Difference between revisions of "07/07/2020"
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J. S.: | J. S.: | ||
| − | A. D.: | + | A. D.: |
| + | *Remembered why so far the GDH data on neutron have not been published in spite of the available deuteron data from MAMI/ELSA. This is because of the very large contribution from the deuteron 2-body breakup at very low nu. With the 1/nu weighting, it cancels almost exactly the resonance+large nu part and accurately correcting for it to get the neutron information is very delicate without a very good model of the deuteron structure, see [https://arxiv.org/abs/nucl-th/0006083 the study by H. Arenhövel] concluding that ''It is very doubtful, if not impossible, that one can extract in a simple manner the neutron spin asymmetry from the spin asymmetry of the deuteron.'' | ||
| + | This is a problem for the MAMI/ELSA data, but less for us for the following reason: | ||
| + | **The problematic contribution from the 2-body breakup | ||
Revision as of 09:22, 8 July 2020
Present: M.D., A.D., J.S., S.Š
General:
M.D:
S.Š: (By email):
I have coded the unpolarized Bethe-Heitler cross-section for a nuclear (carbon) target, and the result is shown in the attached figure in orange: the elastic part (dotted), the elastic+quasielastic (dashed) and the elastic+quasielastic+inelastic = total (full curve). I used the parameterizations (B49), (B52,53) and (B56,57) from Tsai (1974).
Now how does that compare to the free proton case? For now the only meaningful comparison I was able to make was in the elastic part. Assuming (B49) for the nuclear elastic form-factor I learn that this scales as mass*Z^2 (!!!). So I have multiplied the elastic part of free-proton Bethe-Heitler by 12*36 (!!!) and got the green dotted line.
Am I telling you that we should have multiplied the proton BH cross-sections by 12*36 = 432 instead of 6 or 12 ?? I was hoping that the target mass ("mi" in the paper) cancels somewhere, but it does not seem so. In Eq. (2.1), for instance, there is a mi in the numerator, but (k*pi) in the denominator is just Egamma*mi, so both mi cancel. So there is this unfortunate 12 in the form-factor and there is a 36 factor due to the charge in (B49), I am afraid, unless I am missing something.
It is harder to compare the inelastic parts on the same plot, as these scale linearly with Z, as in (B52,53) and (B56,57).
J. S.:
A. D.:
- Remembered why so far the GDH data on neutron have not been published in spite of the available deuteron data from MAMI/ELSA. This is because of the very large contribution from the deuteron 2-body breakup at very low nu. With the 1/nu weighting, it cancels almost exactly the resonance+large nu part and accurately correcting for it to get the neutron information is very delicate without a very good model of the deuteron structure, see the study by H. Arenhövel concluding that It is very doubtful, if not impossible, that one can extract in a simple manner the neutron spin asymmetry from the spin asymmetry of the deuteron.
This is a problem for the MAMI/ELSA data, but less for us for the following reason:
- The problematic contribution from the 2-body breakup
