Using the Lorentz transformations of momentum/energy of the 4-momentum along the x-direction, derive an expression for the laboratory angle of emission of a ?-ray by an unstable particle in terms of the angle of emission in the rest-frame of the particle. Thus show that for a beam of very high-energy particles, half the ?-rays will be concentrated inside a forward cone of opening angle theta of order 1/?, with of course ? = 1 / sqrt(1-v^2/c^2).
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