Nucleons – protons and neutrons, the composite particles that make up the atomic nucleus – are critically important building blocks of matter, yet much of their detailed nature remains mysterious. We know that each nucleon is made up of three “valence” quarks (up-up-down for a proton and up-down-down for a neutron); a “sea” of quark-antiquark pairs, continually popping in and out of existence; and innumerable gluons, the particles that carry the strong nuclear force and hold the whole thing together. But we don't fully understand that internal structure. Just in the last few years, for example, a novel way of measuring the proton size has revealed that we don't know that value nearly as well as we had thought. And the “proton spin crisis” remains unresolved almost thirty years after it initially arose.
Spin is a quantum-mechanical quantity that doesn't lend itself to classical metaphors. It's an intrinsic angular momentum associated with particles. Every photon has spin 1; every proton, and every neutron, has spin 1/2. But the quarks and gluons inside a nucleon have spin, as well, and their spins and orbital angular momenta must add up to the total nucleon spin of 1/2. That doesn't actually pose an obvious mathematical problem – not in quantum mechanics – but we would like to know how much each part contributes to the whole. At first, physicists thought the valence quarks must be the biggest contributors, but in 1989 the European Muon Collaboration surprised the world by showing experimentally that this was not the case: the valence quark spins are largely canceled out by the sea quark spins. This is the proton spin crisis: if the proton spin doesn't come from the quark spins, then how does it arise?
Over the last thirty years, physicists have developed better and better theoretical and experimental methods for studying this problem, but it's still not entirely solved -- and we know more about the proton spin structure than we do about the neutron spin structure. We know how to use electromagnetic fields to accelerate proton beams for scattering experiments, but neutrons have no electric charge so those techniques don't work for them. Neutrons are also unstable outside nuclei, with a half life about 15 minutes, so it's more challenging to make a spin-polarized neutron target. It's not impossible, though! One nice trick is to polarize a relatively simple nuclear target like 3He, in which most of the polarization is carried by the neutron. Then we do the measurement, and finally we use theoretical models to extract the effect due solely to the polarized neutron within.
My thesis experiment, E06-014 in Hall A of Jefferson Lab , was part of an extensive program to shed light on neutron spin structure. Like many of our sibling experiments, we used a longitudinally polarized electron beam and a 3He target that could be longitudinally or transversely polarized. We took data in the deep inelastic regime, where the incident electron can be said to interact with individual quarks inside the nucleon. By measuring the probability of this process irrespective of spins, and the asymmetry of the scattering probability between different spin configurations, we extracted a quantity called d2n that is sensitive to higher-twist contributions from the standard model and is therefore a good test of several theories. In contrast to a previous experiment, we found good agreement between the experimental result and the prediction from a theoretical method called Lattice QCD. (The paper is also available on arXiv.)
By combining our asymmetry measurements with world data on the proton, we could examine the way in which up and down quarks of different spins are distributed within the nucleon. We found that down quarks prefer spins opposite to the nucleon spin, even when they’re carrying a relatively large fraction of the nucleon momentum. Our results, added to those of some previous experiments, put significant tension on some theoretical calculations.
Jefferson Lab has recently upgraded its accelerator, approximately doubling the maximum possible electron energy. This will allow the next generation of experiments to probe the distribution functions of quarks with much higher momenta relative to the neutron, for a more stringent test of the available theoretical calculations. In the next several years, depending on scheduling, hardware development and analysis work, we can look forward to new d2n results from E12-06-121 and precise new double-spin asymmetry results from E12-06-110 and E12-06-122. When we consider recent, exciting results from the STAR and PHENIX detectors at the Relativistic Heavy Ion Collider, finding a significant contribution from gluon spin, these are thrilling times for the proton spin crisis!