Interaction without T-Channel: S-Channel Only

[touqra]

Just wondering whether there's any interaction, where the t-channel is prohibited, only the s-channel ?

 

[Timo]

If you meant "there is only the s-channel": e+ e- -> tau+ tau-, as one example. A more general class: Every reaction in which the result products do not couple to the incoming particles. Another example (either unfitting or better fitting, depending on what you are looking for): Compton scattering.