Neutrinos, they are very small.

They have no charge and have no mass

And do not interact at all.

Cosmic Gall, John Updike

The 20th century was the golden era for particle physics. A plethora of particles were discovered and catalogued, from the common electron to the bizarrely named $J/psi$ (discovered by two experiments at the same time). Hundreds of particles, in fact: a `particle zoo’. So many appeared that scientists began to wonder if they really were all fundamental. As it was to be found, they weren’t. As we dissected matter further, we learnt more, and the particle zoo grew smaller and smaller until we reached what is now known as the `Standard Model of Particle Physics’. The protons and neutrons in the nucleus of an atom were found to be made of quarks, governed by the strong and electromagnetic forces. The electron was found to be one of a group of similar particles, made up of the electron, the muon and the tau (the latter two acting like heavier versions of the electron). These were the building blocks of everyday life, the stuff of atoms. But they were not all the particles that were to make up the Standard Model. There was one last particle, so elusive, so obscure that it took decades to get from its conception to detection. Pauli even bet a case of champagne that it would never be found. Lucky for us, he was wrong. That particle was to be the neutrino; the Cheshire Cat of particles, appearing and disappearing seemingly at will.

The story of the neutrino begins with Pauli. He was the first to postulate the neutrino – or as he called it at the time, the neutron (a name which was later given to the neutral nuclear particle) – but why did he? It was to solve a problem so large that Fermi himself had thought it could indicate a violation of the conservation of energy – one of the fundamental laws of physics – on a nuclear level. That problem was beta decay.

In 1896, Henri Becquerel stumbled upon radioactivity when he left a photographic plate and a uranium sample in a drawer together over a cloudy week. Having the plate lying around, he decided to get it developed out of curiosity; a curiosity that turned out to be not unfounded when it came back with particle tracks all over it. Just like that, out of the blue, Becquerel had accidentally discovered radioactivity. He was to open the door to a whole new area of physics.

It was Ernest Rutherford who had the task of defining the different kinds of radioactivity. There were alpha rays, resembling a helium nucleus; beta decay, which contained an electron; and gamma rays, which resembled a high energy X-ray. With further study, it was discovered that the energy of this radioactivity was the discrete difference of the final and initial energy levels of the decaying, which lined up with atomic theory. Well, it did for alpha and gamma rays: beta decay refused to comply, with electrons being emitted with any energy between zero and an upper bound. This was the problem that had Fermi so worried about energy conservation – this seemed to defy that on a nuclear level. But there was nowhere else the energy could have gone, was there?

It took a stroke of genius as crazy as it was clever to solve the problem. Writing to a physics conference in 1930, Pauli proposed a solution. Perhaps, he thought, there was another particle being released with the electron: a particle with energy that, when added to that of the electron, gave the total energy of beta decay to be the discrete difference between the final and initial energy states of the atom, keeping the laws of nature intact. For this particle to not have been noticed by Rutherford, it would have to have zero mass and neutral charge. He called this particle the `neutron’, meaning `neutral one’ (it was later to become the `neutrino’ – small neutron). It was an inspired solution, but it seemed farfetched – a whole new particle just to solve one problem seemed a bit over the top. It was this `neutron’ that Pauli wagered that case of champagne over, and bet that it would never be found. He shouldn’t have been so hard on himself; it was going to take 30 years, but the elusive neutrino would be found.

letter

The letter Pauli wrote to the nuclear physicists at the conference in Tubingen. It was imaginatively addressed to the `Radioactive Ladies and Gentlemen!’

 With Pauli having postulated the `neutron’ neutrino, Fermi got to work on seeing how it worked. One of the major problems of beta decay was there was no known force that could facilitate it. The process of changing a neutron to a proton (letting off an electron and a (anti)neutrino as by-products) couldn’t be explained by any of the known forces. There was a new fundamental force at play, a force that would come to be known as the `weak’ force, and governed radioactive decay and nuclear fusion. This was to be the only force that governed the neutrino, impervious as it was to all others.

Fermi gave up theorising about neutrinos in 1934, and the mantle passed to a student of his, Bruno Pontecorvo. At the time, nuclear power was garnering interest, and nuclear reactors became more common. Beta decay occurred in them, and Pontecorvo realised that they could be used as a source of many neutrinos. He wrote a paper about possible neutrino detection; the only problem was, what was actually been released in beta decay were antineutrinos, making Pontecorvo’s method invalid (as it needed the neutrino).

Fred Reines and Clyde Cowan took inspiration from Pontecorvo, deciding to detect neutrinos from nuclear reactors. However, their method was different, and relied more on observing the reverse of beta decay, so they did not need to know that the neutrino was actually an antineutrino. All they had to do was detect the by-products, which could be seen by observing two gamma ray bursts about 5 microseconds apart (the delay was due to the neutron by-product needing collisions to slow down). And in the summer of 1956, they saw just that; `Poltergeist’, their detector, recorded two gamma ray burst with 5.5 microseconds between them. The neutrino had been found, and Reines and Cowan celebrated with Pauli’s case of champagne.

Whilst all this was going on, there was a problem happening in astronomy. As we learnt more about our world, the question of how long the Sun had been burning (and would burn) came up. The Sun was thought of as a ball of fire, and fire needs fuel. But this model gave the age of the Sun to be about 25 million years, not at all in accordance with the 4.5 billion years that the Earth was calculated to have existed. There was no way intelligent life could have formed in this limited time range. Luckily, help was on the way, once more from nuclear physics. Nuclear reactors ran on nuclear ssion – splitting the atom – but there was also nuclear fusion (fusing two atoms together) that could release far more energy. It required immense temperatures to do so, but this was exactly what the Sun had. If nuclear fusion was what had kept the Sun running for so long, then it solved all the problems surrounding its age.

There was no way to crawl into the heart of the Sun and check this theory, so another way had to be found. Finally, the neutrino came to the stage. A particle many had previously ignored, it suddenly had signi cance. If nuclear fusion was indeed what was happening in the heart of the sun, there would be many excess protons lying around; protons which could then undergo inverse beta decay, letting o neutrinos. Neutrally charged, these neutrinos could zip straight out of the Sun and head to Earth. The detection of these `solar neutrinos’ could prove the theory right or wrong.

It sounded easy, but once again there was a spanner in the works. Whilst Fermi had been studying neutrinos and the weak force, he had calculated the neutrino’s rate of interaction with matter. It was measly. In fact, if a million neutrinos from beta decay were to enter the Earth, only one would actually interact with something, the rest passing through it like a bullet through fog.

Enter Ray Davies and John Bahcall. Davies had previously attempted to detect (anti)neutrinos from a nuclear reactor and failed, but when he heard about the concept of solar neutrinos as a way to prove nuclear fusion in the Sun he was enthused. In 1966 he nally had a detector, based at the Homestake mine. Bahcall, his partner, had worked out how many solar neutrinos they should be detecting, inventing a whole new unit to do so. It was called the SNU, and stood for `solar neutrino unit’, and was equivalent to 1 in 10-36 per second, Bahcall predicted that they should see 7 SNU, and so Davies got to searching.

After two years, Davies was able to release his results of his experiment. On the good side, he had de nitely detected solar neutrinos, a win. But the amount he was detected was far below that which was predicted: 3 SNUs at most. There was something clearly going wrong. The problem would come to be known as the `solar neutrino problem’.

Whilst all these problems were occurring with solar neutrinos, success with measuring terrestrial ones was increasing. Whilst neutrinos with low energy were likely to pass straight through the earth, the higher the energy of the neutrino, the higher its chance of interacting. As it would turn, the perfect source of high-energy neutrinos was to be just above our heads: the atmosphere.

Before the difficult detection of the neutrino, scientists were having success discovering many other particles. One of these was the pion, a particle that had been predicted as a solution to the fact that atomic nuclei exist. The surprise with the pion was what it decayed into – something that looked just like a heavier version on the electron. This was the muon, a particle that caused Isadore Rabi to famously declare `Who ordered that?’

As a heavier version of the electron, when the muon decayed it should give off an electron (with discrete energy) and a photon. Yet, in a situation that was to resemble that of beta decay, the electron it gave off was released on a continuum of energies. Could the answer to this be just the same as that for beta decay? At a time when the first neutrino was nearly a decade from being detected, this seemed like a big jump, but it turned out to be a correct. When a muon decayed into an electron, it gave off two neutrinos with it. It was once again Pontecorvo who had a thought that could change the entire stage of neutrino physics: that somehow, the neutrinos produced in beta decay and pion decay were different. This theory was given credence when it was realised the muon had some distinct `muon-ness’ about it that made it more than just a heavy electron. This came to be called a `flavour’, and when the tau particle was discovered in the 1970s, this became another of those `flavours’. Perhaps the neutrinos given off in beta decay and muon decay also had flavours, corresponding to the particles that decayed from (or with). Over time it would be found that there were three flavours of the neutrino: the electron-neutrino, the muon-neutrino, and the tau-neutrino.

One of the great advances in neutrino physics came with the advent of a new kind of detector. Cherenkov detectors used Cherenkov radiation – radiation given off when a charged particle in a medium travels faster than the speed of light in that medium – to detect particles. These detectors were able to differentiate between different kinds of neutrinos, and often consisted of huge tanks of water surrounded by photomultiplier tubes (PMTs).

cherenkov

An image of Cherenkov radiation caused by a charged lepton, as seen by the photomultiplier tubes. This was caused by a neutrino being absorbed by a nuceli and giving off a charged lepton, which then causes Cherenkov radiation.

Kamioka mine in Japan had a Cherenkov detector, and in the 1980s they observed something that came to be called the `atmospheric neutrino anomaly’. When cosmic rays caused a pion to decay in the upper atmosphere, the decay chain should produce two muon-neutrinos and one electron-neutrino. Instead, the ratio consistently measured seemed to be much closer to 1:1. Wanting to observe this closer, they went through an upgrade, giving them ten times more water and PMTs. The new experiment was called SuperKamiokande, or SuperK, and was able to not just measure what types of neutrinos there were, but exactly where they came from. This uncovered something new and exciting about the atmospheric neutrino anomaly. When they measured the number of neutrinos coming through the Earth, the de cit in muon neutrinos was far larger than those coming from directly above. In fact, it seemed that further the neutrinos travelled, the greater the deficit became. If muon-neutrinos were somehow disappearing as they travelled, this solved the problem: muon neutrinos and electrons neutrinos were being produced with a ratio of 2:1 in the atmosphere, but then vanishing.

This could also solve Davies’ problem: if neutrinos were disappearing when they travelled only the length of the Earth, surely they could also disappear in the 15 million kilometre journey from the Sun. It seemed like a wonderful solution, but yet, where on earth could the neutrinos be going?

It was once again Pontecorvo who thought up the inspired solution years earlier, and by throwing away one of the earliest tenements about the properties of neutrinos. If neutrinos did not actually have zero mass, and were instead massive (however small that mass may be) then they could oscillate between different states as they travelled. This is due to quantum physics, where particles act as probability waves. If the probability wave of the neutrino was a superposition of two (or three) different waves, then measurement of that wave at di erent points in its journey would give different states.

oscil_graph

The graph shows the oscillation probabilities between states for an initial electron-neutrino (with arbitrary initial conditions), with the muon-neutrino represented in red, and the tau-neutrino by blue.

This could solve both the solar neutrino problem and the atmospheric neutrino anomaly, and so had to be checked out. SuperK buddied up with a new experiment, the Sudbury Neutrino Observatory (SNO), to see if it could be true. Between the two, they could measure the in ux of electron-neutrinos from the Sun, and the total in flux of all kinds of neutrinos. Their data collection spanned the turn of the millennium, but when they were done, their results were perfect. About one third of the total neutrinos detected were electron-neutrinos. Davies’ earlier experiment had been working, and Bahcall’s prediction correct, but they had only been able to measure electron neutrinos, and therefore had found a major deficit. The solar neutrino problem and the atmospheric neutrino anomaly had nally been solved!

From when it was only a twinkle in Pauli’s eye to its full development as a massive oscillating particle, the truth about the neutrino had been found. But its journey is far from over.

Only recently have we been able to measure some of the final parameters of neutrino oscillation1, and their measurements will continue to higher and higher precision. As for the mass of the neutrino, it’s been found that the three mass states are actually separate to the three flavour states. All we really know of these three mass states is that they are ridiculously small (the neutrino is at least a thousand times lighter than the electron, the next lightest particle), and a bit about how they relate to the flavour states. Our knowledge of how the mass states relate to each other is restricted to speculations about `mass hierarchies’2; either `normal’ ($nu_{1}<nu_{2}<nu_{3}$) or `inverted’ ($nu_{3}<nu_{1}<nu_{2}$)).

masshierarchy

This diagram shows the two di erent mass hierarchies, and how the di erent mass states are fractionally comprised of the di erent neutrino flavour states.

These little particles could answer many of the questions of the universe. We already know the weak force (and therefore the neutrino) to be parity (space) symmetry violating, and if it was discovered to also violate charge symmetry, it could have produced an abundance of matter in the early universe. This could potentially help explain the confusing matter-antimatter asymmetry in a universe which otherwise seems symmetrical. When it comes to cosmology, neutrinos could hold the key to the continuing problem of dark matter. One of the possible candidates for dark matter is a sterile neutrino3; an ultramassive neutrino that doesn’t interact via the weak force, so that it interacts with no forces at all (and so is nearly undetectable). This would be physics beyond the Standard Model: dangerous and exciting. This is just a taster of what the neutrino may have in store for us; we are still in our infancy of even attempting to understand the universe and its constituents on a fundamental level, and the neutrino could arguable be seen as one of the most intriguing fundamental particles to exist.

The journey of the neutrino has spanned nearly a century, and seems to have no view to slowing down. One of the last fundamental particles to be common knowledge, it is the secret ace in our back pocket that could answer so much. Sometimes, it’s the smallest players who make the biggest moves of the game. Pauli never could have imagined the journey his impossible `neutron’ would travel when he wagered that champagne back in 1930. Just like bubbles rising to the surface of the glass, infusing the room with their scent, they have slowly and surely gained import and interest within the physics world, unfolding their secrets one by one. Whilst John Updike may have gotten the specifics wrong in Cosmic Gall4, he encapsulated the thrall that captured those few but important neutrino scientists. From Pauli to Fermi, Davies to Reines, Pontecorvo and more: without them we would be nowhere. So let us raise a glass of Pauli’s champagne to that particle: the flighty, elusive, and yet oh-so-important neutrino.


References

  1. Pawel Przewlocki. Recent measurements of 13 mixing angle in neutrino oscillation experiments. Acta Physica Polonica B, 44, November 2013.
  2. Kenneth Long. Neutrino production moves to an industrial scale. Cern Courier, April 2012.
  3. Pawel Przewlocki. Recent measurements of 13 mixing angle in neutrino oscillation experiments. Acta Physica Polonica B, 44, November 2013.
  4. Scott Dodelson and Lawrence Widrow. Sterile neutrinos as dark matter. Phys. Rev. Lett., 72:17{20, Jan 1994.
  5. John Updike. Collected Poems, 1953-1993. Knopf Australia, Australia, May 1993.
  6. Frank Close. Neutrino. Oxford University Press, United Kingdom, May 2012.
  7. Brian Martin and Graham Shaw. Particle physics. Wiley-Blackwell (an imprint of John Wiley & Sons Ltd), United Kingdom, January 2008.
  8. Lincoln Wolfenstein and Joao Silva. Exploring fundamental particles. Taylor & Francis Inc, United States, August 2010.