Section 6: The Little Neutral Ones: Neutrinos
We have leapfrogged ahead in our story, ignoring an important but easily overlooked particle: the neutrino. German theorist Wolfgang Pauli first proposed the concept of neutrinos in the 1930s to explain a puzzling feature observed in nuclear beta decay. A beta decay happens when a neutron in the nucleus converts (decays) into a proton, an electron, and (for reasons outlined below) an anti-neutrino. The proton remains bound to the nucleus by the strong nuclear force, but the electron and the anti-neutrino escape as radiation. These radioactive decays emit a negative beta ray (that is, an electron). As a result, the nucleus gains one unit of positive charge, which transforms it into the next element in the periodic table. Because energy is conserved, the electron should carry off a well-defined amount of kinetic energy corresponding to the mass difference between the two nuclear states. However, the emitted electrons did not exhibit a sharp peak in energy. Instead, the measured electron energies were seen to spread over a broad range, rather than the single value that would correspond to the electron being the only emitted particle.
Figure 15: Beta decay spectrum: The puzzling process explained by the detection of the neutrino.
Source: © Michelle Leber, 2009. More info
It appeared that the sacrosanct principle of energy conservation was violated in beta decay. Niels Bohr even suggested that perhaps energy conservation did not hold inside the nucleus. Pauli offered an alternative suggestion: An undetected, electrically neutral particle could be emitted, so that it and the emitted electron could share the energy of the decay process between them.
At the time, nobody regarded this ghost particle explanation as satisfactory, though it was certainly better than Bohr's alternative. But in 1932, British physicist James Chadwick discovered the neutron and confirmed that the electrons emitted in beta decay do not have a well-defined energy. Chadwick's work prompted the great Italian physicist Enrico Fermi to write down what turned out to be the correct theory of beta decay: A neutron decays into a proton, an electron—and a ghost. Fermi named the ghost a "neutrino." This particle possessed no mass and no charge, and hardly ever interacted—just like Pauli's ghost particle. Fermi's theory worked not only for beta decay, but also for a variety of other processes with missing energy, including decays of pions and muons. The process would later be called the weak interaction, because of the very low probability that it would occur.
Figure 16: The inverse beta decay that revealed the neutrino.
Physicists did not directly detect the neutrino until 1956, using the standard technique of fixed target scattering that had previously led to the discoveries of the nucleus and later the quark. In this case, the challenge was not to probe inside the target but to detect the neutrino beam, which could be discovered only by detecting the products of its scattering interaction. A single neutrino with 1 GeV of energy will travel, on average, through one million earths before interacting; so to catch one in the act requires both a copious source of neutrinos and a massive detector to increase the odds. Frederick Reines and Clyde Cowan Jr. designed an experiment to do just that. They used a large water tank located next to the Savannah River nuclear reactor in South Carolina, which produced about 1012– 1013 neutrinos per square centimeter per second. Reines and Cowan looked for evidence of the "inverse beta decay reaction" that occurs when a neutrino interacts with a proton, producing a neutron and a positron: . See the math
Figure 17: Aerial view of South Carolina's Savannah River nuclear reactor.
Source: © NASA, visibleearth.nasa.gov. More info
In the water tank, the positron will immediately annihilate with an electron, emitting two photons, each with the same characteristic energy. Cadmium dissolved in the water absorbs the neutron and undergoes gamma decay, which emits a third photon with a different energy a few microseconds later. Reines and Cowan devised a way of distinguishing this characteristic signature—two photons of the same energy, followed by a third photon at a different energy—from the many accidental background coincidences caused by cosmic rays and other extraneous signals. Despite the huge flux of neutrons, they observed only a handful of events per day. So as a check, they verified that the signal went away when the reactor was turned off. Technically, the pair discovered the anti-neutrino. However, as we shall see later in this unit, certain types of neutrinos may be identical to their anti-neutrinos.
Many open questions
This experiment conclusively established the existence of the elusive neutrino, but many open questions remained. It would take several more decades of challenging experiments using neutrinos from reactors, cosmic rays, the Sun, and accelerators to establish the existence of three different kinds, or flavors, of neutrinos, corresponding to the three different types of lepton: electron neutrinos, muon neutrinos, and tau neutrinos. All three neutrino flavors are light in mass. Indeed, they were originally assumed to be massless.
Figure 18: Three generations of quarks and leptons.
Source: © Wikimedia Commons. License: CC 3.0 Unported. Author: MissMJ, 27 June 2006. More info
A positron-electron collider called LEP at the European Organization for Nuclear Research (CERN) played a critical role in putting neutrinos into the broad context of the Standard Model. CERN scientists studied the "invisible" decays of the Z boson—the neutral carrier of the weak force that we shall meet in the next unit—to a pair of neutrinos. The observed rate of these decays showed that only three generations of light neutrinos exist. This important result suggests that there are only three generations of particles in the Standard Model, organized in the "periodic table" of fundamental particles shown in the accompanying figure.
This is the happy family of quarks and leptons that all particle physicists know and love. But in it, there lurked a big surprise in the neutrino sector. Some call it the first evidence of physics beyond the Standard Model.
The evidence first showed up in experiments conducted deep underground in South Dakota's Homestake Gold Mine, away from cosmic ray backgrounds, to detect the neutrino flux from the Sun. This started as a way to study the properties of the Sun, by monitoring the neutrinos from the nuclear reactions that power the Sun's energy. The initial experiments, pioneered by Raymond Davis Jr. of Brookhaven National Laboratory, reported far too few neutrinos. The shortfall wasn't trifling. Davis detected only one-third as many neutrinos as expected.
Figure 19: Drawing of the underground Brookhaven Solar Neutrino Observatory.
Source: © Courtesy Brookhaven National Laboratory. More info
This discrepancy spurred new questions—was the solar model wrong, or was something strange going on with neutrinos? It also generated new types of experiments to unravel the puzzle. Studies that used neutrinos produced in the decay of cosmic rays provided the surprising answer: Neutrinos could change from one flavor into another.
A Japanese-led experiment called Super-Kamiokande showed that the flux of muon neutrinos from cosmic rays differed depending on whether the detected neutrinos were moving down or up. Upward-moving neutrinos are produced by cosmic rays that impact the atmosphere on the opposite side of the Earth to the detector. They travel all the way through the Earth before being detected. That gives them more time to change flavors. During this time, about half of the muon neutrinos had changed into tau neutrinos. The same effect explained the reduced neutrino flux from the Sun: Electron neutrinos produced in the Sun were changing into muon and tau neutrinos before they reached the Earth. Since the early solar neutrino experiments were sensitive only to electron neutrinos, they could not detect the two-thirds that had mutated. Later, more sophisticated experiments sensitive to all three flavors of neutrinos confirmed that all three types of neutrinos can change, or oscillate, into one another.
The mass of neutrinos
Physicists had already observed this type of mixing behavior in neutral mesons, but they had no reason to expect it in neutrinos. After all, the Standard Model assumed that neutrinos had no mass. However, oscillation between neutrino flavors, which means that individual neutrinos change their identities, is theoretically possible only if different flavors of neutrinos have different masses. Physicists still do not know the absolute mass scale of neutrinos, but they have measured the mass differences between pairs of neutrino flavors through careful study of their oscillation properties. These differences are very tiny, suggesting that neutrinos may be a million times lighter than the electron. Now theorists face the challenge of explaining why nature should have given neutrinos such miniscule masses.
Figure 20: The Sudbury Neutrino Detector led to the discovery of neutrino mass.
Source: © Lawrence Berkeley National Laboratory. More info
Experiments to make more accurate measurements of neutrinos' mass differences and their mixing rates are under way in several countries. Some use nuclear reactors as the sources of neutrino beams. Others rely on neutrinos produced in accelerators by the decay of a secondary beam of mesons produced when high-energy protons smash into a target. The results of both types of studies may make it experimentally feasible for the next generation of projects to look for CP violation in neutrinos—a phenomenon that we shall explain in the next section.
Some experimenters are trying to pin down the absolute mass scale of neutrinos by making precision measurements of the highest-energy electrons emitted in beta decay. These experiments are performed on large and small scales, using a spectrometer as large as a house, or making careful measurements of a single atom as a neutron in its nucleus decays. Other experimenters are trying to measure the absolute mass scale of the neutrino through a process called "neutrino-less double-beta decay." In this phenomenon, two beta decays occur simultaneously; the neutrino emitted in one decay is absorbed in the second, so that only two electrons emerge. This type of decay is possible only if neutrinos are their own antiparticles, otherwise known as Majorana neutrinos. (If neutrinos and anti-neutrinos are distinct from one another, they are called "Dirac neutrinos.") Because neutrino-less double-beta decay is extremely rare, experiments intended to differentiate between the Majorana and Dirac scenarios take place deep underground, insulated from cosmic rays and other radioactive backgrounds. The distinction is significant because it might have played a role in the asymmetry between matter and antimatter, as we shall discuss in the following section.