Corralling the Mass Maker: Hunting ground shifts for elusive particle

It was the year 2000, and scientists at a European particle collider observed possible traces of the subatomic particle known as the Higgs boson-the presumed origin of mass itself and the most-wanted quarry in high-energy physics today. Despite a feverish search for confirming evidence, they came up empty-handed (SN: 12/9/00, p. 381). Perhaps they were looking in the wrong place.

GROUND DZERO. This detector, known as DZero, has yielded a new clue in the search for the mass-giving particle, the Higgs boson. Fermilab Photo

In the June 10 Nature, researchers at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., report that the mass that physicists had assigned to a different subatomic particle, the top quark, is too low. That may have misled scientists in their hunt for the Higgs boson.

Using a newly refined value for the top quark’s mass, the Fermilab scientists now calculate that the Higgs’ most likely mass is equivalent to an amount of energy about 20 percent higher than previously predicted. Even so, an existing accelerator and another one soon to be completed should be powerful enough to create the Higgs particle at the mass now considered most likely, say the Fermilab researchers, who are known as the DZero team.

The revised energy value for the Higgs particle is “a tremendously exciting result for particle physics,” says Christopher G. Tully of Princeton University. It could explain why some past searches for the particle have failed even though they were guided by long-accepted theories.

Some physicists downplay the upshift’s impact. The energy of a particle is only one of many factors that determine whether it can be found with a particular accelerator, notes Peter B. Renton of Oxford University in England.

What’s more, comments Fermilab theorist Marcela Carena, who is not part of the DZero team, the range of possible energies of the Higgs particle remains broad. So, the newly calculated “central value does not mean much,” she says.

In the standard model of particle physics, the top quark is one of 16 particles that constitute matter and govern many of the interactions among matter’s constituents. Scientists theorize that the only standard- model particle that hasn’t yet been observed experimentally—the Higgs boson—performs a vastly important and mysterious service for its cousin particles: It somehow bestows mass upon them (SN: 3/10/01, p. 152: Jiggling the Cosmic Ooze).

“To get the most information on the Higgs, we have to deal with the heaviest particle around,” which is the top quark, notes DZero physicist Greg Landsberg of Brown University in Providence, R.I. The more massive a particle, the more strongly it would interact with Higgs bosons.

Ordinarily, physicists would improve a mass measurement by making more observations of the particle in accelerator experiments. In this case, however, the DZero team reexamined its data with a more powerful method of distinguishing top-quark appearances from mere look-alikes. This achieved the same effect as running Fermilab’s most powerful collider, called the Tevatron, for 3 years at a cost of more than $100 million, Landsberg says. The new top-quark mass is about 2 percent larger and its measurement is about 15 percent more precise than the best previous value.

Many future particle physics experiments will apply the powerful analytical technique, predicts DZero-team member B. Paul Padley of Rice University in Houston. Two other Fermilab measurements have already benefited from it.

The newly calculated mass for the top quark could also render the Tevatron more relevant to studies of an expanded theory of particles known as supersymmetry. The theory predicts yet-undiscovered particles that would be partners to standard-model particles, plus a family of five Higgs bosons.

Some previously dismissed ways of detecting supersymmetric particles at the Tevatron are now back on the table. That, says Landsberg, “makes life at the Tevatron quite exciting for the next few years.”

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