Weak Interaction
February 22, 2011

Jefferson Lab has an accelerator designed to do incisive medium energy physics. This program is dominated by experiments aimed at developing our understanding of the strong interaction. This is the force which generates much of the complexity in the world around us. On the finest distance scales, the strong interaction acts between quarks and quarks, quarks and gluons, gluons and gluons. The mass of the proton (which is built of quarks and gluons) comes essentially from this interaction, the intrinsic mass of the quarks is small. At a slightly larger scale, the strong interactions are transformed to describe the way protons and neutrons are bound into nuclei. Jefferson Lab studies all of these things; this is our raison d’etre.

But the world we know has other interactions.

When we describe charged particles or magnets, we invoke electromagnetism. This interaction is also the mechanism we use to probe the strongly interacting systems like the proton or the neutron using electron scattering; our CEBAF accelerator provides the electrons.

Are these all the interactions? No! There is gravity, the force that generates our own weight, that which keeps our feet on the ground. For our purposes at Jefferson Lab, it is a force which is indispensible for everyday life but which doesn’t play a physics role in the experiments we do. And lastly, in our current understanding there is one more force, the weak interaction, which controls beta decay of nuclei.

We describe weak interactions by saying there is an exchange of a vector boson, a W boson or a Z boson. The Z and W bosons are about a 100 times the mass of the proton or neutron. In the most advanced theories, the weak interaction and the electromagnetic interactions are different aspects of the electroweak force. The fact that the weak interaction is much weaker comes from the mass of the bosons.

So can we forget about it?

Well, let’s ask it differently. If we don’t forget about it, how could it affect our electron scattering experiments? If we had a really intense beam with a very small spot size and with the spins of the electrons in the beam aligned, and we could flip the orientation of the spin, then maybe we could see the effect of this weak interaction. After all, the weak interaction depends on whether the electrons are right handed or left-handed, spins forwards or backwards. But the effects must be very small. Indeed, taking into account the squares of the masses and the specifics of the scattering, we expect an effect of a hundred parts in a billion. But if our teams do the math, they find that we do have a beam that has all those attributes and that can be controlled exquisitely by our accelerator physicists and operators. To maximize the data taking we also need a toroidal magnetic spectrometer with nearly full azimuthal coverage, which is the responsibility of the physics collaboration.

OK, so let’s measure the weak charge of the proton! Let me see, we will need a hydrogen target that can handle the heating from 180 microamperes of beam, a lot of particles per second. And it needs to be 35 cm long. That will require a couple of kilowatts of cooling at a few Kelvin, it would be a world record for a cryogenic target. Cool! Don’t forget that we will need to handle a whole slew of new ASME code procedures for pressure vessels as the engineers and target group build the device. Yes, and to avoid the effects of just a little boiling as a result of the heating from the powerful beam, we will need to flip the spin of the beam 960 times per second without moving the beam. No sweat, that’s only 16 times faster than we did last year.

Put altogether, that’s a pretty unlikely story. The most amazing thing is that, in essence it’s true and has been achieved. Q-weak, an experiment in Hall C of Jefferson Lab, is operational. The Q-weak experimenters think they have things, at least on good days, pretty much under control. The experimental group showed some plots to the lab, a couple of weeks ago and, if they are able to keep things working through the end of their allocated data taking up to May 2012, they should approach the level of precision discussed in their proposal.

A not so Weak Interaction!