It represents a photon in a Feynman diagram, as well as the quotient of measured time over proper time in relativity equations, the Lorentz factor, and the nuclear emission that results from an annihilation.  
The lower-case version of this symbol can denote the ratio of heat capacity at constant pressure to that at constant temperature, the adiabatic index, while its upper-case version can denote a Christofel symbol or a namesake probability distribution governed by the parameters k and theta.  It represents a photon in a Feynman diagram, as well as the quotient of measured time over proper time in relativity equations, the Lorentz factor, and the nuclear emission that results from an annihilation.  For 10 points, name this Greek letter that names a function which is a generalization of the factorial and a ray that has a wavelength shorter than x-rays.
The lower-case version of this symbol can denote the ratio of heat capacity at constant pressure to that at constant temperature, the adiabatic index, while its upper-case version can denote a Christofel symbol or a namesake probability distribution governed by the parameters k and theta.  
gamma