| Topic: |
Science > Physics |
| User: |
"robert j. kolker" |
| Date: |
26 Mar 2005 06:43:55 AM |
| Object: |
What is the spin of a proton |
What is the spin of a proton. How is it measured and what is its value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Bob Kolker
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| User: "Schoenfeld" |
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| Title: Re: What is the spin of a proton |
26 Mar 2005 09:31:26 PM |
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robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its
value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Protons are fermions and fermions are elementary particles of, 1/2
spin. Spin-statistics complexity of proton intrinsic angular momentum
is due to empirical antisymmetric invariance of time-evolving fermion
quantum state, giving rise to Pauli-exclusion.
Bob Kolker
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| User: "Bjoern Feuerbacher" |
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| Title: Re: What is the spin of a proton |
29 Mar 2005 02:33:54 AM |
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Schoenfeld wrote:
robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its
value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Protons are fermions
Right.
and fermions are elementary particles
In general wrong.
of, 1/2 spin.
Right (if you mean here spin 1/2, 3/2, 5/2, ...).
[snip]
Bye,
Bjoern
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| User: "Schoenfeld" |
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| Title: Re: What is the spin of a proton |
30 Mar 2005 12:57:51 PM |
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Bjoern Feuerbacher wrote:
Schoenfeld wrote:
robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its
value?
From the little I have read on the matter, it is non-trivial to
make
this determination since the proton is not an elementary particle.
Protons are fermions
Right.
and fermions are elementary particles
In general wrong.
Take out the 'elementary'.
of, 1/2 spin.
Right (if you mean here spin 1/2, 3/2, 5/2, ...).
Yes.
[snip]
Bye,
Bjoern
Say Bjoern, if I have 2 fermions in a system with 6 possible energy
levels how many microstates does this system have and what energy level
is the system most likely to be in?
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| User: "Bjoern Feuerbacher" |
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| Title: Re: What is the spin of a proton |
31 Mar 2005 10:30:21 AM |
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Schoenfeld wrote:
Bjoern Feuerbacher wrote:
[snip]
Say Bjoern, if I have 2 fermions in a system with 6 possible energy
levels how many microstates does this system have and what energy level
is the system most likely to be in?
Is this a test question? Or am I supposed to do your homework for you?
I suppose you mean two identical fermions?
There are four different possible spin states, 3 symmetric and one
antisymmetric.
If the two fermions are in the same energy state, the total spin
function has to be antisymmetric, and there is only one possibility
for that. So there are 6 microstates where the fermions have identical
energies.
There are 15 possibilities for the two fermions to be in two different
states (6*5=30, take the half because the two fermions are
indistinguishable). For each of those, we can have a symmetric or
an antisymmetric combination. In the symmetric case, the spin function
has to be antisymmetric (1 possibility), so there are 15 additional
microstates. In the antisymmetric case, the spin function has to be
symmetric (3 possibilities), so there are 45 additional microstates.
All in all: 66 microstates. (I hope I didn't fool up somewhere
- I never was very good in combinatorics).
What energy level the system is most likely in depends on the
environment. If we have an isolated system (microcanonical ensemble),
all states are equally likely; if we have a closed system, i.e.
a system in a heatbath (canonical ensemble), the state for which
the product of the degeneracy and the Boltzmann factor is highest
is the most likely. Which state that is depends on the energy levels.
Bye,
Bjoern
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| User: "Bjoern Feuerbacher" |
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| Title: Re: What is the spin of a proton |
29 Mar 2005 02:36:21 AM |
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robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its value?
The hyper-finestructure of hydrogen comes to mind. We see only two
different levels there (spin of electron and proton parallel or
anti-parallel). If the proton had another spin than 1/2, we should see
more (or less) energy levels.
Other evidence probably comes from scattering.
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
I think such determinations are done in cases where the proton can be
considered as "elementary", i.e. by looking at results which don't
depend on its inner structure.
Bye,
Bjoern
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| User: "Sam Wormley" |
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| Title: Re: What is the spin of a proton |
26 Mar 2005 07:19:45 AM |
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robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Bob Kolker
Some background
http://en.wikipedia.org/wiki/Spin_%28physics%29
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| User: "Andrew" |
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| Title: Re: What is the spin of a proton |
26 Mar 2005 08:17:16 AM |
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robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Bob Kolker
http://pdg.lbl.gov/2004/listings/s016.pdf gives its spin as 1/2. You're
right about it being non-trivial :-)
http://search.arxiv.org:8081/paper.jsp?p=hep-ph/0304225&qid=1111845650002639204033
As far as we can tell only about 20% of the proton's spin is carried by
the valence quarks. The HERMES experiment at DESY intends to answer
your question by scattering leptons off protons, the so-called
Generalised Parton Distributions provide a way of measuring the quark
and gluon angular momentum.
http://www-hermes.desy.de/notes/pub/POSTER/arne.santorini2003.jpg
Cheers,
Andrew
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| User: "Andrew" |
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| Title: Re: What is the spin of a proton |
26 Mar 2005 09:16:04 AM |
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robert j. kolker wrote:
What is the spin of a proton. How is it measured and what is its value?
From the little I have read on the matter, it is non-trivial to make
this determination since the proton is not an elementary particle.
Bob Kolker
http://pdg.lbl.gov/2004/listings/s016.pdf gives its spin as 1/2. You're
right about it being non-trivial :-)
http://search.arxiv.org:8081/paper.jsp?p=hep-ph/0304225&qid=1111845650002639204033
As far as we can tell only about 20% of the proton's spin is carried by
the valence quarks. The HERMES experiment at DESY intends to answer
your question by scattering leptons off protons, the so-called
Generalised Parton Distributions provide a way of measuring the quark
and gluon angular momentum.
http://www-hermes.desy.de/notes/pub/POSTER/arne.santorini2003.jpg
Cheers,
Andrew
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