Evidence for the so-called "sterile" neutrino, a "flavor" of neutrino which does not interact with other matter through radioactive decay (also known as "the weak force") today comes in the form of high-speed pulsars as well as x-rays from the edge of our own galaxy. Meanwhile, the Karlsruhe Tritium Neutrino (KATRIN) experiment seeks to learn more about neutrino mass and neutrino oscillation between various types or "flavors" of neutrinos.
Mass, Oscillation, and "Sterile" Neutrinos
Neutrino mass and oscillation are actually related to one another and to the question of sterile neutrinos. Neutrino oscillation, into what physicists dub "different flavors," which travel at slightly different speeds, can only occur if at least one of the neutrino particles has mass. Oscillation does seem to occur, and, says Michael Moyer of Scientific American (2011), it does so in a way that seems to require one of the neutrino flavors to be "sterile."
Originally it was thought that the "neutrino" (Italian for "little neutron"), so-named by physicist Enrico Fermi because it had no charge, also had no mass. While Wolfgang Pauli in 1931 calculated the angular momentum of the neutrino, then only a conjectured particle, in order to account for the change in angular momentum when hydrogen atoms (protons and electrons) compressed to form neutrons, physicists were not able to measure any change in mass.
However, since 1998, when a team of Japanese and U.S. physicists measured a major difference in the number of muon neutrinos apparently detected by the Superkamiokande Detector's underground water collection, depending on whether the neutrinos had come from just above or had passed through the earth (about half as many neutrinos were detected that had passed through the earth as were detected coming from just above the collector; the missing muon neutrinos thus may have "oscillated" into another flavor of neutrino), it's generally been assumed that neutrinos oscillate from one flavor to another, and therefore have mass. The exact mass has yet to be determined.
It's also been assumed that there are at least three flavors of neutrinos. Research in the 1990s suggested that there might also be a fourth "sterile" flavor, one that does not even interact with matter via the "weak" force (as noted above, this is the force associated with wavelength and nuclear decay), but only via gravity.
Because neutrinos are so much lighter than electrons, physicists speculate that a particle of intermediary mass might be involved. Because there are already three "flavors" of neutrinos that do interact via the weak force, many physicists believe that any additional flavors must be "sterile," and thus this intermediary neutrino is believed to be "sterile," as well as perhaps heavier than the three current neutrino particles. Recent evidence suggests that perhaps some antineutrinos but not apparently neutrinos, may change briefly into the sterile neutrinos before changing into other flavors.
Evidence from the Sky
Additional recent evidence for sterile neutrinos comes from the sky. According to both Michael Moyer (2011) and Eric Hand (2010), University of California astrophysicist Alexander Kusenko, and N.A.S.A.'s Michael Loewenstein believe they have found possibly the wavelength of x-rays that would be expected if the apparently heavier "sterile" neutrinos were decaying (or rather oscillating) into the lighter neutrinos. The data comes from the "orbiting Chandra X-ray Observatory" in the direction of "a dim dwarf galaxy that orbits the Milky Way."
Still more evidence includes pulsars, says Moyer, "whizzing through the universe at speeds of thousands of kilometers a second." Moyer explains that sterile neutrinos, if these existed, would be expected to shoot out from supernovae "in a tight stream," with the "recoil from this blast" shooting pulsars "through the cosmos at such speeds."
Relationship Between Oscillation and Mass
Whether or not a "sterile" flavor of neutrino is confirmed, how is the oscillation of neutrinos, from one flavor to another, related to neutrino mass? According to string theorist Matt Strassler, for the neutrinos to oscillate between three flavors, any flavor of neutrino must be made up of three different underlying particles, all traveling at slightly different speeds, because of their slightly different masses.
The "Uncertainty Principle"
This is part of the so-called "uncertainty principle" in quantum mechanics, says Strassler, that is you cannot know everything at once, that is you cannot specify the exact "flavor" of neutrino you have (that is the way the neutrino interacts via the "weak force," which includes the wavelength associated with it when it is involved in reactions) if you can specify the neutrino's mass. Instead, you have a "composite" of "flavors," of weak force interactions, with a probability that one or the other will surface depending on the neutrino's mass (and thus the neutrino can change from one to the other flavor). (Similarly, physicists only describe the probabity that a particle traveling at a particular speed can be found at a particular point in space. And, in the case of a "quantum field," which essentially is the waves associated with a particle, physicists talk about the probability that a particle is there.)
The Higgs Mechanism, "The Weak Force," and Mass
The Higgs Mechanism or field is a way for a composite particle, consisting of two or more not-quite-like particles, or rather, for a particle oscillating between various not-quite-like particle "flavors," to become a composite with mass. The "Higgs difference" is a "weak force" vector. The various flavors of "particles" that can be said to make up a particle with mass thus have different interactions with the "weak force," different wavelengths associated with them, but not of course different charges. This is one way neutrinos might get mass.
Dirac Versus Majorana Mass
Instead of having different "flavors" interacting in the composite particle, flavors that interact with matter differently via the "weak force," it's possible in the case of neutrinos that particles and antiparticles interact instead. The particle in this case of course must have a neutral charge, as otherwise the antiparticle would necessarily have a charge opposite to that of the particle and thus a particle could not be its own antiparticle; that is particles and antiparticles, when the particle has a charge, do have different charges. Particles and antiparticles that "decay into one another, instead of getting a Dirac Mass, which makes use of the Higgs Mechanism, get a Majorana Mass. It's also speculated that some particles that are their own antiparticles and that come in multiple flavors, such as neutrinos, can "seesaw" between Dirac and Majorana masses.
Majorana Mass and "Neutrinoless Double Beta Decay"
According to physicist Paul Langacker (University of Pennsylvania), "[a] Majorana mass . . . , makes use of the right-handed antineutrino, rather than a separate" neutrino flavor. Langacker explains that this involves "a transition from an antineutrino into a neutrino," or, "[e]quivalently, it can be viewed as the creation or annihilation of two neutrinos . . . ." Thus Langacker explains, if this transition occurs, it should " . . . lead to neutrinoless double beta decay." (In this kind of beta decay, a neutrino and antineutrino are produced; these of course can annihilate one another.)
The KATRIN experiment is actually looking for evidence of such "neutrinoless double beta decay" in radioactive hydrogen (tritium), where tritium nuclei (with a proton and two neutrons) decay into he3 (helium 3) nuclei (2 protons and a neutron) plus antineutrinos. Similarly physicist Lily Schrempp (discussed in "Elusive Neutrinos: Clues to the Universe?") is looking for evidence of such double beta decay in cosmic rays. Neutrinoless double beta decay is of course something that no one has yet seen. KATRIN as noted hopes to learn more about both neutrino mass and oscillation.
R. M. Santilli: Non-Believer
R.M. Santilli is a physicist who has abandoned more "mainstream" quantum mechanics and who argues for a theory he dubs "hadron mechanics" (hadrons are observable particles, neutrons and protons). Santilli argues that when the number of so-called "neutrinos" measured, based on radioactive decay, does not add up to what's expected, more flavors are postulated to explain the discrepancies, which Santilli believes occur because theories of the atom and atomic particle collisions that involve neutrinos are off in the first place. Santilli explains that no one has actually "seen" a neutrino.
Santilli argues that the neutrino is no more proven to exist than other conjectured particles. Santilli explains that, when it combines with the proton, the electron plunges deep into the dense matter of the proton, and that thus calculations assuming that when a proton and electron come together, discrete "quantum" particles are simply crossing paths, bouncing against one another, and being "captured" in orbit will be "off" (quantum theories argue that protons are composed of discrete particles, "quarks"). Quantum mechanics, says Santilli, fails inside of the dense matter of the proton, and thus angular momentum should not be conserved. The neutrino as noted was conjectured to "conserve angular momentum."
"The proton and other hadrons," according to what Santilli says is relatively "non-controversial" data from "deep inelastic scattering" ("inelastic scattering" is where one of the particles loses energy when two particles such as the proton and electron make contact), are "composed of a somewhat homogeneous and isotropic hyperdense medium in which the search for the remnants of an atomic structure has no scientific sense." However, Santilli himself has not of course yet "seen" electrons actually plunge inside of the matter that makes up protons; he only conjectures that they do so.
Evidence for Neutrinos
The experimental evidence for neutrino existence includes the discrepancies in expected and measured energy from beta decay, when a neutron decays to a proton (or when electrons are captured by a proton to form a neutron). Evidence that neutrinos where involved in such decay was first observed in 1970.