Information for the General Public
What are neutrinos?
During the 20th and 21st century, physicists have attempted to understand the fundamental building blocks of our universe. They have found evidence for many different particles that seem to make up the world about us. These particles come in several different sorts.
Bosons transmit forces. A photon is a type of boson.
Quarks often have high mass and interact very strongly with each other. They make up the heavy nuclei that sit at the centre of atoms.
Leptons are usually lower in mass and don't interact as strongly as quarks. Electrons that make up the outside of atoms are a sort of lepton. Neutrinos are also a sort of lepton.
The Standard Model of particle physics. Neutrinos are a type of lepton, shown with their charged partner-lepton in green.
Leptons come in two different types. There are charged leptons - electrons, muons and taus; and neutrinos - one for each of the charged leptons. Electrons are the most familiar lepton - these are the particles that sit on the outside of an atom to make up the rich tapestry of chemicals in the universe. Their cousins, the neutrinos, are ghost particles, which the Neutrino Factory is designed to study. In particular, neutrinos have several special properties that we would like to understand.
- Neutrinos hardly interact with matter at all. If I were to try to catch a neutrino it would take a lot of material before it slowed down.
- Neutrinos have hardly any mass. If I could give a neutrino a push, it wouldn't take much effort to make it travel very quickly.
- Neutrinos can change from one type to another. This strange flipping between different types of neutrino is called neutrino oscillation. Most physicists think that this oscillation occurs because neutrinos have mass.
- Anti-neutrinos may have different properties to neutrinos - we just don't know. This might partly explain why the universe is made mostly of matter and we don't see much antimatter.
- Thousands of billions of neutrinos pass through you every second! These neutrinos originate from cosmic rays travelling through space.
- Neutrinos are one of the least-studied particles. They are so difficult to make and detect that it is really difficult to do experiments with neutrinos.
More about the different sorts of particles can be found in the particle adventure: http://www.particleadventure.org/
Neutrino oscillations occur when a neutrino changes from one type to another. Neutrino oscillations are thought by many physicists to occur because neutrinos have mass. By measuring neutrino oscillations closely, we can calculate the mass of the neutrinos and other fundamental parameters such as whether there is an asymmetry between matter and antimatter in neutrinos. These parameters are fundamental constants of the universe because there is no known way of calculating them from other parameters. That means that it is important that we can measure them and understand them - and by understanding them we may be able to understand better why they take the values that they have.
What is a Neutrino Factory?
A Neutrino Factory is a special facility that we hope to build, designed for the study of neutrino oscillation. In a Neutrino Factory, physicists will make a beam of high energy muons. This is a sort of lepton, like a heavy electron. Muons are unstable particles that decay into neutrinos. By pointing the muons in the right direction as they decay, we can fire neutrinos at detectors on the far side of the earth. If the neutrino beam is measured before as it leaves the end of the muon accelerator and again as it comes out of the far side of the world, we can look to see if the mix of the beam has changed; and hence observe neutrino oscillations.
Neutrinos are manufactured at the muon facility, here shown in North America, and pass through the earth's mantle to a detector, here shown in India.
By using muons to make neutrinos, we can make a source of neutrinos that is pure, intense and high energy, and both neutrinos and anti-neutrinos can be produced. Each of these characteristics makes our measurements more precise; a pure beam means that we can understand the mixture of neutrinos in our beam very well; an intense beam means that we can look for oscillation with lots of neutrinos; and a high energy beam means that the chance of seeing each neutrino is higher. The presence of neutrinos and anti-neutrinos means that we can measure the difference between matter and antimatter. Together these factors make for a very precise experiment.
How do you build a Neutrino Factory?
A Neutrino Factory is constructed of many different parts. We need to make a beam of muons and then accelerate them up to high energy. Then we need to store the muons while they decay into neutrinos, and build detectors to measure the neutrino beam.
A schematic of the particle accelerator facility for muon production and acceleration. Protons are created in the proton driver, shown at the top, before being accelerated onto a target where pions are created. These decay to muons, which are accelerated through a number of sections before entering the muon storage ring where they can decay.
How can I make a muon beam?
Muons can be made from protons - one of the constituents of atomic nuclei. The protons are accelerated to high energy and then fired into a high energy target, where they make particles called pions, which quickly decay into muons.
The muon beam is initially very messy - when pions decay they fire muons in all sorts of directions with a broad range of energies. The muons are captured and controlled using very powerful magnets. Initially we seek to control the big range in energies using a special technique called energy-phase rotation to make the muon beam into many smaller bunches of particles with a smaller range in energies. After that we seek to make the muon beam more parallel. We do this using a technique called ionisation cooling. By controlling the energy spread of the beam and making it more parallel, we can make the beam ready to be accelerated.
How can I accelerate muons?
Acceleration is achieved using cavities filled with very intense electromagnetic fields that are oscillating very quickly. The cavities have millions of volts between one side and the other and this voltage flips hundreds of millions of times per second. The voltage needs to be as large as possible in order to accelerate the particles as quickly as possible and the field needs to flip so that muons are not slowed down by the voltage once they have left the far side of the cavity.
Three sorts of accelerators are used. It is harder to control the muons at low energy so we choose to accelerate in a straight line to start with. Once the muons get to higher energy, we can re-use the linear accelerators by recirculating muons through special tear-drop shaped accelerators. This allows us to use the same equipment several times making the accelerator cheaper. Finally we can accelerate the beam in rings, which is an even less expensive technology.
How do I make the muons into neutrinos?
The last part of the muon facility are storage rings. Muons decay at random times in the storage ring and most of the neutrinos fire along the direction of motion of the muons. We try to point the muons for as much time as possible to the neutrino detectors so that we can get as many neutrinos as possible into the detectors. This leads us to design rings that are racetrack-shaped and sloping steeply so that neutrinos travel into the mantle of the earth. While the top end of the rings is at ground level, the bottom end is many hundreds of metres below the earth's surface.
Typically we would want to measure the neutrinos at more than one distance from the muon facility, so we would split the muon beam after acceleration and divert it into two or more storage rings. Also we can calculate the properties of the neutrino beams as muons decay by carefully measuring the muon beams so the storage ring also contains several instruments to precisely measure the muon beams.
How do I detect neutrinos?
There are several sets of detectors that measure the properties of the neutrinos. The properties of the neutrino beams at the muon facility using a Near Detector. Then the neutrino beams are measured where they leave the earth in Far Detectors.
The detectors work by measuring particles such as electrons that are produced when neutrinos hit the material. These particles can be detected in a number of different ways. One method is to put material called scintillator into the detector. Scintillator emits a tiny flash of light every time a particle passes through, which can be detected using very sensitive cameras. Another method is to measure Cerenkov light. This is a form of light that some particles emit in a cone along their direction of travel; it can be measured in another sort of sensitive camera. Also it is possible to put photographic emulsion in the path of the particles. The particles will leave a tiny defect in the emulsion, similar to the process which makes a photograph using light photons. This defect can be seen by eye or measured using automated robots. A final method to detect these particles is to put high voltage cables through some material. The passage of particles causes sparks to form at the cables which produces an electric signal that can be measured.
A schematic of a Magnetised Iron Neutrino Detector. Alternating plates of steel and scintillator are used to study particles created as neutrinos interact with the detector material.
These detectors share several features. They are usually magnetised so that we can tell the difference between matter and anti-matter. Charged particles travel in circles in magnetic fields. Anti-particles of charged particles have the opposite charge, which means that they circulate in the opposite direction. By measuring the direction of circulation of particles we can assess the charge and so examine whether particles are matter or anti-matter, and so assess whether the original particle in the neutrino beam was a neutrino or anti-neutrino.
In addition the detectors have very large mass. Neutrinos only interact with matter very rarely. By making the detectors very massive, we can increase the chance that a neutrino will interact with the detector and can be observed. This increases the number of neutrinos that we can measure and so improves the sensitivity of our measurement.