The Standard Model and Neutrinos
The Standard Model
The discovery of the electron in 1897 by J J Thomson [1] led to the birth of what is currently known as elementary particle physics. Previously it was believed that atoms were the fundamental building blocks of matter but by probing the atom to smaller and smaller length scales, it was discovered that the atom itself had structure. These experiments and their discoveries have led to the development of the standard model of particle physics. The Standard Model attempts to explain all known matter particles and their interactions [2]. According to this model, all matter is composed of quarks and leptons and their antiparticles (all being spin- 1/2 fermions) and they interact by exchanging force carrier particles (which are all bosons, i.e. integer-spin particles) . A summary of the quarks and leptons can be seen in Table 1 and Table 2. A summary of the four fundamental forces and their force carrier particles can be seen in Table 3.
Table 1: Properties of quarks (u, up; d, down; c, charm; s, strange; b, bottom; t, top): I, isospin; S, strangeness; C, charm; Q, charge; B, baryon number; B*, bottom; T, top [2].
| Flavour | Spin | B | I | I3 | S | C | B∗ | T | Q[e] |
| u | 1/2 | 1/3 | 1/2 | 1/2 | 0 | 0 | 0 | 0 | 2/3 |
| d | 1/2 | 1/3 | 1/2 | -1/2 | 0 | 0 | 0 | 0 | -1/3 |
| c | 1/2 | 1/3 | 0 | 0 | 0 | 1 | 0 | 0 | 2/3 |
| s | 1/2 | 1/3 | 0 | 0 | -1 | 0 | 0 | 0 | -1/3 |
| b | 1/2 | 1/3 | 0 | 0 | 0 | 0 | -1 | 0 | -1/3 |
| t | 1/2 | 1/3 | 0 | 0 | 0 | 0 | 0 | 1 | 2/3 |
Table 2: Properties of leptons (e−, electron; νe, electron neutrino; μ−,muon; νμ, muon neutrino; τ−, tau; ντ , tau neutrino); Q[e], charge; Le, Lμ,Lτ, flavour-related lepton number; L = Le+Lμ+Lτ, total lepton number [2].
| Lepton | Q[e] | le | Lμ | Lτ | L |
| e- | -1 | 1 | 0 | 0 | 1 |
| νe | 0 | 1 | 0 | 0 | 1 |
| μ− | -1 | 0 | 1 | 0 | 1 |
| νμ | 0 | 0 | 1 | 0 | 1 |
| τ− | -1 | 0 | 0 | 1 | 1 |
| ντ | 0 | 0 | 0 | 1 | 1 |
Table 3: Phenomenology of the four fundamental forces [2].
| Interaction | Exchange Particle | Example |
| Gravitation | Graviton | Mass attraction |
| Weak | W+, W-, Z0 | β-decay |
| Electromagnetic | Photon | Force between electric charges |
| Strong (nuclear) | Gluons | Nuclear forces |
| Strong (colour) | Gluons | Forces between the quarks |
The elementary forces along with these elementary building blocks seem sufficient to describe the underlying physics of all phenomena observed so far. Since the model was formulated, there appears to be a number of fundamental problems with the model’s details: ”The theory just has far too many arbitrary parameters and mysterious relationships to be the final one.” [3], thus indicating that new Physics beyond the Standard Model is required.
Neutrinos
A neutrino (denoted by the symbol ν) is a type of lepton (i.e. it does not participate in strong interactions) and has zero charge (so does not undergo electromagnetic interactions) which makes them very difficult to detect. However, neutrinos do interact via the weak force and are created as a result of certain types of radioactive decay or nuclear reactions such as those in the sun and nuclear reactors. There are three types, or ”flavors”, of neutrinos: electron neutrinos (νe), muon neutrinos (νμ) and tau neutrinos (ντ); each type also has an antimatter partner, an antineutrino. Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, two forms of beta decay.
Neutrinos have provided evidence that new physics beyond the standard model exists. According to the standard model, these neutrinos (little neutral ones) are massless. The model does not predict that neutrinos have a non-zero mass and mix among the neutrinos in different families, although mixing had long been anticipated as it parallels the behaviour seen in quarks. The fact that neutrinos do have a non-zero mass and mix has been shown to be true experimentally.
The Standard model held up to some scrupulous testing over the years. Recently though, there has been evidence contrary to the predictions made by the Standard Model. Neutrino oscillation experiments, along with solar- and atmospheric-neutrino experiments suggest that muon neutrinos can transmute in to electron neutrinos and vice versa on their travels. It appears that electron neutrinos arriving at the Earth from the centre of the sun would be too few in number, although the total number of expected neutrinos would be present. This phenomena of neutrinos ”oscillating” from one flavour to another is only possible if different neutrino types have different masses. Therefore, oscillation experiments are proof that neutrinos must have mass.
Neutrino oscillation experiments are unable to measure the absolute mass of the neutrino but can only measure the square of the mass differences. Although neutrino oscillation experiments are unable to determine the absolute mass scale of the neutrino, double beta decay may offer the capabilities to do so.
References
[1] E. A. Davis and I. J Falconer, J. J. Thomson and the Disovery of the Electron, CRC Press (1997)
[2] Kai Zuber, Neutrino Physics, Institute of Physics (2004)
[3] N. Cooper ed., Los Alamos Science Magazine, Celebrating the Neutrino, 25 (1997); http://library.lanl.gov/cgi-bin/getfile?number25.htm
