Particle physics is the study of the elementary constituents of matter and radiation, and the reactions between them. It is also known as 'high energy physics', as many of these particles do not occur naturally, but can be created and observed during energetic collisions of other particles.
Neutrinos belong to the family of particles known as leptons. They are electrically neutral, i.e. they have no charge, and have a very small mass in comparison to other particles. They only interact via the weak force and gravitation. Neutrinos can contain one of three different lepton 'flavours' – electron; tau and muon.
Their existence was first predicted by Wolfgang Pauli in 1930, to solve the energy conservation problem during beta decay - he suggested that the missing energy was carried off by this new, neutral particle. Because they interact so weakly with matter, Pauli bet a case of champagne that nobody would ever find one. This held true until 1956, when Clyde Cowan and Fred Reines observed antineutrinos being emitted from a nuclear reactor in South Carolina, USA. Pauli kept his promise when they announced their findings.
The name 'neutrino' literally means 'little neutral one' in Italian.
Trillions of neutrinos actually pass through your body every second, although we don't see or feel them.
If, for some reason, you wanted to completely stop a neutrino you would need to accumulate at least one light-year's worth of lead.
The Super KamiokaNDE detector in Japan, shown below, was the first to provide evidence of neutrino oscillations in atmospheric neutrinos, in 1998. This also implied that neutrinos had mass, as otherwise, such oscillations would not be possible.
The Super-Kamiokande detector consists of 50,000 tons of ultrapure water in a tank 1000 metres below ground in central Japan. When a neutrino reacts with the electrons or nuclei of water molecules, a particle that moves faster than the speed of light in water may be produced. This creates a cone of light known as Cerenkov radiation. This light is detected by the PMT's which line the inside of the detector, and such measurements by the PMT's are used to reconstruct the energy, direction and starting point of the particle. Details of the pattern of light are used to distinguish between muon-neutrino and electron-neutrino interactions.
Neutrino oscillation is a phenomenon, whereby a neutrino with a certain lepton flavour is later observed to have changed flavour, for example a muon neutrino may be later observed as an electron neutrino. Neutrino oscillations provide proof that neutrinos have mass – for a long time they were believed to have no mass at all.
Atmospheric neutrinos are neutrinos which are produced when cosmic rays collide with the nuclei of the atoms which make up the Earth's atmosphere.
(The image on the right is of the inside of the Super Kamiokande detector - see 'neutrinos' from above )
Beta decay is a type of radioactive decay in which a beta particle ( an electron or positron ) is emitted.
When an electron is emitted, it is known as "beta minus" or β -. During this, the weak interaction converts a neutron into a proton, emitting an electron and an anti-neutrino:
When a positron is emitted, it is known as "beta plus" or β +. In this decay, a proton is converted into a neutron, a positron and a neutrino:
Cosmic rays originate from outer space, and are composed of high-energy particles ( for example, protons, helium nuclei and leptons ), as well as gamma rays.
A cosmic ray shower, shown on the right, is when a primary particle interacts with dense matter, creating a 'shower' of secondary particles with less ebergy. These secondary particles go on to react in the same way, and so on, until many low-energy particles are produced, which are then stopped and absorbed by matter.
The Standard Model is the current theory of particle physics, which can be used to describe most of the experimental results so far. It is based on three assumptions:
- That matter is made up of two groups of fundamental particles: 'quarks' and 'leptons'.
- That all particles have characteristic charges which control their interactions, or the forces acting between them. These forces are mediated by exchange particles specific to each force.
- That all particles are given mass by the 'Higgs mechanism' – although this has not yet been proven. The theory behind this mechanism requires the presence of the 'Higgs boson', which has not yet been found. Coming experiments at the new LHC ( large hadron collider ) at CERN will hopefully provide evidence of the Higgs Boson.
As can be seen from the chart, there are 6 quarks ( shown in green ) and 6 leptons ( shown in blue ). The exchange particles, or force carriers, are shown in red.
Often referred to as PMTs, they are very sensitive devices which are used to detect light. Single photons can be detected due to the PMTs' ability to multiply the signal produced by the incident light. Various PMT's used for the Aberdeen Tunnel Experiment:
Flux is the amount of 'flow' per unit time, so for example, neutron flux would refer to the number of neutrons detected in a certain unit of time.
A proportional counter, or PropTube, is a gas-filled radiation detector which is used to count particles of ionizing radiation, and to measure their energy. The signal produced by the PropTube is proportional to the energy deposited by the ionizing radiation which passes through it.
A device or substance which absorbs high energy elctromagentic, or charged particle radiation, and then in response, fluoresces photons to release the energy it has absorbed from the radiation. The light produced can then be detected using apparatus such as photomultiplier tubes.