Knocking on Heaven's Door (39 page)

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[
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 between the bottom quark that came in and the decay product that came out.
⁵⁵
With the fine segmentation of the silicon detectors, experimenters can view detailed tracks in the region close to the beam, and successfully identify bottom quarks a significant fraction of the time.

The other type of quark that is distinctive from an experimental vantage point is the top quark, which is special because it is so heavy. The top quark is the heaviest of the three quarks that have the same charge as the up quark (the other one is called charm). Its mass is about 40 times heavier than the differently charged bottom quark and more than 30,000 times the mass of the up quark, which has the same charge as the top.

Top quarks are sufficiently heavy that their decay products leave distinct tracks. When lighter quarks decay, the decay products, like the initial particle, travel so close to the speed of light that they are rushed along together into what appears to be a single jet—even if the jet had its origin in two or more distinct decay products. Unless they are extremely energetic, top quarks, on the other hand, visibly decay into bottom quarks and
W
bosons (the charged weak gauge bosons) and can be identified by finding both of them. Because the top quark’s heavy mass implies that it interacts most closely with the Higgs particle and other particles involved in weak scale physics that we are hoping to soon understand, the properties of top quarks and their interactions might provide valuable clues to physical theories underlying the Standard Model.

FINDING THE WEAK FORCE CARRIERS

Before we finish discussing how to identify Standard Model particles, the final particles to consider are the weak gauge bosons, the two
W
s and the
Z
, that communicate the weak nuclear force. The weak gauge bosons have the peculiar property that, unlike the photon or gluons, they have nonvanishing mass. The masses associated with the weak gauge bosons that communicate the weak force pose some major fundamental mysteries. The origin of this mass—as with the masses of the other elementary particles this chapter has discussed—is rooted in the Higgs mechanism that we will get to shortly.

Because the
W
s and
Z
are heavy, these gauge bosons decay. This means that the
W
and Z bosons, as with the top quark and other un-stable heavy particles, can be identified only by finding the particles into which they decay. Because heavy new particles are also likely to be unstable, we’ll use the weak gauge boson decays to exemplify one other interesting property of decaying particles.

A
W
boson interacts with all particles that are sensitive to the weak force (namely, all the particles we have discussed). That gives the
W
plenty of decay options. It can decay into any charged lepton (the electron, the muon, or the tau) and their associated neutrino. It can also decay into an up and down quark or into a charm and strange quark pair, as illustrated in Figure 44.