Edited by Sanaya Narula.
Particle Accelerator. https://aeon.co/_next/image?url=https%3A%2F%2Fomicron.aeon.co%2Fimages%2F25f77085-5ca4-40d0-a711-9383ff491a06%2Fidea_sized-hadron-collider.jpg&w=3840&q=75
In high school, we are taught that all matter is made up of atoms which are composed of subatomic particles:protons, neutrons and electrons.
That’s all true, but it’s not the full picture. There are actually a multitude of more subatomic particles such as bosons and fermions which are a mystery still being explored today! This is called particle physics and it can answer our deepest questions, such as the origin of the universe, or tell us about the craziest distant supernovae.
Neutrino Who?
Here’s a fun fact: at this very moment, there are trillions of subatomic particles called neutrinos passing through you. This bizarre particle is one of the biggest mysteries in particle physics because of its incredible properties.
Neutrinos don’t interact with matter, which means they can penetrate through objects- similar to how Casper the Ghost can fly through a wall. At the same time, they’re lightweight, having a mass of only a millionth of the mass of an electron¹ and they travel at amazing speeds near to the speed of light².
All of these properties packed together into one little particle allow neutrinos to travel for billions or trillions of years while carrying valuable information about where they came from.
Origin Story
You might be wondering: where do these particles come from? The most prominent source of neutrinos is nuclear reactions that take place in the cores of stars like our Sun.³ In a nutshell, neutrinos are generated when a neutron in an unstable nucleus splits to form a proton and an electron (known as beta decay). Therefore, anytime a cosmic ray strikes an atom or a stellar explosion happens, you are likely to find neutrinos flying all over the place.
Human advancements in technology and the understanding of physics have opened doors for us to generate our own neutrinos through particle accelerators such as the Large Hadron Collider at CERN in Switzerland⁴. In these mega machines, particles are made to accelerate until they approach the speed of light and collide with each other, causing other forms of subatomic particles to be produced. These human-generated ‘Accelerator Neutrinos’⁵ allow us to further research some of the big questions surrounding neutrinos.
Studying Neutrinos
The only way we can begin to understand and research neutrinos is by detecting them — that’s why billions of dollars have gone into producing neutrino detectors all over the world. However, remember how I mentioned that neutrinos don’t interact with matter? This makes it extraordinarily difficult to detect them. If trillions of neutrinos pass through Earth, without leaving a trace, how would we ever go about detecting them?
Technically neutrinos do sometimes interact with matter- just not ordinary matter. They are able to interact with weak forces and gravity³ in extremely rare conditions which makes detection nearly impossible because of the minuscule probability of a collision taking place. 20th century physicists solved this problem by coming up with a few nifty tricks as to how we can increase the likelihood of neutrino collision and be able to abstract data from them.
A common neutrino detection method is carried out using a huge tank of water. And by huge, I mean 50 000 tonnes worth of water huge. How does this work? Light travels at a slower speed in water than in a vacuum. If a neutrino bumps into an electron, it causes the electron to travel through the tank at a speed faster than the light travelling in water. This causes a “light shockwave” to be produced, similar to how a sonic boom is produced when a fighter jet exceeds the speed of sound.⁶
An example of this approach is the Super Kamiokande in Japan, which is a remarkable feat of engineering. The tank is surrounded by thousands of beach ball sized light-bulbs which illuminate when “light shockwaves” are present.
There is also a radiochemical method to detect neutrinos, as a collision between a neutrino and an unstable isotope causes a new isotope to be formed.³ However, this approach is less common and not widely used.
Solar Neutrino Drama
Everyone was off to work to research neutrinos when detectors started providing readings of neutrino emissions from the Sun which were only 1/3 or 2/3 of the expected amount. This posed a huge dilemma in particle physics as it directly disproved our understanding of neutrinos.
After 30 years of the community drafting conspiracy theories and formulating new equations, a team from the Super Kamiokande and Sansbury Neutrino Detector⁷ realised that the problem was because our detectors were only detecting muon neutrinos(which are emitted by the Sun).
They discovered a phenomenon which was dubbed neutrino oscillation and won the 2015 Nobel Prize in Physics.⁷ Let me explain.
Shape-Shifters
There are actually three types of neutrinos which are the electron neutrino, muon neutrino and tau neutrino, and they can shapeshift into each other! This means that a muon neutrino emitted by the Sun continuously ‘oscillates’ between the three types during its flight to Earth. This is another incredible property of neutrinos as no other particle can do this!
Hypothetical Particles
Those were all the solved mysteries in the world of neutrinos, so what’s left hidden?
This is what's called the Standard Model which is basically a periodic table but for particle physics.
If you were to peer at the neutrinos section attentively, which is under Leptons, the types of neutrinos are limited to the three that we discussed. However, in reality and in theory, these are only left-handed spin neutrinos.
What does this mean? Imagine a neutrino as a football flung into the air. The football is likely to spin in one direction as it flies. If it spins with clockwise rotation, it’s a left-handed spin but it is possible that it can spin in an anticlockwise rotation, forming a right-handed spin.⁸
However, we are only able to detect left-handed spin neutrinos because they interact with weak forces and gravity, as we discussed. These right-handed spin neutrinos are a complete mystery because we are unable to detect them as they don’t interact with weak forces.⁸ This is called the Sterile Neutrino problem.
Remember how neutrinos oscillate between their flavours? Some scientists have suggested that neutrinos may also be able to oscillate between types (ie. left-handed and right-handed).⁹
There have been abnormalities in three neutrino detection results, especially the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab, that might be pointing towards this theory. However, a second attempt by the Micro Booster Neutrino Experiment (MicroBoonE), didn’t record any abnormalities, which sparked confusion in the scientific community(9). More tests are being carried out today to investigate sterile neutrinos, and it is likely those abnormalities were caused by something much more complicated that we have yet to discover.
“If they exist, sterile neutrinos could help solve several mysteries in physics, such as why neutrinos have mass when theories predicted they should not and what the invisible dark matter pervading the cosmos is made of.”- Scientific American⁹
As a takeaway, we remain optimistic and excited about future discoveries in the awesome world of particle physics in the hope that we gain a greater understanding and appreciation of the universe we live in. In fact, it just may happen that by the time you’re reading this, the table of the Standard Model might look completely different.
References:
Mertens, S. Direct Neutrino Mass Experiments. (2016). NASA/ADS. https://ui.adsabs.harvard.edu/abs/2016JPhCS.718b2013M/abstract
Wikipedia contributors. (2022, April 16). Measurements of neutrino speed. Wikipedia. https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed
Wikipedia contributors. (2022, May 12). Neutrino. Wikipedia. https://en.wikipedia.org/wiki/Neutrino
CERN. (2022, May 18). CERN Neutrino Platform. https://home.cern/science/experiments/cern-neutrino-platform
Wikipedia contributors. (2022, May 31). Accelerator neutrino. Wikipedia. https://en.wikipedia.org/wiki/Accelerator_neutrino
Minute Physics. (2022). Know about the properties and ways to detect neutrinos. Encyclopedia Britannica. https://www.britannica.com/video/185553/detection-properties-neutrino
Taroni, A. (2015, October 6). Nobel Prize 2015: Kajita and McDonald. Nature. https://www.nature.com/articles/nphys3543?error=cookies_not_supported&code=ac2cb91d-82c9-4e57-94bd-e177580e7ce7
PBS Space Time. (2018, July 4). YouTube. Will A New Neutrino Change The Standard Model? https://www.youtube.com/watch?v=0mXW1zPlxEE
Moskowitz, C. (2021, November 4). Can Sterile Neutrinos Exist? Scientific American. https://www.scientificamerican.com/article/can-sterile-neutrinos-exist/
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