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Scientists Measure the Weak Charge of the Proton

An international collaboration of scientists has for the first time directly measured the strength of the weak nuclear force acting between a single electron and a single proton. Published in Nature (https://goo.gl/qZ1hdP), the measurement supports the Standard Model of particle physics, and places constraints on the possibilities for new types of forces beyond our present knowledge.

“This is the first time a dedicated experiment has directly measured the strength of the weak nuclear force acting between a single electron and a single proton,” said A/Prof Ross Young of the University of Adelaide’s School of Physical Sciences, who collaborated on the experiment. “The strength of this force is governed by the ‘weak charge’ of the proton, in much the same way as the strength of the electromagnetic force is governed by the proton’s electric charge. Measuring this effect has proven difficult because the weak force is so much weaker than the electro­magnetic.”

To measure the proton’s weak charge, an intense beam of highly polarised electrons was directed onto a target containing cold liquid hydrogen, and the electrons scattered from this target were then detected.

The key to the experiment was that the electrons in the beam were mostly spinning in one direction, parallel or anti-parallel to the beam’s direction. When the direction of polarisation was rapidly reversed, the experimenters were able to detect a scattering rate that differed between the two polarisation states.

“The difference between the two helicity configurations amounts to less than 300 for every billion electrons scattered,” Young explained. “By measuring this tiny difference very precisely, we’ve been able to determine the weak charge of the proton.”

The proton’s weak charge was in close agreement with predictions of the Standard Model, which takes into account all known subatomic particles and the forces that act on them. Because the proton’s weak charge is so precisely predicted in this model, the new result provides insights into predictions of hitherto unobserved heavy particles, such as those that may be produced by the Large Hadron Collider.

For example, the result has set limits on the possible existence of leptoquarks. These hypothetical particles can reverse the identities of two very different fundamental particles, turning quarks (the building blocks of nuclear matter) into leptons (electrons and their heavier counterparts) and vice versa.

“From our understanding of the Standard Model of particle physics, the value of the weak charge of the proton is predicted quite precisely theoretically,” Young said. “The new measurements therefore act to test this theory.

“If the measurement had deviated from the prediction, it would be a strong signature for a new type of as-yet unknown force that is acting between fundamental particles. Given that we found excellent agreement with the theoretical expectations, this places new bounds on the types of new forces that may exist in nature.”