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The Froissart–Stora equation describes the change in polarization which a high energy charged particle beam in a storage ring will undergoas it passes through a resonance in the spin tune. It is named after the French physicists Marcel Froissart and Raymond Stora.The polarization following passage through the resonance is given by
where ϵ {\displaystyle \epsilon } is the resonance strength and α 0 {\displaystyle \alpha _{0}} is the speed at which the resonance is crossed. P y 0 {\displaystyle P_{y0}} is the initial polarization before resonance crossing.
The resonance may be crossed by raising the energy so that the spin tune passes through a resonance, or driven with a transverse magnetic field at a frequency that is in resonance with the spin oscillations.
The Froissart–Stora equation has a direct analogy in condensed matter physics in the Landau–Zener effect.