Neutrino Physics


It is an exciting time for neutrino physics. Many experiments are currently under way — or are being constructed or planned — to put the evidence for neutrino mass on a more solid footing. Physicists prefer to use "man-made" neutrinos produced by accelerators or in nuclear reactors because these neutrinos can be controlled, unlike atmospheric or solar neutrinos.

The difficulty is that neutrinos only appear to oscillate over long distances, thereby motivating a series of so-called long-baseline experiments. The K2K experiment in Japan has already been running for a few years. It involves firing a beam of muon neutrinos produced in an accelerator at the KEK laboratory towards the SuperKamiokande detector, some 250 km away. So far the experiment has detected the disappearance of muon neutrinos due to neutrino oscillations, which is completely consistent with what we have learned from atmospheric neutrinos. An even better experiment called MINOS will extend the search for neutrino oscillations. Currently under construction, the neutrinos produced at Fermilab will be sent a distance of 750 km to the Soudan mine in Minnesota, and there are similar plans to fire muon neutrinos produced at CERN towards detectors at the Gran Sasso Laboratory in Italy. Particle physicists there are also hoping to detect tau leptons produced by the oscillation of muon neutrinos into tau neutrinos.

Last year the SNO collaboration upgraded its detector in an effort to detect muon neutrinos or tau neutrinos directly. On the rare occasions when these neutrinos interact in the detector, they break up the deuterium nuclei in the heavy water to release neutrons. In order to count the muon neutrinos and tau neutrinos, the SNO team added purified sodium chloride, which captures the neutrons. And another experiment called KamLAND in Japan is studying antineutrinos from commercial nuclear-power plants some 175 km away. Researchers there are hoping to establish that electron neutrinos do indeed convert to other types of neutrinos.

In the longer term, there are serious discussions about sending neutrinos thousands of kilometres. Beams produced at Fermilab or Brookhaven, for example, could be fired towards experiments in Japan or Europe. Also, a serious effort is being made to observe the conversion of matter and antimatter using a rare process in nuclei called neutrinoless double beta decay. In this reaction, which is forbidden by the Standard Model, two neutrons decay into two protons and two electrons without emitting any antineutrinos. Recently Hans Klapdor-Kleingrothaus and co-workers at the Max Planck Institute for Nuclear Physics in Heidelberg claimed to have observed such a process, but the evidence is far from conclusive (see Physics World March p5).


We are at an amazing moment in the history of particle physics. The Higgs boson, the mysterious object that fills our universe and disturbs particles, will be found sometime this decade, and evidence for neutrino mass appears very strong. The Standard Model, which was established in late 1970s and has withstood all experimental tests, has finally been found to be incomplete. To incorporate neutrino mass into the theory — and to explain why it is so small — requires major changes to the Standard Model. We may need to invoke extra dimensions or we may need to abandon the sacred distinction between matter and antimatter. If the latter is the case, neutrino mass may reveal the very origins of our existence. One thing is certain, we are sure to learn a lot more about neutrinos in the coming years.

  • 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.