Matter/antimatter imbalenc - forked from AMA ask a physicist

[freyar]

First off, ask away!

Can you produce neutrinos such as, but not necessarily the same as, an electron is produced in a tube?

then focus the neutrino with a deflector shield array?
sorry about my inner Trekkie

When you say "an electron is produced in a tube," I guess you're talking about electron guns found in old cathode ray tube televisions, etc. In those, free electrons are produced by heating a piece of metal to a high enough temperature that the electrons "boil off." Then an electric field is used to pull the electrons away from the metal and accelerate them.

Neutrinos, on the other hand, are trickier. They are produced in nature; for example, the sun makes a lot of them, some radioactive decays produce neutrinos, a supernova produces a tremendous number, etc. But we also want to make neutrinos for experiments to study them. Some experiments use nuclear reactors, since various nuclear reactions including radioactive decay of unstable "daughter" nuclei can make neutrinos. There are also experiments that need a high intensity beam of neutrinos, which takes a multi-step process. First, you accelerate protons to very high energies, and then you smash them into a fixed target (like a piece of graphite). This produces a bunch of junk, including some short-lived subatomic particles called pions. Some pions are charged, so you can separate them from the other particles by sending everything through a magnetic field and selecting only the pions. Then the pions decay, producing neutrinos and electrons/positrons, which you then just let hit a wall or something. Meanwhile, the neutrinos just go right through the wall to your detector, which can sometimes be hundreds of miles away through the earth.

 

[Scott DeWar]

A crt or any vacuum tube is exactly what I was thinking of.

Ah, so it is possible, being done and being experimented with. Very interesting.

behaving my self.

ony one question.

Gah! Why graphite?

 

[freyar]

do opposite sides of quark pairs attract each other? such as up/down, does an up quark attract a down quark anywhere like a proton and elecytron pull at each other?

if so, is this the binding force that keeps a nucleus together [protons/neutrons]?

For quarks, up/down, charm/strange, and top/bottom are sort of like "weak force charges" for quarks (very very roughly, think of the first of the pair having one charge and the second a different charge), though it's somewhat more complicated than that. But the weak nuclear force is extremely short range and can't really reach even across a nucleus very well. To understand the binding of a nucleus, we need to talk about the strong nuclear force.

First off, protons and neutrons are made of fundamental particles called quarks. Quarks carry "strong force charge" known as color. There are three colors (red, green, and blue) for quarks (anti-quarks have anti-colors, which you might call cyan, magenta, and yellow or less imaginatively anti-red, etc). The strong force is actually strong enough that you can't have a colored object like a quark sitting around by itself. If you tried to pull the quarks of a proton apart (by, for example smashing the proton with something), the energy you'd need to do that would actually be sufficient to produce (lots of) quark/antiquark pairs from the vacuum, which would the assemble with the original quarks from the proton. So, in the end, every particle we see on it's own is color-neutral: you can either have a quark/antiquark bound together (so it would have a color plus the corresponding anti-color, which adds to no color) or three quarks with one red, one green, one blue so the colors add to "white." Quark/antiquark states are called mesons and include the pions I mentioned earlier, and the three-quark states, including protons and neutrons, are baryons. Together, mesons and baryons are called hadrons, as Umbran already said. Short version: the quarks in protons and neutrons are held together by the strong force, and protons and neutrons are therefore strong-force-neutral.

But even though protons and neutrons don't carry color, the strong force still holds them together. The analogy is the van der Waals force between electrically neutral atoms: if two atoms are close together, the electrons and nuclei inside can move around so there is a net attraction between the atoms. It's much much more complicated (to the point where we don't have a good way to calculate it from first principles), but protons and neutrons in a nucleus are held together by the strong force version of the van der Waals force. And, yes, that is strong enough to overcome the electromagnetic repulsion among the protons (which all have the same electric charge).

Last note: there have also recently been discoveries of 4-quark and 5-quark bound states (tetraquarks and pentaquarks) but not a lot is known about them yet. Tetraquarks have two quarks and two antiquarks; pentaquarks have 4 quarks and one antiquark. The question is whether they are one big blob of quarks or something like a meson orbiting another meson/baryon. These qre quite unstable, though.