Quarks Know Their Left From Their Right
How an electron interacts with other matter depends on which way it's spinning as it zips along—to the right like a football thrown by a right-handed quarterback or the left like a pigskin thrown by a lefty. Now, physicists have confirmed that quarks—the particles that join in trios to form the protons and neutrons in atomic nuclei—exhibit the same asymmetry.
The result could give physics a new weapon in the grand hunt for new particles and forces. Right now, scientists can try to blast massive new particles into existence, as they do at the world's biggest atom smasher, the Large Hadron Collider (LHC) in Switzerland. Or they can search for subtle hints of exotic new things beyond their tried-and-true standard model by studying familiar particles in great detail. In the latter approach, the new experiment gives physicists a way to probe for certain kinds of new forces, says Frank Maas, a nuclear physicist at Johannes Gutenberg University Mainz and the GSI Helmholtz Centre for Heavy Ion Research in Germany. "For a specific type of model, this type of experiment is much, much more sensitive than the experiments at the LHC," Maas says.
Matter interacts through four forces: the electromagnetic force that creates light and chemical bonds, the strong nuclear forces that binds quarks and nuclei, the weak nuclear force that produces a type of radioactive decay called beta decay, and gravity. (There could be others; some theorists have speculated that a second version of the weak force may also exist.) At one time, physicists assumed that all the forces obeyed a handful of symmetries. So, for example, a physical system should behave exactly like its mirror image, a symmetry known as parity.
However, in 1957, physicists discovered that parity does not hold in particle interactions mediated by the weak force. For example, suppose you aim right-spinning electrons at nuclei and watch them bounce off. If you look at the tiny shooting gallery in a mirror, you'll see left-spinning electrons bouncing off the target. So if the interaction between electron and nucleus were mirror-symmetric, then the scattering of right- and left-spinning electrons should be the same. And, indeed, that’s exactly what would happen if the negatively charged electrons interacted with the positively charged nuclei only through the electromagnetic force.
But the electrons also interact with the nuclei through the weak force, which violates parity and is not mirror symmetric. As a result, right-spinning and left-spinning electrons ricochet off the target differently, creating a slight asymmetry in their scattering pattern. That effect was seen at SLAC National Accelerator Laboratory in Menlo Park, California, in 1978 in an experiment called E122 that helped cement physicists' then-emerging standard model. A second weak force, if it exists, ought to give similarly lopsided results.
But what about the quarks? Like electrons, they can spin one way or the other as they zip around inside protons and neutrons. And, according to the standard model, the right- and left-spinning quarks should interact slightly differently with an incoming electron, producing an additional asymmetry, or parity violation, when the spin of the incoming electrons is flipped. Now, Xiaochao Zheng, a nuclear physicist at the University of Virginia in Charlottesville, and colleagues have observed that smaller contribution, as they report today in Nature.
That was no mean feat. To see the extra asymmetry, the incoming electron must strike the nucleus hard enough to blast out a single quark, setting off a shower of particles, as was done in E122 but not in subsequent experiments. Researchers must take great care to ensure that they alternately shine equally intense beams of right- and left-spinning electrons on the target. Using the electron accelerator at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, the researchers shined 170 billion electrons on a target of liquid deuterium over 2 months in 2009. After crunching the data, they were able to measure the part-in-10,000 scattering asymmetry precisely enough to pull out the contribution from the quarks, albeit with a large uncertainty. The result agrees with the standard model prediction.
"They've measured something fundamental at the quark level that wasn't measured before," says William Marciano, a theorist at Brookhaven National Laboratory in Upton, New York. Maas notes that the result is not as exciting as it could have been, however. "They have not observed any new physics at the level of their precision," he says. The new result does place tighter limits on models that assume a second weak force exists, Maas says.
The measurement is not the end of the road. The 101 members in the experimental team intend to repeat their measurement and hope to improve their precision by at least a factor of 5, Zheng says. That should enable them to test for new forces with far more sensitivity, she says. Marciano agrees that "this is just the first step." He notes that it might be beneficial that the asymmetry from the quarks is so small in the standard model, as that will make any deviation look relatively large.