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.
