How is the semi-decay period calculated?

Oliver Gutsche, Dissertation, Department of Physics at the University of Hamburg, 2005:

"Beauty quark cross-section measurement in photo production with the ZEUS experiment at the HERA electron-proton accelerator"


"Measurement of Beauty Quark Cross Sections in Photoproduction with the ZEUS Experiment at the Electron-Proton Collider HERA"



Tags:
PACS: 14.65.Fy
Full text

Summary

short version

The standard model of elementary particle physics has proven to be a great success for the description of the microscopic structure of matter over the last few decades. However, there are still unanswered questions. The area of ​​heavy quarks in particular shows differences between experiments and theoretical predictions. In this thesis, beauty quark cross-section measurements in the photoproduction area, measured in ep collisions at the HERA accelerator with the ZEUS experiment, are presented in order to help clarify these differences.

The measurement of beauty quark cross sections requires a reliable beauty quark identification mostly through the utilization of the high mass or the long lifespan of the beauty quarks. The ptrel method is an established method for statistical identification by using the harder spectrum of the transverse momentum of muons from semi-leptonic decays of beauty quarks relative to the original quark compared to lighter quarks. But since the expansion of the ZEUS detector during the HERA II conversion program 2000/2001, there has been the possibility of using another method for beauty quark identification using the long lifespan of the beauty quark relative to lighter quarks. By installing the micro-vertex detector, the impact parameter of the muon relative to the origin of the event can be determined with sufficient accuracy to use the resulting also harder spectrum of the semi-leptonic beauty quark decay for statistical purposes. In this work, jets experimentally represent the quarks through the collimated shower of their decay products and serve as a reference for the ptrel and impact parameter method. In addition, they define a hard scale for the pertubative calculation of the theoretical predictions. This work presents beauty quark cross-section measurements in the photoproduction area in events with two jets and a muon:

ep -> e b bbar X -> e jj mu X

The ptrel method is used to identify beauty quarks in the data collection periods before and after the renovation. These results will then be used to study the applicability of the impact parameter method in the data collection periods after the renovation.

The technical part of the work includes the development of a new event display for the ZEUS detector which reflects the changes in the detector structure after the conversion and the integration of the micro-vertex detector. The new visualization solution offers event visualizations for the data acquisition periods before and after the renovation and is presented through the description of the concept and its implementation.

A global comparison of beauty quark cross-section measurements from different experiments at different accelerators, carried out during a study stay abroad at University College London, describes the framework in which the measurements of this work can be interpreted. The comparison uses leading order plus parton shower Monte Carlo simulations and provides an answer to the question of the compatibility between the individual measurements. The answer to this question is important because the individual measurements do not give a uniform answer on compatibility with theoretical predictions.

The structure of the cross-section measurements and the study of the impact parameter method are briefly described below. First a record of 110 pb-1 of the data acquisition periods before the renovation was used to measure cross sections with the prel T method. A new improved muon reconstruction is established to increase the statistics significantly. Then the experiences from the data acquisition periods before the conversion to the data record of 40 pb-1 Transferred from the new data collection periods in order to determine the first beauty quark cross-sections after the renovation. A comparison of the cross-sections between the data acquisition periods before and after the conversion shows that the data set from the new period with its beauty quark portion determined by the prel T method is suitable for the comparison of the two identification methods. A first feasibility study then shows the applicability of the impact parameter method. At the end, future plans for beauty quark cross-section measurements with the ZEUS detector are described, taking advantage of the long lifespan of the beauty quarks.

title

short version

Summary

The Standard Model of elementary particles has proven to be a big success over the last decades in describing the microscopic structure of matter. Despite this success, it still poses open questions to theory and experiment. In particular the heavy quark sector still shows discrepancies between measurement and theoretical prediction. This thesis presents measurements of beauty quark cross sections in the photoproduction regime in ep collisions with the ZEUS experiment at the HERA collider to contribute to the clarification of these questions.

The measurement of beauty quark cross sections requires a reliable identification of beauty quarks mostly exploiting their heavy mass or long lifetime. A well established procedure for the identification is the ptrel method which exploits statistically the harder spectrum of the transverse momentum of the muon from semi-leptonic beauty quark decays relative to the originating quark compared to lighter quarks. With the upgrade of the ZEUS detector in the course of the HERA II upgrade program 2000/2001, an additional method exploiting the long lifetime of the beauty quark became accessible. With the integration of a micro-vertex detector in the ZEUS detector, a precise measurement of the impact parameter of the muon with respect to the interaction origin can statistically identify semi-leptonic beauty quark decays by their harder spectrum compared to lighter quarks. For both methods, jets experimentally represent the beauty quarks by their collimated shower of decay products and act as a reference for the ptrel and impact parameter methods. In this thesis, they also provide a hard scale for the perturbative calculation of theoretical predictions. This thesis measures photoproduction cross sections of semi-leptonically decaying beauty quarks into muons in dijet events

ep -> e b bbar X -> e jj mu X

The ptrel method is used in pre- and post-upgrade running periods to identify beauty quarks. Afterwards, these results are used to explore the applicability of the impact parameter method in the post-upgrade running periods.

The technical task of this thesis consisted of the development of a new event display for the ZEUS detector incorporating the changes in the detector layout after the upgrade including the new micro-vertex detector. The new solution providing event displays both for the pre- and post-upgrade running periods is presented by describing the visualization concept and the implementation.

A global comparison of beauty quark production measurements from different experiments at different high energy particle colliders, carried out during a research stay at the University College London, describes the frame in which the cross section measurements of this thesis can be interpreted. Using leading order plus parton shower Monte Carlo generators for the comparison, the question of compatibility between the different results is of larger importance because the measurements under investigation do not provide a consistent picture about their compatibility to theoretical predictions.

In the following, a brief description of the structure of the cross section measurements and the study of the impact parameter method is given.

In the pre-upgrade running periods, a data sample of 110 pb-1 is used to measure cross sections with the ptrel method. A new general muon reconstruction is established to increase the statistics significantly. The experience from the pre-upgrade running periods is then transferred to the post-upgrade running periods to measure first beauty quark cross sections using the ptrel method with the upgraded detector in a data sample of 40 pb-1. The comparison between pre- and post-upgrade results establishes a post-upgrade event sample with known beauty content which is suitable for comparisons of the two beauty quark identification methods. A first study of the applicability of the impact parameter method concludes the investigation of beauty quark production after the upgrade. In the end, future plans for measurements of beauty quark cross sections at ZEUS exploiting the long lifetime of the beauty quark are outlined.