The case of the purloined identity

	Ghostly particles--in flavors! It has been said that a neutrino can pass through a wall of lead stretching from the earth to the moon and not interact with a single atom. The electrically neutral and almost incomprehensibly small neutrino – it is dwarfed in size by an electron – was theorized in 1930 by Wolfgang Pauli but not detected until 1956. Neutrinos were created in the Big Bang and are now created in the nuclear fusion process that powers the sun and other stars. They come in three different types, or what physicists call "flavors" – the electron neutrino, the muon neutrino, and the tau neutrino. Originally the flavors were thought to be distinct, but scientists now believe the picture is more complicated and that neutrinos mix and change between flavors. That neutrinos are more like wraiths than physical particles makes them extremely valuable in efforts to map the universe. Neutrinos can come to us from regions in space too densely packed with matter for light to penetrate. They also come unaltered by any of the myriad magnetic fields that permeate the cosmos, which means their journey can be traced as a straight line back to their point of origin. Neutrinos have always been an enigma. For decades, scientists believed neutrinos were massless. When it was discovered that neutrinos do have mass – albeit very tiny – scientists were left scratching their heads. Said Leon Lederman, who shared the 1988 Nobel Prize in physics for showing there was more than one type of neutrino, said, “It shows us that we really don't know nothing.” Neutrinos have also inspired commentary from nonscientists. In Cosmic Gall, an ode to neutrinos, Pulitzer-Prize-winning author John Updike said: “Neutrinos they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass. They snub the most exquisite gas, Ignore the most substantial wall, Cold-shoulder steel and sounding brass, Insult the stallion in his stall, And, scorning barriers of class, Infiltrate you and me! Like tall And painless guillotines, they fall Down through our heads into the grass..."

The case of the sun’s missing neutrinos has been solved. Like many good mysteries, there was a surprising twist involving switched identities and the possibility of one or more sequels.

According to the predictions of the Standard Model, the scientific theory that for the past three decades has served as our basis for understanding the fundamental particles and forces of nature, neutrinos have no mass. However, based on the Standard Model and our understanding of thermonuclear reactions, scientists were only able to detect about a third of the number of electron neutrinos (the type the sun produces) that they were expecting. Where were the missing solar neutrinos?

The arrival in the year 2000 of the Sudbury Neutrino Observatory (SNO) gave scientists their first chance ever to directly measure not only electron neutrinos, but muon and tau neutrinos as well, even though they weren’t expected based on the Standard Model. A little more than a year later, they had an answer to the mystery:

there were no "missing" solar neutrinos; the electron neutrinos had been changing their type or “flavor” during their 93 million mile journey from the sun to the earth.

Two years of SNO observations confirmed that contrary to the Standard Model’s predictions, neutrinos do have a mass – by some estimates about 1/60,000th that of an electron. Having a mass enables neutrinos to oscillate in transit, meaning they can change flavor from electron to muon or tau neutrinos.

The SNO observations have since been independently confirmed by another major neutrino observatory, KamLAND, which is located in central Japan. Observations at KamLAND showed that antineutrinos (the mirror image counterpart to neutrinos) emanating from nearby nuclear reactors are also “disappearing," which indicates they, too, have mass and can change from one flavor to another.

The results from SNO and KamLAND are enough to convict neutrinos of having mass, which means changes are in store for the Standard Model.