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BackwardsProduction.md

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Backwards Production

$\omega \rightarrow \pi^0+\gamma$ Decay

Default decay for $\omega$ is $\omega \rightarrow 2\pi$ with branch ratio 0.0153 $\omega \rightarrow \pi^0+\gamma$ decay (branching ratio 0.0828) can be enabled with PROD_PID = 223022 This neutral decay of the $\omega$ currently assumes an unpolarized $\omega$, as it is not clear whether $s$-channel helicity conservation holds in backwards production. This assumption can be changed in src/gammaavm.cpp

Added Code

Added check in src/e_narrowResonanceCrossSection.cpp for backwards production.

Switched to two vertices, gamma and electron, in src/hepmc3writer.cpp

Added targetBeam, electronBeam and backwards_production tag for slight.in src/inputParameters.cpp and include/inputParameters.h
(Removed TAU+TAU decay)

Changed src/starlightpythia.cpp to produce eXEvent instead of UPCEvent

Backwards Production Code in src/gammaavm.cpp

if (_backwardsProduction) {
  //Fill the intial state in the lab frame
	double pz_tar_i_lab = -1.0*target_pz;  //Flip the pz of the target
	double E_tar_i_lab = sqrt(pow(target_pz,2) + pow(starlightConstants::protonMass,2.)); //Calculate proton energy
	double pz_gam_i_lab = Egam; //Photon energy
	double E_gam_i_lab = Egam;  //Photon energy

	//Calculate the final state target energy and pz
	double pz_tar_f_lab = (pz_tar_i_lab + pz_gam_i_lab) - pz;
	double E_tar_f_lab = sqrt( pow(pz_tar_f_lab,2) + pow(starlightConstants::protonMass,2.) );

	//Calculate the boost (beta,gamma) to the center of mass frame
	double beta = (pz_tar_i_lab + pz_gam_i_lab)/(E_gam_i_lab + E_tar_i_lab);
	double lorentzGamma = 1.0/sqrt( 1 - pow(beta,2.));
	  
	//Boost to the final state target to the CM frame
	double cm_frame_target_pz = lorentzGamma*( pz_tar_f_lab + beta*E_tar_f_lab );
	double cm_frame_gamma_pz  = lorentzGamma*( pz + beta*pz );
	 
	//Recalculate target energy in the CM frame (assign to VM) 
	double vm_mass = sqrt( E*E - ( px*px + py*py + pz*pz ) );
	double vm_E_cm = sqrt( vm_mass*vm_mass + px*px + py*py + cm_frame_target_pz*cm_frame_target_pz);

  //Calcuate photon energy in CM frame (assign to target)
  double cm_E = sqrt( pow(starlightConstants::protonMass,2.)  + pow(t_px,2.) + pow(t_py,2.) + pow(cm_frame_gamma_pz,2.));	  
  //Boost photon and target back to lab frame
  //target:
  t_pz = lorentzGamma*(cm_frame_gamma_pz - beta*cm_E);
  t_E = lorentzGamma*(cm_E - beta*cm_fame_gamma_pz);
  //Photon:
  pz = lorentzGamma*(cm_frame_target_pz - beta*vm_E_cm);
  E = lorentzGamma*(vm_E_cm - beta*cm_frame_target_pz);

  //Calculate the rapidity
  Y = 0.5*std::log( (E+fabs(pz))/(E-fabs(pz)) );
}

Removed STARlight Code (Conflicted with backwards production)

DPMJET

Removed the option to include DPMJET in CMakeLists.txt

BeamSystem

Changed the beam1/beam2 -> electronBeam/targetBeam. Defined in include/beambeamsystem.h

UPCEvent

Removed all references to UPCEvents in:

include/e_starlightStandalone.h 
include/e_starlight.h 
include/eventchannel.h
include/eventfilewriter.h
include/filewriter.h
include/gammaavm.h
include/starlightpythia.h
include/pythiadecayer.h

Updated kinematics:

src/gammaavm.cpp