You never get to see a naked quark (what, you thought you were going to see one just then? Patience, patience – the next two links have VERY revealing photos!). Scientists think there are six kinds of quarks (up, down, top, bottom, charm and strange are their names, and one of them is an adjective that certainly applies to their selected names) and they are bound together in various combinations to make different other subatomic particles like protons and neutrons. A bunch of quarks move around inside the particle they create like peas in a pod. But try to take one of the quarks away from the other(s) – make it “naked” – and an incredibly strong attractive force appears as if by magic to keep them all together. When quark-composite particles like protons ARE finally split with powerful particle accelerators, in the wink of an eye they reform into some different particle – another batch of quarks, not necessarily the same ones you started with, in a different kind of “pod”. That’s particle physics in a nutshell.
If you can’t see a naked quark, sometimes you can see it sort of half-naked, like it was doing a fan dance. (See, who said physics-by-analogy had to be boring!) Two experiments discussed this week have done just that. The first experiment, by Gerald Miller of University of Washington, leads physicists to the conclusion that protons aren’t always spherically shaped, like a basketball. Turns out only a proton at rest can be shaped like a spherical ball – the expected shape and the only one described in physics textbooks. A moving proton, however, can be shaped like a peanut, like an (American) football or even something similar to a bagel. The variety of shapes is nearly limitless and depends on the speed of the quarks inside the proton and what direction they are spinning, said Miller. This is an important new clue trying to tell us something about quarks and the force between them – but what?
The second experiment offers clues to what the individual weights of the six quarks are – hey, how can you ever know the body weight of someone so shy they never get naked? By smashing together either two deuterium atoms or a neutron and a proton, a long-sought but never-before-seen pion particle was produced exhibiting something called charge symmetry breaking – a fancy name for a quark fan dance. Two months of day-and-night work yielded just a few dozen data points. Yet: “This is a new and significant discovery, and an immense technical achievement,” comments physicist Gerald Miller (yeah, the guy that did the first experiment). Theoreticians will study the pion debris from these collisions, and the rate at which the reactions occurred. They hope that this will lead them backwards into the intact nucleus, to understand how much of the up/down difference is due to mass and how much to electrical charge. From this they could work out the quarks’ masses. Inconsequential? Nope. Quark mass differences decided the original composition of the Universe, explains theoretical physicist Bira van Kolck of the University of Arizona, Tucson. “When protons and neutrons combined to form the elements in the first few minutes after the Big Bang, the resulting abundances [of chemical elements] depended on the differences in their [quark] masses.”
So don’t let anybody ever fool you, physicists are human too – always peeking through cracks, trying to see a pretty thing naked.
