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Higgs Boson Gets New Mass Limit

Ashutosh Kotwal and his Fermilab team offer more precise prediction for Higgs boson mass

The W boson is squeezing in on what the mass of the Higgs can be. Image courtesy of symmetry magazine/plush toys by artist Julie Peasley/photo by Robert Tilden.

New, more precise measurements of a particle called the W boson are again suggesting that physicists' prized Higgs boson is lighter than previously predicted. Using detectors at the Fermi National Accelerator Laboratory in Illinois, Duke physicist Ashutosh Kotwal and his collaborators have made the world's most precise mass measurement of the W boson, a key quantity in the Standard Model -- the theory physicists use to explain the dynamics of subatomic particles.According to the results of the experiment, the W boson's new mass was measured at 80387 million electron volts divided by the speed of light squared, give or take 19 MeV (80387 +- 19 MeV/c^2), with a precision of 0.02 percent. "This result significantly improves the world knowledge of the Higgs boson," Kotwal said, explaining that the precise measurement of the mass of the W boson makes a precise prediction for the mass of the Higgs.Based on the new W boson mass, Kotwal's team has calculated that the Higgs boson mass is roughly 90 GeV, or billion electron volts, with a precision of 30 percent."The best way to think of this is to present the upper limit on the Higgs boson mass with 95 percent probability, which means that there is only one 1 in 20 chance of the Higgs being heavier than this upper limit," Kotwal said. The new upper limit is 145 GeV.When Kotwal presented his last measurement of the W boson's mass in 2007, it put the upper limit of the Higgs boson mass at 161 GeV. With the new measurement of the W boson mass, the new upper limit of the Higgs has dropped to 145 GeV, "so it is now more likely that the Higgs boson is light," Kotwal said. He announced the finding in a talk at Fermilab on Feb. 23 and will submit the results of the experiment for publication in the near future.Last December, physicists from two experiments at CERN's Large Hadron Collider announced data that hinted the Higgs boson mass was between 115-130 GeV. Kotwal's new W boson mass measurement and predicted Higgs mass within 90-145 GeV suggests that the physicists are narrowing in on the energy region where the Higgs particle may be hiding.CERN and Fermilab physicists are expected to present their latest direct-search Higgs measurements at the annual conference on Electroweak Interactions and Unified Theories known as Rencontres de Moriond in Italy next month."If the Higgs boson is soon discovered and its mass agrees with the precise prediction given by the W boson mass measurement, then the Standard Model will be proven correct once again," Kotwal said "But, if the Higgs boson is not found where it is predicted, then this model will be proven wrong decisively." He explained that physicists ultimately want to know whether or not their model of the behavior of fundamental particles is correct. If the model is proven wrong, "we would have learned that there is something really new and more fundamental going on in nature, and this will be a major scientific advance of the last 50 years," Kotwal said.His team included Duke graduate student Yu Zeng, post-doctoral researcher Bodhitha Jayatilaka and former students Ravi Shekhar and Siyuan Sun.