I am working on a Geant4-based project using version geant4-v11.2.2,
focusing on spin tracking simulations of Lambda and Anti-lambda particles
in a uniform magnetic field.
During these simulations, I noticed that the spin precession directions
of Lambda and Anti-lambda with the same momentum appear to be the same,
which seems to contradict the expected behavior based on PDG data.
As we know, Lambda and Anti-lambda have magnetic moments
of approximately ±0.613 nuclear magnetons, respectively.
To investigate further, I reviewed the implementation of the G4Mag_SpinEqRhs.cc file,
particularly the G4Mag_SpinEqRhs::EvaluateRhsGivenB function,
which uses the BMT equation for spin precession.
For neutral particles like Lambda and Anti-lambda,
the Thomas precession is absent (due to no acceleration),
so the equation simplifies to only the Larmor precession.
However, I observed that in the Geant4 code,
the g-factor calculation uses the absolute value,
and the variable “pcharge” is set to 1 for neutral particles.
This setup makes the spin precession directions
of Lambda and Anti-lambda appear identical,
which seems inconsistent with their known opposite magnetic moments.
I’m unsure if others have encountered this issue before,
but it has been puzzling me greatly.
I hope to receive help from Geant4 users, enthusiasts, or even developers
who have insights into this problem.
Thank you very much!
*The detailed code of G4Mag_SpinEqRhs.cc in version-11.2.2 is as follows
#include "G4Mag_SpinEqRhs.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4MagneticField.hh"
#include "G4ThreeVector.hh"
G4Mag_SpinEqRhs::G4Mag_SpinEqRhs( G4MagneticField* MagField )
: G4Mag_EqRhs( MagField )
{
}
G4Mag_SpinEqRhs::~G4Mag_SpinEqRhs() = default;
void
G4Mag_SpinEqRhs::SetChargeMomentumMass(G4ChargeState particleCharge,
G4double MomentumXc,
G4double particleMass)
{
G4Mag_EqRhs::SetChargeMomentumMass( particleCharge, MomentumXc, mass);
charge = particleCharge.GetCharge();
mass = particleMass;
magMoment = particleCharge.GetMagneticDipoleMoment();
spin = particleCharge.GetSpin();
omegac = (eplus/mass)*c_light;
G4double muB = 0.5*eplus*hbar_Planck/(mass/c_squared);
G4double g_BMT;
if ( spin != 0. )
{
g_BMT = (std::abs(magMoment)/muB)/spin;
}
else
{
g_BMT = 2.;
}
anomaly = (g_BMT - 2.)/2.;
G4double E = std::sqrt(sqr(MomentumXc)+sqr(mass));
beta = MomentumXc/E;
gamma = E/mass;
}
void
G4Mag_SpinEqRhs::EvaluateRhsGivenB( const G4double y[],
const G4double B[3],
G4double dydx[] ) const
{
G4double momentum_mag_square = sqr(y[3]) + sqr(y[4]) + sqr(y[5]);
G4double inv_momentum_magnitude = 1.0 / std::sqrt( momentum_mag_square );
G4double cof = FCof()*inv_momentum_magnitude;
dydx[0] = y[3] * inv_momentum_magnitude; // (d/ds)x = Vx/V
dydx[1] = y[4] * inv_momentum_magnitude; // (d/ds)y = Vy/V
dydx[2] = y[5] * inv_momentum_magnitude; // (d/ds)z = Vz/V
if (charge == 0.)
{
dydx[3] = 0.;
dydx[4] = 0.;
dydx[5] = 0.;
}
else
{
dydx[3] = cof*(y[4]*B[2] - y[5]*B[1]) ; // Ax = a*(Vy*Bz - Vz*By)
dydx[4] = cof*(y[5]*B[0] - y[3]*B[2]) ; // Ay = a*(Vz*Bx - Vx*Bz)
dydx[5] = cof*(y[3]*B[1] - y[4]*B[0]) ; // Az = a*(Vx*By - Vy*Bx)
}
G4ThreeVector u(y[3], y[4], y[5]);
u *= inv_momentum_magnitude;
G4ThreeVector BField(B[0],B[1],B[2]);
G4double udb = anomaly*beta*gamma/(1.+gamma) * (BField * u);
G4double ucb = (anomaly+1./gamma)/beta;
// Initialise the values of dydx that we do not update.
dydx[6] = dydx[7] = dydx[8] = 0.0;
G4ThreeVector Spin(y[9],y[10],y[11]);
G4double pcharge;
if (charge == 0.)
{
pcharge = 1.;
}
else
{
pcharge = charge;
}
G4ThreeVector dSpin(0.,0.,0.);
if (Spin.mag2() != 0.)
{
dSpin = pcharge*omegac*(ucb*(Spin.cross(BField))-udb*(Spin.cross(u)));
}
dydx[9] = dSpin.x();
dydx[10] = dSpin.y();
dydx[11] = dSpin.z();
return;
}
_Geant4 Version: geant4-v11.2.2