Feb 20, 2013
From the March 1970 issue
Important experiment on the cheap
On 5 December 1969 the University of Chicago-Berkeley neutrino neutral current experiment concluded its run at the 6 GeV Bevatron. The experimenters looked for the reaction K+→ π+ + ν + ν which, had it been observed, would have been evidence of the existence of such currents. During the course of the experiment a total of 1.5 × 109 positive kaons decayed within the detecting apparatus but no example of the neutral current reaction was observed.
Preliminary analysis of the data indicates that the branching ratio for K+ decay in this manner is less than 1.2 × 10–6. The experiment is important because the absence of this decay mode (and of the related mode K+ → π+ + e+ + e–) is an unsolved puzzle in weak interaction theory. One possible interpretation of this observation is that electrons and neutrinos carry a quantum number which forbids the creation of isolated electron or neutrino pairs by weak interactions.
An unusual feature of the experiment is that the apparatus was put together almost entirely from "odds and ends". One crucial part consisted of two 15-year-old oscilloscopes, used to display the π+ → μ+ → e+ decay sequence. No computers, either online or off-line, were used in the reduction of the data.
• Compiled from texts on p87.
Computer takes over
On 29 January, a rather unusual test was made during one of the machine development periods on the CERN proton synchrotron.
All of the dipolar corrections guiding the beam in the horizontal plane were switched off and it was impossible to accelerate the beam to full energy. The beam would make only thirty revolutions in the machine (lasting about 200 μs). The IBM 1800 machine control computer was then asked to bring the beam back on. After successive optimization procedures, the computer succeeded in accelerating a beam with an intensity of 96 × 1010 protons per pulse. The intensity before beginning the tests had been about 80 × 1010 protons per pulse.
The test was part of the research programme of the controls group concerning the use of online computers in accelerator control. Though the experiment does not foreshadow computer-controlled operation in the near future, it does show how the use of computers can help in accelerator control. This is believed to be the first time that the intensity of an accelerator in use for physics has been optimized by computer.
• Compiled from texts on p84.
Feeding the PS with nitrogen
Following successful tests carried out on sector 4 of the PS vacuum chamber, it has been decided to use dry nitrogen, before atmospheric air, to fill the vacuum chambers when the vacuum has to be broken for maintenance or modification of equipment. This trick, which is already in use in other laboratories and by certain CERN groups, has the advantage of shortening pumping time by a factor of more than three when the high vacuum has to be re-established.
The reduction in pumping time is due to absorption of dry nitrogen by the chamber walls, which inhibits the subsequent absorption of water vapour when atmospheric air is allowed in. This is of special importance since the PS oil diffusion pumps are at present being replaced by ion pumps, which are particularly sensitive to moisture.
• Compiled from texts on p86.
The K+ is an up quark plus antistrange quark; the π+ is an up quark plus an antidown quark. Thus the decay K+→ π+ + ν + ν– violates flavour conservation and is forbidden in any first-order Standard Model process. However, it can proceed via a flavour-changing, second-order weak interaction, involving two force carrier bosons and other quarks. This ultrarare decay, sensitive to effects predicted in extensions to the Standard Model, is one of the most challenging interactions in particle physics.
In 1995, a quarter of a century after the null result reported here, experiment E787 at the Brookhaven National Laboratory made the first sighting of the decay. A second event appeared in 1998 and by 2008 a total of seven events had been observed by E787 and its successor E949 – statistics that could only provide limitations on possible physics beyond the Standard Model.
Now, experiment NA62 at CERN is preparing to measure the branching ratio of this decay with a precision of 10%, similar to that of current Standard Model predictions. Data-taking is planned to start in the North Area at the SPS in 2014, with the aim of collecting some 100 events within two years.
Physicists are nothing if not patient.
About the author
Compiled by Peggie Rimmer.