Neutrinos: the basics and why they matter

1. What is a neutrino?

neutrino_icecube_diagramA neutrino is a subatomic particle, grouped with other leptons that include the electron, muons and taus.The charged leptons and uncharged neutrino are all classified as leptons for a number of reasons, one being that neither is affected by the strong force, which keeps quarks together as either neutrons or protons. Due to their small size, leptons have an extremely weak interaction with gravity. But unlike charged leptons, neutrinos are not affected by the electromagnetic force. This is why they come to us straight from their source. We’re showered with them everyday from the sun and from the rest of the universe. If we are in the Northern hemisphere they also come at us from the Southern sky, through flesh and land without leaving a trace, and through the planet.

a) Part 1 : the theory

Before experimental evidence for a type of neutrino was obtained in 1956, their existence was proposed by Pauli in 1930 to secure energy, momentum and spin conservation in weak interactions. (His statement was preceded by a funny formal address “Dear radioactive ladies and gentlemen”. ) In beta decay, electrons are made in and emitted from the nucleus. But when the kinetic energy distribution, or spectrum, of beta particles was measured and compared to that predicted by relativistic energy, it seemed like energy was not conserved. But a small, neutral unknown particle, the neutrino, could be carrying off the unaccounted kinetic energy.

spaghetti
In Italian, the “one” suffix signifies that something is big. The plural becomes oni, as in spaghettoni. The opposite is ino (plural ini) as in thin spaghetti or spaghettini. Because the Italian word for neutron is neutrone, Fermi proposed the name neutrino for the smaller neutral particle.

What also confused physicists at the time is that for a while they did not immediately realise that another neutral particle, the neutron, existed. They believed that “nuclear electrons” along with protons were part of the nucleus. For example, the nitrogen nucleus, with a mass of fourteen and a charge of +7, was thought to consist of 14 protons and 7 nuclear electrons for a total of 21 particles, each known to have a fractional (1/2) spin number. Yet a proposed sum of 21 contradicted the evidence. The nuclear spin of nuclei with an even-numbered nuclear mass such as nitrogen-14 had been measured, and it was known to be an integer. Odd-numbered nuclei would have fractional spin numbers of 1/2, 3/2 etc. But if a neutron existed and its mass was respectively similar to that of a proton and its spin number was also fractional, then it implied that nitrogen actually had 7 protons and 7 neutrons. With an even number of particles, its spin number could indeed be +1.

But how is all this related to the neutrino?

Consider the beta decay of  carbon 14, a radioactive isotope in our bodies. If we assume that the even-numbered carbon-14 ( 14C ) isotope is only turning into the stable nitrogen isotope, 14N, and a beta particle -1e, not only is there kinetic energy missing, but we seemingly have an odd number of particles produced (14 for nitrogen and the beta particle for a total of 15), each having fractional spin numbers . That would not conserve spin number. So another particle must be carrying some of the energy and that particle must have a fractional spin number. All leptons do, and in this case, the other particle produced is the electron antineutrino ( antielectrino), a form of antimatter:

14C → 14N +-1e + antielectrino

a) Part 2 : the experiment  Actually first, let’s dish out a little more theory. The lepton number is a conserved quantum number in an elementary particle reaction. All leptons have assigned a value of +1, antileptons −1, and non-leptonic particles like the neutron and proton, 0. In case you wondered why an antineutrino was produced in the  14C reaction, it was to preserve the overall lepton number of zero for the 14 nuclear particles of 14C .

The following reaction is sound in the sense that it also conserves lepton number.

{\displaystyle {\bar {\nu }}_{e}+p\to n+e^{+}}

Among the reactants we have the antineutrino with a lepton number of -1. And a positron should be produced, as opposed to a beta particle, which not only conserves the lepton number of -1 but also conserves charge. The neutron produced will be captured by some nucleus and in a second reaction, the positron will be annihilated by an electron, creating gamma. To identify the observed signal as neutrino-induced, Cowan and Reines used nuclear reactors to compare energies of the two pulses, their time-delay spectrum, the dependence of the signal rate on reactor power and its magnitude. They used two different detectors (a water based one and cadmium) for neutrons and a number of other experiments to make sure they were not fooling themselves. And it turned out that the data could only be explained by the presence of neutrinos.

3. Types of Neutrinos and the Solar Neutrino Problem Solved

Every flavour of neutrino has its antimatter counterpart. We’ve seen the electron antineutrino whose counterpart in matter is the electron neutrino. There are also muon and tau neutrinos and their respective antimatter counterparts. At one point theoretical physicists were questioning the neutrino detecting measurements because they did not confirm their theoretical models of the sun’s fusion reactions. Based on their understanding of fusion reactions, not enough electron neutrinos were being detected from the sun. ( Electron neutrinos are produced in the first part of the sun’s proton-proton chain reaction.

proton_proton

In this reaction a proton is being converted into a neutron,positron and neutrino. Notice how the lepton number of zero is conserved by having a neutrino (L= +1) accompanying the emission of the positron (L= -1).

Meanwhile, others were wondering if the experiments were revealing some flaw in the theory.

It turned out no one was actually wrong. They knew that the original chlorine-37 detectors were only sensitive to electron neutrinos — upon being captured by the unstable chlorine isotope they would be converted into a measurable argon-37 isotope. But this was done deliberately because the sun is not a direct source of either tau or muon neutrinos. Eventually measurements of other flavors of neutrinos, especially those from the Sudbury, Canada reactor, revealed that 2/3 of the sun’s electron neutrinos flipped flavors on their way to earth. Eventually investigators revealed a connection between the oscillations and mass differences of the different types of neutrinos and another neutrino-related Nobel was awarded in 2015.neutrinooscillation21

4. High Energy Neutrinos

In Antarctica , there is a detector of high-energy neutrinos named IceCube. It’s a cubic kilometre of ice equipped with 5160 optical sensors that collect a shower of charged particles radiating blue light known as Cherenkov radiation. The shower is created after rare collisions between neutrinos and the nuclei of pure ultra-transparent ice, and the radiation can travel hundreds of meters to the detectors.  So far IceCube has detected high energy neutrinos with one to two billion times the energy of solar neutrinos. The suspected sources of these particles are supernovae, gamma ray bursts from large collapsing stars, supermassive black holes or even other exotic possibilities. It’s been postulated for example that the decay of heavy, long-lived dark matter particles, if they exist,  could also produce such signatures.

The largest pool of almost 100 000 photons was created by neutrinos dubbed Ernie and Bert, named after the Sesame Street characters. Their energies were 1.07 PeV and 1.24 PeV, respectively. When the number of such high-energy neutrinos totalled 54, the signals’ significance exceeded 5 standard deviations, implying that it’s highly unlikely that the observations are not atmospheric phenomena. (At sigma-5  the probability is 1 in 3.5 million that if the high energy-particles do not exist, the data that IceCube collected in would be at least as extreme as what was observed.) Some of the neutrinos were not from our galaxy, but Bert was within a degree of the galactic plane, which is rich in supernova remnants and is also the host of a giant black hole.

icecubeneutrinos
The signal from “Ernie”. Each sphere represents one optical sensor; the coloured spheres show modules that collected light from this event. The sizes of the spheres are proportional to how many photons each module recorded. Finally, the arrival time of the first photon is symbolised by colour from red (earliest) to blue (latest). Source: AntarcticaNeutrinos.blogspot.ca

They are hoping to build a larger collector, one that could conceivably collect GZK- neutrinos, made by interactions with the big bang’s afterglow. GZK could have energies 1000 times bigger than Bert’s. So far none have been found, and this blog from astronomer Spencer Kline,who is on site in Antarctica, provides updates.

Sources :

Detection of the Free Neutrino: A Confirmation Science Vol. 124, No. 3212, Jul. 20, 1956

NEUTRINO OSCILLATIONS compiled by the Class for Physics of the Royal Swedish Academy of Sciences

A Brilliant Darkness. Joao Magueijo. Basic books. 2009

Neutrinos at the Ends of the Earth. Francis Halzen (chief investigator of IceCube project). Scientific American. October 2015

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