1 // BoseEinstein.cc is a part of the PYTHIA event generator.
2 // Copyright (C) 2008 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 BoseEinsten class.
8 #include "BoseEinstein.h"
12 //**************************************************************************
14 // The BoseEinstein class.
18 // Constants: could be changed here if desired, but normally should not.
19 // These are of technical nature, as described for each.
21 // Enumeration of id codes and table for particle species considered.
22 const int BoseEinstein::IDHADRON[9] = { 211, -211, 111, 321, -321,
24 const int BoseEinstein::ITABLE[9] = { 0, 0, 0, 1, 1, 1, 1, 2, 3 };
26 // Distance between table entries, normalized to min( 2*mass, QRef).
27 const double BoseEinstein::STEPSIZE = 0.05;
29 // Skip shift for two extremely close particles, to avoid instabilities.
30 const double BoseEinstein::Q2MIN = 1e-8;
32 // Parameters of energy compensation procedure: maximally allowed
33 // relative energy error, iterative stepsize, and number of iterations.
34 const double BoseEinstein::COMPRELERR = 1e-10;
35 const double BoseEinstein::COMPFACMAX = 1000.;
36 const int BoseEinstein::NCOMPSTEP = 10;
40 // Find settings. Precalculate table used to find momentum shifts.
42 bool BoseEinstein::init(Info* infoPtrIn) {
48 doPion = Settings::flag("BoseEinstein:Pion");
49 doKaon = Settings::flag("BoseEinstein:Kaon");
50 doEta = Settings::flag("BoseEinstein:Eta");
52 // Shape of Bose-Einstein enhancement/suppression.
53 lambda = Settings::parm("BoseEinstein:lambda");
54 QRef = Settings::parm("BoseEinstein:QRef");
56 // Multiples and inverses (= "radii") of distance parameters in Q-space.
59 R2Ref = 1. / (QRef * QRef);
60 R2Ref2 = 1. / (QRef2 * QRef2);
61 R2Ref3 = 1. / (QRef3 * QRef3);
63 // Masses of particles with Bose-Einstein implemented.
64 for (int iSpecies = 0; iSpecies < 9; ++iSpecies)
65 mHadron[iSpecies] = ParticleDataTable::m0( IDHADRON[iSpecies] );
67 // Pair pi, K, eta and eta' masses for use in tables.
68 mPair[0] = 2. * mHadron[0];
69 mPair[1] = 2. * mHadron[3];
70 mPair[2] = 2. * mHadron[7];
71 mPair[3] = 2. * mHadron[8];
73 // Loop over the four required tables. Local variables.
74 double Qnow, Q2now, centerCorr;
75 for (int iTab = 0; iTab < 4; ++iTab) {
76 m2Pair[iTab] = mPair[iTab] * mPair[iTab];
78 // Step size and number of steps in normal table.
79 deltaQ[iTab] = STEPSIZE * min(mPair[iTab], QRef);
80 nStep[iTab] = min( 199, 1 + int(3. * QRef / deltaQ[iTab]) );
81 maxQ[iTab] = (nStep[iTab] - 0.1) * deltaQ[iTab];
82 centerCorr = deltaQ[iTab] * deltaQ[iTab] / 12.;
84 // Construct normal table recursively in Q space.
86 for (int i = 1; i <= nStep[iTab]; ++i) {
87 Qnow = deltaQ[iTab] * (i - 0.5);
89 shift[iTab][i] = shift[iTab][i - 1] + exp(-Q2now * R2Ref)
90 * deltaQ[iTab] * (Q2now + centerCorr) / sqrt(Q2now + m2Pair[iTab]);
93 // Step size and number of steps in compensation table.
94 deltaQ3[iTab] = STEPSIZE * min(mPair[iTab], QRef3);
95 nStep3[iTab] = min( 199, 1 + int(9. * QRef / deltaQ3[iTab]) );
96 maxQ3[iTab] = (nStep3[iTab] - 0.1) * deltaQ3[iTab];
97 centerCorr = deltaQ3[iTab] * deltaQ3[iTab] / 12.;
99 // Construct compensation table recursively in Q space.
100 shift3[iTab][0] = 0.;
101 for (int i = 1; i <= nStep3[iTab]; ++i) {
102 Qnow = deltaQ3[iTab] * (i - 0.5);
104 shift3[iTab][i] = shift3[iTab][i - 1] + exp(-Q2now * R2Ref3)
105 * deltaQ3[iTab] * (Q2now + centerCorr) / sqrt(Q2now + m2Pair[iTab]);
117 // Perform Bose-Einstein corrections on an event.
119 bool BoseEinstein::shiftEvent( Event& event) {
121 // Reset list of identical particles.
124 // Loop over all hadron species with BE effects.
126 for (int iSpecies = 0; iSpecies < 9; ++iSpecies) {
127 nStored[iSpecies + 1] = nStored[iSpecies];
128 if (!doPion && iSpecies <= 2) continue;
129 if (!doKaon && iSpecies >= 3 && iSpecies <= 6) continue;
130 if (!doEta && iSpecies >= 7) continue;
132 // Properties of current hadron species.
133 int idNow = IDHADRON[ iSpecies ];
134 int iTab = ITABLE[ iSpecies ];
136 // Loop through event record to store copies of current species.
137 for (int i = 0; i < event.size(); ++i)
138 if ( event[i].id() == idNow && event[i].isFinal() )
140 BoseEinsteinHadron( idNow, i, event[i].p(), event[i].m() ) );
141 nStored[iSpecies + 1] = hadronBE.size();
143 // Loop through pairs of identical particles and find shifts.
144 for (int i1 = nStored[iSpecies]; i1 < nStored[iSpecies+1] - 1; ++i1)
145 for (int i2 = i1 + 1; i2 < nStored[iSpecies+1]; ++i2)
146 shiftPair( i1, i2, iTab);
149 // Must have at least two pairs to carry out compensation.
150 if (nStored[9] < 2) return true;
152 // Shift momenta and recalculate energies.
153 double eSumOriginal = 0.;
154 double eSumShifted = 0.;
155 double eDiffByComp = 0.;
156 for (int i = 0; i < nStored[9]; ++i) {
157 eSumOriginal += hadronBE[i].p.e();
158 hadronBE[i].p += hadronBE[i].pShift;
159 hadronBE[i].p.e( sqrt( hadronBE[i].p.pAbs2() + hadronBE[i].m2 ) );
160 eSumShifted += hadronBE[i].p.e();
161 eDiffByComp += dot3( hadronBE[i].pComp, hadronBE[i].p)
165 // Iterate compensation shift until convergence.
167 while ( abs(eSumShifted - eSumOriginal) > COMPRELERR * eSumOriginal
168 && abs(eSumShifted - eSumOriginal) < COMPFACMAX * abs(eDiffByComp)
169 && iStep < NCOMPSTEP ) {
171 double compFac = (eSumOriginal - eSumShifted) / eDiffByComp;
174 for (int i = 0; i < nStored[9]; ++i) {
175 hadronBE[i].p += compFac * hadronBE[i].pComp;
176 hadronBE[i].p.e( sqrt( hadronBE[i].p.pAbs2() + hadronBE[i].m2 ) );
177 eSumShifted += hadronBE[i].p.e();
178 eDiffByComp += dot3( hadronBE[i].pComp, hadronBE[i].p)
183 // Error if no convergence, and then return without doing BE shift.
184 // However, not grave enough to kill event, so return true.
185 if ( abs(eSumShifted - eSumOriginal) > COMPRELERR * eSumOriginal ) {
186 infoPtr->errorMsg("Warning in BoseEinstein::shiftEvent: "
187 "no consistent BE shift topology found, so skip BE");
191 // Store new particle copies with shifted momenta.
192 for (int i = 0; i < nStored[9]; ++i) {
193 int iNew = event.copy( hadronBE[i].iPos, 99);
194 event[ iNew ].p( hadronBE[i].p );
204 // Calculate shift and (unnormalized) compensation for pair.
206 void BoseEinstein::shiftPair( int i1, int i2, int iTab) {
208 // Calculate old relative momentum.
209 double Q2old = m2(hadronBE[i1].p, hadronBE[i2].p) - m2Pair[iTab];
210 if (Q2old < Q2MIN) return;
211 double Qold = sqrt(Q2old);
212 double psFac = sqrt(Q2old + m2Pair[iTab]) / Q2old;
214 // Calculate new relative momentum for normal shift.
216 if (Qold < deltaQ[iTab]) Qmove = Qold / 3.;
217 else if (Qold < maxQ[iTab]) {
218 double realQbin = Qold / deltaQ[iTab];
219 int intQbin = int( realQbin );
220 double inter = (pow3(realQbin) - pow3(intQbin))
221 / (3 * intQbin * (intQbin + 1) + 1);
222 Qmove = ( shift[iTab][intQbin] + inter * (shift[iTab][intQbin + 1]
223 - shift[iTab][intQbin]) ) * psFac;
225 else Qmove = shift[iTab][nStep[iTab]] * psFac;
226 double Q2new = Q2old * pow( Qold / (Qold + 3. * lambda * Qmove), 2. / 3.);
228 // Calculate corresponding three-momentum shift.
229 double Q2Diff = Q2new - Q2old;
230 double p2DiffAbs = (hadronBE[i1].p - hadronBE[i2].p).pAbs2();
231 double p2AbsDiff = hadronBE[i1].p.pAbs2() - hadronBE[i2].p.pAbs2();
232 double eSum = hadronBE[i1].p.e() + hadronBE[i2].p.e();
233 double eDiff = hadronBE[i1].p.e() - hadronBE[i2].p.e();
234 double sumQ2E = Q2Diff + eSum * eSum;
235 double rootA = eSum * eDiff * p2AbsDiff - p2DiffAbs * sumQ2E;
236 double rootB = p2DiffAbs * sumQ2E - p2AbsDiff * p2AbsDiff;
237 double factor = 0.5 * ( rootA + sqrtpos(rootA * rootA
238 + Q2Diff * (sumQ2E - eDiff * eDiff) * rootB) ) / rootB;
240 // Add shifts to sum. (Energy component dummy.)
241 Vec4 pDiff = factor * (hadronBE[i1].p - hadronBE[i2].p);
242 hadronBE[i1].pShift += pDiff;
243 hadronBE[i2].pShift -= pDiff;
245 // Calculate new relative momentum for compensation shift.
247 if (Qold < deltaQ3[iTab]) Qmove3 = Qold / 3.;
248 else if (Qold < maxQ3[iTab]) {
249 double realQbin = Qold / deltaQ3[iTab];
250 int intQbin = int( realQbin );
251 double inter = (pow3(realQbin) - pow3(intQbin))
252 / (3 * intQbin * (intQbin + 1) + 1);
253 Qmove3 = ( shift3[iTab][intQbin] + inter * (shift3[iTab][intQbin + 1]
254 - shift3[iTab][intQbin]) ) * psFac;
256 else Qmove3 = shift3[iTab][nStep3[iTab]] *psFac;
257 double Q2new3 = Q2old * pow( Qold / (Qold + 3. * lambda * Qmove3), 2. / 3.);
259 // Calculate corresponding three-momentum shift.
260 Q2Diff = Q2new3 - Q2old;
261 sumQ2E = Q2Diff + eSum * eSum;
262 rootA = eSum * eDiff * p2AbsDiff - p2DiffAbs * sumQ2E;
263 rootB = p2DiffAbs * sumQ2E - p2AbsDiff * p2AbsDiff;
264 factor = 0.5 * ( rootA + sqrtpos(rootA * rootA
265 + Q2Diff * (sumQ2E - eDiff * eDiff) * rootB) ) / rootB;
267 // Extra dampening factor to go from BE_3 to BE_32.
268 factor *= 1. - exp(-Q2old * R2Ref2);
270 // Add shifts to sum. (Energy component dummy.)
271 pDiff = factor * (hadronBE[i1].p - hadronBE[i2].p);
272 hadronBE[i1].pComp += pDiff;
273 hadronBE[i2].pComp -= pDiff;
277 //**************************************************************************
279 } // end namespace Pythia8