Quantum Universe

Hitoshi Murayama, of Princeton/ University of California, Berkeley, explains how neutrinos can potentially answer questions about how we survived the Big Bang.
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Ubiquitous, elusive and full of surprises, neutrinos are the most mysterious of the known particles in the universe. They interact so weakly with other particles that trillions of them pass through our bodies each second without leaving a trace. The sun shines brightly in neutrinos, produced in the internal fusion reactions that power the sun. These reactions produce neutrinos of only one kind, but they mysteriously morph into two other kinds on their way to earth. Neutrinos have mass, but the heaviest neutrino is at least a million times lighter than the lightest charged particle.

The existence of the neutrino's tiny nonzero mass raises the possibility that neutrinos get their masses from unknown physics, perhaps related to unification. Detailed studies of the properties of neutrinos-their masses, how they change from one kind to another, and whether neutrinos are their own antiparticles-will tell us whether neutrinos conform to the patterns of ordinary matter or whether they are leading us toward the discovery of new phenomena.


	DARK MATTER MAP The total mass within giant galaxy cluster CL0025+1654 is the sum of the galaxies themselves, seen in yellow as ordinary luminous matter, plus the cluster's invisible dark matter shown in blue. The cluster's dark matter is not evenly distributed, but follows the clumps of luminous matter closely. Credit: J.-P. Kneib Observatoire Midi-Pyrenees, Caltech. ESA, NASA View larger image

The discovery that neutrinos have mass opens a window on physics beyond the Standard Model. The Standard Model cannot accommodate neutrino masses without the introduction of new particles, which themselves raise new questions. In fact, the size of the neutrino masses is consistent with expectations from unified theories that require the new particles for the unification itself.

The most pressing question about neutrinos involves how many different kinds there are. Results from the LSND experiment suggest that there may be more than the canonical three families. If so, that would require a major revision of current understanding. The Mini-BooNE experiment, now running at Fermilab, will settle this issue by mid-2005.

Even if there are only three kinds of neutrinos, questions remain. What generates the neutrino masses and what are their values? Are neutrinos their own antiparticles? How do different kinds of neutrinos mix? Answering these questions requires precision measurements of the neutrino masses and mixings. Physicists are now studying neutrino mixing at the SNO, KamLAND, K2K and SuperKamiokande experiments. A big step will occur in 2005, when the NuMI/MINOS program at Fermilab begins to probe nu-mu/nu-tau neutrino mixing in a controlled accelerator experiment. In 2006, the CERN-to-Gran Sasso long-baseline neutrino program will begin. The neutrino beam at JPARC is being developed in Japan. In the longer-term future, these experiments and their upgrades, possibly using an off-axis beam, or dedicated reactor neutrino experiments may tell us if a measurement of CP violation in the neutrino sector is feasible. Then researchers might use a neutrino superbeam or neutrino factory to search for it. The detector in such an experiment could also search for proton decay, if located deep underground in a facility such as a National Underground Science and Engineering Laboratory.

The Main Injector Neutrino Oscillation Search experiment, half a mile underground in the historic Soudan iron mine in northern Minnesota, uses a neutrino beam from the Main Injector... read more

Accelerator and reactor oscillation experiments measure mass differences. The masses themselves must be determined by different methods. Neutrinoless double beta decay experiments such as EXO and Majorana can be used to measure the electron neutrino mass to ~0.01eV, if neutrinos are their own antiparticles. The observation of neutrinoless double beta decay would have far-reaching consequences, raising the possibility that the matter and antimatter could have transformed to each other in the early universe.