From the November 1966 issue

The next neutrino experiments

In 1931, Pauli postulated the existence of neutral, very light and very feebly interacting particles [to resolve the energy crisis in nuclear beta-decay]. Now called electron anti-neutrinos (ν^–_e), they are produced, with an electron, e.g:

n → p + e^– + ν^–_e

The most prolific sources of ν^–e are nuclear reactors; each of us is bombarded by thousands per second from the world’s reactors. Cowan and Reines detected “neutrino” interactions for the first time in 1955 at the Savannah River reactor, USA, winning a wager made by Pauli that they would never be observed.

The Sun is our most powerful source of electron neutrinos (ν_e), bathing the Earth in more than 10¹⁰/cm²

p + p → d + e⁺ + ν_e

To study solar neutrinos, very large detectors have been installed deep in the earth to escape confusing background [e.g. in the Homestake Mine, USA]. Will the solar neutrino flux be measured at last and lead to a more precise understanding of how the Sun transforms mass into energy?

‘Laboratory’ neutrinos

The ν_μ and ν^–_μ, generated from pions or kaons, are especially suitable for experiments at high-energy accelerators [see later].

μ⁺ or k⁺→ μ⁺ + ν_μ, μ^– or k^– → μ^– + ν^–_μ

In 1963–64, about 1000 interactions of ν_μ were detected in the CERN heavy-liquid bubble chamber (HLBC) at the Proton Synchrotron (PS). The previous Brookhaven results, which demonstrated the existence of the two neutrinos, ν_e and ν_μ, were confirmed, with increased evidence that ν_μ interacts to produce muons and never electrons. No evidence was found that ν_μ from kaons differ from those from pions, nor that the type of particle called “strange” is often produced in ν–_μ interactions.

Perhaps most interesting was the lack of evidence for the intermediate boson, W, postulated to be the carrier of the weak force. If the W exists at all, its mass must be greater than 2 GeV/c², much higher than first thought.

Plans for the next step

The principal information from the first experiments came from “elastic” interactions:

ν_μ + n → μ^– + p

where both the muon and the proton had a high probability of escaping from the target nucleus.

Inelastic interactions are much more difficult to investigate, the simplest being:

ν_μ + p –> μ^– + μ⁺ + p

where the pion is likely to be absorbed in the target nucleus, so the kinematic analysis will be less accurate.

This problem would not occur for interactions with “free” protons. Therefore an obvious next step was to replace freon, CF3Br, in the HLBC by a fluid like propane, C₃H₈. To obtain the same event rate in propane as in freon requires a ten-fold increase in neutrino flux.

The HLBC volume has been increased from 500 to 1180 litres to give more than a three-fold increase in event rate. When “Gargamelle” is available, its effective volume should be some seven times more than the present HLBC.

The new neutrino beam

To produce a neutrino beam, the accelerated proton beam is brought out of the PS and directed onto a target to produce secondary particles that include neutrino “parents”, pions and kaons, which decay after a short time.

In the first experiments, the neutrino parents were focused into a parallel beam and directed towards the detector by a magnetic horn, invented by S van der Meer at CERN. In the new system, the parents will be partially focused by a new horn, designed by D H Perkins of the University of Oxford and W Venus, after which two magnetic correcting elements, reflectors, developed by A Asner and Ch Iselin, will converge them towards the detector, significantly improving the neutrino flux. To filter out all particles except the feebly interacting neutrinos, shielding has been rebuilt from 6000 tonnes of steel ingots generously lent by the Swiss authorities. The new horn, reflectors and steel block will produce, for each proton incident on the target, a neutrino flux about six times greater than before.

Behind the shielding, the HLBC will operate with propane for the first time. A few neutrino events on “free” protons are expected per day, with about five times as many events on carbon nuclei. A magnetic field of 27 kG will select candidate interactions so that less than 10% will be on carbon. The “carbon” events, being in propane, can be measured with much greater precision than in the first experiment, allowing an invaluable re-examination of previous conclusions.

Spark chambers will be associated with the HLBC to test the law that a ν_μ always transforms to a μ^–, and a ν^–_μ to a μ⁺. The chamber array will be able to detect a violation as small as one μ⁺ per thousand muons produced by ν_μ.

Neutrinos carry a significant fraction of the total energy of the universe. What is the cosmological role of these tantalizing, all-permeating particles? It may be interesting to review later, what has been the outcome of this programme and to assess what contributions have been made to increase our understanding of the nature of neutrino interactions.

• Compiled from the article by C A Ramm pp211–217.

Compiler’s Note

In the Homestake Mine the measured solar ν_e flux was only a third of that predicted. This “problem” was eventually solved as being due to neutrino identity-changing flavour oscillations: quantum mixing that requires neutrinos to have non-zero mass, currently estimated to have an upper limit in the 1 eV region.

Using ν_μ at the PS, neutral currents were observed in Gargamelle in 1973, providing the first clear evidence for a neutral weak force carrier, the Z⁰. This gave a strong boost to electroweak theory, which by then predicted much higher masses for both the Z⁰ and the charged W than was first thought. Discovered at CERN in 1983, the W (80.4 GeV/c²) and Z⁰ (91.2 GeV/c²) are almost as heavy as the silver nucleus.