In high energy physics there are different channels of particle production: t-channel, s-channel. What do they mean? And are there any other channels besides "t" and "s"?
These are basically ways of describing which leading diagram contributes the largest term to the cross-section; they are named after which of the Mandelstam variables characterizes the 4-momentum of the virtual particle.
In t- and u-channel processes the 4-momentum of the exchange particle is space-like (has a negative norm).
In the s-channel the 4-momentum of the exchange particle is time-like (has a positive norm). This necessitates the annihilation of the incident particles.