Up and Down, but Not Strange

What's matter really made of? Physicists know surprisingly little about what goes on in even one of the most basic parts of an atom--the proton. Now, in a clever experiment, researchers have taken the first clean look at the chaotic sea of quarks, the basic particles that make up matter, inside the proton. The findings could give physicists a much better idea of what goes into a proton.

Particle physicists know protons are made of two up quarks and one down quark. But though the charges of the three quarks add up to the charge of the proton, their combined magnetic moments (the strengths and directions of their magnetic fields) don't add up to nearly enough, and their mass adds up to barely 1% of the proton's. There's obviously something else going on.

That something else is a "quark sea," a bubbling ocean of quarks popping in and out of existence within the proton. Pairs of quarks--a strange and an antistrange, for example--materialize and then annihilate each other. But for the brief moment they exist, they could add to the proton's mass, magnetic moment, and the charge inside the proton.

At the April 2006 meeting of the American Physical Society, Paul Souder, a particle physicist at Syracuse University in New York, and colleagues described how they tested the influence of strange quarks on the proton. Using the particle accelerator at Jefferson Laboratory in Newport News, Virginia, they shined a 3-giga electron volt beam of electrons at two targets; one hydrogen, the other helium. By comparing how the electrons scattered off the hydrogen atom and the helium, they untangled the effects of the proton's magnetic moment and its charge. They then reversed the polarization of the electrons in the beam and repeated the experiment. The tiny asymmetries in the way the electrons scattered in the original beam versus the reversed polarization beam told the tale: Pairs of strange and antistrange quarks popping out of the sea cancel each other so effectively that they have almost zero impact on the proton's magnetic moment, charge, or mass.

"It's a very precise, elegant measurement," says Larry Cardman, assistant director of physics at Jefferson Lab. If strange quarks don't contribute much to the proton, he adds, then heavier quarks contribute even less. That means that the missing magnetic moment of the proton must be due to pairs of up and down quarks popping around in the quark sea. And the missing mass, Cardman says, probably comes from the huge amount of energy that glues the proton together.