Measuring a fractional charge

At left is the apparatus used by Robert Millikan and his student to measure the charge of the electron in the early 20th century. At right is the apparatus used by more than 300 physicists — including students — to measure the charge of the top quark in the early 21st century.

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In 1909, Robert Millikan and Harvey Fletcher performed a Nobel Prize-winning experiment that showed that elementary particles always have a specific amount of electric charge and that fractional charges do not exist. Now, a century later, we measure fractional charges.

To be fair, in Millikan’s time, there were few known particles to work with. His experiment was done using electrons, although by inference the result also applies to protons. Particles called quarks were unknown to Millikan and are quite different from electrons. They should have charges of either +2/3 or -1/3 times that of the electron.

We say “should have charges” because quarks are (nearly) always found inside hadrons, such as protons and neutrons, where they are bound very tightly to other quarks. Unlike electrons, you can’t find just one and measure it. Consequently, the charge assignments for quarks are inferred from the charges of hadrons and from our model of the various quarks that are inside hadrons.

The top quark is a little different though. Because of its large mass, the top quark decays before becoming bound up in a hadron. Electrical charge being conserved, we know that if we identify the particles that a top quark decays into and add up their electrical charges, then we have the charge of the top quark.

The tricky part is identifying which particles measured at the Tevatron are the ones that come from the decaying top quark. There is so much energy in Tevatron collisions that the available energy E results in the mc2 of many particles, not all of which are the products of top quark decay.

In “lepton + jets” events, there is always a very energetic electron or muon, which very likely is the product of the top decay. The other “particle” produced in that top quark decay is a jet, a narrow spray of particles that are all going in the same direction. They are produced from the decays of a single particle; in the Standard Model, that progenitor is a bottom quark. To determine the charge of the progenitor, we count the charges of all the particles in a jet. Because the more energetic particles are more likely to reflect the progenitor’s charge, we count their charges more heavily in our summation.

The prediction of the Standard Model is quite clear: the top quark’s charge should be positive and 2/3 times that of the electron. However, it is still possible that the top quarks that are produced by the Tevatron are not what we think they are. There is some possibility that it is instead an exotic particle with a negative charge 4/3 times that of the electron.

To check on this possibility, the DZero collaboration used these methods to measure the electrical charge of the top quark. Because there are only two possible answers, +2/3 and -4/3, the primary result is to determine which of the two options is more likely and how much more likely it is. We find with a very high level of confidence that the +2/3 answer is the right one. The probability that an exotic top quark could have given our result is only one in about 16 million. As far as its electric charge is concerned, the top quark looks just as predicted in the Standard Model of elementary particles.

Leo Bellantoni

These DZero members all made significant contributions to this result.
The Tevatron is no longer running, but the river of results it produces still is. The DZero collaborators who presented DZero or Tevatron results at the International Conference of High Energy Physics recently in Valencia, Spain, are: Top row, from left: Gregorio Bernardi (LPNHE/Paris), Don Lincoln (Fermilab), Rick Van Kooten (U Indiana). Middle row, from left: Marj Corcoran (Rice U), Ken Herner (Fermilab), Oleg Brandt (U Göttingen), Christian Schwannenberger (U Manchester). Bottom row, from left: Regina Demina (U. Rochester), Hengni Li (U Virginia), Siqi Yang (USTC, China).