What are neutrons made of?

[gib65]

I've been visiting this site which gives a really cool overview of the basics of particle physics:

 

http://particleadventure.org/particleadventure/

 

It confused me at one point, however, when it talked about what neutrons were made of. At this page:

 

http://particleadventure.org/particleadventure/frameless/hadrons.html

 

it says that neutrons are a type of hadron which are defined as a composite of 3 quarks, and in the case of the neutron, it is 2 up quarks and 1 down. But then on this page:

 

http://particleadventure.org/particleadventure/frameless/neutrinos.html

 

it says that when neutrons reduce their momentum to near zero, they decay into an electron, a proton, and a neutrino. So which is it?

 

Gib

[granDODyssey]

A neutron can be either bound in an atomic nucleus or free.

it could have 2 kinds of momentum : angular momentum(spin) or momentum like any other object when they are moving ( for free neutron ...like in nuclear reaction ).

 

A free neutron is not stable and decayed into a proton, an electron, and a neutrino having Half-life of about 1*10^3 sec

[Severian]

It is both. The neutron is udd (on up-quark and two down-quarks). The proton is uud. The down-quark can decay into an up-quark by emitting a 'W-boson', and the W-boson can then decay to an electron and neutrino. So the down's decay looks like:

 

[math]d \to W^{-} u \to e^- \bar \nu_e u[/math]

 

The electron and neutrino escape but the up is sucked back in, making a uud state, a proton.

[Ophiolite]

Severian, your explanations of the sub-atomic world are always clear. Can you recommend some excellent web-sites (or books) that offer a current, well structured, lucid and maybe even entertaining digest of the field for an educated layman?

 

I've googled through scores, but am not always sure about their reliability.

 

(gib65, sorry for hi-jacking your thread, but I think these could be useful to all of us.)

[Severian]

To be honest I am probably not the person to ask, since I would not visit sites for the 'layman'. It is rather difficult to find anything which is suitable because once one goes beyond the simple '3 quarks in a proton' sort of thing, one really needs quite abstract maths.

 

On a simple level, the particle adventure is a good place to start, but might be too basic. There are some links to other sites too. Perhaps you could also try CERN too? There is also an equiring minds site at Fermilab.

 

For something more advanced you need to go to text books. 'Quarks and Leptons' by Halzen and Martin is a good place to start, but bear in mind that it is a textbook. This is the simplest textbook that I can think of which is reasonably rigourous. It doesn't really do justice to the beauty of the Standard Model though; for that you would need something much more advanced, like 'An Introduction to Quantum Field Theory' by Peskin and Schroeder. This latter is quite advanced mind - it is a book I would give to a starting PhD student.

[Ophiolite]

Thanks for the prompt and comprehensive reply. I'll start delving into these. If I ever make it as far as Peskin & Schroeder I'll let you know! Thanks again.

[swansont]

it says that when neutrons reduce their momentum to near zero' date=' they decay into an electron, a proton, and a neutrino. So which is it?

[/quote']

 

That's not really what it says. The momentum doesn't need to be near zero for the decay. It's when the momentum is near zero that it's easy to observe that momentum wouldn't be conserved by only the observed proton and electron, which is how the antineutrino was inferred. (and it's antineutrino in this case; the electron is a particle and has to be accompanied by an antiparticle to conserve lepton number. Leptons being the classification of particle to which electrons and neutrinos belong)

[gib65]

I see, makes sense. Thanks

[Bernstein]

It is both. The neutron is udd (on up-quark and two down-quarks). The proton is uud. The down-quark can decay into an up-quark by emitting a 'W-boson'' date=' and the W-boson can then decay to an electron and neutrino. So the down's decay looks like:

 

[math']d \to W^{-} u \to e^- \bar \nu_e u[/math]

 

The electron and neutrino escape but the up is sucked back in, making a uud state, a proton.

 

An excellent and lucid account of the reactions contained in such a decay.

[ed84c]

does this happen before or after the 10^34 years it takes a proton to decay?

[Severian]

Before, obviously! The 10^34 years is only a limit. No-one has ever seen a proton decay, but that doesn't mean that it does not - just that it is incredibly unlikely.

[ed84c]

is that 10^34 years the begginging of decay or a half life for the proton?

[YT2095]

protons being basic particles don`t have a "half life" Atoms (more specificaly Isotopes of atoms) do.

[Severian]

Yes they do, and that is what that is. The half life is defined by imagining you had a large number of protons, and counting how long it takes for half of them to have decayed. Since no-one has seen even one proton decay, the half-life must be veeery long. When you consider that the age of the universe so far 1.4x10¹⁰ years, the limit on the half life is pretty strong...

 

Protons aren't fundamental particles by the way.

[YT2095]

I didn`t say or mean Fundemental (like quarks bosons neutrinos etc...), I said basic parts when refering to an Atom.

in atomic decay of an isotope, proton decay isn`t mentioned.

 

Isotopes have Half Lives.

[swansont]

I didn`t say or mean Fundemental (like quarks bosons neutrinos etc...)' date=' I said basic parts when refering to an Atom.

in atomic decay of an isotope, proton decay isn`t mentioned.

 

Isotopes have Half Lives.[/quote']

 

Isotopes are just different versions of atoms (different number of neutrons). Some are stable and some aren't. "Isotopes have Half Lives" really doesn't make sense. O-16 is an isotope of oxygen, and it's stable. No half-life.

 

Protons do decay when they are in nuclei - it's called beta plus decay (there's also electron capture). But there has to be an available neutron energy level that allows a release of energy, i.e. a lower energy state. Then the proton changes into a neutron, a positron and a neutrino. (The up quark changes to down)

 

It's forbidden (i.e. very long half life) for a free proton because there isn't a lower energy state available.

[YT2095]

aha! thnx

 

I guess that`s the diference between a Chemist and a Phsicist

we never have to deal with the deeper intricacies.

[fermions]

anyway does it mean that a proton will not decay in a stable nucleus just like neutrons in stable nucleus ? or the protons will eventually all decay to pi+ and neutrinos?

[Guest Mr. Safety]

YT2095.. that is merely the difference between chemistry and quantum mechanics.

Physics is a whole different ball park.

[Gilded]

"The down-quark can decay into an up-quark by emitting a 'W-boson', and the W-boson can then decay to an electron and neutrino."

 

Nice explanation. But how does the quark decay work in the case of positron emission? I know that one of the up-quarks becomes a down-quark, emitting a positron and an anti-neutrino, but is there a boson or something between?

[Severian]

Yes. It is the conjugate (antiparticle) of the W^-, the W⁺. so you have [math]u \to W^{+}d\to e^+ \nu_e d[/math]

 

PS: Notice that is a neutrino which is emitted, not an anti-neutrino, since in this case it is the e⁺ which is the anti-particle.

 

PPS: CP invariance tells us that the matrix element for this decay is the same as the previous decay (of the d). So the only reason why this only happens in special cases is because of the restricted phase space (as Swanston pointed out earlier).