And not even then. There are also loop diagrams.mfb said:They all contribute if (!) they can happen for a given process. Their "physical significance" is the same as for the s channel. You cannot point to an observed process and say "it happened via this channel!" unless all but one channel are impossible.
The S, U, and T channels refer to the three different types of particles that are exchanged in interactions between particles. The S channel involves exchanging scalar particles, the U channel involves exchanging vector particles, and the T channel involves exchanging tensor particles.
The S, U, and T channels play important roles in determining the strength and behavior of particle interactions. Different types of particles interact differently depending on which channel they are exchanging particles through.
The S, U, and T channels are all interconnected and can influence each other. For example, some interactions may involve a combination of all three channels, with particles exchanging different types of particles at different points during the interaction.
Understanding the S, U, and T channels is crucial for studying and predicting particle interactions and the behavior of subatomic particles. This knowledge is essential for fields such as particle physics, quantum mechanics, and cosmology.
While the primary use of understanding the S, U, and T channels is for theoretical and scientific research, there are some practical applications. For example, this knowledge has helped in the development of technologies such as particle accelerators and medical imaging devices like PET scanners.