It weighs less than 1,000,000,000,000,000,000,000,000th* of a grain of sand. Now think of something smaller

DANNY PENMAN

and CHARLES ARTHUR

Scientists in the US think they have found evidence that the basic constituent of matter - long thought to be particles called "quarks" - is itself made of something smaller.

Experiments carried out in Chicago suggest that quarks - whose mass is a million, billion billion, times less than that of a grain of sand - are not the tiniest things the Universe has to offer. At present, most scientists think that quarks are the building blocks of the protons and neutrons in atomic nuclei, which make up all matter. And, says the prevailing theory, quarks are indivisible.

The experiments consisted of crashing protons - the nuclei of hydrogen atoms - into their mirror images, known as anti- protons, at almost the speed of light. Present theory suggests that the two should always eliminate each other completely. But when the outcomes were studied, a few had produced odd results which could lead to a revolution in thinking.

The present theory of matter, known as the Standard Model, has been built up since 1930 as physicists have been able to conduct advanced experiments like this, and explains most of the present knowledge on how matter is constructed. It starts by suggesting that atoms are built from neutrons, which have no charge, and protons, which together form the nucleus. Outside the nucleus are negatively charged electrons.

Quarks, according to the theory, occupy no space. But when the scientists at Fermilab smashed the protons and anti-protons together, the results suggested that quarks may occupy some space after all. If that is the case, they say, then quarks must have some kind of internal structure, which may be constructed from even more fundamental building blocks of matter. The identity of this component - if it exists - remains unclear.

Scientists at Cern, the European particle physics laboratory in Switzerland, reacted cautiously to the results. But Brenna Flaugher, one of the physicists who performed the experiment said: "This is where the fun begins."

Fate of the Universe, page 6