Speaker
Description
Summary
We have conducted to assess the feasibility of measuring the polarization of
lambda_b hyperons in the CERN ATLAS experiment by making the first successful
adaptation of the generation package EvtGen for polarized spin-1/2 particles in a
large hadron detector. The simulations using EvtGen have permitted us to develop
an efficient method to generate polarized hyperons in the ATLAS detector and to
carry out a number of precision tests which replicate many of the real challanges
that will be faced in the actual experiment. The physics focus of our studies is
our plan to acquire over 75,000 inclusively produced lambda_b hyperons in the first
three years of ATLAS running, permitting polarization measurements to be made at
the level of a few percent. Because of the simplicity of the final state, with only
four charged particles, and because of the two displaced vertices involved in the
reconstruction, we expect minimal combinatorial backgrounds. Since detector
acceptance corrections can dominate the polarization analysis, accurate simulations
using polarized particles are required. With the capabilities of EvtGen we have
simulated several hundred thousand lambda_b events with fixed polarization and
fixed alpha parameter. Using an independent Maximum Likelihood approach we have
analyzed the resulting angular distributions from simulated lambda_b decays and
extracted the values of polarization and alpha and found them to be in good
agreement with the values that were originally given as input. We have been greatly
encouraged by our success in developing simple procedures for extracting the
polarization and alpha parameter from simulated data. EvtGen allows to specify the
complete amplitude information and hence to perform a complete simulation of the
angular distributions in a way that properly takes into account the relevant
particle spin structure. The lambda_b polarization was introduced exploiting the
EvtGen capability to manipulate spinor algebra.
We will report on the details of the techniques we used to implement EVTGen for our
physics application and on the various test we conducted to validate its performance
