The TRIUMF Weak Interaction Symmetry Test

A collaboration of RRC "Kurchatov Institute" (Russia), Texas A&M, TRIUMF, University of Alberta, University of British Columbia, University of Montreal, University of Regina, Valparaiso University

To contact our spokesperson, email Glen Marshall (glen dot marshall at triumf dot ca).

Physics Background Description of TWIST
(with pictures!)
Publications and Presentations TWIST Participants

Searching for Nature's Right Hand

Elementary particles such as electrons are labeled as either "right-handed" or "left-handed". A surprising observation is that only left-handed particles seem to feel the weak nuclear force, whereas left- and right-handed particles feel the other fundamental forces equally. This difference, called parity violation, was predicted and observed in the 1950's, and led to a Nobel Prize. It is now a cornerstone of the modern Standard Model of subatomic physics. However, there are strong reasons to believe that this Standard Model is incomplete. "Left-right symmetric" theories explain parity violation by assuming that right-handed particles also respond to the weak force, but we would need to be in a much hotter world - with a temperature well above 10^15 degrees - for this to be obvious.

The TWIST Collaboration is testing these theories with high-precision measurements of the decay of the muon, a particle that appears identical to an electron in all regards except that it is heavier. The Collaboration has just reported the first improvement since 1966 in the measurement of the energy spectrum of positrons produced in muon decay. This new result agrees with theoretical predictions in which right-handed particles do not feel weak forces. Thus, it is less likely that left-right symmetric theories are the correct explanation for the parity violation of the weak force.

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