FourVector.cc

/**************************************************************************
 * basf2 (Belle II Analysis Software Framework)                           *
 * Author: The Belle II Collaboration                                     *
 *                                                                        *
 * Forked from https://github.com/iLCSoft/MarlinKinfit                    *
 *                                                                        *
 * Further information about the fit engine and the user interface        *
 * provided in MarlinKinfit can be found at                               *
 * https://www.desy.de/~blist/kinfit/doc/html/                            *
 * and in the LCNotes LC-TOOL-2009-001 and LC-TOOL-2009-004 available     *
 * from http://www-flc.desy.de/lcnotes/                                   *
 *                                                                        *
 * See git log for contributors and copyright holders.                    *
 * This file is licensed under LGPL-3.0, see LICENSE.md.                  *
 **************************************************************************/
 
#ifdef MARLIN_USE_ROOT
 
#include "analysis/OrcaKInFit/FourVector.h"
 
#include <cmath>
 
#undef NDEBUG
#include <cassert>
 
#include <TRandom3.h>
 
namespace Belle2 {
  namespace OrcaKinFit {
 
    static TRandom* rnd = 0;
 
    FourVector& FourVector::boost(const FourVector& P)
    {
      // See CERNLIB U101 for a description
 
      double pP = -(p * P.p);
      double e = getE();
      double M = P.getM();
 
      E = (e * P.getE() - pP) / M;
      p = p - ((pP / (P.getE() + M) - e) / M) * P.p;
 
      return *this;
    }
 
    void FourVector::decayto(FourVector& d1, FourVector& d2) const
    {
      // Let this particle decay isotropically into 4-vectors d1 and d2;
      // d1 and d2 must have definite mass at beginning
      using std::abs;
      using std::sqrt;
      using std::pow;
 
      double M2 = getM2();
      double M  = getM();;
      double m1 = d1.getM();
      double m2 = d2.getM();
 
      double randoms[2];
      if (rnd == 0) rnd = new TRandom3();
      rnd->RndmArray(2, randoms);
 
 
      assert(m1 + m2 <= M);
 
      double pstar = 0.5 * sqrt(abs((M2 - pow(m1 + m2, 2)) * (M2 - pow(m1 - m2, 2)))) / M;
      double phistar = 2 * M_PI * randoms[0];
      double costhetastar = 2 * randoms[1] - 1;
      double sinthetastar = sqrt(abs(1 - costhetastar * costhetastar));
      double E1 = sqrt(m1 * m1 + pstar * pstar);
      double E2 = sqrt(m2 * m2 + pstar * pstar);
 
 
      d1 = FourVector(E1, pstar * sinthetastar * cos(phistar),
                      pstar * sinthetastar * sin(phistar),
                      pstar * costhetastar);
      d2 = FourVector(E2, -pstar * sinthetastar * cos(phistar),
                      -pstar * sinthetastar * sin(phistar),
                      -pstar * costhetastar);
 
      d1.boost(*this);
      d2.boost(*this);
 
    }
 
  }// end OrcaKinFit namespace
} // end Belle2 namespace
 
#endif // MARLIN_USE_ROOT