[Frost] Neutron Measurements with FROST

[Volker Crede]

Hi all,

last Wednesday in the FROST meeting, we started discussing the possible 
physics case for a short, three-week long FROST run for measurements off 
the neutron using deuterized butanol. In my opinion, the most interesting 
physics topic to advertize would be the study of the 1650-1700 MeV mass 
region in eta photoproduction. Using a linearly-polarized beam at 1.1 GeV 
coherent edge position would be ideal; in combination with transverse 
target polarization, the observables H and P can be measured.

The reason why this interesting is the relatively narrow structure that 
has been observed at 1680 MeV off neutrons bound in the deuteron at:

* GRAAL (width < 30 MeV): Kuznetsov et al., Phys. Lett. B647 (2007) 172.
* ELSA (width < 60 MeV): I. Jaegle et al., PRL 100 (2008) 252002.
* MAMI (width about < 40 MeV): not yet published
* Tohoku-LNS (width < 40 MeV):
    F. Miyahara et al., Prog. Theor. Physics Supplement 168 (2007) 90.

A pronounced bump appears in the total cross section. Although the nucleon 
resonance, D15(1675), is not a likely cause of the narrow structure, it's 
role in this reaction is not entirely understood; it cannot be ruled out 
that significant contributions from this state in addition to the narrow 
structure cause the much slower fall-off of the neutron cross section in 
this energy region compared to the proton. I have attached a picture with 
sensitivity studies on the D15(1675) using MAID at 1 GeV. The solid, red 
curves indicate the full model; the dashed, blue curves without D15(1675). 
The model predicts asymmetries of measurable size for basically all pol. 
observables. The cross section data are from ELSA, the beam asymmetry was 
measured at GRAAL.

Taking data for all other reactions simultaneously is certainly also 
useful, but since we have only three weeks, I think eta photoproduction 
offers this particular physics case. A dedicated run at 1.1 GeV for both 
transverse target polarizations (to get H and P) would be very useful.

What do you think?

    Volker
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