Experiment measures strength of weak force on protons

New Scientist : The weak force is one of the four fundamental forces along with gravity, electromagnetism, and the strong force. It is significant only at subatomic levels, where it controls radioactive decay. And it is the only one of the forces that violates mirror or parity symmetry. Shelley Page of the University of Manitoba, Canada, and her colleagues in the Q-weak project took advantage of this quirk of the weak force to measure its strength on protons. They shot a beam of electrons at a target of protons and switched the spin of the electrons 1000 times each second. Electromagnetism is the dominant force governing electron–proton interactions, but because it ignores spin, the researchers were able to remove the measurable electromagnetic forces from the resulting data and leave only the influence of the weak force. The resulting measurement of the proton’s “weak charge” was consistent with the standard model ‘s predictions. However, the measurement is still preliminary; it uses only 4% of the data the team has collected. It is possible that the rest of the data could shift the measurement away from the predicted value.