Correlation Measurement of Lambda-anti-Lambda, Lambda-Lambda and anti-Lambda-anti-Lambda with the ATLAS detector at s=7 TeV

Abstract

This thesis summaries the measurements of correlations between Lambda0-anti-Lambda0, Lambda0-Lambda0, and anti-Lambda0-anti-Lambda0 hyperon pairs produced inclusively at the LHC, which are useful for a better understanding of the quark-antiquark pair production and jet fragmentation and hadronization processes. The analysis is based on hyperon pairs selected using the muon and minimum bias data samples collected at the ATLAS experiment from proton-proton collisions at a center-of-mass energy of 7 TeV in 2010. Excess Lambda0-anti-Lambda0 are observed near the production threshold and are identified to be originated from the parton system in the string model in the MC sample, decaying either directly or through heavy strange resonances such as Sigma0 and Sigma∗(1385). Dynamical correlations have been explored through a correlation function defined as the ratio of two-particle to single-particle densities. Positive correlation is observed for Lambda0-anti-Lambda0 and anticorrelation is observed for Lambda0-Lambda0 and anti-Lambda0-anti-Lambda0 for Q in [0,2] GeV. The structure replicates similar correlations in p-pbar, p-p, and pbar-pbar events in PYTHIA generator as predicted by the Lund string fragmentation model. Parameters of the “popcorn” mechanism implemented in the PYTHIA generator are tuned and are found to have little impact on the structure observed. The spin composition of the sample is extracted using a data-driven reference sample built by event mixing. Appropriate corrections have been made to the kinematic distributions in the reference sample by kinematic weighting to make sure that the detector effects are well modeled. A modified Pearson’s chi-squared test statistics is calculated for the cosθ∗ distribution to determine the best-fitted A-value for data. The results are consistent with zero for both like-type and unlike-type hyperon pairs in Q in [0,10] GeV and Q in [1,10] GeV respectively. The data statistics in the range of Q in [0, 1] GeV is currently too low for the estimation of the emitter size for Fermi-Dirac correlation.