How do we study neutrinos?
In order to measure the fundamental properties of neutrinos, it is necessary to observe neutrinos interacting, and for accurate measurements, it is necessary to observe a LARGE number of neutrino interactions. To achieve this, there are two key ingredients:
- a powerful source of neutrinos, like an artificial beam of neutrinos, which permits to control the properties of the incoming neutrinos
- a sensitive detector to capture neutrino interactions and study the physics behind it, and if the detector is big it is also easier to catch more neutrinos
Let’s see these two elements in more detail.
Creating an artificial beam of neutrino is as easy as pie:
- Protons from the accelerator smash into a graphite target – the same material of pencils – that is 95cm long, creating many new particles, called pions. These collisions make the target very hot, so it must be constantly cooled using Helium gas.
- The pions must then be collected and focussed into a narrow beam, which eventually will become a beam of neutrinos. This is done using a series of magnetic cones, called horns. The magnetic field is created by passing a large current of 320kA around the horn – this is enough to boil 32,000 electric kettles at the same time!
- The beam of pions travels into a 100m long steel tunnel. In this tunnel, the pions decay into two different particles: a muon μ, and a muon neutrino νμ. Now the neutrino beam is sent across Japan to be studied by the T2K experiment.
- At the end of the beamline there is a huge block of graphite weighing 75 tons – the same as 6 double decker buses! This absorbs all particles except neutrinos, acting in this way as a particle filter.
Extra credit: do you know why the neutrinos are not also absorbed?
When neutrinos interact with matter particles, they can create new particles. We detect these new, secondary particles and their properties can tell us about the original neutrino energy and direction. For instance, if a charged lepton is produced with one of the three existing flavours ( e, μ, or τ ), then we can easily determine the flavour of the associated neutrino flavour ( νe, νμ, ντ ).
The T2K experiment (Tokai-to-Kamioka) provides a great example of an experiment that produces a beam of neutrinos and detects them in a huge tank of water called Super-Kamiokande (SK), more than 180 miles away. As the muon neutrinos travel through the earth, they mysteriously change (or oscillate) to a different kinds of neutrino, called an electron neutrino, and back again. The proportion of the two kinds of neutrinos at any point depends on:
- the energy of the neutrinos,
- and how far they have travelled, so the distance.
We want to see the highest number of oscillations possible, so we must find the optimum distance for the detector location. For the T2K, the beam energy is of 600MeV and the best distance is about 300km away, so Super-Kamiokande is located near Toyama City in Western Japan. It takes about 3 hours to make this journey on the Japanese Bullet Train, but it only takes the neutrinos 1/100th of a second!
You can learn more with acceleratAR, an augmented reality app to investigate how neutrinos change based on the distance from their origin point. The cubes are your detectors – move them around Japan to see how the proportion of electron neutrinos (blue) and muon neutrinos (red) changes based on the distance from Tokai, where the neutrino beam is produced. Try changing the energy of the beam to see how this affects the oscillation of neutrinos. If you set the beam to half its energy, can you find where we would need to put Super-Kamiokande to detect the most electron neutrinos?
There are many types of neutrino detectors, using different materials for the target. One common type is a big water tank that detects Cherenkov radiation. When a particle travels faster than then speed of light in a given medium, it emits Cherenkov radiation. This is light that is light blue in colour and propagates in a cone shape. The light forms a ring shape when projected onto detector walls, from which precious information about the particle can be extracted.
**The particles only travel faster than light inside some materials… NOT in vacuum. So, this does not defy Einstein’s theory of special relativity.**
Neutrino detectors that look for Cherenkov radiation must be very efficient at collecting this light. The tanks of water have PMTs covering the walls. A Photomultipliers (PMTs) is a device that releases electrons when hit by a photon, which can in turn release other electrons and so on. A cascade of electrons is produced by a few photons forming a current that can be measured as an electrical signal proportional to the amount of light collected.
Super-Kamiokande is an example of detector of Cherenkov radiation in water. Built in 1991 in Japan, it consists of a larg..big..super big stainless-steel tank, 39.3m diameter and 41.4m tall, filled with 50,000 tons of ultra pure water.
The detector is located 1,000 meters underground in the Kamioka mine, in order to shield it from external radiation. About 13,000 PMTs are installed on the tank wall. These PMTs are 20 inches in diameter and you can see one in real life at our stand, as well as a scale model of the Super-K detector! Currently 150 people from Japan, the United States, Korea, China, Poland, Spain, Canada, UK, Italy and France all work on the detector and on analysing its output.
The detector was designed to study neutrinos from the sun, our atmosphere and supernovae as well as man made sources like the particle beams from the T2K experiment.
To date it has been running for 22 years and has made many discoveries about the nature of neutrinos and was instrumental in proving that each of the 3 types of neutrino oscillate. Today the experiment is being upgraded and will continue to operate, furthering our understanding of neutrinos and other interesting areas of Physics.
Hyper-Kamiokande is the next generation of water tank based neutrino experiment in Japan, due to start in 2026. The tank is 10 times larger with twice the number of PMTs and use new generation PMTs with greater sensitivity. This will allow us to make precision measurements of the oscillation properties discovered with Super-K as well as probe new phenomena like proton decay and detect much more distant supernova. We are currently designing and planning this next exciting step.
You can learn more playing with the Super-K VR experience: it will take you on a journey from the Sun to deep underground to see the Super-K tank. Here you see how many neutrinos pass through the tank every second and learn how neutrinos are detected from interactions with water and watch as the cone of Cherenkov light created from these interactions produces rings on the PMTs of the tank.