Aug23 200

Overall background (tail)- efforts to understand it.

a) What is it? See in Tom's page , as discussed last week. We refer to this analysis as "tom1".
If you look at the y=0 spectra (yellow), the background (tail) we are talking about is that part of the spectra that is not fitted by the 4 Gaussians.
It's worth emphasizing that this is a small effect, barely noticible in the (lower statistics and worst resolution) earlier analyses.

b) The thought was that this "low intensity" tail could be due to the TPC central membrane.
To test that, we made a run on the (new) flow runs (P01he) cutting on ...
mVertexZ>0 and z-momentum>0
In this way, no track is allowed to cross the central membrane.
A comparison with tom1 shows the same features, that is, the ratios of the spectra are constant within statistics.

c) At this point, Steph observed that the word mDca (distance of closest approach for primary tracks) was essencially 0 (<1e-6) for all events in the (new) flow runs (P01he). See Steph's page
This is interesting, we thought, because we require mDca<3 (cm) in our analysis. So this cut has no effect on the new data set. (Steph found out later from Raimond that indeed the new set should have mDca =~ 0.)

In the old flow files (P00hm), mDca has values as large as 6, so we applied the cut mDca>0 to the old data. A comparison with mDca<3 shows no significant difference.

d) In summary, we conclude that this background (tail) is most likely not due to either the presence of the central membrane or the restriction in the new anaylysis for all the primary tracks to have mDca < 1e-6 cm.