I can answer this, most of the jets are going to be of charged particles so when they fly into the HCAL they ionize the particles that make up the detector. Which is held in an electric field causing these electrons to drift to the sensor to tell us that there is a hit. Depending on the energy (momentum and mass) of the particle it will only go a certain distance into the HCAL. Based on these measurements we can determine what it is.

Neutral particles are the tricky ones, for these we have to look for missing energy. By measuring a couple charged particles we can see that they don't add up to what we are supposed to have.

I hope that all makes sense, if you have any more questions or if things are still unclear let me know