As its name states, the top quark is a quark. Quarks are one of the fundamental building blocks of which the entire universe is constructed. Scientists know of six different quarks, although only two of them (the up and down quarks) are found inside the protons and neutrons at the center of atoms. The other four quarks (strange, charm, bottom and top) are all unstable and decay in fractions of a second.
The top quark is the heaviest subatomic particle ever observed, with a mass that is about as heavy as an entire atom of gold. Top quarks are also among the most fleeting of particles, with a lifetime of about a trillionth of a trillionth of a second.
Top quarks were first observed at Fermilab in 1995 and the two experiments at Fermilab’s Tevatron collider each observed about 150,000 top quarks over their lifetimes. Top quarks are now produced in even greater quantities at the Large Hadron Collider (LHC). When the LHC is running at design luminosity, it will produce a pair of top quarks every second. As of June 2012, over two million top quarks have been created in each of the big experiments at the LHC, and the rate is rapidly increasing.
The original studies concentrated on collisions in which a top quark and an antimatter top quark were simultaneously produced. Experiments at Fermilab were also the first to observe top quarks produced one at a time. This was a tricky thing to accomplish because ordinary processes can produce top quark lookalikes. Making two lookalikes at the same time is rare, which makes it easier to separate real top quark events from fakes. To find singly made top quarks required a detailed understanding of the detector.
Though 17 years have passed since the top quark was discovered, it remains a topic of intense research interest. Perhaps the most pressing mystery in contemporary particle physics is the origin of mass. While the mechanism that gives particles their mass is not yet known, a very popular theory is that the Higgs boson is responsible. Given that the top quark is the heaviest particle yet discovered, it is a natural laboratory in which to investigate how the Higgs boson interacts with matter. Further, if the Higgs boson is real, the theory makes very precise predictions on how the mass of the Higgs boson and the top quark are related. In fact, one of the ways we will be certain that an observation of a Higgs boson is the real Higgs boson (as opposed to a competitor theory) will be to measure the mass of the top quark with sufficient precision to see if the prediction is correct.
After years of intense studies, the top quark has revealed a mystery or two of its own. At Fermilab’s Tevatron, measurements by both of its collider experiments showed that the top quark prefers to be created in the same direction the proton beam is travelling. This measurement disagrees with predictions and could indicate new physical phenomena or it could just mean that the regular theory needs to be tweaked.
In addition, top quarks, produced singly or in pairs, have rich panoply of decay products with signatures that can mimic collisions of even rarer phenomena. Thus it is imperative to understand in great detail how top quarks are produced.
The top quark is an intriguing particle, massive and fleeting, and one that continues to provide a broad range of interesting research topics.