## Problem 3.5: Determine the mass of a particle that decays into two photons

Please wait for the animation to completely load.

An unknown particle decays into two photons with wavelengths given in the animation.^{3} You can change the incident speed of the unknown particle. You can use the pink protractor to measure angles: click-drag on the gray circles to move and adjust the angle of the protractor. **(Angles are given in degrees and wavelengths in nm.)** A relationship you may find useful is *hc* = 1240 eV·nm. Restart.

- Develop a relationship for the angle the photons leave as a function of v
*/c*. Verify your relationship using the animation. - Find the mass (in MeV/
*c*^{2}) of the unknown particle. (What is it?)

^{3}Based on Problems 2.37 and 2.39, J. Taylor, C. Zafiratos, and M. Dubson, *Modern Physics*, Prentice Hall (2004).

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