Neutrinos have been observed originating from a supernova just as the light arrives. However, a neutrino does have a small, detectable mass. If its arrival coincides with light from an exploding star, how is that possible?

Neutrinos are actually emitted prior to the explosion. They have a very small, but non zero mass. Being that small, any push sends them flying off at near the speed of light. Neutrinos travel slower than light, but not greatly so.

Neutrino is QNX's name for it's microkernel architecture. It is POSIX compilant, so code developed on UNIX will be relatively easy to port. It is designed to run well on lower-end hardware and provides Photon for a low-memory GUI. It also runs on a variety of CPUs: x86, PowerPC, and MIPS.

The cyber-punk creatures from Dimention X who are friends with the Teenage Mutant Ninja Turtles.

They have rainbow hair and a flying car and the turtles help them fight Kreing when he takes over their world. There are two boys who remained unnamed, and a girl named Kala, who Michaelangelo is in love with.

Neutrino is the name given to three different but related elementary particles, all with small mass and zero charge. These are the electron neutrinoe), the muon neutrinoμ), and the tau neutrinoτ), which in the Standard Model are paired with the electron, muon, and tau lepton, respectively, to form the three 'families' of leptons. Like all fundamental fermions, the neutrinos have spin 1/2.

Leptons are distinguished from the other elementary fermions, the quarks, by not interacting through the strong nuclear force. Neutrinos, through their lack of charge, also do not interact through the electromagnetic force, leaving only the weak nuclear force (and also gravity, but the effect of gravity on elementary particles is negligible). Since the weak nuclear force is, after all, weak, this means that neutrinos interact very weakly with matter. It is said that a neutrino could pass through a light-year of solid lead and have its trajectory and momentum unchanged.

Neutrinos were originally postulated by Wolfgang Pauli in relation to beta decay. When a nucleus undergoes beta decay, it spits out an electron (or a positron). Conservation of momentum would have that there be a characteristic direction and momentum for the emitted electron, but the electrons were observed to have a large spectrum of momenta and angles of emission. Hence, some momentum appeared to be 'disappearing'. Pauli postulated than an unobserved, massless particle would also be emitted to carry away the missing momentum. Enrico Fermi named this particle the 'neutrino'.

The particle postulated by Pauli and Fermi was the electron neutrino. When the muon and tau lepton were discovered, corresponding neutrinos were postulated and eventually observed. Each of these neutrinos has a corresponding antineutrino, and, in fact, the original 'neutrino' of beta decay is actually an antineutrino. For reasons explained at the W particle node, antineutrinos are produced along with negative charged leptons, and neutrinos are produced along with positively charged leptons (antileptons).

Neutrinos and antineutrinos have the interesting property of having only one helicity state; all antineutrinos have positive helicity meaning that their spins are parallel to their momentum, while all neutrinos have negative helicity, meaning that their spins are anti-parallel to their momentum. This is a consequence of parity violation in the weak interaction. It has been suggested that neutrinos are actually their own antiparticles and what are observed as antineutrinos are simply right-handed (positive helicity) neutrinos. This is an elegant theory, but it is by no means proven as of yet.

In the Standard Model, neutrinos are generally approximated as massless, but experiments, until recently, were unclear as to whether or not the neutrino has mass. Recent results from Super Kamiokande and the Sudbury Neutrino Observatory have shown convincing evidence for the phenomenon of neutrino oscillation, which requires that the neutrinos have mass. The actual masses have been constrained to be beneath 5 eV/c^2 (1/100,000 of an electron mass), but the nature of the oscillations makes assigning the masses complicated. Essentially, the oscillations predict the existence of three 'mass eigenstate' neutrinos, ν1, ν2, and ν3, which combine to create the three 'flavour eigenstate' neutrinos νe, νμ, and ντ that each interact with their corresponding charged lepton. The mass eigenstates each have a definite mass, but the flavour eigenstates would then not have definite masses.

The Universe is suffused with neutrinos. Every square centimetre of the Earth's surface has billions of neutrinos pass through it every second, usually without any effect whatsoever. This makes them the closest thing to dark matter currently known in particle physics, although there are not nearly enough neutrinos to make them a viable dark matter candidate. Nevertheless, their role in modern particle physics is greatly expanded from their original postulation as an explanation for beta decay.

This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at .

The Particle Formerly Known as Massless

Why did everyone think the neutrino was massless? It was clear from the fact that neutrinos seemed to go very fast when given a relatively small amount of energy that they have a very small mass, but why did many physicists think it must be exactly zero? Sure, we think the photon has exactly zero mass too, but there's a good reason. If the photon has mass then the theories of relativity and electromagnetism would have to be different as well. So, why should we think the neutrino is massless?

In a recent talk at the University of Maryland given by Hitoshi Murayama of UC Berkley I was offered a pretty good answer. In 1958, Goldhaber, Grodzins, and Sunyar did an experiment that determined experimentally that neutrinos are always left-handed (have negative helicity) and anti-neutrinos are always right-handed (have positive helicity). Other experiments confirmed this result. This was remarkable, because other particles, like electrons, can have either helicity.

The reason this has to do with the mass of the neutrino is the following: Helicity is the property of how much the spin of the particle is aligned with the momentum. Suppose an electron traveling in the x-direction has a positive helicity as measured in one frame of reference. If we look in another frame of reference that's traveling faster than the electron in the x-direction, then in that frame of reference the electron will appear to be moving in the negative x-direction; its momentum will be reversed. The spin is the intrinsic angular momentum, though. If you imagine the electron as a ball spinning around the x-axis in the right-handed sense, in the other reference frame it will still be spinning around the x-axis in the right handed sense. The direction of the spin remains the same while the direction of the momentum switches, which means that the helicity has switched. To put it another way, if we look along the new direction of momentum, the rotation is now left-handed. The component of spin along the momentum has switched signs because the momentum switched directions.

Once we know that helicity is a frame dependent property, then the experimental results of Goldhaber look pretty fishy. How can all neutrinos be left handed if a left-handed particle looks right-handed to a different observer. The principle of relativity tells us that physics is supposed to be the same in all inertial frames of reference, so if the laws of physics we observe say there can't be right-handed neutrinos, that should be true in all frames of reference. This appears to be a contradiction.

There is a way out of the contradiction. One finds a frame of reference in which the helicity is reversed by going faster than the particle. If neutrinos are massless, then they move at the speed of light and we can't go faster than the particle to switch the helicity. If neutrinos are massless then it makes sense that they can all be left-handed, and anitneutrinos can be right-handed.

Of course, more recent experiments have detected neutrino oscillation, which tells us that neutrinos don't move at the speed of light and do have masses. So, it seems that we're back to the same contradiction we started with. How do we get out of it? It seems we are not entirely sure yet. Murayama discussed two basic paths one can follow. One can consider that maybe those things we thought were antineutrinos are actually just right handed neutrinos, meaning that a neutrino and an antineutrino are the same particle. The other path is the idea that right handed neutrinos do exist, but for some reason we don't see them. One can then come up with different reasons why we don't see these right-handed neutrinos. This is exciting, at least, because it requires us to consider physics beyond the standard model in order to reconcile these facts.


  • Big World of Small Neutrinos, Hitoshi Murayama physics colloquium at the University of Maryland
  • Classes, etc. etc.

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