[Frost] Neutron Measurements with FROST

[Igor Strakovsky]

Barry, let me clarify physics (experimental part is hard, no questions).

First of all, the structure in the cross section of the gn-->etaN at W =
1680 MeV has been observed by three (correctly four as Volker pointed out) 
groups independently.  The cross section on the neutron is larger than on
the proton by a factor of 7 (for the P11 case, u-spin can explain this
large factor).  Sigma-beam asymmetry changes a sign at the same place.
That is not a hunting for a bump.  Something is going on here and it is
good to figure out what is going on

Mike, if our feeling is correct and we met a non-strange narrow resonance
(say 20-40 MeV) that is something.  We do not know any so narrow and
non-strange baryons.

Pi-p case is bad for the P11 (BR P11 into pi-p is too small) but we found
a suspect via piN modified PWA.  So, there is a chance that polarized 
measurements will allow to catch up something

Igor

On Mon, 24 May 2010 22:47:02 -0400 (EDT), Michael Dugger <dugger at jlab.org> 
wrote:

> 
> Just to clarify something that I think Eugene forgot to mention: The pi- 
> on the neutron probably won't show the bump. The physics motivation is 
> different. The good part is that we should be able to get this channel 
> easily. The bad part is that the physics motivation is not as strong.
> 
> -Michael
> 
> On Mon, 24 May 2010, Eugene Pasyuk wrote:
> 
>> Eta-production on the neutron is hardly possible with FROST or even
>> HDIce for that matter. I looked in to that when HDIce proposal was
>> written. One would need to detect two charged pions, two photons from
>> pi0 decay and a neutron. The acceptance is way below 10^-3, more like
>> 10^-4. That's on the top of a large background. Acceptance of CLAS to
>> 2gamma decay of eta is tiny as well. This case is even worth, no charged
>> particles in final state at all! That's ok for detectors like Crystal
>> Ball or Crystal Barrel, but not for CLAS.
>> What seems to be doable is K0Lambda on the neutron. This is four charged
>> particles final state.
>> And of course pi- on the neutron.
>>
>> -Eugene
>>
>> On 5/24/10 5:33 PM, Volker Crede wrote:
>>> 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