DISCOVERING THE NEUTRINO
Savannah River, South Carolina, 14 June 1956. Frederick Reines had spent much of the 1950s exploding nuclear bombs in the Pacific Ocean. One, with 700 times the destructive power of the Hiroshima bomb, had vaporised an entire island, creating a radioactive mushroom cloud 150 kilometres across and gouging a hole in the ocean floor more than two kilometres wide and as a deep as a 16-storey building. But Reines was sick to death of it. He persuaded the leader of the theoretical division at the bomb lab in Los Alamos, New Mexico, to give him time off from testing weapons to think about physics. For several months, he sat in a bare office staring at a blank piece of paper, asking himself the question: “What do you want to do with your life?”
Reines flew to a conference in Princeton, New Jersey, and the plane developed engine trouble, forcing a stopover in Kansas City. On the plane was another bomb scientist called Clyde Cowan, who Reines had met but never talked to properly. In Kansas City, the pair hit it off. When their conversation turned to fundamental physics, the question that came up was: “What is the hardest experiment in the world?” Both instantly agreed: detecting the neutrino. There and then, they decided to use their ‘can do’ attitude, developed during the bomb programme, to try and bag nature’s most elusive particle.
Their first idea was to place a neutrino detector 50 metres from Ground Zero of a nuclear explosion. They had a 50-metre-deep vertical shaft dug at the Nevada bomb testing site. If, at the instant of detonation, the detector were dropped into the shaft, it would be in freefall and isolated from the shockwaves thundering through the ground. At the bottom of the shaft, the detector’s fall would be cushioned by a thick bed of foam rubber and feathers. Reines and Cowan intended to retrieve the detector several days later when radiation levels were deemed low enough to risk a quick in-and-out foray.
Fortunately, it never came to this. Reines and Cowan decided to use a nuclear reactor rather than a bomb. Although a nuclear reactor was a source of neutrinos 1,000 times weaker than a nuclear bomb, it had the advantage that a detector might soak up neutrinos for months or even years, rather than the few seconds available at a bomb blast. Reines and Cowan eventually found an ideal reactor at the Savannah River Plant, a facility used to make the tritium and plutonium for nuclear bombs. As a tribute to the local cuisine of South Carolina, they even planned to shield their experiment from stray neutrons with sacks of black-eyed peas. Sadly, wet sawdust was easier and cheaper to obtain in the required quantities. It was at P Reactor at the Savannah River Plant that, on 14 June 1956, Reines and Cowan finally bagged neutrinos (strictly speaking, antineutrinos).
Their telegram to Wolfgang Pauli read: “We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing the inverse beta decay of protons… Frederick Reines, Clyde Cowan.”
The next day, Pauli sent a reply from ETH Zürich, the Swiss Federal Institute of Technology: “Frederick REINES, and Clyde COWAN,
Box 1663, LOS ALAMOS, New Mexico. Thanks for the message. Everything comes to him who knows how to wait. Pauli.”
is a different mix of all three masses. Imagine an animal that is 25 per cent cat, 25 per cent dog and 50 per cent giraffe. This conveys some idea of the weirdness of neutrinos. As each type flies through space, its individual mass components travel at different speeds, and consequentially the relative proportions of each mass state changes. This results in a neutrino ‘oscillating’ between an electron-, muon- and tau-neutrino.
Measurements of neutrino oscillations will provide estimates of the differences between masses of the three neutrinos. Importantly, KATRIN pins down an upper limit on one mass. Crucially, however, we still don’t know the neutrino-mass hierarchy – whether electron-, muon- and tau-neutrinos get progressively more massive as do electrons, muons and taus.
Underdstanding neutrino oscillations and neutrino masses is vitally important. If the ‘mixing’ between neutrino mass states is big enough, it could indicate that nature permits a process that, in the jargon ‘violates charge-parity symmetry’. This would make antineutrinos behave differently from neutrinos. By favouring the production of matter over antimatter, this could solve one of the outstanding puzzles of cosmology: why we live in a Universe of matter. “According to the Standard Model, all fundamental particle processes create equal quantities of matter and antimatter,” says
Uchida. “We therefore should not exist [because when matter and antimatter particles meet, they annihilate]!”.
RETHINKING THE EARLY UNIVERSE
Neutrino oscillations may reveal the existence of a fourth, ‘sterile’ neutrino, interacting with matter so rarely it makes the other three flavours appear positively sociable. The total mass of all three (or more) types of neutrino has consequences for the Universe because neutrinos are the second most common subatomic particle, after photons. In the early Universe, their considerable gravity would have helped matter clump together to make the first galaxies. The more massive neutrinos are, the earlier they would have slowed down after the Big Bang and the clumpier our Universe should be. Consequently, knowing the masses of the neutrinos helps pin down the cosmological model that best describes our Universe.
If astronomers’ observations of clumpiness contradict that model, then it will be strong evidence of physics beyond the Standard Model.