Newsletter: Ghost Particles

Guide to the Cosmos
Making the Wonders of our Universe Accessible to Everyone.

Ghost Particles

For 80 years, physicists have been tricked by nature’s most subtle treat. Ubiquitous but nearly undetectable, neutrinos are the ghostliest fundamental particles.

In the bewitching hours, as you seek safety in your bed, 500 trillion neutrinos slice through your body every second. Produced by the Sun, they pass through the Earth as if it were tissue paper, striking us from below, and continuing upward into the heavens. And when you think the rising Sun has vanquished vampires and apparitions, neutrinos strikes you from above. Of this vast horde, less than one neutrino per year will actually “hit” you; the rest pass through without even noticing you’re there.

Neutrinos are so ethereal that 90% can pass through a block of lead one light-year thick without the slightest interaction.

Yet neutrinos play an essential role in some supernovae, in powering stars, and in the creation of the atoms of which all living things are composed.

With the discovery of the Higgs boson, physicists are now casting about for the next Big Thing. For many, it’s time to search for ghosts, or at least for neutrinos.

Neutrinos have spooked physicists since Wolfgang Pauli first postulated them in 1930. Physicists had found that energy and momentum seemed to be lost in the decays of certain radioactive elements. In the equivalent of a quarterback’s Hail Mary pass, Pauli proposed that some unobserved particles were carrying away the missing energy and momentum. Fermi later named them “neutrinos” — Italian for “little neutral one.” (No true Italian is that demure.)

Neutrinos are the only particles of matter with no electric charge; hence they don’t ionize material along their paths, so they escape direct detection. Like ghosts, we can only infer their existence from the mess left behind in their rare interactions with other matter, as seen below.

Muon neutrino entered a hydrogen bubble chamber from right, struck a proton, and produced a muon and pion.

Neutrinos are the only fundamental particles (those not made of anything “smaller”) whose masses remain unknown. We think they are a million times lighter than any other particle of matter. Why so different? It’s another mystery.

In 1956, 26 years after Pauli’s Hail Mary, the first neutrino was actually detected by Clyde Cowan and Fredrick Reines, for which they won the 1995 Nobel Prize.

As if one neutrino wasn’t spooky enough, Leon Lederman, Jack Steinberger, and Melvin Schwartz (my thesis advisor) discovered a second type in 1962, for which they won the 1988 Nobel Prize.

Another mystery is why the Nobel committee rewarded the discoverers of the second neutrino type seven years before rewarding the discoverers of the first type.

Yet a third neutrino type was discovered by the DONUT collaboration at the U.S. Fermi National Lab in 2000. The name DONUT seems odd. It stands for Direct Observation of NU Tau, but doesn’t DONUT sound like they’re missing something? No Nobel for DONUT, at least not yet.

All of that adds up to three types of ghostly neutrinos, named for their associated charged cousins: the electron, muon, and tau, in the order of their discovery.

While all other fundamental particles are content with one identity, neutrinos schizophrenically morph from one type to another and back again. Such “flavor oscillations” would be impossible if all neutrino types had zero masses, as physicists long believed.

Particle physicists haven’t successfully measured the mass of the neutrino. But strangely, cosmologists have. By measuring the total energy distribution of the universe, cosmologists have shown that the total mass of one neutrino of each of the three types is 2.5 million times less than the mass of one electron, the next lightest particle. Amazingly, the best way to measure the mass of the lightest particles is to measure the mass of universe and subtract everything else (well, sort of).

Immense numbers of neutrinos are produced when massive stars explode in spectacular supernovae and form neutron stars. In 1987, three neutrino detectors (in Japan, in a Morton Salt mine near Lake Erie, and in Russia’s Caucasus Mountains) detected a total of 24 neutrinos during a 13-second burst. These neutrinos came from SN1987a, the nearest supernovae of the last 400 years. This event created a new field of science: neutrino astronomy.

Supernovae can briefly shine brighter than billions of normal stars. Yet physicists estimate that only 1% of the energy released by supernovae, like SN1987a, is in the form of visible light; 99% is in the form of invisible neutrinos. The neutrinos are produced during the initial collapse, while the supernova’s visible light comes mostly later as radioactive nuclei decay. The result is SN1987a’s neutrinos arrived several hours before its light. Astronomers hope to use neutrino detectors as early warning systems for supernovae.

Stars, including our Sun, produce neutrinos in their cores as they fuse hydrogen into helium. For each helium nucleus produced, two protons transform into two neutrons, two antielectrons, and two electron neutrinos. This nuclear fusion process releases a colossal amount of energy that powers the stars and enables life on Earth. Additional fusion reactions produce carbon and other atoms that life requires.