New precise particle measurement improves subatomic tool for probing mysteries of universe

30-Sep-2015 - USA

Physicists at Southern Methodist University, Dallas, have achieved a new precise measurement of a key subatomic particle, opening the door to better understanding some of the deepest mysteries of our universe.

The researchers calculated the new measurement for a critical characteristic of the top quark. The newly calculated measurement of the top quark will help guide physicists in formulating new theories, said Robert Kehoe, a professor in SMU's Department of Physics. Kehoe leads the SMU group that performed the measurement. Top quark's mass matters ultimately because the particle is a highly sensitive probe and key tool to evaluate competing theories about the nature of matter and the fate of the universe. Physicists for two decades have worked to improve measurement of the top quark's mass and narrow its value.

"Top" bears on newest fundamental particle, the Higgs boson

The new value from SMU confirms the validity of recent measurements by other physicists, said Kehoe. But it also adds growing uncertainty about aspects of physics' Standard Model.

"So the top quark is really pushing both theories," Kehoe said. "The top mass is particularly interesting because its measurement is getting to the point now where we are pushing even beyond the level that the theorists understand."

He added, "Our experimental errors, or uncertainties, are so small, that it really forces theorists to try hard to understand the impact of the quark's mass. We need to observe the Higgs interacting with the top directly and we need to measure both particles more precisely."

SMU measurement achieves surprising level of precision

To narrow the top quark measurement, SMU doctoral researcher Huanzhao Liu took a standard methodology for measuring the top quark and improved the accuracy of some parameters. He also improved calibration of an analysis of top quark data.

"Liu achieved a surprising level of precision," Kehoe said. "And his new method for optimizing analysis is also applicable to analyses of other particle data besides the top quark, making the methodology useful within the field of particle physics as a whole."

The SMU optimization could be used to more precisely understand the Higgs boson, which explains why matter has mass, said Liu.

"This methodology has its advantages - including understanding Higgs interactions with other particles - and we hope that others use it," said Liu. "With it we achieved 20-percent improvement in the measurement. Here's how I think of it myself - everybody likes a $199 iPhone with contract. If someday Apple tells us they will reduce the price by 20 percent, how would we all feel to get the lower price?"

Another optimization employed by Liu improved the calibration precision by four times, Kehoe said.

Original publication

D0; "Precise measurement of the top quark mass in dilepton decays using optimized neutrino weighting"; 2015

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