Measuring the Heaviest Known Particle,
Measuring the Heaviest Known Particle
A quark and an anti-quark annihilate to form a top and an antitop quark. The top and antitop rapidly decay into a W boson and a b quark. One W decays into an electron an a neutrino and the other decays into a pair of quarks, as shown at the left (animation courtesy particleadventure.org).
The building blocks of known matter in the universe are quarks and leptons. The lightest quarks, called up and down, make up the proton and the neutron that comprise the nucleus of the atom. The lightest electrically charged lepton is the electron, which lives outside the positively charged nucleus and keeps the atom electrically neutral. For reasons that are not well understood, there are three families of quarks and leptons, distinguished by their masses. The lightest family consists of the up and down quarks and the electron and its neutral partner the electron neutrino. The next family is the charm and strange quarks, together with the muon and muon neutrino. The heaviest family consists of the top and bottom quarks, the tau lepton and tau neutrino. The top quark, which was discovered at Fermilab in 1994–95, is the heaviest known elementary particle. It has a mass that is 40 times the mass of the next heaviest quark, bottom, and it is hundreds of times heavier than the lightest quarks. High-energy physicists hope that by studying the top quark they can uncover clues that will lead to an explanation of the existence of three families, rather than one (or four!), and of the bizarre pattern of the masses.
Top quarks can be produced in very high-energy collisions between other particles, making use of the 100-year-old equation E = Mc2 to convert the energy of the collisions into the enormous mass of the top quark. Professor Tony Liss is a collaborator on the CDF experiment at the Fermilab Tevatron accelerator, where collisions between protons and antiprotons occur at an energy of 1.96 TeV (that's approximately 2000 times the energy equivalent of the proton's mass!). The Tevatron is currently the world's highest energy accelerator (until the LHC turns on ca. 2007). At the Tevatron collisions between protons and antiprotons occur at a rate of nearly 10 million per second. Every so often, a collision will produce a top quark together with its antimatter counterpart, an anti-top quark. The top quarks do not live very long, about a yactosecond (that's 10–24 s!). That's not long enough to observe them directly. Instead, we look for the decay products of a top quark, which are a W boson and a bottom quark. Unfortunately, the W boson doesn't live much longer than a top quark, so it's not observed directly either. Instead the decay products of the W are measured, and those are either a pair of light quarks that show up as jets of charged particles in the detector, or and energetic lepton and neutrino—such as an electron and an electron neutrino. The bottom quark from the top decay also shows up as a jet of charged particles, but about 10 percent of the time there is a muon in that jet from the decay of the bottom quark. That makes it possible to sift out the relatively rare top quark decays from 10 million collisions per second that don't produce top quarks.
Professor Tony Liss, together with postdoctoral researchers Anyes Taffard and Greg Veramendi, graduate student Ulysses Grundler and former postdoc Lucio Cerrito (now at Oxford) have recently completed a measurement of the rate of top+antitop production at the Fermilab Tevatron. They developed a technique, called "soft lepton tagging" for identifying muons inside jets to aid in sifting out the top–antitop quark events from the huge background of collision events that have nothing to do with top quark production. By carefully measuring the efficiency of their tagging technique and estimating the number of residual background events that mimic the top quark signature, they measured a "cross section" for the production of top-antitop pairs of 5.2 ± 2.7 picobarns. What that means is that top–antitop pairs are produced in less than one out of every 10 billion proton-antiproton collisions at the Tevatron. That's rare! With about 10 million collisons per second and about one top–antitop pair in 10 billion collisions, that's about one top–antitop pair produced every 15 minutes.
If you would like to learn more about this exciting work, contact Professor Tony Liss.
The U of I CDF top quark muon taggers. Left to right: Tony Liss, Lucio Cerrito, Anyes Taffard, Greg Veramendi and Ulysses Grundler.