Making 3D images of the proton
The proton is a surprisingly complicated object. Far from the two up quarks and one down quark you might have heard make up the proton, it is actually a seething sea of quark pairs and gluons that surround the "bare" up and down quarks. In fact, 99 percent of the mass of the proton comes from the "sea". Only one percent comes from the bare quarks.
Kent Paschke of the University of Virginia, speaking at the American Physical Society meeting in Denver, Colorado called it an "exciting QCD vacuum bubbling with quark-antiquark pairs." Quantum Chromodynamics, or QCD, is the theory of the strong force, which governs the interactions of quarks and gluons.
So what does the proton really look like and how can physicists actually observe what happens on the inside? The answer relies on using high-energy particle accelerators, some tricky investigative work, and a lot of data.
The basic technique for looking inside a proton is electron scattering. An accelerator boosts electrons to high energies, such as the 6 GeV energies achieved by the CEBAF experiment at Thomas Jefferson National Accelerator Facility, soon to be upgraded to 11 GeV. Those electrons are slammed into a target atom, and the remnant particles are observed, measured, and analyzed.
In practice, the data that comes out is a series of number that contribute to the "nuclear form factors", which physicists tend to look at as lines on a graph. But Paschke said, "It's the closest thing we can do to take a picture of a subatomic object."
The guts of a proton
The picture that can be pieced together from this data is quite rich. It reveals that the quarks and gluons aren't evenly distributed through the proton, have some orbital motion within the proton, and even reveal the contributions of more than just up and down quarks-strange quarks play an important role in the structure of the proton.
One of the fairly recent innovations in experimental nuclear physics is the use of polarized electron beams and polarization detectors. In this setup, the electrons have their spins preferentially aligned in a certain direction. Because the spin is a magnetic property of an electron, it can be used to tease apart the differences between the electric and magnetic properties of the proton.
Somewhat surprisingly, the electric and magnetic properties of the proton are significantly different. Paschke commented that this has led to a "rethinking of the nucleon wavefunction", the complete 3D quantum description of the proton.
A set of measurements from particle physics and nuclear physics labs around the world have shown that there seems to be orbital motion of the proton's parts. This orbital angular momentum is also critical in describing contributions of other quarks to the proton. Pairs of strange and antistrange quarks popping up temporarily in the "sea" might not survive long but they can leave a mark.
Strange effects in the proton sea
Researchers can fire electrons at a proton and see them collide with the up and down quarks, known as the valence quarks, sitting inside. But how can experimenters observe the sea? There is a lot of it but it is so ephemeral that it is difficult track down.
One property of the sea saves the day, however. The sea is dominated by up, down, and strange quarks because they are the lightest quarks, although other particles can pop out of the quantum vacuum temporarily. So measuring strangeness is a way to really get at measuring the sea's behavior, independent of the up and down valence quarks.
This pushes the investigation one step back to figuring out how to observe strangeness inside a proton. The trick here is not to use the strong force, but to take advantage of a feature of the weak force-parity violation-in which particles can display a handedness, and the right hand behaves subtly differently to the left.
Electrons also see the weak force, so when an electron is fired at a proton the electron can interact on occasion with strange quarks in the sea through processes involving a Z boson. This interaction happens in something like one in each million interactions. It's a small effect but enough to be measured.
Trying to map the strangeness in the proton has become a major goal of nuclear physics research because, Paschke said, "evidence of certain types of strange contributions to the proton would be the first unambiguous failure of the naïve quark model."
At the moment, measurements are consistent with there being no critical contribution of strangeness to the quark, but the precision of the experiments is increasing fast, based on experiments at places including Jefferson Lab, the MIT Bates Linear Accelerator Laboratory, and at the University of Mainz, Germany. JLab's G0 experiment expects to have new data on this topic in the next few weeks.
Putting protons (and neutrons) together
The same kinds of physics that is key to understanding the structure of the proton comes into play when examining how protons and neutrons combine to form atomic nuclei. The long-time textbook model of nuclear structure involves nucleons (either protons or neutrons) filling concentric shells, just like electrons form shells around a nucleus in an atom, with full shells corresponding to the most stable nuclei. The basic shell model, however, assumes that the nucleons are essentially independent.
When scientists examine the distribution of protons inside a nucleus, they discover that they only make up about 60-70 percent of the amount predicted by the basic shell models, and not in the right places. Since this was discovered, physicists have guessed that the difference between the models comes from interactions between the protons and neutrons. Those interactions are fiendishly difficult to measure.
A clever technique to observe those interactions involves firing electrons at a nucleus and looking for protons that are kicked out with too much energy to have come from the incoming electron. If a proton were to interact with another nucleon as the electron slams into them, the exiting proton could steal some of the other nucleon's energy to create these "fast" protons.
When the experiment is formed, physicists observe protons that come out at a stepped set of energies corresponding to the electron interacting with a single proton, but also with either two or three nucleons.
This is very exciting to nuclear physicists because, as Paschke said, "these long-sought clear experimental signatures open the door to detailed testing of nuclear models."
The observation of these short-range interactions inside the nucleus go a long way to closing the gap between experiments and the naïve shell model, but not all the way. Paschke said he expects that the remaining difference comes from some longer-range interactions among the nucleons and more effects from multiple nucleons interacting with each other.
Nuclear physicists are making great progress in cracking open the proton and mapping its insides. As they gain a lever point in the proton, they are planning to use protons as systems to study as much as they can about the strong force, in a way complementary to particle physics experiments. For example, they hope to look for glueballs-collections of gluons that don't have any quark contributions, and which might reside temporarily inside protons or nuclei. They see this as the purest type of system to study the strong force.
Far from the up+up+down quark model that students have seen for decades, the proton is revealing itself to not only be a complex creature with many more mysteries to reveal, but also a testing ground for understanding one of the fundamental forces of nature.