a particle detector

You cannot see a subatomic particle directly, any more than you can see the wind. What you can see is the trail it leaves as it tears through matter: atoms it strips of electrons, flashes of light it kicks off, showers of secondary particles it spawns. A particle detector is an instrument engineered to turn those faint traces into measurable electrical signals, so that from the trail you can reconstruct what passed through, and with what energy and momentum.

Detectors exploit the specific ways particles deposit energy. Charged particles ionize the material they cross, which tracking chambers and silicon detectors turn into a reconstructed path; bending that path in a magnetic field measures momentum through p = q B r, since the radius of curvature grows with momentum. Scintillators and Cherenkov radiators convert deposited energy into light. Calorimeters absorb a particle completely and measure its total energy from the size of the shower it produces. Large experiments nest these into an onion of subsystems, an inner tracker, then electromagnetic and hadronic calorimeters, then outer muon chambers, so that each particle type leaves a characteristic, complementary signature that together identify it and measure its four-momentum.

You meet particle detectors in the giant ATLAS and CMS experiments at the LHC, in neutrino observatories, in dark-matter searches, and, in disguise, in medicine, where a PET scanner is a particle detector. The honest caveat is that no detector is perfect. Each has a finite efficiency, so some particles are simply missed; a finite resolution, so energies and positions come out smeared; and only partial angular coverage. Neutrinos escape entirely and are inferred only from an imbalance in the total measured momentum. Extracting physics therefore means carefully modelling these imperfections, usually with Monte Carlo simulation, and correcting for them.