The Neutrino: The Key to Understanding the Universe

By Kate Britton, Ciaran Furey, Aoife O’Kane Hackett and Sophie Thomson

Everything around you – your laptop screen, your clothes, even your own body – all have one thing in common: they are composed of tiny building blocks called atoms. It was long thought that these were the most fundamental particles in the universe; the name ‘atom’ is derived from the Greek word ‘atomos,’ meaning ‘indivisible.’ However, it was further realised that atoms are composed of even smaller particles; a dense nucleus containing neutrons and protons, with electrons surrounding this nucleus.  Later experiments in 1968 involving the scattering of protons and electrons at extremely high velocities in the Stanford Linear accelerator found that, by examining how the electron ‘bounced’ off the proton, the proton was composed of even smaller particles called quarks. It is believed that quarks, and other particles like them, represent the smallest scale of the universe, so The Standard Model of particle physics has been devised. These elementary particles represent the fundamental LEGO blocks of the universe, if you like. However, the smallest, strangest, and most elusive of these is the neutrino, which will be the topic of this blog. 

What are neutrinos?

Neutrinos are elementary particles produced by the radioactive decay of larger, unstable particles. The nature of neutrinos is described poetically by John Updike in his poem “Cosmic Gall” [1]:

Neutrinos, they are very small.

They have no charge and have no mass

And do not interact at all.

The earth is just a silly ball

To them, through which they simply pass,

Like dustmaids down a drafty hall

Or photons through a sheet of glass.

Figure 1 – The Standard Model

Although well described, he was using a bit of poetic license here.  They are not completely massless, however their mass is very small, with the upper limit of the electron neutrino being 2 eV/c²  (note how I said upper limit; neutrinos are so difficult to detect than only an upper limit may be placed on their mass.) For comparison, the mass of the up quark, which is found in protons and neutrons, is 2.2 MeV/c², roughly one million times heavier than the electron neutrino. Saying they do not interact at all is not true, although it is incredibly rare they do interact with regular matter. This is because they have no electric charge, and so do not interact by the strong nuclear force or electromagnetic force, meaning they only interact with matter via the weak nuclear force and gravity. In fact, with an impressively small cross section of 10^-48 m² , a neutrino would have to pass through a few parsecs of lead before it even has a 50% chance of interacting with another atom!

As well as this, neutrinos come in three different ‘flavours:’ the electron neutrino, as was just discussed, the muon neutrino, and the tau neutrino. These are shown in the bottom row of the Standard Model, which is shown in Figure 1. 

So how are they detected?

Neutrinos are rightfully referred to as one of the most elusive fundamental particles there is. In the space of one second, around 100 trillion neutrinos pass through us. This goes unnoticed by us as the absence of electronic charge in neutrinos and their miniscule mass mean that they barely interact with any matter at all. Only one in every ten billion neutrinos crossing the Earth’s path successfully interacts with an atom. This makes direct neutrino detection extremely difficult with our current level of knowledge about them.

Figure 2 – The Super-Kamiokande Detector

However, the few neutrinos that do interact with atoms can be detected indirectly. This is the principle of the operation of the Super-Kamiokande detector in Japan, shown in Figure 2. The neutrinos in the Super-Kamiokande detector are observed in a 40m tall water tank containing 50,000 tonnes of ultra-pure water. When a neutrino interacts with matter, a charged particle is generated, and this particle can be detected. The vast body of water acts as a huge target and can increase the number of interactions between neutrinos and nucleons or electrons. When the generated charged particle travels faster than the speed of light in water (225,000 kilometers per second), cone shaped Cherenkov light is emitted in the direction of the charged particle. This light is detected by some of the 13,000 light detectors, called photo-multiplier tubes located in the walls of the detector. Depending on the quantity of light detected, and the timing of the detection, useful information can be extracted, such as the energy of the particle, the direction it was travelling and the location of interaction.

Figure 3 – The IceCube Detector

‘IceCube’ is another neutrino detector located in Antarctica. IceCube detects neutrinos based on the same principles as Super-Kamiokande, by detecting the by-products of the interactions between neutrinos and matter. IceCube measures the light produced by secondary particles when neutrinos interact with the South Pole ice. The amount of light and the pattern it produces allows scientists to estimate the energy, direction, and identity of the original neutrino.

What have neutrino detections told us?

Neutrino detection has shown us that the standard model of particle physics shown in Figure 1 is incomplete. Up until recently, the Standard Model had been predicated on massless neutrinos. However, experiments carried out in the Super-Kamiokande detector showed that neutrinos oscillate between their three flavours. This means that there is a chance that one flavour of neutrino may change flavour, or oscillate, into another flavour neutrino.  This implies that they must have mass, so new theories must now be formed to explain this.

This discovery of neutrino oscillation also solved a mystery that lingered in the astronomy community for quite some time: The Solar Neutrino Problem. In 1970, astrophysicist Raymond Davis devised an experiment that would detect neutrinos produced by the Sun, however, only one third of the expected amount were detected. These results were unexplained until recently, when the mechanism of neutrino oscillation was discovered. The reason this capture rate was observed was because this detector was only sensitive to electron neutrinos, so by the time they had reached the detector, they would have transformed into the other neutrino flavours.

In 1987, the detection of neutrinos from the supernova SN1987A allowed the astrophysics community to watch the formation of a neutron star, which had not been done before. This detection represents the first time neutrinos were observed from an astronomical source other than the sun, and also verified the basic theory of core-collapse supernovae.

What next for neutrino physics?

So the detection of neutrinos have already provided us with verification of our current as well as even modifying them. What next?

Figure 4 – The CMB

Scientists have already detected and documented the Cosmic Microwave Background (CMB); the oldest detectable electromagnetic radiation, produced roughly 380,000 years after the Big Bang where matter in the universe cooled enough to become transparent to photons. This detection strongly indicates that our theory of the Big Bang is indeed correct. Given this, if we go further back in time, we can imagine a period of time where the universe cooled enough to become transparent to neutrinos, resulting in the Cosmic Neutrino Background (CNB). Using our current knowledge of cosmology, and assuming the existence of the CNB, calculations [1] reveal that the total number density of CNB neutrinos is 9/11 times the number density of CMB photons. The detection of these neutrinos will further strengthen the Big Bang theory and reveal what conditions were like shortly after the beginning of the universe.

Future neutrino detections may also alter the Standard Model even more. In 2018, the MiniBooNE Short-Baseline Neutrino Experiment at Fermilab produced compelling results that indicate that this may be about to happen [5]. This experiment involved directing a beam of muon neutrinos into a 12.2 m diameter sphere filled with 818 tonnes of pure mineral oil, located 541 m from the neutrino source. However, a large excess of electron neutrinos was observed. This is a result of the muon neutrinos oscillating to the electron neutrino state.  However, to our current understanding, this event is extremely unlikely to occur over this relatively short distance, so at least four neutrino types are required for this oscillation to occur, indicating physics “beyond the three neutrino paradigm”. Even though this is reported to extremely high significance (6σ), these results are currently being further investigated [6]. 

In order to realise these future discoveries and to address the more fundamental questions of the universe, a new generation of neutrino detectors are being proposed, with one of the main ones being the Deep Underground Neutrino Experiment (DUNE), with the first detector planned for installation in 2024. 

This will consist of two detectors, one “near detector” underground at Fermilab, where neutrinos will be produced, and one “far detector” 1.5km underground at the Sanford Underground Research Facility (SURF) in South Dakota USA, 1300km from Fermilab. The far detector will detect neutrinos by means of a liquid argon time capture chamber. This will make it possible to detect neutrinos and document their interactions with image-like precision by producing a bubble-chamber-like image. This project has 3 primary goals [7]. The first is to carry out comprehensive oscillation measurements, which may describe the matter-antimatter asymmetry in the universe. The second is to observe proton decay. With a constrained half life on the order of 10³³ seconds, observation of this decay would satisfy a key requirement for the grand unification of the fundamental forces. And the third is to measure the electron neutrino flux from core-collapse supernovae, which would provide new information about the early stages of core collapse and even indicate the formation of a black hole. 

Hopefully this blog has given you some insight into the elusive nature of the neutrino, as well as an appreciation for the extreme measures physicists have to go to to detect these ghost-like particles. Also, now you should know that understanding neutrinos is the key to understanding the universe on the most fundamental levels, so next time you here of a ground breaking theory surfacing, don’t be surprised when you hear that it was a result of a neutrino detection!

References

[1] Ryden, B., 2017. Introduction to cosmology. Cambridge University Press.

[2] IceCube. 2021. IceCube and Neutrinos. [online] Available at: <https://icecube.wisc.edu/outreach/neutrinos/> [Accessed April 2021].

[3] Www-sk.icrr.u-tokyo.ac.jp. 2021. Experimental Technique | Super-Kamiokande Official Website. [online] Available at: <http://www-sk.icrr.u-tokyo.ac.jp/sk/detector/cherenkov-e.html> [Accessed April 2021].

[4] Bartels, M., 2018. Here’s Why IceCube’s Neutrino Discovery Is a Big Deal. [online] Space.com. Available at: <https://www.space.com/41142-what-are-neutrinos-why-they-matter.html> [Accessed April 2021].

[5] Aguilar-Arevalo, A.A., Brown, B.C., Bugel, L., Cheng, G., Conrad, J.M., Cooper, R.L., Dharmapalan, R., Diaz, A., Djurcic, Z., Finley, D.A. and Ford, R., 2018. Significant excess of electronlike events in the MiniBooNE short-baseline neutrino experiment. Physical review letters, 121(22), p.221801.

[6] Aguilar-Arevalo, A.A., Brown, B.C., Conrad, J.M., Dharmapalan, R., Diaz, A., Djurcic, Z., Finley, D.A., Ford, R., Garvey, G.T., Gollapinni, S. and Hourlier, A., 2021. Updated MiniBooNE neutrino oscillation results with increased data and new background studies. Physical Review D, 103(5), p.052002.

[7] Abi, B., Acciarri, R., Acero, M.A., Adamov, G., Adams, D., Adinolfi, M., Ahmad, Z., Ahmed, J., Alion, T., Monsalve, S.A. and Alt, C., 2020. Deep Underground Neutrino Experiment (DUNE), far detector technical design report, Volume II DUNE physics.