Jets: how we detect them

In the last post I was kind of cryptic about a couple of things - I did not want to make it too long. Let me explain here a bit more what is a jet and how we see it in our detector.

A jet is a well-collimated spray of particles that is most easily created at CDF when a quark or a gluon is knocked off a proton in a high energy collision. The quark, or the gluon, cannot exist as a free particle: the strong force ties it within a hadron - an observable particle such as a proton or a neutron (which are the stable hadrons) or a pion, a kaon, etcetera (which are unstable and decay very quickly into other lighter particles). The quark or the gluon knocked off a proton fragments back into new hadrons, which travel together and share the energy and momentum of the fragmenting body.

Detecting a jet entails reading the electronic signal left on high-voltage conductive wires immersed in suitable gas mixtures when the jet's charged particles pass by, which allows us to reconstruct particle trajectories if we have many such readings through the particle path; or detecting the light of scintillating pulses released when a charged particle traverse a scintillating (light-releasing) material. But measuring the jet energy, which is the most important part, requires destroying the particles it is made up of (not only charged but also neutral ones), by allowing them to smash through large amounts of heavy material, which creates yet more particles: from the correspondence between the number of secondary particles created and the original particles energy, the latter is measured.

    

The picture above shows how a charged particle is detected in a tracking chamber. The particle ionizes the gas, electrons travel to high potential wires, and leave there a signal which indicates where the particle passed. From many such readings, the path of the particle can be reconstructed with great accuracy. In CDF our tracking chamber measures several tens of such points along the track.

As shown above schematically, the CDF detector - not unlike most other particle physics detectors these days - has a inner tracking detector, which measures the trajectory of charged particles, surrounded by a calorimeter. The latter is the device responsible for the jet measurement: both charged and neutral particles hit the lead or iron atoms of the calorimeter, creating many other particles - mostly light hadrons such as pions or kaons, but also protons, neutrons, and photons - as they lose their kinetic energy. It is the collective signal of the passage of all these bodies through the scintillating material filling the gaps between iron or lead sheets in the calorimeter what provides us with a energy measurement.

One word about muons: as depicted in the sketch above, they are special in that they can pass through tons of material without being destroyed in a head-on collision with a heavy atom. To detect muons, wire chambers surround the calorimeter, at a depth where only muons can have survived.