Define neutrino1/31/2024 ![]() Well, for decades that seemed fine there wasn’t any experimental sign that neutrinos have any mass. As far as experiment was able to tell, for several decades after the discovery of the first neutrino in 1956, each of the three neutrinos is a sort of half-neutrino… a Weyl fermion that interacts with the W boson and thus experiences the weak nuclear force (but is unaffected by the electromagnetic and strong nuclear forces.) By the logic I just gave you, this Weyl fermion can only have zero mass it has nothing to marry. Massless Neutrinos? Standard Model 1.0īut the logic for neutrinos has a twist. Each one is really made from two half-particles - two Weyl fermions - of which only one half interacts with W bosons, and each of which would have zero mass were it not for the marriage engineered by the Higgs field. The same logic applies for the heavier cousins of the electron (the muon and the tau), as well as for the six types of quarks. But instead this interaction is very weak, and the marriage is a loose one, resulting in an electron whose mass is much smaller than the top quark’s. If the Higgs field’s interaction with the electron’s halves were very strong, then this would be a strong marriage and the newly formed electron would have a very large mass, like the top quark. The resulting electron is a “ Dirac fermion“, with a mass. Along the way, as Weinberg and Salam pointed out, it allows these two discrepant electron-halves to be married into one. So on the face of it, these two half-electrons must remain unmarried, and with zero mass, forever.īut when the Higgs field switches on, it changes the rules, giving the W boson its mass. They have to behave the same way with respect to all the elementary forces of nature. You can’t “marry” two half-particles into one if they behave in fundamentally different ways. Although both halves of the electron have “electric charge” (meaning they are affected by electric and magnetic forces), only one half of the electron interacts with the particle known as the W boson, a crucial component of the weak nuclear force. Without the Higgs field, there is a fundamental obstruction to the electron having any mass at all. Without the marriage arranged by the Higgs field, neither “half” of the electron could have any mass. The two parts of the electron differ only in their interaction with the weak nuclear force, especially though the W field and its boson. These half-electrons are not half-particles, though they are particles in and of their own right, except that a particle like this (called a “ Weyl fermion“) must have zero mass.įigure 2: An electron is a Dirac fermion, formed through the marriage of two Weyl fermions, made possible by the Higgs field. The idea is that particles like the electron are really put together from two particles, not one. It’s a weird, awkward idea, certainly at first glance, so much so that if experiment didn’t confirm it, it would be hard to believe. The story begins with Steven Weinberg in 1967 and Abdus Salam in 1968, who first introduced the basic concept of how the Standard Model’s “fermions” (often referred to, to my dismay, as “matter particles”) get their masses. How the Majority of Standard Model Particles Get Mass each of these requires a minor modification of the Standard Model: in one case a new type of particle, in another case a new phenomenon.today we know of two simple solutions to it, but don’t know which one is right.once upon a time it was thought that the Standard Model solved this puzzle.Also, though some of these particles have separate anti-particles, I haven’t shown them it wouldn’t add anything, since the anti-particle of any particle type has exactly the same mass.Īs you can see, the neutrinos are way down at the bottom, far from everyone else? What’s up with that? The answer isn’t known it’s part of ongoing research.For later use, I’ve divided the particles into two classes: “fermions” and “bosons”. ![]() I’ve used a “logarithmic plot”, which compresses the vertical scale if I used a regular “linear” plot, you’d see only the heaviest few masses, with the rest crushed to the bottom.This striking situation is illustrated in Figure 1, in which ![]() The horizontal grey bar shows the maximum masses from cosmic measurements the vertical grey bars give an idea of where the masses might lie based on current knowledge, indicating the still very substantial uncertainty. Figure 1: The masses of the known elementary particles, showing how neutrino masses are much smaller and much more uncertain than those of all the other particles with mass.
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