Tracking Quality Cuts and Ks Selection



 After we have selected our sample of DIS events, we require only events with at least 2 secondary vertices identified.

 A set of quality cuts are applied to the tracks (pions) in order to have a good Ks candidate mass reconstruction. Extra quality requirements are put on the impact parameters of the secondary vertex with respect to the primary vertex. These cuts also play an importante role in cutting some background and are listed bellow:
Number of DoF > 32
Upper most super layer in CTD with hit >= 3
Number of tracks < 60
Impact parameter on XY plane < 0.4 (*)
Impact parameter on YZ plane < 0.6
Collinearity angle of the Ks candidate < 0.085 (*)

 (*) Here we depict the impact parameter dxy and Ks collinearity angle.

 The items bellow are links to the plots where we show MC (Ariadne + Herwig) for true reconstructed K0, non-K0 and (anti-)Lambda for the above variables, along with a study on the efficiency and background contamination after we have applied the selected cut values.
 In addition, the Ks sample purity is improved by removing (anti-)Lambda and photoconversion (e+ e-) candidates. We apply the following cuts:

Armenteros variable > 0.110 (*)

 (*) Click here for a description of the armenteros variable.

 We eliminate most of the extra background by using the cuts defined bellow:

1.1 cm < decay lenght of the Ks candidate < 40.0 cm
Cossine between the 2 Ks candidates < 0.9 (*)

 (*) Here you find the reason for this cut.

 A further requirement on the mass of the reconstructed Ks to be inside a 2*sigma region around the fitted mass peak (click here for the mass of the Ks after all cuts) leads to a final clean sample of Ks candidates.

 Similar studies as the ones done for the quality of the tracks ( links above ) are done for the "purity" cuts. Click on the links to see the plots:
 The correlation between the cuts are also studied, where all cuts but the one under study are applied to our sample of candidates. You can obtain the postscript files for each study by clicking on the items bellow: