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‘Weak charge’ measurement holds up SM prediction

Various dark matter detectors around the world, massive particle accelerators and colliders, powerful telescopes on the ground and in space all have their distinct agendas but ultimately what unites them is humankind’s quest to understand what the hell this universe is on about. There are unanswered questions in every branch of scientific endeavour that will keep us busy for millennia to come.

Among them, physics seems to be sufferingly uniquely, as it stumbles even as we speak through a ‘nightmare scenario’: the most sensitive measurements we have made of the physical reality around us, at the largest and smallest scales, don’t agree with what physicists have been able to work out on paper. Something’s gotta give – but scientists don’t know where or how they will find their answers.

The Qweak experiment at the Jefferson Lab, Virginia, is one of scores of experiments around the world trying to find a way out of the nightmare scenario. And Qweak is doing that by studying how the rate at which electrons scatter off a proton is affected by the electrons’ polarisation (a.k.a. spin polarisation: whether the spin of each electron is “left” or “right”).

Unlike instruments like the Large Hadron Collider, which are very big, operate at much higher energies, are expensive and are used to look for new particles hiding in spacetime, Qweak and others like it make ultra-precise measurements of known values, in effect studying the effects of particles both known and unknown on natural phenomena.

And if these experiments are able to find that these values deviate at some level from that predicted by the theory, physicists will have the break they’re looking for. For example, if Qweak is the one to break new ground, then physicists will have reason to suspect that the two nuclear forces of nature, simply called strong and weak, hold some secrets.

However, Qweak’s latest – and possibly its last – results don’t break new ground. In fact, they assert that the current theory of particle physics is correct, the same theory that physicists are trying to break free of.

Most of us are familiar with protons and electrons: they’re subatomic particles, carry positive and negative charges resp., and are the stuff of one chapter of high-school physics. What students of science find out quite later is that electrons are fundamental particles – they’re not made up of smaller particles – but protons are not. Protons are made up of quarks and gluons.

Interactions between electrons and quarks/gluons is mediated by two fundamental forces: the electromagnetic and the weak nuclear. The electromagnetic force is much stronger than the aptly named weak nuclear force. On the other hand, it is agnostic to the electron’s polarisation while the weak nuclear force is sensitive to it. In fact, the weak nuclear force is known to respond differently to left- and right-handed particles.

When electrons are bombarded at protons, the electrons are scattered off. Scientists at measure how often this happens and at what angle, together with the electrons’ polarisation – and try to find correlations between the two sets of data.

An illustration showing the expected outcomes when left- and right-handed electrons, visualised as mirror-images of each other, scatter off of a proton. Credit: doi:10.1038/s41586-018-0096-0
An illustration showing the expected outcomes when left- and right-handed electrons, visualised as mirror-images of each other, scatter off of a proton. Credit: doi:10.1038/s41586-018-0096-0

At Qweak, the electrons were accelerated to 1.16 GeV and bombarded at a tank of liquid hydrogen. A detector positioned near the tank picked up on electrons scattered at angles between 5.8º and 11.6º. By finely tuning different aspects of this setup, the scientists were able to up the measurement precision to 10 parts per billion.

For example, they were able to achieve a detection rate of 7 billion per second, a target luminosity of 1.7 x 1039 cm-2 s-1 and provide a polarised beam of electrons at 180 µA – all considered high for an experiment of this kind.

The scientists were looking for patterns in the detector data that would tell them something about the proton’s weak charge: the strength with which it interacts with electrons via the weak nuclear force. (Its notation is Qweak, hence the experiment’s name.)

At Qweak, they’re doing this by studying how the electrons are scattered versus their polarisation. The Standard Model (SM) of particle physics, the theory that physicists work with to understand the behaviour of elementary particles, predicts that the number of left- and right-handed electrons scattered should differ by one for every 10 million interactions. If this number is found to be bigger or smaller than usual when measured in the wild, then the Standard Model will be in trouble – much to physicists’ delight.

SM’s corresponding value for the proton’s weak charge is 0.0708. At Qweak, the value was measured to be 0.0719 ± 0.0045, i.e. between 0.0674 and 0.0764, completely agreeing with the SM prediction. Something’s gotta give – but it’s not going to be the proton’s weak charge for now.

Paper: Precision measurement of the weak charge of the proton

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