Physicists Confirm Quark That `Had' to Be There
AN international collaboration of more than 440 scientists has announced the most reliable ``sightings'' yet of an elusive member of the first family of elementary particles thought to make up matter. They call it the ``top'' or ``truth'' quark.
It is the only one of 12 basic particles called for in the current standard theory of matter that had not been produced, weighed, and measured in the laboratory. Now the collaboration has enough evidence, produced at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., to specify its probable mass. It weighs in at about 187 times the mass of a hydrogen atom -
about the same as the element rhenium and a little lighter than gold. It's the heaviest particle known.
Sense of relief
Physicists are greeting this long-anticipated discovery with more of a sense of relief than of surprise. Theorist John Bahcall at the Institute for Advanced Study in Princeton, N.J., notes that the mass is in the range expected. But if the top quark had not been found as expected, he says, ``then everything we know [about the basic nature of matter] wouldn't make sense.'' Seen in this perspective, Dr. Bahcall calls the work ``a magnificent achievement ... [marking] an end of an era and beginning of a new era'' for research that goes beyond what the standard theory covers.
Briefly, that theory involves two sets of six basic particles. There are three electron-like particles - the ordinary electron and the muon and tau particles. These are associated with three neutrinos - electrically neutral particles having little or no mass. There are also six quarks called up, down, charm, strange, bottom (or beauty) and top (or truth). The new knowledge about the top quark, if confirmed by further experiments, completes the evidence for this scheme.
Most matter, as we know it, is made up of the ordinary electron and the up and down quarks. The other particles exist only fleetingly in high-energy cosmic-ray interactions and in particle collisions in laboratory accelerators. That is why the top quark is so elusive.
Because it's the most massive basic particle known, it takes a lot of energy to produce it. As announced Tuesday at Fermilab in the United States and in Canada, Italy, Japan, and Taiwan, the physicists from those countries put the energy equivalent of the top quark mass at 174 billion electron volts, give or take 17 billion electron volts. That's the energy an electron would gain when accelerated by a voltage difference of 174 billion volts. It took the Fermilab accelerator, which can smash protons and antiprotons together at a combined energy of 1.8 trillion electron volts, to produce collisions with enough energy in the resulting debris to make a few top quarks.
Once made, the top quark has only a fleeting existence. Physicists must infer its presence from the subsequent particles into which it decays.
Many other processes in the collisions mimic the top quark decay signature. It has taken something like two decades for quark hunters to get as far as they have today.
More work to do
Work still needs to be done to confirm the findings. In reporting ambiguous top quark sightings a year ago, Melvyn Shochet, a collaboration spokesman from the University of Chicago, warned it would take ``something like five to 20'' unambiguous events in each of several top quark decay modes to claim a solid discovery. So far, the collaboration has about 15 such events.
Meanwhile, physicists look ahead to the next challenge - explaining why matter has mass. They think a presently unknown type of interaction may be involved.
Uncovering that interaction now is the ``golden ring'' prize for particle physicists, Bahcall explains.