How do physicists know that particles observed during collisions are actual particles and not random pieces of

Answer 1

Put a magnetic field and some receptors around the collision site. You can tell when any charged particle hits a receptor because there's an electric signal. The size of the signal tells you about the charge. Measure the time between signals and you can trace the path the particle took. When you know the charge and you see how much the particle is bent by the magnetic field, you can tell its mass. From the speed, you can derive the energy. Then you can recreate the collision by looking at all the particles that came out of the collision.

Answer 2

One of the earliest detectors was the cloud chamber. If you perform the collisions in calibrated electric and magnetic fields, you can learn a lot about the mass and charge of product particles by watching their ionization trail in the cloud chamber and seeing the direction and amount of their bending.

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Answer 3

By repeating the experiment many times.