Why can't a neutron be thought of as a proton plus an electron and neutrino?

[Chi Meson]

Wow. Clean slate! I like the format! And I've got a question.

In elementary physics, it is said that the neutron can "sorta" be thought of as a proton plus an electron together. The mass of the neutron is slightly higher than a proton, by approximately the mass of an electron; in beta decay, a neutron decays into a proton, electron and neutrino; the neutron is neutral, since the two charges cancel.

My question is: I know it is incorrect to say that a neutron is a "Proton plus Electron and Neutrino," but why is it wrong to say this? I'm familiar with the HUP explanation that since the mass of an electron is so small, then it's position is too vague to be contained in the tiny neucleus, but is there some other reason?

 

[FZ+]

For a start, because neutron decay produces a proton, and electron and an electron ANTI-neutrino?

 

[mathman]

For starters: A proton consists of 2 up quarks, 1 down quark, and gluons holding it together. A neutron consists of 1 up quark, 2 down quarks, and gluons.

The decay process in simple terms occurs because a down quark changes into an up quark and emits an electron and an anti-neutrino. The important thing to note is that the electron and anti-neutrino weren't there before. They were created at the time this (weak force) reaction took place.

 

[chroot]

It it correct to say:

Proposition 1: The neutron is composed of two up and one down quark.

but NOT correct to say:

Proposition 2: The neutron is composed of a proton, electron, and anti-electron-neutrino.

Why is this incorrect? It's simple, really. Let's say you put the neutrons in a particle accelerator and bounce electrons off of them. You will see scattering events. If Proposition 1 is correct, you will see by analyzing the scattering events that there are three different scattering centers inside the neutron -- each is one of the quarks. If Proposition 2 is correct, you would instead see only two scatterting centers, one of which is much, much "stronger" than the other, corresponding to the internal electron and proton. You would probably not get enough scattering off of the neutrino to be measurable.

Experiment confirms Proposition 1, and disproves Proposition 2. The neutron is really composed of three quarks.

The electron and anti-electron-neutrino appear when one of the down quarks decays via the weak force into an up quark, and a W- boson. The W- boson then decays into an electron and an anti-electron neutrino, conserving charge and lepton number.

- Warren

 

[Chi Meson]

Originally posted by FZ+
For a start, because neutron decay produces a proton, and electron and an electron ANTI-neutrino?

yes, okay, for someone as picky as me, I should have caught that.

Thanks to Chroot & Mathman. I was searching my old texbooks, but I could not find a direct answer. Now, a follow up question:

If I were explaining this to advanced high school students, would it be a bad analogy if I were to say that this explanation is similar to: "It is incorrect to say that the uranium atom is composed of a barium plus a krypton atom and a few neutrons" ?

In both situations (beta decay and uranium fission) there is a conservation of a set of quantum numbers, and in both situations there are smaller components that are "shuffled" to create significantly different larger particles. Should I just avoid this analogy?

 

[selfAdjoint]

Well about "It is incorrect to say that the uranium atom is composed of a barium plus a krypton atom and a few neutrons" it is! Because you could think of many other ways to compose it = uranium is made up of 92 Hydrogen atoms, plus some neutrons. If you mean that Barium and Krypton were found in the first (recognized) fission experiments, be aware that there are many ways fission can happen.

 

[Chi Meson]

I was about to elaborate further, but I'm seeing now that this analogy is gettingout of hand. I'm leaning to the explanation that the composition of fundamental particles as an arrangement of quarks is NOT analogous to the compsition of nuclei as an arrangement of protons and neutrons.

Would you agree?

 

[chroot]

In atomic fission, no particles are changing identities. There is no decay or transmutation. A big nucleus splits up into some smaller ones. While I don't particularly care for the explanation that uranium = barium + krypton + neutrons, it's really no more incorrect than a dozen eggs = four eggs + eight eggs.

However, it is entirely incorrect to talk about a neutron = proton + electron + anti-electron-neutrino, since these particles only appear after a down quark decays.

- Warren

 

[bdkeenan00]

what is the charge of an anti-electron-neutrino,anti-muon-neutrino,and the anti-tau-neutrino?

 

[chroot]

The neutrinos and and their antiparticles all have no electrical charge.

- Warren

 

[Chi Meson]

Thank you chroot. That clears it up.

 

[bdkeenan00]

what's the difference between the neutrinos and their anti particles.

 

[chroot]

Their interaction with the weak force.

- Warren

 

[bdkeenan00]

Okay,thank you very much.

 

[jcsd]

That's if neutrinos aren't their own antiparticles which is still a matter of conteion among particle physicists.

 

[garytse86]

hey mathman, you said that the elctron and the electron-antineutrino were created when the weak force decay took place. How can matter be actually created? Mustn't there be some sort of energy being converted in the particles?

 

[chroot]

The neutron's rest mass is equal to the sum of the kinetic energy and rest masses of the decay products.

- Warren

 

[garytse86]

no I mean where does the electron come from?

can't have been released from the neutron because the neutron is composed of 2 down quarks and 1 up quark. therefore it is not from the neutron.

 

[Nereid]

The electron is a lepton, and the anti-neutrino is an anti-lepton, so lepton number is conserved. Baryon number is also conserved. Energy is conserved. Since no conservation laws are broken, the neutron must decay (ergodic principle; what is not forbidden is compulsory).

A down quark is transformed into an up quark, via the weak interaction (force carrier is the W^- particle).

So, yes, it did 'come from' the neutron; but it didn't exist until the decay happened!

 

[chroot]

Originally posted by garytse86
no I mean where does the electron come from?

can't have been released from the neutron because the neutron is composed of 2 down quarks and 1 up quark. therefore it is not from the neutron.

Did you not read this thread? The beta decay is a result of a down quark decaying into an up quark.

- Warren

 

[FZ+]

Originally posted by garytse86
no I mean where does the electron come from?

Promotion of virtual particles, IIRC.

 

[garytse86]

yes, but I didn't know where that lepton (electron) came from.

 

[1100f]

Originally posted by bdkeenan00
what's the difference between the neutrinos and their anti particles.

The neutrino has a property called helicity, which is like the spin for massless particles. The neutrino's helicity is the opposite of the antineutrino's.
One is called left-handed, the other is right-handed

 

[jcsd]

As I said before though, it's not 100% certain that neutrinos aren't their own anti-particles.

 

[theriddler876]

how can these particles have no mass? even light has mass

 

[theriddler876]

one more thing if the charge of an up quark is 2/3, and one down is -1/3 then a couln't a neutron be any combination that cancels them out, IE 2 up and 4 down, or any 1:2 ratio that stays within the confines of the mass of a neutron

 

[chroot]

Originally posted by theriddler876
how can these particles have no mass? even light has mass

This is incorrect. Light has no mass.

- Warren

 

[chroot]

Originally posted by theriddler876
one more thing if the charge of an up quark is 2/3, and one down is -1/3 then a couln't a neutron be any combination that cancels them out, IE 2 up and 4 down, or any 1:2 ratio that stays within the confines of the mass of a neutron

The quarks interact not only electromagnetically, but also via the strong force. The strong force requires all hadrons (particles composed of quarks) to be color-neutral. Therefore, there are only two kinds of hadrons:

1) The mesons, composed of a quark-antiquark color-neutral pair. The two quarks must be a combination like red + anti-red, making the composite particle color-neutral. The colors are transmuted by strong force interactions -- when quarks exchange gluons, they change color -- and thus (red, anti-red), (blue, anti-blue) and (green, anti-green) are all indistinguishable.

2) The baryons, composed of three quarks or three antiquarks, also color-neutral. Baryons must be made of one each red, green, and blue quark or anti-red, anti-green, and anti-blue, and are therefore also color neutral.

There is no way for a composite particle of more than three quarks to be color neutral. There is also no way for a single quark to be color-neutral. Therefore, there are no particles with more than three quarks, or fewer than two. Quarks cannot be found in isolation.

- Warren

 

[theriddler876]

Originally posted by chroot
This is incorrect. Light has no mass.

- Warren

You're reffering to rest mass

 

[Nereid]

chroot: The strong force requires all hadrons (particles composed of quarks) to be color-neutral.

Can anyone recommend a good, non-technical overview of QCD, its origins, how the ideas developed, and the landmark experiments which validated it?

 

[jcsd]

Originally posted by theriddler876
You're reffering to rest mass

mass = rest mass

 

[theriddler876]

do these flavours have a charge, and if so, do they have a charge to mass ratio?

 

[jcsd]

No, neutrinoes are electrically neutral and possibly massless (if you look at a chart of elementary particles their mass will usually be given as zero, however a couple of experiments have suggested they may have a small but non-zero mass so the issue is still up in the air at the momen. It seems to me though that more people are coming down on the side that they have mass which is useful as they are a candidate for non-baryonic dark matter). They are generally cofined to weak interactions.

 

[Nereid]

Originally posted by jcsd
No, neutrinoes are electrically neutral and possibly massless (if you look at a chart of elementary particles their mass will usually be given as zero, however a couple of experiments have suggested they may have a small but non-zero mass so the issue is still up in the air at the momen. It seems to me though that more people are coming down on the side that they have mass which is useful as they are a candidate for non-baryonic dark matter). They are generally cofined to weak interactions.

At least one variety (mixture) must have mass ("rest mass"), otherwise there'd be no neutrino oscillations; these are now firmly established from observation. There's furious work going on to re-do core-collapse supernova models, to incorporate these oscillations.

Neutrinos are not likely to be more than a small fraction of non-baryonic dark matter as they're too hot; dark matter seems to be quite cold.

Here's an image of the Sun, in 'neutrino light'; it's fuzzy (image is >20^o cf <1^o actual) because we can't (yet) focus neutrinos as we can light:
http://antwrp.gsfc.nasa.gov/apod/ap980605.html

 

[jcsd]

It's still not 100% sure but IIRC neutrinoes cannot have both a well-defined flavour and a well-defined mass.