The challenge of neutrinos by Christine Sutton
Neutrino pioneers - left to right, Paul Dirac, Wolfgang Pauli., Rudolf Peierls. (Photo MP Niels Bohr Library)
Earlier this year, a team of physicists working at Los Alamos claimed a possible sighting of neutrino oscillations (June, page 13). However even before the claim appeared in the scientific literature, newspaper reports had leaked the possible findings and their implications. Detecting particles which can pass unhindered through the Earth provides the ultimate physics challenge. Neutrinos have always captivated public imagination and make compelling reading. In the wake of the latest neutrino episode, Oxford physi cist and science writer Christine Sutton looks back on the short but volatile history of the neutrino and its attendant publicity. strangers to controversy, for they beta-decay electrons varies continu With the availability of high were in a sense born amidst it, and ously up to a maximum, with peaks energy neutrino beams in the as recent work on neutrino oscilla ("lines") at only a few energies. And 1970s, physicists eagerly tions demonstrates they continue to he was able to explain how the scoured each new batch of data fuel debate. This is of course largely photographic technique could "fake" for signs of exciting new physics. to do with the fact that neutrinos have lines through its great sensitivity to But few initial 'sightings' proved no electric charge and experience small changes in intensity. to be of substance. One of the only the weak nuclear force, making The First World War interrupted major problems faced by such them supremely difficult to detect. these investigations, but afterwards experiments, lies with neutrons The debates surrounding neutrinos arguments between a continuous produced outside the apparatus, began in the first decades of the 20th energy spectrum and discrete lines either by beam particles or by century, before the particle had even continued until 1927, when Charles cosmic rays. been thought of. Studies pioneered Drummond Ellis and William Wooster by Use Meitner and Otto Hahn at Cambridge published results from Further reading: Spaceship suggested that the electrons emitted a definitive experiment in which they Neutrino, by Christine Sutton, in beta-decay emerged with discrete measured the total electron energy in Cambridge University Press, energies. In these experiments a a single decay process. If the elec 1992, ISBN 0-521-36404-3 (he), spectrometer bent the electrons ac trons always started out with the 0-521-36703-3 (pb). cording to their energy, and a photo same energy but lost varying graphic plate detected the electrons amounts in subsidiary processes to emerging through a movable slit, so give many lines, as Meitner believed, yielding "lines" at various energies. then the result would equal the eutrinos", Maurice Goldhaber But when James Chadwick used a maximum at the end of the spectrum. Nonce remarked, "are remark point counter instead of a photo However Ellis and Wooster meas able particles: they induce courage in graphic plate, he could not find "the ured an energy equal to the average theoreticians and perseverance in ghost of a line". Instead, he con energy of the spectrum, so confirm experimenters". They are also no vinced himself that the energy of the ing that the electrons started out with
36 CERN Courier, September 1995
The challenge of neutrinos
In 1953 Clyde Cowan and Fred Reines used a newly invented supply of copious numbers of neutrinos - a nuclear reactor - in the 'Project Poltergeist' experiment. This provided the first experimental evidence for interactions from a neutrino source. (Photo Los Alamos)
a range of energies. But this raised a new problem. If the electron shares the decay energy with the nucleus, this "two-body" process should always yield elec trons of the same energy - provided that energy and momentum are conserved. No less a physicist than Niels Bohr suggested that these cherished laws of physics might well break down within the confines of the nucleus, but Wolfgang Pauli would have none of this. He suggested instead that an invisible third party was involved - the particle Enrico Fermi later dubbed the neutrino, or "little neutral one". In his famous letter of 4 December 1930 to Meitner and friends, in which he proposed the idea of the neutrino, Pauli said "For the time being I dare not publish anything ... and address myself confidentially first to you He nuclear force, so the probability for nos - a nuclear reactor - together with remained cautious, and although he interactions is typically 20 orders of what was then a large detector with presented the ideas at a meeting in magnitude smaller. But anything that 300 litres of liquid scintillator. At first Pasadena in July 1931, he did not is created can in principle be de it seemed that the rate for the neu have his talk printed. However tected. "Obviously", as Leon trino signal was as high when the (interestingly enough, in the light of Lederman has written, "a particle that reactor was off as when it was on. more recent debates) a report did reacted with nothing could never be But then they realized that they had appear in the New York Times. detected. It would be a fiction. The the hint of a signal above a back Pauli realized that his particle would neutrino is just barely a fact." ground five times greater. Later be difficult to detect, and in 1934 To be sure of detecting even a few studies, with the detector under Hans Bethe and Rudolf Peierls neutrino interactions you need as ground, showed that the background calculated an interaction cross- many neutrinos as possible and as was due mainly to cosmic rays. So section of less than 10~44 cm2, sug large a detector as possible. But you Cowan and Reines went to a more gesting that "there is no practically also need to understand completely powerful reactor and built an im possible way of observing the neu the backgrounds - the processes due proved detector to reduce the back trino". But as Peierls commented 50 to other particles that mimic the ground. Here they found counts of years later, they were not allowing for signal you expect for the neutrino. typically 1.6 per hour with the reactor "the existence of nuclear reactors (or Undoubtedly, in all cases where on, and 0.4 when it was off - and they particle accelerators) producing results in neutrino experiments declared the neutrino discovered. neutrinos in vast quantities" or for disagree with each other or do not Cosmic rays are a major problem "the ingenuity of experimentalists". stand the test of time, it is because with neutrino experiments, in particu Almost any other particle imagina the backgrounds have not been lar because they give rise to neutrons ble is easier to detect than the properly understood. outside the apparatus, which then neutrino. Being uncharged, it leaves In their first attempt to detect the look like neutrinos when they interact no tracks in a detector such as a neutrino in 1953 Clyde Cowan and inside the detector. Neutron back bubble chamber; and unlike the Fred Reines used a newly invented grounds played a major role in the neutron it does not feel the strong supply of copious numbers of neutri efforts to understand neutrino inter-
38 CERN Courier, September 1995 The challenge of neutrinos
actions in the bubble chamber spectrometer to measure precisely tected in the experiment. To oscillate Gargamelle, which revealed "neutral the high energy end of the spectrum in this way, the two types of neutrino currents" at CERN in 1973. This of electrons from beta decay. The would have to differ slightly in mass, discovery proved to be a keystone in shape here depends critically on implying in turn that at least one type the development of the present whether the neutrinos that share the must have a finite mass. Standard Model of particle interac energy with the electrons have any Neither of these experiments turned tions, but for a while it too became rest-mass energy. Lyubimov and out to stand the test of time. Nor did the centre of controversy. colleagues claimed to have evidence the excitement generated several The main difficulties arose with the for a mass between 14 and 46 eV - years later by suggestions of a neu classification of events in which a an exciting prospect as it implied that trino with a mass of 17 keV, again in spray of hadrons (protons, pions etc) neutrinos from the big bang might an experiment using a spectrometer appeared as if from nowhere and together have sufficient mass eventu to measure the energy spectrum of were not associated with an identifi ally to halt the expansion of the beta-decay. In this case, Andrew able muon. While these could be Universe! Hime and his colleagues realized neutral current events due to neutri Meanwhile, across the world in eventually that scattering effects nos, they could also be due to California, Fred Reines claimed were faking the appearance of this neutrons, and the onus was on the evidence of a different kind for massive neutrino in several experi Gargamelle team to show that they neutrino mass (which again made the ments. fully understood the ways in which newspapers before publication in the So in a sense, the story comes full neutrons could enter the detector scientific literature). In studying circle, echoing the early problems from the surrounding material. neutrinos from the Savannah River that Hahn and Meitner faced. But the The team was put under additional reactor, Reines and colleagues important point to remember is that pressure when conflicting signals believed they had detected a smaller none of the "wrong" results with came from the HPWF (Harvard, ratio between charged current and neutrinos is a waste of time. They Pennsylvania, Wisconsin, Fermilab) neutral current events than theory inspire theoretical work that may neutrino experiment at Fermilab. predicted. This suggested that the eventually lead to new insights and Although the preliminary results from electron-neutrinos emitted by the even new discoveries. Indeed, this is HPWF supported Gargamelle's reactor might have changed to muon- the way that any scientific research findings of hadronic neutral currents, neutrinos which would not take part works, with many wrong turns on the "improvements" to the HPWF detec in the charged-current events de route to a proper understanding. tor, led to the conclusion that the neutral currents were not there after all, as the events now seemed to include muons, indicative of charged currents. This "un-discovery" caused great concern at CERN outside the Gargamelle collaboration, but in the end it was the HPWF team who realized that the muons were not in fact muons at all, but hadrons that had managed to "punch through" a reduced shield into the muon-detec- tion apparatus. The following decade began with more excitement when in 1980 two experiments suggested that the neutrino might have a small but finite Leon Lederman - "a particle that reacted with mass. In Moscow, Valentin Lyubimov nothing could never be detected, it would be a and colleagues were using a fiction. The neutrino is just barely a fact."
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