[G12] Target Position

[Michael Paolone]

Hi Volker,

What type of acceptance correction was applied originally (if any)?

-Michael

> Good Morning Everyone,
>
> I would like to raise another challenging aspect of the g12 experiment
> that we are currently dealing with at FSU and that may possibly affect
> other people. For a long time, we have considered the double-pion channel
> a by-product of Zulkaida's ω analysis. It is the same measured final
> state and given the good agreement of the g12-ω cross section with the
> published g11a-ω cross section, we did not expect any serious problems.
> Note that Zulkaida's g12-ω cross section is however about 7-8% larger on
> average. The ratio distribution of corresponding g12 / g11a (cross
> section) data points shows an almost perfect Gaussian; it is shifted by
> 7-8%, though.
>
> We have accepted that we may not be able to understand this remaining
> discrepancy. While working with the Bonn-Gatchina group on the PWA of the
> double-pion channel, we noticed that we are loosing the (usually) dominant
> Δ-peaks in the invariant pπ mass distributions. This was puzzling since
> the Δ's show up as pronounced features in the g11a data. Upon
> investigating this, Zulkaida discovered that the target position plays a
> crucial role. The center-of-mass angular distributions of our final-state
> particles (in the ω and ππ channels) depend strongly on the z-vertex;
> we see huge holes when we plot c.m. angles versus the z-vertex. Cutting
> out these holes when analyzing final states with more than two particles
> is not an option (since it does not remove the effect of integrating in
> higher dimensions and will require a higher-dimensional acceptance
> correction). I think this was briefly discussed yesterday (but I had to
> leave the meeting early).
>
> We looked back at the ω channel and compared the cross sections from the
> forward half of the target with the cross section of the backward half.
> The angular distributions are in good agreement (the ratio distribution
> looks Gaussian) but unfortunately, we observe a 10-15% normalization
> effect. It appears that this could be the missing piece in understanding
> the discrepancy in the g11a / g12 ω comparison.
>
> Zulkaida will prepare some slides for next week's g12 meeting. I am afraid
> we will have to substantially increase the systematic uncertainty of the
> ω cross section (unless we apply an event-based acceptance correction
> using some kind of PWA). It also remains to be seen if and how the
> two-pion channel can be partial-wave analyzed. This is an issue that could
> affect all multi-particle final states, e.g. η, η', ω, …
>
> Has anybody else noticed the problem?
>
> Best wishes,
>
> 	Volker
>
>
>
>
> _______________________________________________
> G12 mailing list
> G12 at jlab.org
> https://mailman.jlab.org/mailman/listinfo/g12
>