Scientists have produced the most precise measurement of a fundamental particle called the W boson. It will help them search for the elusive Higgs boson, the discovery of which would be an epoch-making event.
The W boson’s new mass is 80.387 giga electron volts, or GeV, plus or minus 0.019 GeV. (Scientists often give a particle’s mass in units of energy because, according Einstein’s famous E=MC² equation, the two are interchangeable.) The most precise previous measurement had an uncertainty of about 0.060 GeV.
At the subatomic scale, such little differences are immense.
The new result is “exquisite” and places the uncertainty “in another category with respect to the past results,” wrote physicist Tommaso Dorigo in his blog. The finding was presented Feb. 23 at the Fermi National Accelerator Laboratory in Illinois.
Researchers with the CDF collaboration at Fermilab produced the estimate using data from the now-closed Tevatron, formerly the world’s premier particle accelerator, where measurements of collisions between protons and antiprotons fired around a 4-mile-long track provide insight into the subatomic world. Though CERN’s Large Hadron Collider has eclipsed the Tevatron, the result shows that the U.S. lab still has a few tricks up its sleeve.
The W boson, along with its counterpart the Z boson, are responsible for carrying the weak force, much the same way that photons convey electromagnetic force. Together with gravity and strong nuclear force, these comprise the four fundamental forces of nature. The W boson’s discovery in 1983 was a major success for the Standard Model, developed by physicists to explain the interactions of all subatomic particles and forces, and its mass is an important input for many nuclear and astrophysical calculations.
It’s also intimately linked to two other subatomic particles: the top quark, the heaviest of the six types of quarks, and the Higgs boson. “If you know the mass of any two, you know the mass of the third,” said physicist Rob Roser, co-spokesman for the CDF collaboration.
'It's basically make it or break it for the Standard Model.' That potential extrapolation is crucial. While the Higgs boson has been theoretically predicted to exist, and is believed integral to the very essence of mass, it hasn’t actually been spotted. Last December, researchers at the Large Hadron Collider saw hints of what may be the Higgs boson, and pegged its mass at about 125 GeV. The extra-precise measurement of the W boson fits with this measurement of the Higgs. The result also means that physicists shouldn’t expect to find the Higgs anywhere higher than 145 GeV.
All eyes are now on this final sliver of energy where the Higgs may be hiding, said physicist Ashutosh Kotwal of Duke University in North Carolina, who presented the latest results from the CDF collaboration. If the Higgs turns up there, it will confirm scientists’ theories. If it doesn’t, they will have to start looking for new, more exotic ways to explain the universe.
“It’s basically make it or break it for the Standard Model,” said Kotwal.
Though the Large Hadron Collider has progressed further in the Higgs search, Fermilab scientists still hope to be part of the discovery. Next month they will present their latest Tevatron data, which may include a Higgs signal. And even if Fermilab doesn’t find the Higgs themselves, the LHC may never be able to measure the W boson with comparable precision. Its mass may be one of the Tevatron’s great legacy calculations, said Roser.
In another three or four years, the CDF collaboration will use the remaining Tevatron data to produce a final estimate, which could go down in history as the most precise W boson measurement ever.
Image: Fermilab physicist Pat Lukens stands in front of the CDF detector. CDF/Fermilab
