A Quantum Diaries Survivor

I just finished reading a very interesting piece by Donald Perkins (professor of Physics at Oxford University, and author of one of the most appreciated books on particle physics ever written) on he discovery of weak neutral currents. He discusses in detail the events that brought CERN to announce the discovery in 1973, and the ensuing debate following a negative result by the HPWF experiment, operating at Fermilab.

The history of the discovery is complex, so my quick and dirty summary below is going to leave much wanting. I do it anyways because I wish to introduce some considerations on the inferiority complex which plagued CERN in those years.

A paper by Weinberg and Salam in 1967 hypothesized a unification of weak and electromagnetic interaction by postulating the existence of both charged and neutral weak currents. The latter, however, had never been observed, while their effect was predicted to be comparable in size to that of charged currents. Because of that the paper by Weinberg and Salam was basically ignored for four years. In 1971, however, Gerald ‘t Hooft proved that the unified electroweak theory was renormalizable, and things started to change. Physicists started believing in the existence of neutral currents, and set out to actively seek them.

To search for neutral current interactions one could look for neutrino collisions with atomic nuclei. In a charged current interaction, the neutrino would change into a charged lepton -typically a muon, given the composition of neutrino beams saw the predominance of muon neutrinos. In a neutral current, instead, one would not observe any lepton downstream, but just the remnants of the nucleus and other light hadrons. These events were studied with the Gargamelle bubble chamber at CERN, which used a neutrino beam obtained from a 26-GeV proton beam. The typical signal, the appearance of a star of hadronic tracks, could be mocked by neutrons produced upstream, and the difficulty in calculating the rate of those events made the discovery of true neutral current events hard.

Another way one could observe neutral current interactions of neutrinos was through the collision of the neutrino with an electron: from the reaction one would only observe a energetic electron coming out of the blue, with a very small angle from the beam direction. Those events were however very rare, and in 1973 only three were found in more than a million pictures of the bubble chamber at CERN.

At the Bonn conference in August 1973 Gargamelle reported a ratio between neutral and charged current interactions from the neutrino and antineutrino beams of 0.21+-0.03 and 0.45+-0.09, respectively. The different behavior of antineutrinos was expected in the unified electroweak theory, but the results were initially greeted with skepticism, and for a while the CERN experimentalists were under a considerable amount of heat.

The reason was that the Fermilab experiment, which had initially reported (by Rubbia, at the same conference) a value of R=0.29+-0.09 for the mixed effect of neutrino and antineutrino interactions (which were not separable in the wide-band beam of Fermilab), had later claimed (although not published) a result consistent with zero contribution from neutral currents.

Let me now quote Perkins, because I find his account of the situation enlightening:

Today, CERN prides itself on being the world’s leading high-energy physics laboratory. Whether or not this is so, it is clear that 20 years ago [the article by Perkins was written in the nineties -TD], things were quite different [...] CERN unfortunately did not have a similar reputation in its physics, and was still recovering from disasters [...]. And during the 1960s it had been repeatedly beaten to the ground, for examples, over the discovery of the hyperon, the two types of neutrinos, and CP violation in decay. All these things could and should have been found first at CERN, with its far greater technical resources, but the Americans had vastly more experience and know-how. Even today, the scoreline in Nobel laureates in high-energy physics (counting from the end of WWII) tells the story: United States 26, Europe 6.

It is important to understand this legacy in inferiority in considering the attitudes at the time of people in CERN over the Gargamelle experiment. When the unpublished (but widely publicized) negative results from the HPWF experiment started to appear in late 1973, the Gargamelle group cane under intense pressure and criticism from the great majority of CERN physicists. [...] many people believed that, once again, the American experiments must be right. One senior CERN physicist bet heavily against Gargamelle, staking (and eventually losing) most of the contents of his wine cellar! [...] It is indeed a dramatic testimony to the rapidly changing fortunes in the world of high-energy physics that wat was undoubtedly the principal discovery during the first 25 years of the CERN laboratory was to be greeted initially with total disbelief by the vast majority of CERN physicists.

I wonder how the matter is perceived nowadays, fifteen years after the above words were written. In the meantime the top quark has been discovered, B_s mixing has been measured, new baryons have been found: all of that at Fermilab. By contrast, the LEP II experiments have basically been a fiasco, adding little to our knowledge of subnuclear physics except maybe a precise W mass measurement which is going to be surpassed by CDF alone very soon.