The equations for the conservation of energy and momentum in this system show that if you know the mass of the stationary target particle then you can calculate the mass of the incoming particle simply by measuring the angles made by the tracks of the particles after the collision.

To "measure" an angle rollover it with the mouse and left click. Left click on the red ball to insert its mass into the formula.

Measure both angles, insert the mass of the red ball and then calculate the mass of the incoming particle.

Does it match your expectation deduced from the rough size of the angles?

Particle Physicists need to identify the particles that they create in their experiments. The method you have just used enables you to find the mass of an unknown particle. This helps in its identification.