Discovered in 1983, the W boson is one of four known types of particles that transmit the forces of nature. It weighs about 86 times the mass of a proton.
For many years, only proton and antiproton accelerators, or hadron colliders, at CERN and Fermilab had enough beam energy to create W bosons and enable mass measurements. But in 1996, after an energy upgrade, the Large Electron Positron collider at CERN, a lepton collider, started to produce pairs of W bosons and delivered the world’s best W mass measurements. Few people believed that hadron colliders, with their messy collisions spewing out hundreds of particles, could ever again provide better information on the W boson mass than the clean collisions of a lepton collider.
But Ashutosh Kotwal thought otherwise. In 1999, the Duke University physicist began to study proton-antiproton collisions with CDF, one of two collider detectors at Fermilab’s Tevatron. He started a long-term program to obtain a precise W mass measurement, using the experience he had gathered by measuring the W mass with the other Tevatron detector, DZero. I had the feeling that someday the Tevatron could have the best result in the world again, Kotwal recalls.
The project wouldn’t be easy. Obtaining precise measurements of the energy and other properties of the particles emerging from a single proton-antiproton collision at the Tevatron was a daunting task.
In July 2004, Kotwal and his collaborators began preparing for meetings with their CDF godparents, three CDF scientists who would independently scrutinize their analysis before blessing it for publication. In this July 15 logbook entry, Kotwal documented the long list of uncertainties that he and his collaborators would have to understand to make the world’s best W mass measurement and convince other scientists that their result was trustworthy.
Two and a half years later, Kotwal presented the result in a talk at Fermilab: the W mass was 80,413 +/- 48 MeV/c2, measured with a world-record precision of 0.06 percent. In a press release, CDF scientists announced that the new value suggested the Higgs particle to be lighter than previously thought.
The result changed how scientists thought about the ability of hadron colliders to make precision measurements, says Kotwal, who now works on the Large Hadron Collider. Our experience now helps with the analysis of LHC data.
