Consider for example we perform the electron-proton scattering experiment.
If we accelerate the incident electron with a certain energy, then it means that we can control the center-of-mass energy of this process. In terms of Mendelstam variables, we know the value of $s$.
But, I wonder how can we control (or measure) the momentum transfer, $Q^2$, which stands for the resolution. If we want to investigate the internal structure of the proton deeply, we need a high $Q^2$ value.
Since $s$ and $t$ are independent variables, it seems that we can change $s$ and $t$ independently.
How can we do that in experiments?