Particle production channels [s,t] in HEP?

Question

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"?

Answer 1

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.