Neutrino Physics

The Standard Model

We now know that all the elementary particles — six quarks and six leptons — are grouped into three families or generations. Indeed, precision experiments at the Large Electron Positron (LEP) collider at CERN in Switzerland have demonstrated that there are exactly three generations. Everyday matter is built from members of the lightest generation: the up and down quarks that make up protons and neutrons; the electron; and the electron neutrino involved in beta decay. The second and third generations comprise heavier versions of these particles with the same quantum numbers. The analogues of the electron are called the muon and the tau, while the muon neutrino and tau neutrino are equivalent to the electron neutrino. Each particle also has a corresponding antiparticle with opposite electric charge. In the case of neutrinos, the antineutrino is neutral but right-handed.

The Standard Model also includes a set of particles that carry the forces between these elementary particles. Photons mediate the electromagnetic force; the massive W+ and W- particles carry the weak force, which only acts on left-handed particles and right-handed antiparticles; and eight gluons carry the strong force.

All the particles that make up matter have mass — from the lightest, the electron, to the heaviest, the top quark — and can be left- or right-handed. Although the Standard Model cannot predict their masses, it does provide a mechanism whereby elementary particles acquire mass. This mechanism requires us to accept that the universe is filled with particles that we have not seen yet.

No matter how empty the vacuum looks, it is packed with particles called Higgs bosons that have zero spin (and are therefore neither left- or right-handed). Quantum field theory and Lorentz invariance show that when a particle is injected into the "vacuum", its handedness changes when it interacts with a Higgs boson (figure 2a). For example, a left-handed electron will become right-handed after the first collision, then left-handed following a second collision, and so on. Put simply, the electron cannot travel through the vacuum at the speed of light; it has to become massive. Similarly, muons collide with Higgs bosons more frequently than electrons, making them 200 times heavier than the electron, while the top quark interacts with the Higgs boson almost all the time.

This picture also explains why neutrinos are massless. If a left-handed neutrino tried to collide with the Higgs boson, it would have to become right-handed. Since no such state exists, the left-handed neutrino is unable to interact with the Higgs boson and therefore does not acquire any mass. In this way, massless neutrinos go hand in hand with the absence of right-handed neutrinos in the Standard Model.


(a) According to the Higgs mechanism in the Standard Model, particles in the vacuum acquire mass as they collide with the Higgs boson. Photons (γ) are massless because they do not interact with the Higgs boson. All particles, including electrons (e), muons (μ) and top quarks (t), change handedness when they collide with the Higgs boson; left-handed particles become right-handed and vice versa. Experiments have shown that neutrinos (ν) are always left-handed. Since right-handed neutrinos do not exist in the Standard Model, the theory predicts that neutrinos can never acquire mass. (b) In one extension to the Standard Model, left- and right-handed neutrinos exist. These Dirac neutrinos acquire mass via the Higgs mechanism but right-handed neutrinos interact much more weakly than any other particles. (c) According to another extension of the Standard Model, extremely heavy right-handed neutrinos are created for a brief moment before they collide with the Higgs boson to produce light left-handed Majorana neutrinos.

  • Introduction
    Neutrinos are everywhere. Trillions of them are passing through your body every second,but they are so shy and we do not see or feel them. They are the least understood elementary particle we know that exist.
  • Birth of Neutrinos
    Existing of neutrinos was suggested as a "desperate remedy" to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei.
  • The Standard Model
    The Standard Model of particle physics can describe everything we know about elementary particles. It says that neutrinos do not have mass. Neutrinos do not have mass because they are all "left-handed" and do not bump on the mysterious "Higgs boson" that fills our entire Universe.
  • Evidence for neutrino mass
    In 1998, a convincing evidence was reported that neutrinos have mass. The Standard Model has fallen after decades of invicibility. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines.
  • Implications of neutrino mass
    Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lighest elementary particle: electron. How do we need to change the Standard Model to explain the neutrino mass? Some argue that our spacetime has unseen spatial dimensions, and we are stuck on three-dimensional "sheets". Other argue that we need to abandon the sacred distinction between matter and anti-matter.
  • Why do we exist?
    When Universe started with the "Big Bang", there were almost equal amount of matter and anti-matter. Most of matter was annihilated by anti-matter when Universe cooled. We are leftover of one part in ten billions. Why was there a small excess matter over anti-matter so that we can exist? Once we abandon the sacred distinction between matter and anti-matter, it provides a key to understand why we exist.
  • Outlook
    The mysteries about neutrinos are now being unraveled dramatically. We will learn much more in the coming years.
This page is based on Feature Article "Origin of Neutrino mass" in Physics World, May 2002, by Hitoshi Murayama. The whole article can be download as a PDF file.