CustomPhysics/src/Decay3Body.cc

0001 #include <cmath>
0002 #include "SimG4Core/CustomPhysics/interface/Decay3Body.h"
0003
0004 #include "CLHEP/Units/GlobalSystemOfUnits.h"
0005 #include "CLHEP/Units/GlobalPhysicalConstants.h"
0006 #include "DataFormats/Math/interface/Vector3D.h"
0007 #include "DataFormats/Math/interface/LorentzVector.h"
0008 #include "Math/GenVector/Rotation3D.h"
0009 #include "Math/GenVector/EulerAngles.h"
0010 #include "Math/GenVector/Boost.h"
0011 #include <vector>
0012 #include <cmath>
0013
0014 #include "Randomize.hh"
0015
0016 Decay3Body::Decay3Body() {}
0017
0018 Decay3Body::~Decay3Body() {}
0019
0020 void Decay3Body::doDecay(const G4LorentzVector& mother,
0021                          G4LorentzVector& daughter1,
0022                          G4LorentzVector& daughter2,
0023                          G4LorentzVector& daughter3) {
0024   double m0 = mother.m();
0025   double m1 = daughter1.m();
0026   double m2 = daughter2.m();
0027   double m3 = daughter3.m();
0028   double sumM2 = m0 * m0 + m1 * m1 + m2 * m2 + m3 * m3;
0029   double tolerance = 1.0e-9;
0030
0031   math::XYZTLorentzVectorD mmm(mother.px(), mother.py(), mother.pz(), mother.e());
0032
0033   if (m0 < m1 + m2 + m3) {
0034     std::cout << "Error: Daughters too heavy!" << std::endl;
0035     std::cout << "M: " << m0 / GeV << " < m1+m2+m3: " << m1 / GeV + m2 / GeV + m3 / GeV << std::endl;
0036     return;
0037   } else {
0038     double m2_12max = sqr(m0 - m3);
0039     double m2_12min = sqr(m1 + m2);
0040     double m2_23max = sqr(m0 - m1);
0041     double m2_23min = sqr(m2 + m3);
0042
0043     double x1, x2;
0044     double m2_12 = 0.0;
0045     double m2_23 = 0.0;
0046     double E2_12, E3_12;
0047     double m2_23max_12, m2_23min_12;
0048
0049     do {
0050       // Pick values for m2_12 and m2_23 uniformly:
0051       x1 = G4UniformRand();
0052       m2_12 = m2_12min + x1 * (m2_12max - m2_12min);
0053       x2 = G4UniformRand();
0054       m2_23 = m2_23min + x2 * (m2_23max - m2_23min);
0055
0056       // From the allowed range of m2_23 (given m2_12), determine if the point is valid:
0057       // (formulae taken from PDG booklet 2004 kinematics, page 305, Eqs. 38.22a+b)
0058       E2_12 = (m2_12 - m1 * m1 + m2 * m2) / (2.0 * sqrt(m2_12));
0059       E3_12 = (m0 * m0 - m2_12 - m3 * m3) / (2.0 * sqrt(m2_12));
0060       m2_23max_12 = sqr(E2_12 + E3_12) - sqr(sqrt(sqr(E2_12) - m2 * m2) - sqrt(sqr(E3_12) - m3 * m3));
0061       m2_23min_12 = sqr(E2_12 + E3_12) - sqr(sqrt(sqr(E2_12) - m2 * m2) + sqrt(sqr(E3_12) - m3 * m3));
0062     } while ((m2_23 > m2_23max_12) || (m2_23 < m2_23min_12));
0063
0064     // Determine the value of the third invariant mass squared:
0065     double m2_13 = sumM2 - m2_12 - m2_23;
0066
0067     // Calculate the energy and size of the momentum of the three daughters:
0068     double e1 = (m0 * m0 + m1 * m1 - m2_23) / (2.0 * m0);
0069     double e2 = (m0 * m0 + m2 * m2 - m2_13) / (2.0 * m0);
0070     double e3 = (m0 * m0 + m3 * m3 - m2_12) / (2.0 * m0);
0071     double p1 = sqrt(e1 * e1 - m1 * m1);
0072     double p2 = sqrt(e2 * e2 - m2 * m2);
0073     double p3 = sqrt(e3 * e3 - m3 * m3);
0074
0075     // Calculate cosine of the relative angles between the three daughters:
0076     double cos12 = (m1 * m1 + m2 * m2 + 2.0 * e1 * e2 - m2_12) / (2.0 * p1 * p2);
0077     double cos13 = (m1 * m1 + m3 * m3 + 2.0 * e1 * e3 - m2_13) / (2.0 * p1 * p3);
0078     double cos23 = (m2 * m2 + m3 * m3 + 2.0 * e2 * e3 - m2_23) / (2.0 * p2 * p3);
0079     if (fabs(cos12) > 1.0)
0080       std::cout << "Error: Undefined angle12!" << std::endl;
0081     if (fabs(cos13) > 1.0)
0082       std::cout << "Error: Undefined angle13!" << std::endl;
0083     if (fabs(cos23) > 1.0)
0084       std::cout << "Error: Undefined angle23!" << std::endl;
0085
0086     // Find the four vectors of the particles in a chosen (i.e. simple) frame:
0087     double xi = 2.0 * pi * G4UniformRand();
0088     math::XYZVectorD q1(0.0, 0.0, p1);
0089     math::XYZVectorD q2(sin(acos(cos12)) * cos(xi) * p2, sin(acos(cos12)) * sin(xi) * p2, cos12 * p2);
0090     math::XYZVectorD q3(-sin(acos(cos13)) * cos(xi) * p3, -sin(acos(cos13)) * sin(xi) * p3, cos13 * p3);
0091
0092     // Rotate all three daughters momentum with the angles theta and phi:
0093     double theta = acos(2.0 * G4UniformRand() - 1.0);
0094     double phi = 2.0 * pi * G4UniformRand();
0095     double psi = 2.0 * pi * G4UniformRand();
0096
0097     ROOT::Math::EulerAngles ang(phi, theta, psi);
0098     ROOT::Math::Rotation3D rot(ang);
0099
0100     math::XYZVectorD q1rot = rot * q1;
0101     math::XYZVectorD q2rot = rot * q2;
0102     math::XYZVectorD q3rot = rot * q3;
0103
0104     math::XYZTLorentzVectorD daughter1_orig(q1rot.X(), q1rot.Y(), q1rot.Z(), e1);
0105     math::XYZTLorentzVectorD daughter2_orig(q2rot.X(), q2rot.Y(), q2rot.Z(), e2);
0106     math::XYZTLorentzVectorD daughter3_orig(q3rot.X(), q3rot.Y(), q3rot.Z(), e3);
0107
0108     ROOT::Math::Boost cmboost(mmm.BoostToCM());
0109
0110     // Check of total angle and momentum:
0111     if (acos(cos12) + acos(cos13) + acos(cos23) - 2.0 * pi > tolerance)
0112       std::cout << "Error: Total angle not 2pi! " << acos(cos12) + acos(cos13) + acos(cos23) - 2.0 * pi << std::endl;
0113     if (fabs(daughter1_orig.px() + daughter2_orig.px() + daughter3_orig.px()) / GeV > tolerance)
0114       std::cout << "Error: Total 3B Px not conserved! "
0115                 << (daughter1_orig.px() + daughter2_orig.px() + daughter3_orig.px()) / GeV << std::endl;
0116     if (fabs(daughter1_orig.py() + daughter2_orig.py() + daughter3_orig.py()) / GeV > tolerance)
0117       std::cout << "Error: Total 3B Py not conserved! "
0118                 << (daughter1_orig.py() + daughter2_orig.py() + daughter3_orig.py()) / GeV << std::endl;
0119     if (fabs(daughter1_orig.pz() + daughter2_orig.pz() + daughter3_orig.pz()) / GeV > tolerance)
0120       std::cout << "Error: Total 3B Pz not conserved! " << (daughter1.pz() + daughter2.pz() + daughter3.pz()) / GeV
0121                 << std::endl;
0122
0123     // Boost the daughters back to the frame of the mother:
0124
0125     math::XYZTLorentzVectorD temp1(cmboost(daughter1_orig));
0126     math::XYZTLorentzVectorD temp2(cmboost(daughter2_orig));
0127     math::XYZTLorentzVectorD temp3(cmboost(daughter3_orig));
0128
0129     daughter1.setPx(temp1.Px());
0130     daughter1.setPy(temp1.Py());
0131     daughter1.setPz(temp1.Pz());
0132     daughter1.setE(temp1.e());
0133
0134     daughter2.setPx(temp2.Px());
0135     daughter2.setPy(temp2.Py());
0136     daughter2.setPz(temp2.Pz());
0137     daughter2.setE(temp2.e());
0138
0139     daughter3.setPx(temp3.Px());
0140     daughter3.setPy(temp3.Py());
0141     daughter3.setPz(temp3.Pz());
0142     daughter3.setE(temp3.e());
0143
0144     return;
0145   }
0146 }
0147
0148 double Decay3Body::sqr(double a) { return a * a; }