = flavSelPtr->pick( flavOld);
58 idHad = flavSelPtr->combine( flavOld, flavNew);
61 // Pick its transverse momentum.
62 pair<double, double> pxy = pTSelPtr->pxy();
65 pxHad = pxOld + pxNew;
66 pyHad = pyOld + pyNew;
68 // Pick its mass and thereby define its transverse mass.
69 mHad = particleDataPtr->mass(idHad);
70 mT2Had = pow2(mHad) + pow2(pxHad) + pow2(pyHad);
74 //--------------------------------------------------------------------------
76 // Fragment off one hadron from the string system, in momentum space,
77 // by taking steps from positive end.
79 Vec4 StringEnd::kinematicsHadron( StringSystem& system) {
81 // Pick fragmentation step z and calculate new Gamma.
82 zHad = zSelPtr->zFrag( flavOld.id, flavNew.id, mT2Had);
83 GammaNew = (1. - zHad) * (GammaOld + mT2Had / zHad);
85 // Set up references that are direction-neutral;
86 // ...Dir for direction of iteration and ...Inv for its inverse.
87 int& iDirOld = (fromPos) ? iPosOld : iNegOld;
88 int& iInvOld = (fromPos) ? iNegOld : iPosOld;
89 int& iDirNew = (fromPos) ? iPosNew : iNegNew;
90 int& iInvNew = (fromPos) ? iNegNew : iPosNew;
91 double& xDirOld = (fromPos) ? xPosOld : xNegOld;
92 double& xInvOld = (fromPos) ? xNegOld : xPosOld;
93 double& xDirNew = (fromPos) ? xPosNew : xNegNew;
94 double& xInvNew = (fromPos) ? xNegNew : xPosNew;
95 double& xDirHad = (fromPos) ? xPosHad : xNegHad;
96 double& xInvHad = (fromPos) ? xNegHad : xPosHad;
98 // Start search for new breakup in the old region.
103 // Each step corresponds to trying a new string region.
104 for (int iStep = 0; ; ++iStep) {
106 // Referance to current string region.
107 StringRegion& region = system.region( iPosNew, iNegNew);
109 // Now begin special section for rapid processing of low region.
110 if (iStep == 0 && iPosOld + iNegOld == iMax) {
112 // A first step within a low region is easy.
113 if (mT2Had < zHad * xDirOld * (1. - xInvOld) * region.w2) {
115 // Translate into x coordinates.
116 xDirHad = zHad * xDirOld;
117 xInvHad = mT2Had / (xDirHad * region.w2);
118 xDirNew = xDirOld - xDirHad;
119 xInvNew = xInvOld + xInvHad;
121 // Find and return four-momentum of the produced particle.
122 return region.pHad( xPosHad, xNegHad, pxHad, pyHad);
124 // A first step out of a low region also OK, if there are more regions.
125 // Negative energy signals failure, i.e. in last region.
128 if (iInvNew < 0) return Vec4(0., 0., 0., -1.);
130 // Momentum taken by stepping out of region. Continue to next region.
131 xInvHad = 1. - xInvOld;
133 pSoFar = region.pHad( xPosHad, xNegHad, pxOld, pyOld);
137 // Else, for first step, take into account starting pT.
138 } else if (iStep == 0) {
139 pSoFar = region.pHad( 0., 0., pxOld, pyOld);
140 pTNew = region.pHad( 0., 0., pxNew, pyNew);
143 // Now begin normal treatment of nontrivial regions.
144 // Set up four-vectors in a region not visited before.
145 if (!region.isSetUp) region.setUp(
146 system.regionLowPos(iPosNew).pPos,
147 system.regionLowNeg(iNegNew).pNeg, true);
149 // If new region is vanishingly small, continue immediately to next.
150 // Negative energy signals failure to do this, i.e. moved too low.
151 if (region.isEmpty) {
152 xDirHad = (iDirNew == iDirOld) ? xDirOld : 1.;
154 pSoFar += region.pHad( xPosHad, xNegHad, 0., 0.);
156 if (iDirNew + iInvNew > iMax) return Vec4(0., 0., 0., -1.);
160 // Reexpress pTNew w.r.t. base vectors in new region, if possible.
161 // Recall minus sign from normalization e_x^2 = e_y^2 = -1.
162 double pxNewTemp = -pTNew * region.eX;
163 double pyNewTemp = -pTNew * region.eY;
164 if (abs( pxNewTemp * pxNewTemp + pyNewTemp * pyNewTemp
165 - pxNew * pxNew - pyNew * pyNew) < PT2SAME) {
170 // Four-momentum taken so far, including new pT.
171 Vec4 pTemp = pSoFar + region.pHad( 0., 0., pxNew, pyNew);
173 // Derive coefficients for m2 expression.
174 // cM2 * x+ + cM3 * x- + cM4 * x+ * x- = m^2 - cM1;
175 double cM1 = pTemp.m2Calc();
176 double cM2 = 2. * (pTemp * region.pPos);
177 double cM3 = 2. * (pTemp * region.pNeg);
178 double cM4 = region.w2;
179 if (!fromPos) swap( cM2, cM3);
181 // Derive coefficients for Gamma expression.
182 // cGam2 * x+ + cGam3 * x- + cGam4 * x+ * x- = Gamma_new - cGam1;
187 for (int iInv = iInvNew; iInv <= iMax - iDirNew; ++iInv) {
189 if (iInv == iInvNew) xInv = (iInvNew == iInvOld) ? xInvOld : 0.;
190 for (int iDir = iDirNew; iDir <= iMax - iInv; ++iDir) {
191 double xDir = (iDir == iDirOld) ? xDirOld : 1.;
192 int iPos = (fromPos) ? iDir : iInv;
193 int iNeg = (fromPos) ? iInv : iDir;
194 StringRegion& regionGam = system.region( iPos, iNeg);
195 if (!regionGam.isSetUp) regionGam.setUp(
196 system.regionLowPos(iPos).pPos,
197 system.regionLowNeg(iNeg).pNeg, true);
198 double w2 = regionGam.w2;
199 cGam1 += xDir * xInv * w2;
200 if (iDir == iDirNew) cGam2 -= xInv * w2;
201 if (iInv == iInvNew) cGam3 += xDir * w2;
202 if (iDir == iDirNew && iInv == iInvNew) cGam4 -= w2;
206 // Solve (m2, Gamma) equation system => r2 * x-^2 + r1 * x- + r0 = 0.
207 double cM0 = pow2(mHad) - cM1;
208 double cGam0 = GammaNew - cGam1;
209 double r2 = cM3 * cGam4 - cM4 * cGam3;
210 double r1 = cM4 * cGam0 - cM0 * cGam4 + cM3 * cGam2 - cM2 * cGam3;
211 double r0 = cM2 * cGam0 - cM0 * cGam2;
212 double root = sqrtpos( r1*r1 - 4. * r2 * r0 );
213 if (abs(r2) < TINY || root < TINY) return Vec4(0., 0., 0., -1.);
214 xInvHad = 0.5 * (root / abs(r2) - r1 / r2);
215 xDirHad = (cM0 - cM3 * xInvHad) / (cM2 + cM4 * xInvHad);
217 // Define position of new trial vertex.
218 xDirNew = (iDirNew == iDirOld) ? xDirOld - xDirHad : 1. - xDirHad;
219 xInvNew = (iInvNew == iInvOld) ? xInvOld + xInvHad : xInvHad;
221 // Step up to new region if new x- > 1.
223 xInvHad = (iInvNew == iInvOld) ? 1. - xInvOld : 1.;
225 pSoFar += region.pHad( xPosHad, xNegHad, 0., 0.);
227 if (iInvNew < 0) return Vec4(0., 0., 0., -1.);
230 // Step down to new region if new x+ < 0.
231 } else if (xDirNew < 0.) {
232 xDirHad = (iDirNew == iDirOld) ? xDirOld : 1.;
234 pSoFar += region.pHad( xPosHad, xNegHad, 0., 0.);
236 if (iDirNew + iInvNew > iMax) return Vec4(0., 0., 0., -1.);
240 // Else we have found the correct region, and can return four-momentum.
241 return pSoFar + region.pHad( xPosHad, xNegHad, pxNew, pyNew);
243 // End of "infinite" loop of stepping to new region.
248 //--------------------------------------------------------------------------
250 // Update string end information after a hadron has been removed.
252 void StringEnd::update() {
254 flavOld.anti(flavNew);
265 //==========================================================================
267 // The StringFragmentation class.
269 //--------------------------------------------------------------------------
271 // Constants: could be changed here if desired, but normally should not.
272 // These are of technical nature, as described for each.
274 // Maximum number of tries to (flavour-, energy) join the two string ends.
275 const int StringFragmentation::NTRYFLAV = 10;
276 const int StringFragmentation::NTRYJOIN = 30;
278 // The last few times gradually increase the stop mass to make it easier.
279 const int StringFragmentation::NSTOPMASS = 15;
280 const double StringFragmentation::FACSTOPMASS = 1.05;
282 // For closed string, pick a Gamma by taking a step with fictitious mass.
283 const double StringFragmentation::CLOSEDM2MAX = 25.;
284 const double StringFragmentation::CLOSEDM2FRAC = 0.1;
286 // Do not allow too large argument to exp function.
287 const double StringFragmentation::EXPMAX = 50.;
289 // Matching criterion that p+ and p- not the same (can happen in gg loop).
290 const double StringFragmentation::MATCHPOSNEG = 1e-6;
292 // For pull on junction, do not trace too far down each leg.
293 const double StringFragmentation::EJNWEIGHTMAX = 10.;
295 // Iterate junction rest frame boost until convergence or too many tries.
296 const double StringFragmentation::CONVJNREST = 1e-5;
297 const int StringFragmentation::NTRYJNREST = 20;
299 // Fail and try again when two legs combined to diquark (3 loops).
300 const int StringFragmentation::NTRYJNMATCH = 20;
301 const double StringFragmentation::EEXTRAJNMATCH = 0.5;
302 const double StringFragmentation::MDIQUARKMIN = -2.0;
304 // Consider junction-leg parton as massless if m2 tiny.
305 const double StringFragmentation::M2MAXJRF = 1e-4;
307 // Iterate junction rest frame equation until convergence or too many tries.
308 const double StringFragmentation::CONVJRFEQ = 1e-12;
309 const int StringFragmentation::NTRYJRFEQ = 40;
311 //--------------------------------------------------------------------------
313 // Initialize and save pointers.
315 void StringFragmentation::init(Info* infoPtrIn, Settings& settings,
316 ParticleData* particleDataPtrIn, Rndm* rndmPtrIn, StringFlav* flavSelPtrIn,
317 StringPT* pTSelPtrIn, StringZ* zSelPtrIn) {
321 particleDataPtr = particleDataPtrIn;
323 flavSelPtr = flavSelPtrIn;
324 pTSelPtr = pTSelPtrIn;
327 // Initialize the StringFragmentation class.
328 stopMass = zSelPtr->stopMass();
329 stopNewFlav = zSelPtr->stopNewFlav();
330 stopSmear = zSelPtr->stopSmear();
331 eNormJunction = settings.parm("StringFragmentation:eNormJunction");
333 = settings.parm("StringFragmentation:eBothLeftJunction");
335 = settings.parm("StringFragmentation:eMaxLeftJunction");
337 = settings.parm("StringFragmentation:eMinLeftJunction");
339 // Joining of nearby partons along the string.
340 mJoin = settings.parm("FragmentationSystems:mJoin");
342 // Initialize the b parameter of the z spectrum, used when joining jets.
343 bLund = zSelPtr->bAreaLund();
345 // Initialize the hadrons instance of an event record.
346 hadrons.init( "(string fragmentation)", particleDataPtr);
348 // Send on pointers to the two StringEnd instances.
349 posEnd.init( particleDataPtr, flavSelPtr, pTSelPtr, zSelPtr);
350 negEnd.init( particleDataPtr, flavSelPtr, pTSelPtr, zSelPtr);
354 //--------------------------------------------------------------------------
356 // Perform the fragmentation.
358 bool StringFragmentation::fragment( int iSub, ColConfig& colConfig,
361 // Find partons and their total four-momentum.
362 iParton = colConfig[iSub].iParton;
364 if (iPos < 0) iPos = iParton[1];
365 int idPos = event[iPos].id();
366 iNeg = iParton.back();
367 int idNeg = event[iNeg].id();
368 pSum = colConfig[iSub].pSum;
370 // Reset the local event record.
373 // For closed gluon string: pick first breakup region.
374 isClosed = colConfig[iSub].isClosed;
375 if (isClosed) iParton = findFirstRegion(iParton, event);
377 // For junction topology: fragment off two of the string legs.
378 // Then iParton overwritten to remaining leg + leftover diquark.
379 pJunctionHadrons = 0.;
380 hasJunction = colConfig[iSub].hasJunction;
381 if (hasJunction && !fragmentToJunction(event)) return false;
382 int junctionHadrons = hadrons.size();
384 idPos = event[ iParton[0] ].id();
385 idNeg = event.back().id();
386 pSum -= pJunctionHadrons;
389 // Set up kinematics of string evolution ( = motion).
390 system.setUp(iParton, event);
391 stopMassNow = stopMass;
394 // Fallback loop, when joining in the middle fails. Bailout if stuck.
395 for ( int iTry = 0; ; ++iTry) {
396 if (iTry > NTRYJOIN) {
397 infoPtr->errorMsg("Error in StringFragmentation::fragment: "
399 if (hasJunction) ++nExtraJoin;
400 if (nExtraJoin > 0) event.popBack(nExtraJoin);
404 // After several failed tries join some (extra) nearby partons.
405 if (iTry == NTRYJOIN / 3) nExtraJoin = extraJoin( 2., event);
406 if (iTry == 2 * NTRYJOIN / 3) nExtraJoin += extraJoin( 4., event);
408 // After several failed tries gradually allow larger stop mass.
409 if (iTry > NTRYJOIN - NSTOPMASS) stopMassNow *= FACSTOPMASS;
411 // Set up flavours of two string ends, and reset other info.
412 setStartEnds(idPos, idNeg, system);
415 // Begin fragmentation loop, interleaved from the two ends.
419 // Take a step either from the positive or the negative end.
420 fromPos = (rndmPtr->flat() < 0.5);
421 StringEnd& nowEnd = (fromPos) ? posEnd : negEnd;
423 // Construct trial hadron and check that energy remains.
425 if ( energyUsedUp(fromPos) ) break;
427 // Construct kinematics of the new hadron and store it.
428 Vec4 pHad = nowEnd.kinematicsHadron(system);
429 int statusHad = (fromPos) ? 83 : 84;
430 hadrons.append( nowEnd.idHad, statusHad, iPos, iNeg,
431 0, 0, 0, 0, pHad, nowEnd.mHad);
432 if (pHad.e() < 0.) break;
434 // Update string end and remaining momentum.
438 // End of fragmentation loop.
441 // When done, join in the middle. If this works, then really done.
442 if ( finalTwo(fromPos) ) break;
444 // Else remove produced particles (except from first two junction legs)
445 // and start all over.
446 int newHadrons = hadrons.size() - junctionHadrons;
447 hadrons.popBack(newHadrons);
450 // Junctions & extra joins: remove fictitious end, restore original partons.
451 if (hasJunction) ++nExtraJoin;
452 if (nExtraJoin > 0) {
453 event.popBack(nExtraJoin);
454 iParton = colConfig[iSub].iParton;
457 // Store the hadrons in the normal event record, ordered from one end.
465 //--------------------------------------------------------------------------
467 // Find region where to put first string break for closed gluon loop.
469 vector<int> StringFragmentation::findFirstRegion(vector<int>& iPartonIn,
472 // Evaluate mass-squared for all adjacent gluon pairs.
473 vector<double> m2Pair;
475 int size = iPartonIn.size();
476 for (int i = 0; i < size; ++i) {
477 double m2Now = 0.5 * event[ iPartonIn[i] ].p()
478 * event[ iPartonIn[(i + 1)%size] ].p();
479 m2Pair.push_back(m2Now);
483 // Pick breakup region with probability proportional to mass-squared.
484 double m2Reg = m2Sum * rndmPtr->flat();
486 do m2Reg -= m2Pair[++iReg];
487 while (m2Reg > 0. && iReg < size - 1);
489 // Create reordered parton list, with breakup string region duplicated.
490 vector<int> iPartonOut;
491 for (int i = 0; i < size + 2; ++i)
492 iPartonOut.push_back( iPartonIn[(i + iReg)%size] );
499 //--------------------------------------------------------------------------
501 // Set flavours and momentum position for initial string endpoints.
503 void StringFragmentation::setStartEnds( int idPos, int idNeg,
504 StringSystem systemNow) {
506 // Variables characterizing string endpoints: defaults for open string.
510 double xPosFromPos = 1.;
511 double xNegFromPos = 0.;
512 double xPosFromNeg = 0.;
513 double xNegFromNeg = 1.;
515 // For closed gluon loop need to pick an initial flavour.
518 int idTry = flavSelPtr->pickLightQ();
519 FlavContainer flavTry(idTry, 1);
520 flavTry = flavSelPtr->pick( flavTry);
521 flavTry = flavSelPtr->pick( flavTry);
524 } while (idPos == 0);
526 // Also need pT and breakup vertex position in region.
527 pair<double, double> pxy = pTSelPtr->pxy();
530 double m2Region = systemNow.regionLowPos(0).w2;
531 double m2Temp = min( CLOSEDM2MAX, CLOSEDM2FRAC * m2Region);
533 double zTemp = zSelPtr->zFrag( idPos, idNeg, m2Temp);
534 xPosFromPos = 1. - zTemp;
535 xNegFromPos = m2Temp / (zTemp * m2Region);
536 } while (xNegFromPos > 1.);
537 Gamma = xPosFromPos * xNegFromPos * m2Region;
538 xPosFromNeg = xPosFromPos;
539 xNegFromNeg = xNegFromPos;
542 // Initialize two string endpoints.
543 posEnd.setUp( true, iPos, idPos, systemNow.iMax, px, py,
544 Gamma, xPosFromPos, xNegFromPos);
545 negEnd.setUp( false, iNeg, idNeg, systemNow.iMax, -px, -py,
546 Gamma, xPosFromNeg, xNegFromNeg);
548 // For closed gluon loop can allow popcorn on one side but not both.
550 flavSelPtr->assignPopQ(posEnd.flavOld);
551 flavSelPtr->assignPopQ(negEnd.flavOld);
552 if (rndmPtr->flat() < 0.5) posEnd.flavOld.nPop = 0;
553 else negEnd.flavOld.nPop = 0;
554 posEnd.flavOld.rank = 1;
555 negEnd.flavOld.rank = 1;
562 //--------------------------------------------------------------------------
564 // Check remaining energy-momentum whether it is OK to continue.
566 bool StringFragmentation::energyUsedUp(bool fromPos) {
568 // If remaining negative energy then abort right away.
569 if (pRem.e() < 0.) return true;
571 // Calculate W2_minimum and done if remaining W2 is below it.
572 double wMin = stopMassNow
573 + particleDataPtr->constituentMass(posEnd.flavOld.id)
574 + particleDataPtr->constituentMass(negEnd.flavOld.id);
575 if (fromPos) wMin += stopNewFlav
576 * particleDataPtr->constituentMass(posEnd.flavNew.id);
577 else wMin += stopNewFlav
578 * particleDataPtr->constituentMass(negEnd.flavNew.id);
579 wMin *= 1. + (2. * rndmPtr->flat() - 1.) * stopSmear;
580 w2Rem = pRem.m2Calc();
581 if (w2Rem < pow2(wMin)) return true;
583 // Else still enough energy left to continue iteration.
588 //--------------------------------------------------------------------------
590 // Produce the final two partons to complete the system.
592 bool StringFragmentation::finalTwo(bool fromPos) {
594 // Check whether we went too far in p+-.
595 if (pRem.e() < 0. || w2Rem < 0. || (hadrons.size() > 0
596 && hadrons.back().e() < 0.) ) return false;
597 if ( posEnd.iPosOld > negEnd.iPosOld || negEnd.iNegOld > posEnd.iNegOld)
599 if ( posEnd.iPosOld == negEnd.iPosOld && posEnd.xPosOld < negEnd.xPosOld)
601 if ( posEnd.iNegOld == negEnd.iNegOld && posEnd.xNegOld > negEnd.xNegOld)
604 // Construct the final hadron from the leftover flavours.
605 // Impossible to join two diquarks. Also break if stuck for other reason.
606 FlavContainer flav1 = (fromPos) ? posEnd.flavNew.anti() : posEnd.flavOld;
607 FlavContainer flav2 = (fromPos) ? negEnd.flavOld : negEnd.flavNew.anti();
608 if (flav1.isDiquark() && flav2.isDiquark()) return false;
610 for (int iTry = 0; iTry < NTRYFLAV; ++iTry) {
611 idHad = flavSelPtr->combine( flav1, flav2);
612 if (idHad != 0) break;
614 if (idHad == 0) return false;
616 // Store the final particle and its new pT, and construct its mass.
618 negEnd.idHad = idHad;
619 negEnd.pxNew = -posEnd.pxNew;
620 negEnd.pyNew = -posEnd.pyNew;
621 negEnd.mHad = particleDataPtr->mass(idHad);
623 posEnd.idHad = idHad;
624 posEnd.pxNew = -negEnd.pxNew;
625 posEnd.pyNew = -negEnd.pyNew;
626 posEnd.mHad = particleDataPtr->mass(idHad);
629 // String region in which to do the joining.
630 StringRegion region = finalRegion();
631 if (region.isEmpty) return false;
633 // Project remaining momentum along longitudinal and transverse directions.
634 region.project( pRem);
635 double pxRem = region.px() - posEnd.pxOld - negEnd.pxOld;
636 double pyRem = region.py() - posEnd.pyOld - negEnd.pyOld;
637 double xPosRem = region.xPos();
638 double xNegRem = region.xNeg();
640 // Share extra pT kick evenly between final two hadrons.
641 posEnd.pxOld += 0.5 * pxRem;
642 posEnd.pyOld += 0.5 * pyRem;
643 negEnd.pxOld += 0.5 * pxRem;
644 negEnd.pyOld += 0.5 * pyRem;
646 // Construct new pT and mT of the final two particles.
647 posEnd.pxHad = posEnd.pxOld + posEnd.pxNew;
648 posEnd.pyHad = posEnd.pyOld + posEnd.pyNew;
649 posEnd.mT2Had = pow2(posEnd.mHad) + pow2(posEnd.pxHad)
650 + pow2(posEnd.pyHad);
651 negEnd.pxHad = negEnd.pxOld + negEnd.pxNew;
652 negEnd.pyHad = negEnd.pyOld + negEnd.pyNew;
653 negEnd.mT2Had = pow2(negEnd.mHad) + pow2(negEnd.pxHad)
654 + pow2(negEnd.pyHad);
656 // Construct remaining system transverse mass.
657 double wT2Rem = w2Rem + pow2( posEnd.pxHad + negEnd.pxHad)
658 + pow2( posEnd.pyHad + negEnd.pyHad);
660 // Check that kinematics possible.
661 if ( sqrt(wT2Rem) < sqrt(posEnd.mT2Had) + sqrt(negEnd.mT2Had) )
663 double lambda2 = pow2( wT2Rem - posEnd.mT2Had - negEnd.mT2Had)
664 - 4. * posEnd.mT2Had * negEnd.mT2Had;
665 if (lambda2 <= 0.) return false;
667 // Construct kinematics, as viewed in the transverse rest frame.
668 double lambda = sqrt(lambda2);
669 double probReverse = 1. / (1. + exp( min( EXPMAX, bLund * lambda) ) );
670 double xpzPos = 0.5 * lambda/ wT2Rem;
671 if (probReverse > rndmPtr->flat()) xpzPos = -xpzPos;
672 double xmDiff = (posEnd.mT2Had - negEnd.mT2Had) / wT2Rem;
673 double xePos = 0.5 * (1. + xmDiff);
674 double xeNeg = 0.5 * (1. - xmDiff );
676 // Translate this into kinematics in the string frame.
677 Vec4 pHadPos = region.pHad( (xePos + xpzPos) * xPosRem,
678 (xePos - xpzPos) * xNegRem, posEnd.pxHad, posEnd.pyHad);
679 Vec4 pHadNeg = region.pHad( (xeNeg - xpzPos) * xPosRem,
680 (xeNeg + xpzPos) * xNegRem, negEnd.pxHad, negEnd.pyHad);
682 // Add produced particles to the event record.
683 hadrons.append( posEnd.idHad, 83, posEnd.iEnd, negEnd.iEnd,
684 0, 0, 0, 0, pHadPos, posEnd.mHad);
685 hadrons.append( negEnd.idHad, 84, posEnd.iEnd, negEnd.iEnd,
686 0, 0, 0, 0, pHadNeg, negEnd.mHad);
693 //--------------------------------------------------------------------------
695 // Construct a special joining region for the final two hadrons.
697 StringRegion StringFragmentation::finalRegion() {
699 // Simple case when both string ends are in the same region.
700 if (posEnd.iPosOld == negEnd.iPosOld && posEnd.iNegOld == negEnd.iNegOld)
701 return system.region( posEnd.iPosOld, posEnd.iNegOld);
703 // Start out with empty region. (Empty used for error returns.)
706 // Add up all remaining p+.
708 if ( posEnd.iPosOld == negEnd.iPosOld) {
709 double xPosJoin = posEnd.xPosOld - negEnd.xPosOld;
710 if (xPosJoin < 0.) return region;
711 pPosJoin = system.regionLowPos(posEnd.iPosOld).pHad( xPosJoin, 0., 0., 0.);
713 for (int iPosNow = posEnd.iPosOld; iPosNow <= negEnd.iPosOld; ++iPosNow) {
714 if (iPosNow == posEnd.iPosOld) pPosJoin
715 += system.regionLowPos(iPosNow).pHad( posEnd.xPosOld, 0., 0., 0.);
716 else if (iPosNow == negEnd.iPosOld) pPosJoin
717 += system.regionLowPos(iPosNow).pHad( 1. - negEnd.xPosOld, 0., 0., 0.);
718 else pPosJoin += system.regionLowPos(iPosNow).pHad( 1., 0., 0., 0.);
722 // Add up all remaining p-.
724 if ( negEnd.iNegOld == posEnd.iNegOld) {
725 double xNegJoin = negEnd.xNegOld - posEnd.xNegOld;
726 if (xNegJoin < 0.) return region;
727 pNegJoin = system.regionLowNeg(negEnd.iNegOld).pHad( 0., xNegJoin, 0., 0.);
729 for (int iNegNow = negEnd.iNegOld; iNegNow <= posEnd.iNegOld; ++iNegNow) {
730 if (iNegNow == negEnd.iNegOld) pNegJoin
731 += system.regionLowNeg(iNegNow).pHad( 0., negEnd.xNegOld, 0., 0.);
732 else if (iNegNow == posEnd.iNegOld) pNegJoin
733 += system.regionLowNeg(iNegNow).pHad( 0., 1. - posEnd.xNegOld, 0., 0.);
734 else pNegJoin += system.regionLowNeg(iNegNow).pHad( 0., 1., 0., 0.);
738 // For a closed gluon loop pPosJoin == pNegJoin and the above does not work.
739 // So reshuffle; "perfect" for g g systems, OK in general.
740 Vec4 pTest = pPosJoin - pNegJoin;
741 if ( abs(pTest.px()) + abs(pTest.py()) + abs(pTest.pz()) + abs(pTest.e())
742 < MATCHPOSNEG * (pPosJoin.e() + pNegJoin.e()) ) {
744 = system.regionLowPos(posEnd.iPosOld + 1).pHad( 1., 0., 0., 0.)
745 - system.regionLowNeg(negEnd.iNegOld + 1).pHad( 0., 1., 0., 0.);
750 // Construct a new region from remaining p+ and p-.
751 region.setUp( pPosJoin, pNegJoin);
752 if (region.isEmpty) return region;
754 // Project the existing pTold vectors onto the new directions.
755 Vec4 pTposOld = system.region( posEnd.iPosOld, posEnd.iNegOld).pHad(
756 0., 0., posEnd.pxOld, posEnd.pyOld);
757 region.project( pTposOld);
758 posEnd.pxOld = region.px();
759 posEnd.pyOld = region.py();
760 Vec4 pTnegOld = system.region( negEnd.iPosOld, negEnd.iNegOld).pHad(
761 0., 0., negEnd.pxOld, negEnd.pyOld);
762 region.project( pTnegOld);
763 negEnd.pxOld = region.px();
764 negEnd.pyOld = region.py();
771 //--------------------------------------------------------------------------
773 // Store the hadrons in the normal event record, ordered from one end.
775 void StringFragmentation::store(Event& event) {
777 // Starting position.
778 int iFirst = event.size();
780 // Copy straight over from first two junction legs.
782 for (int i = 0; i < hadrons.size(); ++i)
783 if (hadrons[i].status() == 85 || hadrons[i].status() == 86)
784 event.append( hadrons[i] );
787 // Loop downwards, copying all from the positive end.
788 for (int i = 0; i < hadrons.size(); ++i)
789 if (hadrons[i].status() == 83) event.append( hadrons[i] );
791 // Loop upwards, copying all from the negative end.
792 for (int i = hadrons.size() - 1; i >= 0 ; --i)
793 if (hadrons[i].status() == 84) event.append( hadrons[i] );
794 int iLast = event.size() - 1;
796 // Set decay vertex when this is displaced.
797 if (event[posEnd.iEnd].hasVertex()) {
798 Vec4 vDec = event[posEnd.iEnd].vDec();
799 for (int i = iFirst; i <= iLast; ++i) event[i].vProd( vDec );
802 // Set lifetime of hadrons.
803 for (int i = iFirst; i <= iLast; ++i)
804 event[i].tau( event[i].tau0() * rndmPtr->exp() );
806 // Mark original partons as hadronized and set their daughter range.
807 for (int i = 0; i < int(iParton.size()); ++i)
808 if (iParton[i] >= 0) {
809 event[ iParton[i] ].statusNeg();
810 event[ iParton[i] ].daughters(iFirst, iLast);
815 //--------------------------------------------------------------------------
817 // Fragment off two of the string legs in to a junction.
819 bool StringFragmentation::fragmentToJunction(Event& event) {
821 // Identify range of partons on the three legs.
822 // (Each leg begins with an iParton[i] = -(10 + 10*junctionNumber + leg),
823 // and partons then appear ordered from the junction outwards.)
824 int legBeg[3] = { 0, 0, 0};
825 int legEnd[3] = { 0, 0, 0};
827 // PS (4/10/2011) Protect against invalid systems
828 if (iParton[0] > 0) {
829 infoPtr->errorMsg("Error in StringFragmentation::fragment"
830 "ToJunction: iParton[0] not a valid junctionNumber");
833 for (int i = 0; i < int(iParton.size()); ++i) {
834 if (iParton[i] < 0) {
836 infoPtr->errorMsg("Error in StringFragmentation::fragment"
837 "ToJunction: unprocessed multi-junction system");
840 legBeg[++leg] = i + 1;
842 else legEnd[leg] = i;
845 // Iterate from system rest frame towards the junction rest frame (JRF).
846 RotBstMatrix MtoJRF, Mstep;
847 MtoJRF.bstback(pSum);
854 // Find weighted sum of momenta on the three sides of the junction.
855 for (leg = 0; leg < 3; ++ leg) {
858 for (int i = legBeg[leg]; i <= legEnd[leg]; ++i) {
859 Vec4 pTemp = event[ iParton[i] ].p();
860 pTemp.rotbst(MtoJRF);
861 pWTinJRF[leg] += pTemp * exp(-eWeight);
862 eWeight += pTemp.e() / eNormJunction;
863 if (eWeight > EJNWEIGHTMAX) break;
867 // Store original deviation from 120 degree topology.
868 if (iter == 1) errInCM = pow2(costheta(pWTinJRF[0], pWTinJRF[1]) + 0.5)
869 + pow2(costheta(pWTinJRF[0], pWTinJRF[2]) + 0.5)
870 + pow2(costheta(pWTinJRF[1], pWTinJRF[2]) + 0.5);
872 // Find new JRF from the set of weighted momenta.
873 Mstep = junctionRestFrame( pWTinJRF[0], pWTinJRF[1], pWTinJRF[2]);
874 // Fortran code will not take full step after the first few
875 // iterations. How implement this in terms of an M matrix??
876 MtoJRF.rotbst( Mstep );
877 } while (iter < 3 || (Mstep.deviation() > CONVJNREST && iter < NTRYJNREST) );
879 // If final deviation from 120 degrees is bigger than in CM then revert.
880 double errInJRF = pow2(costheta(pWTinJRF[0], pWTinJRF[1]) + 0.5)
881 + pow2(costheta(pWTinJRF[0], pWTinJRF[2]) + 0.5)
882 + pow2(costheta(pWTinJRF[1], pWTinJRF[2]) + 0.5);
883 if (errInJRF > errInCM) {
884 infoPtr->errorMsg("Warning in StringFragmentation::fragmentTo"
885 "Junction: bad convergence junction rest frame");
887 MtoJRF.bstback(pSum);
890 // Opposite operation: boost from JRF to original system.
891 RotBstMatrix MfromJRF = MtoJRF;
894 // Sum leg four-momenta in original frame and in JRF.
895 Vec4 pInLeg[3], pInJRF[3];
896 for (leg = 0; leg < 3; ++leg) {
898 for (int i = legBeg[leg]; i <= legEnd[leg]; ++i)
899 pInLeg[leg] += event[ iParton[i] ].p();
900 pInJRF[leg] = pInLeg[leg];
901 pInJRF[leg].rotbst(MtoJRF);
904 // Pick the two legs with lowest energy in JRF.
905 int legMin = (pInJRF[0].e() < pInJRF[1].e()) ? 0 : 1;
906 int legMax = 1 - legMin;
907 if (pInJRF[2].e() < min(pInJRF[0].e(), pInJRF[1].e()) ) legMin = 2;
908 else if (pInJRF[2].e() > max(pInJRF[0].e(), pInJRF[1].e()) ) legMax = 2;
909 int legMid = 3 - legMin - legMax;
911 // Save info on which status codes belong with the three legs.
912 int iJunction = (-iParton[0]) / 10 - 1;
913 event.statusJunction( iJunction, legMin, 85);
914 event.statusJunction( iJunction, legMid, 86);
915 event.statusJunction( iJunction, legMax, 83);
917 // Temporarily copy the partons on the low-energy legs, into the JRF,
918 // in reverse order, so (anti)quark leg end first.
919 vector<int> iPartonMin;
920 for (int i = legEnd[legMin]; i >= legBeg[legMin]; --i) {
921 int iNew = event.append( event[ iParton[i] ] );
922 event[iNew].rotbst(MtoJRF);
923 iPartonMin.push_back( iNew );
925 vector<int> iPartonMid;
926 for (int i = legEnd[legMid]; i >= legBeg[legMid]; --i) {
927 int iNew = event.append( event[ iParton[i] ] );
928 event[iNew].rotbst(MtoJRF);
929 iPartonMid.push_back( iNew );
932 // Find final weighted sum of momenta on each of the two legs.
934 pWTinJRF[legMin] = 0.;
935 for (int i = iPartonMin.size() - 1; i >= 0; --i) {
936 pWTinJRF[legMin] += event[ iPartonMin[i] ].p() * exp(-eWeight);
937 eWeight += event[ iPartonMin[i] ].e() / eNormJunction;
938 if (eWeight > EJNWEIGHTMAX) break;
941 pWTinJRF[legMid] = 0.;
942 for (int i = iPartonMid.size() - 1; i >= 0; --i) {
943 pWTinJRF[legMid] += event[ iPartonMid[i] ].p() * exp(-eWeight);
944 eWeight += event[ iPartonMid[i] ].e() / eNormJunction;
945 if (eWeight > EJNWEIGHTMAX) break;
948 // Define fictitious opposing partons in JRF and store as string ends.
949 Vec4 pOppose = pWTinJRF[legMin];
951 int idOppose = (rndmPtr->flat() > 0.5) ? 2 : 1;
952 if (event[ iPartonMin[0] ].col() > 0) idOppose = -idOppose;
953 int iOppose = event.append( idOppose, 77, 0, 0, 0, 0, 0, 0,
955 iPartonMin.push_back( iOppose);
956 pOppose = pWTinJRF[legMid];
958 idOppose = (rndmPtr->flat() > 0.5) ? 2 : 1;
959 if (event[ iPartonMid[0] ].col() > 0) idOppose = -idOppose;
960 iOppose = event.append( idOppose, 77, 0, 0, 0, 0, 0, 0,
962 iPartonMid.push_back( iOppose);
964 // Set up kinematics of string evolution in low-energy temporary systems.
965 systemMin.setUp(iPartonMin, event);
966 systemMid.setUp(iPartonMid, event);
968 // Outer fallback loop, when too little energy left for third leg.
972 for ( int iTryOuter = 0; ; ++iTryOuter) {
974 // Middle fallback loop, when much unused energy in leg remnants.
975 double eLeftMin = 0.;
976 double eLeftMid = 0.;
977 for ( int iTryMiddle = 0; ; ++iTryMiddle) {
979 // Loop over the two lowest-energy legs.
980 for (int legLoop = 0; legLoop < 2; ++ legLoop) {
981 int legNow = (legLoop == 0) ? legMin : legMid;
983 // Read in properties specific to this leg.
984 StringSystem& systemNow = (legLoop == 0) ? systemMin : systemMid;
985 int idPos = (legLoop == 0) ? event[ iPartonMin[0] ].id()
986 : event[ iPartonMid[0] ].id();
987 idOppose = (legLoop == 0) ? event[ iPartonMin.back() ].id()
988 : event[ iPartonMid.back() ].id();
989 double eInJRF = pInJRF[legNow].e();
990 int statusHad = (legLoop == 0) ? 85 : 86;
992 // Inner fallback loop, when a diquark comes in to junction.
994 for ( int iTryInner = 0; ; ++iTryInner) {
995 if (iTryInner > 2 * NTRYJNMATCH) {
996 infoPtr->errorMsg("Error in StringFragmentation::fragment"
997 "ToJunction: caught in junction flavour loop");
998 event.popBack( iPartonMin.size() + iPartonMid.size() );
1001 bool needBaryon = (abs(idPos) > 10 && iTryInner > NTRYJNMATCH);
1002 double eExtra = (iTryInner > NTRYJNMATCH) ? EEXTRAJNMATCH : 0.;
1004 // Set up two string ends, and begin fragmentation loop.
1005 setStartEnds(idPos, idOppose, systemNow);
1009 for ( ; ; ++nHadrons) {
1011 // Construct trial hadron from positive end.
1013 Vec4 pHad = posEnd.kinematicsHadron(systemNow);
1015 // Negative energy signals failure in construction.
1016 if (pHad.e() < 0. ) { noNegE = false; break; }
1018 // Break if passed system midpoint ( = junction) in energy.
1019 // Exceptions: small systems, and/or with diquark end.
1020 bool delayedBreak = false;
1021 if (eUsed + pHad.e() + eExtra > eInJRF) {
1022 if (nHadrons > 0 || !needBaryon) break;
1023 delayedBreak = true;
1026 // Else construct kinematics of the new hadron and store it.
1027 hadrons.append( posEnd.idHad, statusHad, iPos, iNeg,
1028 0, 0, 0, 0, pHad, posEnd.mHad);
1030 // Update string end and remaining momentum.
1034 // Delayed break in small systems, and/or with diquark end.
1041 // End of fragmentation loop. Inner loopback if ends on a diquark.
1042 if ( noNegE && abs(posEnd.flavOld.id) < 10 ) break;
1043 hadrons.popBack(nHadrons);
1046 // End of one-leg fragmentation. Store end quark and remnant energy.
1047 if (legNow == legMin) {
1048 idMin = posEnd.flavOld.id;
1049 eLeftMin = eInJRF - eUsed;
1051 idMid = posEnd.flavOld.id;
1052 eLeftMid = eInJRF - eUsed;
1056 // End of both-leg fragmentation.
1057 // Middle loopback if too much energy left.
1058 double eTrial = eBothLeftJunction + rndmPtr->flat() * eMaxLeftJunction;
1059 if (iTryMiddle > NTRYJNMATCH
1060 || ( min( eLeftMin, eLeftMid) < eBothLeftJunction
1061 && max( eLeftMin, eLeftMid) < eTrial ) ) break;
1065 // Boost hadrons away from the JRF to the original frame.
1066 for (int i = 0; i < hadrons.size(); ++i) {
1067 hadrons[i].rotbst(MfromJRF);
1068 // Recalculate energy to compensate for numerical precision loss
1069 // in iterative calculation of MfromJRF.
1070 hadrons[i].e( hadrons[i].eCalc() );
1071 pJunctionHadrons += hadrons[i].p();
1074 // Outer loopback if combined diquark mass too negative
1075 // or too little energy left in third leg.
1076 pDiquark = pInLeg[legMin] + pInLeg[legMid] - pJunctionHadrons;
1077 double m2Left = m2( pInLeg[legMax], pDiquark);
1078 if (iTryOuter > NTRYJNMATCH || (pDiquark.mCalc() > MDIQUARKMIN
1079 && m2Left > eMinLeftJunction * pInLeg[legMax].e()) ) break;
1081 pJunctionHadrons = 0.;
1084 // Now found solution; no more loopback. Remove temporary parton copies.
1085 event.popBack( iPartonMin.size() + iPartonMid.size() );
1087 // Construct and store an effective diquark string end from the
1088 // two remnant quark ends, for temporary usage.
1089 int idDiquark = flavSelPtr->makeDiquark( idMin, idMid);
1090 double mDiquark = pDiquark.mCalc();
1091 int iDiquark = event.append( idDiquark, 78, 0, 0, 0, 0, 0, 0,
1092 pDiquark, mDiquark);
1094 // Find the partons on the last leg, again in reverse order.
1095 vector<int> iPartonMax;
1096 for (int i = legEnd[legMax]; i >= legBeg[legMax]; --i)
1097 iPartonMax.push_back( iParton[i] );
1098 iPartonMax.push_back( iDiquark );
1100 // Recluster gluons nearby to diquark end when taken too much energy.
1101 int iPsize = iPartonMax.size();
1102 double m0Diquark = event[iDiquark].m0();
1103 while (iPsize > 2) {
1104 Vec4 pGluNear = event[ iPartonMax[iPsize - 2] ].p();
1105 if ( pDiquark.mCalc() > 0.
1106 && (pDiquark + 0.5 * pGluNear).mCalc() > m0Diquark + mJoin ) break;
1107 pDiquark += pGluNear;
1108 event[iDiquark].p( pDiquark );
1109 event[iDiquark].m( pDiquark.mCalc() );
1110 iPartonMax.pop_back();
1112 iPartonMax[iPsize - 1] = iDiquark;
1115 // Modify parton list to remaining leg + remnant of the first two.
1116 iParton = iPartonMax;
1122 //--------------------------------------------------------------------------
1124 // Find the boost matrix to the rest frame of a junction,
1125 // given the three respective endpoint four-momenta.
1127 RotBstMatrix StringFragmentation::junctionRestFrame(Vec4& p0, Vec4& p1,
1130 // Calculate masses and other invariants.
1131 Vec4 pSumJun = p0 + p1 + p2;
1132 double sHat = pSumJun.m2Calc();
1134 pp[0][0] = p0.m2Calc();
1135 pp[1][1] = p1.m2Calc();
1136 pp[2][2] = p2.m2Calc();
1137 pp[0][1] = pp[1][0] = p0 * p1;
1138 pp[0][2] = pp[2][0] = p0 * p2;
1139 pp[1][2] = pp[2][1] = p1 * p2;
1141 // Requirement (eiMax)_j = pi*pj/mj < (eiMax)_k = pi*pk/mk, used below,
1142 // here rewritten as pi*pj * mk < pi*pk * mj and squared.
1143 double eMax01 = pow2(pp[0][1]) * pp[2][2];
1144 double eMax02 = pow2(pp[0][2]) * pp[1][1];
1145 double eMax12 = pow2(pp[1][2]) * pp[0][0];
1147 // Initially pick i to be the most massive parton. but allow other tries.
1148 int i = (pp[1][1] > pp[0][0]) ? 1 : 0;
1149 if (pp[2][2] > max(pp[0][0], pp[1][1])) i = 2;
1154 for (int iTry = 0; iTry < 3; ++iTry) {
1156 // Pick j to give minimal eiMax, and k the third vector.
1157 if (i == 0) j = (eMax02 < eMax01) ? 2 : 1;
1158 else if (i == 1) j = (eMax12 < eMax01) ? 2 : 0;
1159 else j = (eMax12 < eMax02) ? 1 : 0;
1162 // Alternative names according to i, j, k conventions.
1163 double m2i = pp[i][i];
1164 double m2j = pp[j][j];
1165 double m2k = pp[k][k];
1166 double pipj = pp[i][j];
1167 double pipk = pp[i][k];
1168 double pjpk = pp[j][k];
1170 // Trivial to find new parton energies if all three partons are massless.
1171 if (m2i < M2MAXJRF) {
1172 ei = sqrt( 2. * pipk * pipj / (3. * pjpk) );
1173 ej = sqrt( 2. * pjpk * pipj / (3. * pipk) );
1174 ek = sqrt( 2. * pipk * pjpk / (3. * pipj) );
1176 // Else find three-momentum range for parton i and values at extremes.
1178 // Minimum when i is at rest.
1180 double eiMin = sqrt(m2i);
1181 double ejMin = pipj / eiMin;
1182 double ekMin = pipk / eiMin;
1183 double pjMin = sqrtpos( ejMin*ejMin - m2j );
1184 double pkMin = sqrtpos( ekMin*ekMin - m2k );
1185 double fMin = ejMin * ekMin + 0.5 * pjMin * pkMin - pjpk;
1186 // Maximum estimated when j + k is at rest, alternatively j at rest.
1187 double eiMax = (pipj + pipk) / sqrt(m2j + m2k + 2. * pjpk);
1188 if (m2j > M2MAXJRF) eiMax = min( eiMax, pipj / sqrt(m2j) );
1189 double piMax = sqrtpos( eiMax*eiMax - m2i );
1190 double temp = eiMax*eiMax - 0.25 *piMax*piMax;
1191 double pjMax = (eiMax * sqrtpos( pipj*pipj - m2j * temp )
1192 - 0.5 * piMax * pipj) / temp;
1193 double pkMax = (eiMax * sqrtpos( pipk*pipk - m2k * temp )
1194 - 0.5 * piMax * pipk) / temp;
1195 double ejMax = sqrt(pjMax*pjMax + m2j);
1196 double ekMax = sqrt(pkMax*pkMax + m2k);
1197 double fMax = ejMax * ekMax + 0.5 * pjMax * pkMax - pjpk;
1199 // If unexpected values at upper endpoint then pick another parton.
1201 int iPrel = (i + 1)%3;
1202 if (pp[iPrel][iPrel] > M2MAXJRF) {i = iPrel; continue;}
1205 if (iTry < 3 && pp[iPrel][iPrel] > M2MAXJRF) {i = iPrel; continue;}
1208 // Start binary + linear search to find solution inside range.
1211 double pi = 0.5 * (piMin + piMax);
1212 for (int iter = 0; iter < NTRYJRFEQ; ++iter) {
1214 // Derive momentum of other two partons and distance to root.
1215 ei = sqrt(pi*pi + m2i);
1216 temp = ei*ei - 0.25 * pi*pi;
1217 double pj = (ei * sqrtpos( pipj*pipj - m2j * temp )
1218 - 0.5 * pi * pipj) / temp;
1219 double pk = (ei * sqrtpos( pipk*pipk - m2k * temp )
1220 - 0.5 * pi * pipk) / temp;
1221 ej = sqrt(pj*pj + m2j);
1222 ek = sqrt(pk*pk + m2k);
1223 double fNow = ej * ek + 0.5 * pj * pk - pjpk;
1225 // Replace lower or upper bound by new value.
1226 if (fNow > 0.) { ++iterMin; piMin = pi; fMin = fNow;}
1227 else {++iterMax; piMax = pi; fMax = fNow;}
1229 // Pick next i momentum to explore, hopefully closer to root.
1230 if (2 * iter < NTRYJRFEQ
1231 && (iterMin < 2 || iterMax < 2 || 4 * iter < NTRYJRFEQ))
1232 { pi = 0.5 * (piMin + piMax); continue;}
1233 if (fMin < 0. || fMax > 0. || abs(fNow) < CONVJRFEQ * sHat) break;
1234 pi = piMin + (piMax - piMin) * fMin / (fMin - fMax);
1237 // If arrived here then either succeeded or exhausted possibilities.
1241 // Now we know the energies in the junction rest frame.
1242 double eNew[3] = { 0., 0., 0.};
1247 // Boost (copy of) partons to their rest frame.
1252 Mmove.bstback(pSumJun);
1257 // Construct difference vectors and the boost to junction rest frame.
1258 Vec4 pDir01 = p0cm / p0cm.e() - p1cm / p1cm.e();
1259 Vec4 pDir02 = p0cm / p0cm.e() - p2cm / p2cm.e();
1260 double pDiff01 = pDir01.pAbs2();
1261 double pDiff02 = pDir02.pAbs2();
1262 double pDiff0102 = dot3(pDir01, pDir02);
1263 double eDiff01 = eNew[0] / p0cm.e() - eNew[1] / p1cm.e();
1264 double eDiff02 = eNew[0] / p0cm.e() - eNew[2] / p2cm.e();
1265 double denom = pDiff01 * pDiff02 - pDiff0102*pDiff0102;
1266 double coef01 = (eDiff01 * pDiff02 - eDiff02 * pDiff0102) / denom;
1267 double coef02 = (eDiff02 * pDiff01 - eDiff01 * pDiff0102) / denom;
1268 Vec4 vJunction = coef01 * pDir01 + coef02 * pDir02;
1269 vJunction.e( sqrt(1. + vJunction.pAbs2()) );
1271 // Add two boosts, giving final result.
1272 Mmove.bst(vJunction);
1277 //--------------------------------------------------------------------------
1279 // When string fragmentation has failed several times,
1280 // try to join some more nearby partons.
1282 int StringFragmentation::extraJoin(double facExtra, Event& event) {
1284 // Keep on looping while pairs found below joining threshold.
1286 int iPsize = iParton.size();
1287 while (iPsize > 2) {
1289 // Look for the pair of neighbour partons (along string) with
1290 // the smallest invariant mass (subtracting quark masses).
1292 double mJoinMin = 2. * facExtra * mJoin;
1293 for (int i = 0; i < iPsize - 1; ++i) {
1294 Particle& parton1 = event[ iParton[i] ];
1295 Particle& parton2 = event[ iParton[i + 1] ];
1297 pSumNow += (parton2.isGluon()) ? 0.5 * parton1.p() : parton1.p();
1298 pSumNow += (parton2.isGluon()) ? 0.5 * parton2.p() : parton2.p();
1299 double mJoinNow = pSumNow.mCalc();
1300 if (!parton1.isGluon()) mJoinNow -= parton1.m0();
1301 if (!parton2.isGluon()) mJoinNow -= parton2.m0();
1302 if (mJoinNow < mJoinMin) { iJoinMin = i; mJoinMin = mJoinNow; }
1305 // Decide whether to join, if not finished.
1306 if (iJoinMin < 0 || mJoinMin > facExtra * mJoin) return nJoin;
1309 // Create new joined parton.
1310 int iJoin1 = iParton[iJoinMin];
1311 int iJoin2 = iParton[iJoinMin + 1];
1312 int idNew = (event[iJoin1].isGluon()) ? event[iJoin2].id()
1313 : event[iJoin1].id();
1314 int colNew = event[iJoin1].col();
1315 int acolNew = event[iJoin2].acol();
1316 if (colNew == acolNew) {
1317 colNew = event[iJoin2].col();
1318 acolNew = event[iJoin1].acol();
1320 Vec4 pNew = event[iJoin1].p() + event[iJoin2].p();
1322 // Append joined parton to event record and reduce parton list.
1323 int iNew = event.append( idNew, 73, min(iJoin1, iJoin2),
1324 max(iJoin1, iJoin2), 0, 0, colNew, acolNew, pNew, pNew.mCalc() );
1325 iParton[iJoinMin] = iNew;
1326 for (int i = iJoinMin + 1; i < iPsize - 1; ++i)
1327 iParton[i] = iParton[i + 1];
1336 //==========================================================================
1338 } // end namespace Pythia8