Weak is the new strong

In the family of fundamental forces weak interactions are the child we spend time apologizing for because their gifts are not immediately obvious. Electromagnetism gets to bind atoms and be responsible for all of condensed-matter physics. The strong force keeps nuclei together, and governs the interactions that make stars shine. And gravity binds us to the Earth and keeps the celestial spheres moving in their orderly (or not-so-orderly) paths.

But the weak force we usually pass over with a few words about nuclear beta decay, neutrinos, and so forth. We know they are a little peculiar (ooh look, how cute, parity violation) but when pressed, we may have trouble explaining what their purpose in life is. Yesterday's Colloquium provided a nice corrective to this view by explaining how weak interactions are providing a unique window on aspects of proton structure and Standard-Model physics.

These results come out of the parity-violation program at Jefferson Lab which was initiated by a paper written 20 years ago that proposed looking at electrons that scatter from the proton via the weak force (specficially via the exchange of a Z0-boson). The trick was that these electrons would couple to the quarks inside the proton differently than would electrons that scattered from the proton via the exchange of photons. Photon exchange is the usual (i.e. dominant) process by which protons and electrons interact---it mediates the electromagnetic force. But it turns out that one can combine information from photon-exchange and Z0-exchange experiments to infer the distribution of strange quarks inside the proton---something that would be impossible with only electromagnetic-scattering data.

The trick is distinguishing the electrons that scatter via the weak force (Z0-exchange let's say) from those that scatter in the "regular" way (photon exchange). Here the fact that the weak interaction violates parity comes to our aid, since we can eliminate the dominant contribution from the "regular" electrons by constructing the difference of cross sections for electrons with opposite spins. The resulting "asymmetry" has recently been measured at a variety of kinematic points by the G0 collaboration. As a consequence we have important new information on the distribution of strange quarks inside the proton. So we now know for sure (well, at the 68% confidence level) that a proton is not two up quarks and a down quark, like you have been taught. Instead it has a bunch of other stuff in it too, of which the non-zero results measured by G0 are just one manifestation.

But the parity-violating electron scattering people did not stop there. They realized that their new probe gave them access to the "weak charge" of the proton. In just the same way as the charge of the proton determines the scattering of low-energy electrons from it, so too the "weak charge" of the proton determines the low-momentum-transfer result for the parity-violating asymmetry measured by G0 and other groups. (The information on proton structure that these experiments were actually after was obtained at values of the momentum transfer that are not "low"---where details of the proton's weak charge distribution become important.) A new experiment at JLab aims to do the world's best measurement of the proton's weak charge. And since this number---Qweak---is predicted by the Standard Model, they can test if that prediction is correct and perhaps see contributions of beyond-the-standard-model stuff to the proton's weak charge.

Personally I find it amazing that differences in cross sections at the level of 1 ppm are being measured with a few-per-cent-level accuracy. But perhaps more interesting is that the weak force is coming into its own, not just as a peculiarity but as a tool. Since the mass-scale of the weak force is set by the gauge bosons and so is of order 100 GeV, these experiment use 100-GeV-scale physics to learn about proton structure. And ultimately the (we hope) closeness of that 100 GeV scale to the mass of particles that are beyond the standard model is what gives experiments like Qweak the chance to upset the four-force picture of interactions we all grew up hearing about. That would truly be the revenge of the under-appreciated child.