March 3, 1995 in Nation/World

Research Teams Announce Discovery Of Elusive Particle Top Quark Had Baffled Scientists For Years

Curt Suplee Washington Post

Two research teams working independently at the Fermi National Accelerator Laboratory outside Chicago Thursday declared they have finally found the elusive top quark - the last remaining undiscovered member of the set of fundamental particles that make up all matter.

“There is no remaining doubt about the existence of the top quark,” said Bill Carithers, a spokesperson for one of the teams, a collaboration of 450 physicists who use CDF, one of Fermilab’s two particle detectors.

The announcement had been nervously awaited since last April, when the CDF group reported it had probably detected the “top,” an egregious porker of a subatomic particle that does not exist in ordinary matter, is about as heavy as an entire atom of gold, and is so hard to observe that it took modern science nearly 20 years and hundreds of millions of dollars to find it.

As of last spring, CDF’s arch-competitor in the worldwide race to find the top - a team that uses the same accelerator beam, but a separate detector (called DZero) at the Batavia, Ill. facility - had not seen similar evidence. That called CDF’s apparent international coup into question. Months passed as both groups took more measurements.

Thursday, CDF declared that, with three times the data available last year, the team had confirmed its initial findings: “Right down the line,” Carithers said, “we got very good agreement with what we saw before.” They calculate the top’s mass at about 176 billion electron volts (GeV) - about 40 times larger than the next heaviest known particle.

At the same time, the DZero group released its own observation of the top quark at a mass of 199 GeV - a figure “consistent” with the CDF results, considering the margins of error, said Paul Grannis, a cospokesman for the collaboration.

Definitive evidence of the top quark was so eagerly awaited because it was necessary to validate the so-called “Standard Model” of particle physics - the consensus view of how matter and force behave in their elementary forms.

In that view, all matter is made up of only two kinds of particles: leptons (lightweight entities such as the electron) and quarks.

Quarks were unknown as recently as the 1960s, and their discovery ranks as one of the paramount accomplishments of 20thcentury science. They are the smallest indivisible units of heavy matter. Each proton or neutron in an atom is composed of three quarks bound together.

The Standard Model calls for six kinds of quarks, five of which have been detected previously: the “up” and “down” types that make up the nuclear particles in garden-variety atoms, as well as the “charm,” “strange” and “bottom” quarks that are created in atom-smashers.

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