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None Introducing randomization of resonance mass causes problem with decay 

Forum: Particles
Date: 02 Jan, 2015
From: Rafal Sikora <Rafal Sikora>

Greetings to All,

I would like to share with you a problem, which I hope can be resolved with your help.

In short, I'm investigating with Geant4 (version 9.6.p02) the detector response to products of decay of the baryonic resonances e.g. Delta(1232), N(1440). I have noticed, that Geant4 doesn't "smear" the mass of the resonances (though it contains all the information such as the width). So, I decided to randomize the mass of primary resonance following Breit-Wigner distribution by setting it through SetMass() method in G4PrimaryParticle class. I did it because I believed (and still believe) that Geant4 uses this information to properly set the kinematics of the decay products, what is crucial in my problem. However, if the mass of resonance is too low the exception is thrown:

G4PhaseSpaceDecayChannel::TwoBodyDecayIt sum of daughter mass is larger than parent mass parent :N(1440)+ 1.02138 daughter 1 :neutron 0.939565 daughter 2:pi+ 0.13957

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : PART112
      issued by : G4PhaseSpaceDecayChannel::TwoBodyDecayIt
Can not create decay products: sum of daughter mass is larger than parent mass
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

Why Geant4 doesn't check which decay channel is allowed from energy and momentum conservation? How should I proceed to have properly set the kinematics of the products of decay?

I have found, that in Geant4 10.1 some solution to the similiar problem was found (from release notes): "Added new constructor with dynamical mass in G4DynamicParticle. G4PhaseSpaceDecayChannel takes in to account mass width of daughter particles and gives the dynamical mass according to Breit-Wigner distribution. Added daughters_width in G4VDecayChannel. Addressing problem report #1629", but I'm not sure if switching to newer Geant4 version is really the final solution.

Any ideas?

Best regards, Rafal

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