• Choose a category
• All
• Classic-Fiction

By Root 1147 0

that they never appear in isolation. They are always in the middle of a jet that might contain the original particle, but will always include a bunch of others that also experience the strong force. Jets don’t contain single particles, but a spray of strongly interacting particles “protecting” the initial one, as can be seen in Figure 41. Even if not present in the initial event, the strong interactions will create many new quarks and gluons from the quark or gluon that initiated the jet in the first place. Proton colliders produce a lot of jets since protons themselves are made of strongly interacting particles. Such particles produce sprays of many additional strongly interacting particles that travel alongside them. They also sometimes create quarks and gluons that go off in different directions and form their own independent jets.

The quote I used in Warped Passages from the “Jet Song” in West Side Story actually describes hadronic jets quite well:

You’re never alone,

You’re never disconnected!

You’re home with your own:

When company’s expected,

You’re well protected.

cross section view

side view

[ FIGURE 41 ] Jets are sprays of strongly interacting particles that develop around quarks and gluons. The picture shows their detection in trackers and the hadronic calorimeter. (Modified version of photo courtesy of CERN)

Quarks—and most gang members—won’t be found on their own, but in the midst of related strongly interacting companions.

Jets generally leave visible tracks, since some of the particles in jets are charged. And when a jet reaches the calorimeters, it deposits its energy. Careful experimental studies, as well as analytic and computer calculations, help experimenters deduce the properties of the hadrons that created the jets in the first place. Even so, strong interactions and jets make quarks and gluons more subtle. You don’t measure the quark or gluon itself, but the jet in which it resides. That makes most quark and gluon jets indistinguishable from each other. They all deposit lots of energy and leave many tracks. (See Figure 42 for a schematic of how detectors identify key Standard Model particles.)

Neutral particle path

Charged particle track

Lower-res charged particle track

Energy recorded

Particle shower

[ FIGURE 42 ] A summary of how Standard Model particles are distinguished in the detectors. Neutral particles don’t register in the trackers. Both charged and neutral hadrons can leave small deposits in the ECAL but deposit most of their energy in the HCAL. Muons go through to the outer detector.

Even after measuring a jet’s properties, telling which of the different quarks or gluons initiated the jets is challenging if not impossible. The bottom quark—which is the heaviest quark with the same charge as the down (as well as the heavier strange) quark—is an exception to the rule. The reason the bottom quark is special is that it decays more slowly than the other quarks. Other unstable quarks decay essentially immediately after they are produced, so their decay products appear to start their tracks at the interaction point where the protons collided. Bottom quarks, on the other hand, last long enough (about one and a half picoseconds, or enough time to travel about a half millimeter at the light speed at which they travel) to leave a track a noticeably large distance from the interaction point. The inner silicon detectors detect this displaced vertex, as illustrated in Figure 43.

[ FIGURE 43 ] Hadrons made of bottom quarks live long enough to leave a visible track in the detector before decaying into other charged particles. This can leave a kink in the silicon vertex detectors, which can be used to identify bottom quarks. The ones here came from top quark decays.

When experimenters reconstruct a track from a bottom quark decay, it doesn’t extend back to the initial interaction point in the center of the event. Instead the track seems to originate from the place in the inner tracker where the bottom quark decayed, leaving a kink in tracks that is the juncture