What are Neutrinos?
Denoted by the letter “ν” (the Greek letter “nu”), neutrinos are fundamental particles (particles you can’t split up into anything smaller) with a mass of almost zero and a neutral charge. They’re everywhere, with around a thousand trillion of them passing through you every second, and yet they were only discovered in the last century [1].
So, if they’re so insignificant in every way, why does the Universe need them? Well, they’re important in conserving energy in the decay of protons into neutrons and neutrons into protons (called beta plus, β+, and beta minus, β-, decay) [2]. But apart from appeasing the gods of physics, neutrinos help us understand what’s going on in the Sun [3], as well as pinpointing where supernovae are [4].
First Observation of a Neutrino in a hydrogen bubble chamber. Credit: Argonne National Laboratory
Opposites
But some may have noticed the title includes the word “antineutrino”; not “neutrino”. That’s because the main focus is going to be on the neutrino’s evil twin: the antineutrino. It falls under the umbrella of antimatter: something with the exact same properties as its matter counterpart, except for its charge (plus a few other properties), stemming from how the equations set out by the Bristol born physicist Paul Dirac have two solutions: one positive and one negative, like how the square root of something has both a positive an negative outcome (√4 = +2 and -2). His equation was then interpreted as meaning that for every particle there is a corresponding antiparticle (proton and antiproton, electron and positron, neutrino and antineutrino…) [5].
However, neutrinos don’t have a charge, so what’s different about an antineutrino? Apart from charge, antimatter also changes in spin. All particles (and antiparticles) spin: some clockwise; some anticlockwise. It’s called spin since, if you spin a ball bearing very fast in a magnetic field, it’ll get deflected either up or down (hence the names of the spins are “up” and “down”), depending on which way it’s spinning, and this is how particles act when they’re interacting with a magnetic field (although they’re not actually spinning: in fact the way this works isn’t properly understood). So neutrinos ‘spin’ in the opposite direction to antineutrinos [6].
You may have heard about antimatter and matter in other contexts as well, like how the Universe is made of matter, and yet there was an equal amount of antimatter and matter at the start of the Universe. This is because of something we don’t understand yet, but it’s kept this way by how, if a particle meets the antiparticle version of itself, they annihilate, converting their mass into energy, such as light [5]. And since there’s more matter than antimatter, there’s more than enough matter to annihilate the antimatter whilst having matter which hasn’t been annihilated.
First cloud chamber photo of a positron's path. Credit: Carl D. Anderson
A Cosmic Source
Antineutrinos, and neutrinos, are the bi-product of the weak nuclear force: one of the four fundamental forces in our Universe, responsible for converting protons (positive particles in the nuclei of atoms) into neutrons (neutral particles in the nuclei of atoms) and vice-versa. It allows the components of protons and neutrons, quarks, to change their ‘flavour’ or type, from up to down or vice-versa (with the configuration of ‘ups’ and ‘downs’ determining if the particle is a proton or neutron) [7].
This means that neutrinos and antineutrinos are formed from radioactive processes: processes that change what element something is (for example, hydrogen to helium), found everywhere from supernovae to our own Sun and the Earth below us, showing how active something is [8].
However, they don’t interact much with other particles: lucky since so many go through us every second, but not so for the people who want to observe them. However, they do interact in a way which scientists have now made detectors to exploit.
Detectors
One key way to detect them is through having a lot of a liquid that’s very pure (i.e. it has nothing else apart from one molecule or element). Apart from being very dangerous (in one experiment, a hammer was hollowed out when it was dropped in the ultra-pure water used), the liquid (e.g. water) interacts with a particle (e.g. electron) which has interacted with a neutrino, giving a flash of light (known as Cherenkov radiation) every time a neutrino hits a molecule of the liquid. This light is then detected by a photomultiplier tube or PTM (a reverse lightbulb, taking in light and converting it into electricity) mapping out where the neutrinos are located [13][9].
However, one experiment, Super-Kamiokande (in Japan), is adding something else to the liquid (water) to make the signal more distinct: gadolinium [9]. At number 64 on the periodic table, gadolinium is a silvery white element used to absorb neutrons in the core of nuclear reactors [10]. However, when it absorbs a neutron, it makes a flash of light. And the same can be said for neutrinos, hitting water enriched with gadolinium: two distinct flashes of light are emitted per interaction with a neutrino, separated by around 30 millionths of a second: a cosmic heartbeat. This distinct signature is then being used by the team in Japan to take out background noise from other interactions to focus on those from neutrinos of a cosmic origin [9].
That said, purity isn’t the be-all and end-all for some experiments, as seen in the experiment ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch). The experiment is based in the Mediterranean Sea off the coast of Toulon in the south of France [11]. It uses 1,000 photomultiplier tubes over an area of ~0.1km squared, expected to detect less than 100 events per second, seen as minute flashes in the Mediterranean sea water [12].
ANTARES Neutrino Telescope. Credit: François Montanet, CC BY-SA 2.0 FR
Both the previous experiments used liquid to find neutrinos, but an experiment based at the south pole became the first detector to go off piste, with a detector observing one cubic kilometre (a gigaton) of ice [13]. IceCube is a detector made up of the IceTop detector (a detector on top of the ice) and the DeepCore detector (a detector 2,500 meters below the ice), working much like liquid based detectors in that it uses photomultiplier tubes (5,160 of them for DeepCore and 324 of them for IceTop), except it uses ice instead of liquid to get the signal [14][15].
Along with IceCube being a terrific feat of engineering, it’s also brought about a few key discoveries, one of which was published in Nature Magazine on 10th March. It is in light (no pun intended) of the recent discovery of a rare interaction of neutrinos that’s called the ‘Glashow Resonance’, theorised in 1959 by theoretical physicist Sheldon Glashow. The theory goes that the ‘virtual’ particle known as a W- boson (a negatively charged particle which mediates the weak force, specifically in β- decay -neutron to proton-, letting the emission of antineutrinos happen) is formed by an electron and an electron antineutrino (a specific type of neutrino) colliding. This event then creates a characteristic shower of high-energy particles (emitting ~6.05 quadrillion electron volts, or ~969 microjoules -it’s a lot of energy when looking at particles!): evidence not only of Glashow resonance, but of it having a source in Outer Space [16][17].
Diagram of the IceCube Neutrino Observatory. Credit: IceCube Collaboration
We couldn’t talk about particles without having a bit on CERN, the European Organisation for Nuclear Research (it fits in French!). They’re famous for their Large Hadron Collider (LHC): the world’s largest most powerful particle accelerator [19], but, since it’s antimatter we’re talking about, to observe antimatter, they do the exact opposite. The Antiproton Decelerator (AD) slows down antiprotons to become a beam of antimatter to make antiatoms (atoms made of antimatter) in order to make antimatter (made up of antiatoms), having now made antihydrogen, with ambitions to make many more in the future. This will help scientists study this opposite world and find out what new properties antimatter has, so we can learn more about this less understood corner of particle physics, produced across the Universe [18].
CERN Antiproton Decelerator. Credit: Suaudeu, CC BY-SA 4.0
Why?
When neutrinos and antineutrinos interact with the water to produce Cherenkov radiation, a Cherenkov ring is formed. This ring then helps to pinpoint where the neutrino comes from in Outer Space. This then helps scientists get an early alert as to a supernova explosion, for instance, since neutrinos are given out before the main explosion occurs that’s seen from Earth [20]. One such event happened in 1987, where a neutrinos from a supernova explosion in the Large Magellanic Cloud were detected using Japan’s Super Kamiokande [21].
Map of sky with new sources of neutrinos highlighted in circles. Credit: IceCube Collaboration
There are also many other uses, from solar physics to atmospheric science, all helping in extending our understanding of this complex Universe.
By George Abraham, ADAS member.
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References
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