Input: PLUTO simulation gen2

For the following investigations I have selected events from the PLUTO simulation DST, which contain exactly TWO primary lepton tracks originating from the same mother particle created in PLUTO event generator. The reason for this selection was to keep the algorithm for the following investigations as simple as possible. The selected sample contains lepton tracks originating from different decays e.g. pi0, eta,... and conversion as well. The tracks have to produce signal at least in RICH and inner MDC and somewhere behind the magnetic field (requirement on PidTrackCandidate; I do not require full reconstruction of the tracks). Since I'd like to separate "close pairs" and "open pairs" and look into detector signals for these two cases, the selection is correct.

In the following I separate my sample into pairs, where:

• both partners contribute to a single ring or to two rings with some overlap region double-track rings
• both partners build two well defined and well separated rings with NO overlap single-track rings

Properties of selected tracks

In the selected sample of 1.8M events only 205k primary e+/e- pairs from following decays can be found (at the most one pair per event):

Process	Generator Info	Pairs
gamma conversion from pi0	7001	90766
pi0 Dalitz	7051	38462
eta Dalitz	17051	37912
Delta Dalitz	34051	37798

On the following picture the e+/e- pair opening angle distribution for above mentioned sources of e+/e- pairs is depicted. The displayed range is set to 0 degrees to 25 degrees, which corresponds the to the calculation of the angle to the closest candidate in the inner MDC.

e+e- opening angle distribution for different sources

In the following I compare the true opening angle (left) distribution and distribution of the angle to the closest lepton candidate (right) as measured by the inner MDC. The distributions are plotted for single- (blue) and double- (red) track rings. and for all e+e- sources. For the dot-filled spectra, an additional momentum cut p > 50MeV/c on both legs of the pair was applied.

True opening angle distribution for single and double track rings	Angle to closest lepton track for single and double track rings

	All pairs	After momentum cut
Double track rings	164379	147512
Single track rings	2677	21840

The momentum distributions of double- (left) and single- (right) track rings are displayed on the following two pictures. The black lines highlight the p = 50 MeV/c border.

Momentum distribution for double track rings	Momentum distribution for single track rings

Comparing the distribution of the angle to the closest lepton candidate for both legs of the pair agrees in 85% for double-track rings and in 93% for single track ring. The disagreement is visible mainly at small angles, even for cases, when both partners are in different MDC cells.

Angle to closest lepton candidate for double-track rings	Angle to closest lepton candidate for single-track rings

On the following two pictures the inner MDC chi2 distribution is depicted. For double track rings both track of the pair share the same MDC segment in 54%. Out of this amount only in 47% cases the segment is not fitted, the rest has passed the fit procedure. For the single-track rings the situation looks better. Only in 42 cases out of 26.7k pairs both legs share the same segment, but in all cases this segment was unfitted. In only 4.5% cases on of the legs has unfitted segment, while applying the 50 MeV/c momentum cut on both legs, this number is reduced by factor of 2.

Inner MDC chi2 distribution for double-track rings	Inner MDC chi2 distribution for single-track rings

The true opening angle distributions including the segment fit information are displayed on the following two pictures for single- and double- track rings. The inner MDC chi2 cut tend to select tracks with small opening angle. Unfortunately a not negligible amount of track with small opening angle and same MDC segment survive this cut.

Opening angle distribution for double-track rings for different inner MDC cuts	Opening angle distribution for single-track rings for different inner MDC cuts

RICH signal

On the following pictures I plot RICH ring properties each other. They are:

• pattern matrix (PM)
• average charge (AV_charge)
• number of pads (npads)
• ring centroid (centroid)

In the following, different ring properties are displayed for double- (left) and single- (right) track rings. There is no additional cut on the quality of the track applied.

Ring properties for double-track rings	Ring properties for single-track rings

Comparing the both cases one can see, that the distributions are similar except of the number of pads distribution. The 1D distribution is on the following picture. The number of pads distributions for single- (blue) and double- (red) track rings are selected the exactly the same way as all the distributions before. The brown dashed distribution had in addition to the double-track ring selection a cut on inner MDC chi2 applied. It select rings, which at least one leg of the pair has negative chi2.

Number of pads for double (red) and single (blue) rings

Yields for several cuts on number of pads are in the following table:

	Total	<20	<25	<30
Double-track rings	164370	30%	52%	73%
Double-track rings with at least 1 unfitted segment	56267	24%	45%	67%
Single-track rings	26677	65%	86%	96%

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-- MartinJurkovic - 15 Oct 2007