P4JacobianEEtaPhiM2PtEtaPhiM.cxx

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/*
  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
*/
 
#include "FourMom/P4JacobianEEtaPhiM2PtEtaPhiM.h"
 
#include <cmath>
#include <algorithm>
 
P4JacobianEEtaPhiM2PtEtaPhiM::P4JacobianEEtaPhiM2PtEtaPhiM( double E, 
                                double eta, 
                                double /* phi */, 
                                double M) :
  HepMatrix(4,4,0)
{
  double t = exp(-eta); // == tg(theta/2)
  double P = std::sqrt( std::max( 0., E*E - M*M));
  double u = 1+t*t; 
  double cosTheta = (1-t*t)/u;
  double sinTheta = 2*t/u;
 
 
  (*this)[0][0] = E/P * sinTheta;          // dPt/dE
  (*this)[0][1] = -P * cosTheta*sinTheta;  // dPt/deta
  (*this)[0][3] = -M/P * sinTheta;         // dPt/dM
 
  (*this)[1][1] = 1;  // deta/deta
  (*this)[2][2] = 1;  // dphi/dphi
  (*this)[3][3] = 1;  // dM/dM
}