QMU–July 2012

The third lecture was given by Dr Ben Still on the neutrino and particle physics,


Particle physicists try to discover the fundamental elementary particles. Nature’s building blocks can be seen in the standard model. These appeared within a few seconds after the big bang.


Note that the Higgs hasn’t been added yet. Theory needs it to enable subatomic particles to have mass.

The most common quarks are up and down and the most common lepton is the electron. The other quarks and leptons are more massive, have more energy and are unstable. The bosons are force carriers. Photons (γ) carry the electromagnetic force. The weak nuclear force is confined to the nucleus and is carried by the Z and W bosons. Z bosons are neutral and have been likened to a heavy photon. W bosons carry charge. The gluon is the strong force and keeps the positive protons together. The neutrons in the nucleus provide extra force to overcome the electromagnetic force. Without it the protons would repel each other and the nucleus would fall apart.

Beta decay is the weak force in action. The weak interaction converts a neutron into a proton, beta particle and an electron antineutrino clip_image005 At the fundamental level this is caused by the conversion of a negatively charged down quark to a positively charged up quark by the emission of the Wboson. It is the W- boson that decays into the beta particle and the electron antineutrino.


The Feynman diagram for β− decay of a neutron into a proton, electron, and electron antineutrino via an intermediate W− boson.

It was through beta decay (weak nuclear decay) that the neutrino was first proposed by Wolfgang Pauli (in 1930). In reactions energy, momentum and angular momentum must be conserved and Pauli suggested the presence of the neutrino to enable this to happen in. Enrico Fermi, in 1833, coined the term neutrino (Italian for the “little neutral one”) when he developed the theory.

In 1942 Wang Ganchang suggested the use of beta-capture to experimentally detect neutrinos. In 1953 Reines, Cowan and colleagues set up project poltergeist using a nuclear reactor and discovered the neutrino in 1956 (they had to wait nearly forty years for the Nobel prize). In this experiment antineutrinos created by beta decay reacted with protons producing neutrons and positrons:

νe bar + p+ → n0 + e+


Clyde Cowan conducting the neutrino experiment c. 1956

The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events – positron annihilation and neutron capture – gives a unique signature of an antineutrino interaction.

Neutrinos are produced from radioactive materials such as potassium 40 and uranium, particle accelerators, nuclear reactors and the Sun (produces 1000000000000s per second. Supernova 197A produced ridiculous numbers of them. Whatever the source only 2 or 3 are ever detected at a time.

Because subatomic particles are so small their masses are not given in kg but in GeV (this unit is linked to Einstein’s famous equation E = Δmc^2). Proton mass is 0.938GeV, bottom quark mass is 4.2 GeV, Top quark mass is 175 GeV, electron mass is 0.0005GeV and the Tau mass is 1.78 GeV. At one time neutrinos were thought to be massless. We now know this is not true. If something has no mass it has no notion of time.

In 1962 Lederman, Schwartz and Steinberger showed that more than one type of neutrino exists by detecting interactions of the muon neutrino. When the third type of lepton, tau, was discovered in 1975 at the Stanford Linear Accelerator it was expected to have an associated neutrino (tau neutrino). The first detection of tau interactions was announced in 2000 by the DONUT collaboration at Fermilab.

Starting in the late 1960s, several experiments found that the number of electron neutrinos arriving from the Sun was between one third and one half the number predicted by the Standard Solar Model. This discrepancy (known as the solar neutrino problem) was resolved by the discovery of neutrino oscillation and the fact that neutrinos have mass. If neutrinos have mass they could change or oscillate between flavours. In other words an electron neutrino can change to a muon neutrino. The different neutrinos have different wave functions/probabilities etc.

A neutrino is now believed to be an electrically neutral, weakly interacting subatomic particle with half-integer spin. Its mass is so tiny that it has never been measured accurately (yet). It is not affected by electromagnetic forces but is affected by the weak force. This allows it to travel great distances through matter without being affected by it. Standing in a neutrino beam is harmless.

If neutrinoless double-beta decay is possible then it implies that the neutrino is a Majorana particle (antineutrino and neutrino are actually the same particle) and in essence the two neutrinos annihilate each other (the brilliant disappearing neutrino – destructive interference?).

The work on neutrinos has and is taking a very long time. Particles need to be viewed in situations many times. They need to be viewed in many different situations to understand their true nature. Is the dice loaded?

Atmospheric neutrinos results from the interaction of cosmic rays with atomic nuclei in the Earth’s atmosphere creating showers of particles, many of which are unstable and produce neutrinos when they decay.

clip_image010 The vμ bar is the muon neutrino.

A collaboration of particle physicists recorded the first cosmic ray interaction in an underground laboratory in Kolar Gold Fields in India in 1965.

T2K (Tokai to Kamioka, Japan) is a particle physics experiment to gain a more complete understanding of neutrino oscillation. Previous neutrino experiments have observed the disappearance of muon neutrinos in a beam as they oscillate to tau neutrinos, but oscillation from νμ to νe has not been observed. T2K hopes to be the first experiment to measure the appearance of electron neutrinos in a muon neutrino beam. Future upgrades to T2K could yield measurements of the CP violation phase by comparing oscillations of neutrinos to those of antineutrinos. CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry (charge conjugation symmetry) and P-symmetry (parity symmetry). CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle (C symmetry), and left and right were swapped (P symmetry). It plays an important role both in attempts of cosmology to explain the dominance of matter over antimatter in the present Universe, and in the study of weak interactions in particle physics.

At the start of the universe energy became mass (E = mc^2) and it would be expected that equal amounts of matter and antimatter would be produced if CP-symmetry was produced. This would immediately result in annihilation (total cancellation—protons should have cancelled with antiprotons, electrons with positrons, neutrons with antineutrons, and so on) with the production of high energy photons, therefore no galaxies, stars, planets or Earth. Since this is not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, i.e. violating CP-symmetry.

Muons behave differently to electrons. 100% muons will decay into a mixture of particles.


The dominant muon decay mode (sometimes called the michel decay after Louis Michel) is the simplest possible: the muon decays to an electron, an electron neutrino and a muon-neutrino. Antimuons, in mirror fashion, most often decay to the corresponding antiparticles: a positron, an electron-neutrino, and a muon-neutrino. In formualic terms, these two decays are:


References:                                                                                  http://en.wikipedia.org/wiki/Muon http://en.wikipedia.org/wiki/CP_violation http://en.wikipedia.org/wiki/Neutrino http://en.wikipedia.org/wiki/Big_Bang http://en.wikipedia.org/wiki/Standard_Model http://en.wikipedia.org/wiki/Beta_decay http://www.ias.ac.in/pramana/v67/p665/fulltext.pdf http://en.wikipedia.org/wiki/W_and_Z_bosons http://www.phys.psu.edu/~cteq/schools/summer09/talks/morfinCTEQ09.pdf http://en.wikipedia.org/wiki/Kaon_oscillation http://en.wikipedia.org/wiki/Neutrinoless_double_beta_decay http://en.wikipedia.org/wiki/Cosmic_ray http://en.wikipedia.org/wiki/T2K_experiment

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