[G12] Target Position

[Volker Crede]

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




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