In accelerator physics, rigidity is a concept used to determine the effect of particular magnetic fields on the motion of the charged particles.
It is a measure of the momentum of the particle, and it refers to the fact that a higher momentum particle will have a higher resistance to deflection by a magnetic field. It is defined as R = Bρ = p/q, where B is the magnetic field, ρ is the gyroradius of the particle due to this field, p is the particle momentum, and q is its charge. It is frequently referred to as simply "Bρ".[1]
It is important to note that the rigidity is defined by the action of a static, dipole, field, whose field lines are perpendicular to the velocity vector of the particle. This will cause a force perpendicular both to the velocity vector, and to the field lines, defining a plane through which the particle moves. The definition of the Lorenz force implies that the particle's motion will be circular, thus giving a constant radius of curvature.
If the particle momentum, p, is given in GeV/c, then the rigidity, in tesla-metres, is Bρ = 3.3356p/q.