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BY DEFTLY replacing one type of quark with another, scientists in Japan have
shown that they can shrink the nucleus of an atom. The finding, which confirms a
prediction made almost two decades ago, could shed some light on the exotic
conditions inside superdense neutron stars.

鈥淚t is exciting,鈥� says team leader Kiyoshi Tanida of Tokyo University. 鈥淣o
one knew whether such nuclear shrinkage really occurs until we did the
别虫辫别谤颈尘别苍迟.鈥�

Tanida鈥檚 team achieved this feat by altering one of the neutrons in an atom
of lithium. Neutrons are made of one 鈥渦p鈥� quark and two 鈥渄own鈥� quarks. But in
1983, scientists in Japan predicted that if you swapped one neutron in a nucleus
with a 鈥渓ambda鈥濃�攁 particle made of one up, one down and one strange
quark鈥攖he nucleus would shrink.

Neutrons are forced to keep their distance from each other because quantum
rules forbid them from occupying identical energy states. But because the lambda
particle is not identical to a neutron, it can slip into the centre of the
nucleus, taking up no extra space at all. It should also attract the protons and
neutrons, pulling them closer together. Theorists calculated that a lambda
particle would shrink a lithium nucleus by 19 per cent.

To test this, Tanida鈥檚 team fired particles called pions鈥攃omposed of a
quark bound to an antiquark鈥攁t a lithium target. Occasionally, this
sparked a chain reaction in which a neutron in a lithium nucleus changed into a
lambda while the pion mutated into a similar particle called a kaon.

Sensitive particle detectors revealed that each lithium-lambda nucleus
survived for around 5.8 脳 10^-12 seconds before decaying in a flash of
gamma rays. From this lifetime, which depends on the size of the nucleus, the
team calculated that the lambda had indeed shrunk the nucleus by about a
fifth.

鈥淭his was a very successful experiment,鈥� says John Millener of Brookhaven
National Laboratory in New York state.

Tanida hopes that experiments like this will clarify the behaviour of
material deep within superdense stars such as neutron stars, which form when the
cores of very massive stars collapse following a supernova explosion. Some
neutrons in these stars could mutate into 鈥渉yperons鈥濃�攑articles containing
at least one strange quark. 鈥淎 neutron star is an example of where lambdas and
other hyperons play an important role,鈥� says Tanida.