New measurement of another particle’s mass confirms a final missing piece of physics’ puzzle is right where scientists think it is

By Alexandra Witze

Web edition: February 23, 2012

Even as physicists in Europe close in on their most-wanted quarry — a particle known as the Higgs boson — scientists in Illinois are helping narrow the hunt. New measurements of a different particle, one called the W boson, confirm the Higgs is in the mass range that most physicists had thought.

Theory suggests that the Higgs particle must exist in order to imbue many other particles with mass. Experiments at the Large Hadron Collider, at the CERN laboratory near Geneva, have shown that the Higgs’ own mass must be less than 127 billion electron volts. (Though it sounds like a unit of electricity, the electron volt is particle physicists’ fundamental unit of mass. A proton’s mass is about 1 billion electron volts.)

The new W boson findings  confirm that the Higgs must be less than 145 billion electron volts. At the bottom end scientists have long known the Higgs, if it exists, must be at least 114 billion electron volts.

Narrowing the Higgs mass range with different methods helps scientists cross-check and thus have more confidence in their results. The W boson comes into play because it, the Higgs, and a third particle called the top quark are all interrelated. Determine the mass of any two of those, and you can calculate the mass of the third.

The new measurement is the most precise ever of the W boson mass: 80,387 million electron volts, according to scientists with the CDF collaboration at the Fermi National Accelerator Laboratory in Batavia, Ill., who announced the findings February 23 at a lab seminar.

For Fermilab, the W boson report is a sort of swan song from its now-closed particle accelerator, the Tevatron. For CERN, the findings strengthen its all-out hunt for the Higgs. It will boost the energy with which it smashes together beams of protons, from 7 to 8 trillion electron volts, when it switches the LHC on again later this year. (The machine shuts down every winter.)

In December, LHC scientists reported seeing hints of a Higgs around 125 billion electron volts. The next update on the Higgs hunt is expected on March 11, at a conference in the Italian Alps.

"2012 will be the year of experimental data after many years of theories,” Sergio Bertolucci, CERN’s research director, said in Vancouver February 17 at the annual meeting of the American Association for the Advancement of Science. “We’re very much looking forward to it."