Quark Quirk Triggers Nuclear Shrinkage
Charles Seife 

 If atoms had egos, a few lithium nuclei would be nursing bruises right now. By sticking an exotic type of quark where it doesn't belong, physicists have cut the nuclei down to four-fifths normal size. In the process, the scientists are edging toward a theory that can explain nuclear interactions of all varieties.

 "Shrinkage of about 20% is very surprising," says Hirokazu Tamura, a physicist at Tohoku University in Sendai, Japan. "Nuclear physicists know that compressing the nucleus is very, very difficult."

[Figure 1] 
Squeeze play. Gamma rays entering the 14 spokelike detectors of Tohoku University's Hyperball instrument showed evidence of pint-sized lithium nuclei.

So instead of trying to squeeze an atomic nucleus, Tamura and colleagues from Japan, China, Korea, and the United States set out to shrink it from within. In the 5 March Physical Review Letters, the physicists describe how they injected a little dose of strangeness into a lithium-7 nucleus. Through a handful of particle interactions, they substituted a strange quark for a down quark, turning one of the atom's neutrons into a particle called lambda, or L. "It's quite similar to the neutron, but somewhat heavier," says John Millener, a physicist at Brookhaven National Laboratory in Upton, New York. "A proton is two ups and a down, a neutron is two downs and an up, and a L is an up, a down, and a strange." The quark substitution turned lithium-7 into lithium-6-L, a so-called "hypernucleus" with subtly different properties from a garden-variety lithium nucleus.

 The difference stems from the Pauli exclusion principle, the quantum-mechanical rule that forbids certain particles from having the same quantum state. Given the chance, a neutron in a nucleus will occupy the lowest possible energy level, or ground state. Two neutrons can inhabit that level, but only if they have different quantum states. For that to be true, one neutron must have spin +1/2, and the other must have spin -1/2. A third neutron, however, must take a higher energy position farther away from the center of the atom. The same exclusion rules apply, independently, to protons.

 Lithium-6 has three protons and three neutrons; one proton and one neutron are in the higher energy state, loosely bound to the core. Enter the L. Because a L particle is distinct from both protons and neutrons, it is exempt from the Pauli exclusion principle that governs those particles. As a result, it sinks directly into its ground state, joining the low-energy protons and neutrons at the center of the nucleus. "You put the L in the system, and it makes everything more stable by interacting with the [protons and neutrons]," Tamura says. The extra L binds the particles more tightly together but, unlike an added proton or neutron, takes up no additional space. The stabilized nucleus shrinks.

 Tamura's team observed the shrinkage by precisely measuring gamma rays that emanate from lithium-6-L hypernuclei. The gamma rays reflect the shifting of particles' spins within hypernuclei--information that can help scientists determine not only a hypernucleus's size, but also how its components interact with one another. "Nobody's been able to measure this with such high precision," says Millener, who hopes that understanding those interactions will shed light on so-far-obscure aspects of nuclear physics. "We don't really have a theory for these interactions." 


Volume 291, Number 5510, Issue of 9 Mar 2001, pp. 1877-1878.
Copyright © 2001 by The American Association for the Advancement of Science.