The mass of the top quark just got topped off. A new analysis of the fundamental particle implies that it is a little heavier than previously thought. This is heartening news for particle physicists, who are hoping to find several new particles by the end of the decade.
The top quark is the heaviest of the six indivisible quarks that make up composite particles such as protons and neutrons. Scientists can "weigh" the top quark by using particle colliders to smash protons and antiprotons together at enormous speeds and studying the resulting sprays of particles. Earlier data collected mainly at the Tevatron collider at the Fermi National Accelerator Laboratory in Batavia, Illinois, pegged the particle's mass at about 174 billion electron volts (GeV).
But this week in Nature, physicists with the D0 experiment at the Tevatron report that a new mathematical technique for analyzing collision data increases the mass of the top quark to 178 GeV. Even such a small difference in mass can have a large effect on the expected heft of as-yet-undiscovered particles, such as the Higgs boson, which is thought to imbue particles with mass. "If you measure the top, you can set indirect limits on where to look for Mr. Higgs," says Greg Landsberg, co-spokesperson for the D0 experiment.
The new analysis raises the favored mass of the Higgs to about 117 GeV, says Landsberg, a mass just barely out of reach of previous experiments. (The higher the mass, the higher the energy of collisions needed to find the particle.) The new estimate might lessen the anxiety among Higgs aficionados, who were worried because experimentalists had failed to find the Higgs at lower energies. (The higher the mass, the higher the energy of collisions needed to find the particle.) In addition to the new analysis, new data from the Tevatron also indicate a top-quark mass a few GeV higher than previously thought.
However, other scientists downplay the importance of such a small shift in value. "From a statistical point of view, it's not really very different," says physicist Gordon Kane of the University of Michigan, Ann Arbor. Kane says that the new technique "in principle should work better" than previous analyses but he worries that assumptions built into in the computer simulations may have allowed errors to creep into the team's analysis.
The D0 experiment