P4JacobianPxPyPzE2PxPyPzM.cxx

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/*
  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
*/
 
#include "FourMom/P4JacobianPxPyPzE2PxPyPzM.h"
 
#include <cmath>
#include <algorithm>
 
P4JacobianPxPyPzE2PxPyPzM::P4JacobianPxPyPzE2PxPyPzM( double px, 
                              double py, 
                              double pz, 
                              double E) :
  HepMatrix(4,4,0)
{
  double pt2 = px*px + py*py;
  double p2 = pt2 + pz*pz;
  double M = std::sqrt( std::max( 0., E*E - p2));
 
  (*this)[0][0] = 1; // dpx/dpx
  (*this)[1][1] = 1; // dpy/dpy
  (*this)[2][2] = 1; // dpz/dpz
 
  (*this)[3][0] = -px/M;     // dM/dpx
  (*this)[3][1] = -py/M;     // dM/dpy
  (*this)[3][2] = -pz/M;     // dM/dpz
  (*this)[3][3] = E/M;       // dM/dE
}