V0 Reconstruction Using Fitting Techniques

A. C. Saulys

BNL

Work in Progress

The older report can be found here.

Here are some PRELIMINARY results from my analysis of 99 HIJING central Au-Au events from these files:
/star/rcf/test/gstardata/auau200/hijing135/jetq_on/b0_3/year_1b/hadronic_on/set0029_04_50evts.fzd
/star/rcf/test/gstardata/auau200/hijing135/jetq_on/b0_3/year_1b/hadronic_on/set0028_04_49evts.fzd
This is year 1 geometry, using tracks from TPC only refit using the Kalman filter. I used SL00b_3 version of the library and my private code.
Constraint fits to the V0 vertices and the impact parameter of the daughter tracks at the primary vertex were done using Victor's constrained fit code.
The latest version of the code uses Confidence Level cuts instead of Chisq/ndegf.
More details will be coming...

Armenteros plot and effective mass spectra

The cuts used for the K0s are:

Decay length > 5.0 cm
CL of daughter tracks coming from primary vertex < 10**-6
CL of decay vertex pointing to primary vertex > .01

The cuts used for the Lambdas/Antilambdas are:

Decay length > 10.0 cm
CL of daughter tracks coming from primary vertex < 10**-7
CL of decay vertex pointing to primary vertex > .01
Pt(V0)>0.2 GeV/c
Small angle V0s excluded ( <.1)

No Armenteros cuts, no magic, just basic physics principles and statistical analysis used in the cuts.

The Kalman fit assumes pion mass. This ,of course, is the correct hypothesis for K0s. The decay kinematics of the Lambdas/Antilambdas is such that the baryon momentum is high enough so that the multiple scattering contribution difference between pions and baryons does not seem to matter much.

The Lambda/Antilambda background suppression requires the additional cuts, since the backgrounds are worse. The Pt cut is effective, since we basically have no acceptance for low Pt Lambdas/Antilambdas. This is because of the low momentum cutoff caused by the inner radius of the TPC field cage. Yet for more symmetric decays, there is a low Pt background.

More events have to be analyzed to get a measure of resolutions and yields/event.
The advantages of my approach are that the cuts are momentum and model independent, making corrections easier.
The constrained fits should give a better effective mass and V0 momentum resolutions. This will be evaluated with higher statistics.

More to come...

Last update 6 April 2000