Measurements of A0 and A0B Baryon Properties in the Atlas Experiment.

[摘要] To study the bound system of 3 valence quarks, Lambda and Lambda_b baryons represent ideal particles, since they are produced in relatively high quantities at the Large Hadron Collider (LHC) and decay into fully reconstructible final states. This thesis summarizes results of two important measurements with Lambda and Lambda_b baryons: a measurement of the Lambda_b lifetime and mass, and a measurement of the Lambda polarization in minimum bias and di-jet events.The Lambda_b lifetime is predictable within a framework of the heavy quark expansion (HQE) calculations, which is widely and successfully used in b-physics. In past decade, the Lambda_b lifetime has received some special attention due to a discrepancy between the theoretical prediction and experimental results. The measurement presented here improves on the precision of the previous measurements and helps resolving this discrepancy. The Lambda_b lifetime and mass are measured to be tau(Lambda_b) = 1.449 +- 0.036 +- 0.017 ps and m(Lambda_b) = 5619.7 +- 0.7 +- 1.1 MeV. A value of the Lambda_b and Bd lifetime ratio is measured, too, with the value of tau(Lambda_b)/tau(Bd) = 0.960 +- 0.025 +- 0.016. The measured value of the ratio is compatible with the theoretical predictions, which range between 0.86 and 0.97.Large hyperon polarization observed at low-energy fixed target experiments has been puzzling physicists for decades, since their results are incompatible with pQCD calculations. This suggests that some unknown mechanism, which cannot be described by pQCD, is responsible for the hyperon polarization. Measurements at the LHC open up a new avenue for study of the hyperon polarization, since the measurement of polarization of the Lambda hyperon produced within a jet becomes possible for the first time. This measurement probes non-perturbative QCD processes, such as fragmentation and hadronization, as possible sources of the hyperon polarization. Lambda polarization is measured to be -0.007 +- 0.006 +- 0.012 in the minimum bias and 0.007 +- 0.011 +- 0.007 in the di-jet sample. The observed results agree with the extrapolation of the low-energy data.

[发布机构] University of Michigan

[效力级别] Lambda_b Lifetime

[关键词] B-physics;Lambda_b Lifetime;Lambda_b Mass;Lambda Polarization;Atlas;LHC;Physics;Science;Physics