include/Pythia8/ThermalFragmentation.h

0001 // ThermalFragmerntation.h is a part of the PYTHIA event generator.
0002 // Copyright (C) 2025 Torbjorn Sjostrand.
0003 // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
0004 // Please respect the MCnet Guidelines, see GUIDELINES for details.
0005
0006 // This file contains helper classes for thermal fragmentation.
0007 // ThermalStringFlav is used to select quark and hadron flavours.
0008 // ThermalStringPT is used to select transverse momenta.
0009 // ThermalStringEnd described fragmentation from one end of the string.
0010 // ThermalFragmentation is the top-level class for this model.
0011
0012 #ifndef Pythia8_ThermalFragmentation_H
0013 #define Pythia8_ThermalFragmentation_H
0014
0015 #include "Pythia8/FragmentationModel.h"
0016 #include "Pythia8/MiniStringFragmentation.h"
0017 #include "Pythia8/StringFragmentation.h"
0018
0019 namespace Pythia8 {
0020
0021 //==========================================================================
0022
0023 // The ThermalStringFlav class is used to select quark and hadron flavours.
0024
0025 class ThermalStringFlav : public StringFlav {
0026
0027 public:
0028
0029   // Constructor.
0030   ThermalStringFlav() : mesonNonetL1(), temperature(), tempPreFactor(),
0031     nNewQuark(), mesMixRate1(), mesMixRate2(), mesMixRate3(),
0032     baryonOctWeight(), baryonDecWeight(), hadronConstIDs(), possibleHadrons(),
0033     possibleRatePrefacs(), possibleHadronsLast(), possibleRatePrefacsLast(),
0034     hadronIDwin(), quarkIDwin(), hadronMassWin() {}
0035
0036   // Destructor.
0037   ~ThermalStringFlav() {}
0038
0039   // Initialize data members.
0040   void init() override;
0041
0042   // Pick a new flavour (including diquarks) given an incoming one.
0043   FlavContainer pick(FlavContainer& flavOld,
0044     double pT, double kappaModifier, bool allowPop) override;
0045
0046   // Return chosen hadron in case of thermal model.
0047   virtual int getHadronIDwin() { return hadronIDwin; }
0048
0049   // Return hadron mass. Used one if present, pick otherwise.
0050   double getHadronMassWin(int idHad) override { return
0051     ((hadronMassWin < 0.0) ? particleDataPtr->mSel(idHad) : hadronMassWin); }
0052
0053   // Combine two flavours into hadron for last two remaining flavours
0054   // for thermal model.
0055   int combineLastThermal(FlavContainer& flav1, FlavContainer& flav2,
0056     double pT, double kappaModifier);
0057
0058   // Return already set hadron id or combination of the two flavours.
0059   int getHadronID(FlavContainer& flav1, FlavContainer& flav2, double pT = -1.0,
0060     double kappaModifier = -1.0, bool finalTwo = false) override {
0061     if (finalTwo) return combineLastThermal(flav1, flav2, pT, kappaModifier);
0062     if ( (hadronIDwin != 0) && (quarkIDwin != 0)) return getHadronIDwin();
0063     return combine(flav1, flav2); }
0064
0065   // Check if quark-diquark combination should be added. If so add.
0066   void addQuarkDiquark(vector< pair<int,int> >& quarkCombis,
0067     int qID, int diqID, int hadronID) {
0068     bool allowed = true;
0069     for (int iCombi = 0; iCombi < int(quarkCombis.size()); iCombi++)
0070       if ( (qID   == quarkCombis[iCombi].first ) &&
0071            (diqID == quarkCombis[iCombi].second) ) allowed = false;
0072     if (allowed) quarkCombis.push_back( (hadronID > 0) ?
0073       make_pair( qID,  diqID) : make_pair(-qID, -diqID) ); }
0074
0075 protected:
0076
0077   // Settings for thermal model.
0078   bool   mesonNonetL1;
0079   double temperature, tempPreFactor;
0080   int    nNewQuark;
0081   double mesMixRate1[2][6], mesMixRate2[2][6], mesMixRate3[2][6];
0082   double baryonOctWeight[6][6][6][2], baryonDecWeight[6][6][6][2];
0083
0084   // Key = hadron id, value = list of constituent ids.
0085   map< int, vector< pair<int,int> > > hadronConstIDs;
0086   // Key = initial (di)quark id, value = list of possible hadron ids
0087   //                                     + nr in hadronConstIDs.
0088   map< int, vector< pair<int,int> > > possibleHadrons;
0089   // Key = initial (di)quark id, value = prefactor to multiply rate.
0090   map< int, vector<double> > possibleRatePrefacs;
0091   // Similar, but for combining the last two (di)quarks. Key = (di)quark pair.
0092   map< pair<int,int>, vector< pair<int,int> > > possibleHadronsLast;
0093   map< pair<int,int>, vector<double> > possibleRatePrefacsLast;
0094
0095   // Selection in thermal model.
0096   int    hadronIDwin, quarkIDwin;
0097   double hadronMassWin;
0098
0099 };
0100
0101 //==========================================================================
0102
0103 // The ThermalStringPT class is used to select transverse momenta.
0104
0105 class ThermalStringPT : public StringPT {
0106
0107 public:
0108
0109   // Constructor.
0110   ThermalStringPT() : temperature(), tempPreFactor(), fracSmallX() {}
0111
0112   // Destructor.
0113   ~ThermalStringPT() {}
0114
0115   // Initialize data members.
0116   void init() override;
0117
0118   // Return px and py as a pair.
0119   pair<double, double> pxy(int idIn = 0, double kappaModifier = -1.0) override;
0120
0121   // Exponential suppression of given pT2; used in MiniStringFragmentation.
0122   double suppressPT2(double pT2) override {
0123     return exp(-sqrt(pT2)/temperature); }
0124
0125 protected:
0126
0127   // Initialization data, to be read from Settings.
0128   double temperature, tempPreFactor, fracSmallX;
0129
0130 private:
0131
0132   // Evaluate Bessel function K_{1/4}(x).
0133   double BesselK14(double x);
0134
0135 };
0136
0137 //==========================================================================
0138
0139 // The ThermalFragmentation class handles the alternative fragmentation,
0140 // using both the StringFragmentation and MiniStringFragmentation classes.
0141
0142 class ThermalFragmentation : public FragmentationModel {
0143
0144 public:
0145
0146   // Constructor (creates string, string-end and mini-string pointers).
0147   ThermalFragmentation();
0148
0149   // Destructor (deletes string, string-end and mini-string pointers).
0150   ~ThermalFragmentation() override;
0151
0152   // Initialize and save pointers.
0153   bool init(StringFlav* flavSelPtrIn = nullptr,
0154     StringPT* pTSelPtrIn = nullptr, StringZ* zSelPtrIn = nullptr,
0155     FragModPtr fragModPtrIn = nullptr) override;
0156
0157   // Do the fragmentation: driver routine.
0158   bool fragment(int iSub, ColConfig& colConfig, Event& event,
0159     bool isDiff = false, bool systemRecoil = true) override;
0160
0161   // Classes for flavour, pT and z generation.
0162   ThermalStringFlav* thermalFlavSelPtr{};
0163   ThermalStringPT*   thermalPTSelPtr{};
0164   StringZ*           zSelPtr{};
0165
0166   // Internal StringFragmentation and MiniStringFragmentation objects.
0167   StringFragmentation* stringFragPtr{};
0168   MiniStringFragmentation* ministringFragPtr{};
0169
0170 private:
0171
0172   // Parameters controlling the fragmentation.
0173   double mStringMin{};
0174   bool tryMiniAfterFailedFrag{};
0175
0176 };
0177
0178 //==========================================================================
0179
0180 } // end namespace Pythia8
0181
0182 #endif // Pythia8_ThermalFragmentation_H