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Forum: Fields: Magnetic and Otherwise
Re: Question Spin tracking of moving muons (Kevin Lynch)
Date: 18 Feb, 2009
From: Gumplinger Peter <Gumplinger Peter>

Hi Kevin,

Thanks for your detailed and constructive forum posting.

3) As for your last question, the missing third term is an oversight. I noticed it a little while ago when I went back to Jackson. I made mental note to investigate and fix it. Actually, I have no feeling as to when this term (size of E field and beta) becomes sizable relative to the others. Until now, I only thought of E fields inside drift chambers. According to Jackson: "If the particle is relativistic (beta->1), even the presence of an electric field causes the longitudinal polarization to change only very slowly, at a rate proportional to gamma**(-2) times the electric field component perpendicular to v_vector." (I assume you are dealing with 'surface muons' - longitudinal polarized muons?)

2) G4EqEMFieldWithSpin copies from G4EqMagElectricField while G4Mag_SpinEqRhs is a copy of G4Mag_UsualEqRhs. Only if the field can change energy, then the time must be integrated, else:

the time is incremented from the particle's velocity and step length.

1) > Practically, given the way Geant4 handles the energy loss, should I be

> worried about this?

> Do we ever end up in a condition where 1/\beta gets big enough to be a

> problem? If so, is there any good way to deal with this?

You have a very good point. I don't know the answers.... I suggest you flag large final step spin variations - should they occur - in your application. My experience has only ever dealt with muons stopping in a rather homogeneous, albeit strong magnetic field. We never observed a weired spin. We were very sensitive to any false prediction (depolarization) at the 10-3 level and consequently plotted the final spin predicition for each event history for many millions of such histories.

A group who extensively uses spin tracking in G4 is from PSI:

A general paper:
              T. Shiroka et al.,
              "Geant4 as a simulation framework in muSR"
              proceedings of the muSR 2008 conference,  Tsukuba, Japan
              (21-25th July 2008), to be published in Physica B. 

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1 None: Re: Spin tracking of moving muons   (Tom Roberts - 18 Feb, 2009)
2 None: Re: Spin tracking of moving muons   (Kevin Lynch - 18 Feb, 2009)
1 Ok: Re: Spin tracking of moving muons   (Gumplinger Peter - 19 Feb, 2009)
(_ None: Implementation of missing terms and EDM spin evolution   (Kevin Lynch - 25 Feb, 2009)
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