Dsh analysis meeting minutes 13-01-11

Present : Laurence, Sajan, Niels, Nicola, Barbara

On EVO : Piotr, Mariusz

Presentations

Piotr :
Presented status of the fitter. He has tried to fit to Vava's new cocktail samples. The DPi fit (slides 1--3) looks fine, except maybe around 5050MeV. This is thought to be due to the D*Pi template not including the double peak structure. The DK fit (slide 6) does not look good. Even fixing the number of DPi events doesn't help much. This is surprising as the fit to the old cocktails sample worked okay and it is not clear what could have changed in the new sample to cause this problem. The toy fits to DK look similarly bad to the cocktail fit. The pulls (slide 5) are means of order 20, so they are way off (each toy has 15,000evts, much more than in the data). Also strange is the reweighted DPi mass shape on slide 4. Previously the shape was seen to change a lot when reweighted following the DLL cut, but here it hardly changes at all. Barbara and Nico agreed to try to reproduce the toy fits for DK to see what they get. Also it was suggested to try re-including the DRho component in the cocktail fit.

Nico, Barbara:
Some responses to the referee's questions were presented. The main discussions centred around: a) Question 23 (slide 5). Niels proposed that the error on the yield with BDT is larger than with the cut-based because the BDT was optimised to reject combinatorial bkg, not all bkg. This hypothesis could be checked by looking at the S/sqrt(B) for the DK plot for each selection, taking all bkg into account. The DK/DPi ratio was claimed by the referees to be 2.9sigma different in the two selections. But Kazu's calculations (also done by Laurence) show that the difference is only just over 1 sigma (see slide 14). Of more concern is the fact that the number of DPi event varies (by many sigma) between the BDT and cut-based (slide 14 again). Laurence pointed out that the comb bkg shape for the DK fit is flat, which is rather different from the sloped shape seen in the DPi fit. Perhaps fixing the comb bkg shape to that obtained in the DPi fit could help (this shape should be rather insensitive to the PID of the bachelor). b)Question 22 (slide 15). The simplest answer is that the wiggles are due to the low stats used to make the RooHistPDF template. They can in principle be smoothed out by using a RooKeysPDf instead. The question is whether the source of the wiggles is physical or just a fluctuation. While the Lambda_b->Lambda_c Pi could cause the bump at 5430, we have no hypothesis for the bump at 5370. But in any case, whatever is there in the DPi spectrum under the DPI hypothesis is very likely to be there under the DK hypo as well. If the bachelor is a true kaon there would be a larger contribution under the Dk hypo, but this isn't really seen. But in any case it is important to state that the PDF doesn't fit the wiggles in the DK hypo data, the wiggles just happen to (whether for a physical reason or just due to a fluctuation) exist in similar places in the DPi hypo and Dk hypo data.

-- LaurenceCarson - 13-Jan-2011

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