In textbooks, the EPR problem and Bell's Inequality are dealt with for particles. However, when it comes to experimental verification, photons are used. The formula for the expectation value for particles is given, which is $-\cos\alpha$, $\alpha$ being the angle between the two axes used in the experiment. Then one textbook simply says, similarly for photons the expectation value is $\cos 2\alpha$, but no derivation is given. I have look through quite a few big textbooks but I still cannot find the derivation. I wish to go through the derivation myself. Please point me to some papers or textbooks or web pages where I can find the derivation of the expectation value for photon in the EPR experiments. Thank you in advance for your help.