Problem 2072

Summary: Energy is not conserved in the muon decay
Product: Geant4 Reporter: Artur Shaikhiev <artur.shaikhiev>
Component: processes/decayAssignee: kurasige
Status: RESOLVED FIXED    
Severity: minor    
Priority: P4    
Version: 10.1   
Hardware: All   
OS: All   

Description Artur Shaikhiev 2018-08-02 13:30:47 CEST
Greetings,

I've checked energy conservation in the G4MuonDecayChannelWithSpin generator using sum of e+, nu_mu and anti_nue energies. In terms of generator I used the sum of the following variables:
total_energy = energy + daughterparticle1->Get4Momentum().e() + daughterparticle2->Get4Momentum().e();
Then I compared my total_energy variable with EMMU (which is the muon mass) by introducing the difference (total_energy - EMMU)/EMMU. So, the difference is very low ~10^{-5} - 10^{-4}. I checked 193 muon decays with this channel and found 4 events with the high difference: -0.011572, -0.015309, -0.019909, -0.007233. It means that energy is not always conserved in the muon decays (2% of considering events!).
I did the same exercise for the G4MuonRadiativeDecayChannelWithSpin. My total energy in terms of generator now is:
total_energy = E + G + daughterparticle2->Get4Momentum().e() + daughterparticle3->Get4Momentum().e();
And again I compared total energy with muon mass by introducing the same difference:
(total_energy - EMMU)/EMMU.
I checked 193 events, the difference is higher than for the non-radiative case ~(1-3)x10^{-3}, but I found two events with very high difference -0.495478, -0.496089, this is large energy deficit! So ~1% of checked events are not consistent with the energy conservation law.
Is this behaviour expected or the energy conservation check have to be added to the generator?

Regards,
Artur
Comment 1 kurasige 2018-08-08 14:37:22 CEST
Hi, Artur

  Thank you very much for reporting the bug.

  I found that calculation of beta of 2 neutrino system in CM frame was wrong.
  I've committed a bug fix version (particles-V10-04-04)  

Cheers,
    Hisaya2072 by Hisaya'