The top quark, 20 years after its discovery

A top–antitop quark pair can be produced from the collision of energetic protons and antiprotons.

Ever since the days of the Greek philosophers, humankind has sought to identify the elementary building blocks of matter. Over time, the notion has been refined; the original idea that indivisible atoms were the fundamental elements has evolved to the present view that objects called quarks lie at the heart of all matter. So in 1995 the discovery at Fermilab of the top quark—the sixth and possibly last of the quarks—might have been thought to signal the end of one of science’s longest searches.1

But the properties of the top quark are bizarre and raise new questions. In particular, its mass is the largest of any known elementary particle. That weightiness suggests the top quark plays a fundamental role in the breaking of the symmetry of the electroweak interaction, a symmetry that requires that the masses of the elementary particles vanish. If so, the top quark is itself fundamental to the generation of mass.

A MISSING FAMILY MEMBER

In 1964 Murray Gell-Mann and George Zweig independently proposed the quark hypothesis2to account for the explosion of subatomic particles discovered in accelerator and cosmic-ray experiments during the 1950s and early 1960s. More than 100 new particles had been observed, most of them strongly interacting and very short-lived. Those strongly interacting particles, called hadrons, are not elementary; they possess a definite size and internal structure. The quark hypothesis suggested that different combinations of three quarks—the up (u), down (d), and strange (s) quarks—and their antiparticles could account for all the hadrons then known. Each quark had an intrinsic spin of ½ℏ and was presumed to be elementary. To explain the observed spectrum of hadrons, quarks had to have electric chargesthat are fractions of the electron charge.

Read more: 

http://physicstoday.scitation.org/doi/10.1063/PT.3.2749