Twenty-five years of neutral currents (page 2)

Theoretical foundation

With far-sighted physicists such as Julian Schwinger convinced that deeper mechanisms were at work, a continual effort tried to reconcile the weak force with electromagnetism. After many major contributions by a host of theoreticians, this ultimately led to the 1967 prescription by Weinberg and Salam to unify the two forces. This provided a theoretical foundation for a neutral current, a weak force analogue of electromagnetism. But few people took any notice of the theory. Nobody had ever seen a weak neutral current, and anyway people believed the theory was not renormalizable. The Weinberg­Salam seed fell on infertile ground.

Injection system

This changed dramatically in 1971, when Gerard 't Hooft, Martin Veltman's student at Utrecht, showed that the new theory, with its neutral current, was indeed renormalizable. People sat up and took notice, and began calculating its consequences. Weinberg said it had "become urgent to settle the question of the existence of neutral currents", and urged experimentalists to start looking afresh.

Pioneer neutrino experiments at CERN in the 1960s using a 1.2 metre heavy-liquid chamber had looked for neutral currents and unfortunately claimed that the ratio of neutral to charged currents was less than 3% (a value which is several times smaller than the presently accepted ratio!).

The result of a bookkeeping error, this overstatement was eventually discovered and corrected, but the new limit of 12%, augmented by additional data, was not published until 1970. Such a false limit could have deterred imaginative theorists from proposing new ideas, although it clearly had not deterred Weinberg and Salam.

A "neutral current".

Motivated by the news that the theory was renormalizable and that the limits were not as severe as had been supposed, in 1971 experimenters on both sides of the Atlantic set off on a new hunt in possible neutral current territory. The rest is history.

The 1973 European particle physics conference took place in Aix-en-Provence. In his 1979 Nobel lecture, Salam related: "I still remember Paul Matthews and I getting off the train... and foolishly deciding to walk with our rather heavy luggage to the student hostel where we were billeted. A car drove from behind us, stopped, and the driver leaned out. This was [Paul] Musset [of the Gargamelle collaboration] whom I did not know well personally then. 'Are you Salam?... Get into the car. I have news for you. We have found neutral currents.' At the Aix-en-Provence meeting, that great and modest man Lagarrigue was also present [André Lagarrigue, who led the French effort to build the Gargamelle bubble chamber, died in 1975] and the atmosphere was that of a carnival."