Study of b-hadron decays into two hadrons and a photon at LHCb and first observation of b-baryon radiative decays

Abstract

This thesis is divided in different sections. After a short introduction about the thesis contents, the theory chapter presents the Standard Model of Particle Physics (SM), with an emphasis on those properties of flavour physics, such as oscillations. In this chapter a brief comment on radiative decays, the main topic through the thesis, is given, to present the specific theoretical description of these processes.

The European Centre of Nuclear Research (CERN) is introduced in the second chapter, where the nature of such organization and the different experiments it holds are explained. Special emphasis is given for the LHCb experiment, which is the one that collected the data used in the analysis.

The third chapter describes the monitorization and re-calibration of the SPD sub- detector, which is the one in charge of the discrimination between photons and other electromagnetic particles. During the Run 1 (2011-2012), this sub-detector suffered from ageing due to the presence of radiation inside the LHCb cavern. This ageing led to a drop in the SPD cell efficiencies, which could imply a loss in the data quality. The SPD is key in the context of radiative decays due to the presence of photons in the radiative final states and due to the fact that the main sources of background for radiative decays are related to the mis-identification of other particles as photons. With the work presented in this thesis, the performance of the SPD sub-detector reached the same status than it did for the beginning of Run 1, which corresponds to an almost flat cell efficiency around 95%. This recovery in efficiencies was performed by applying a correction factor calculated through the study of cosmic ray data and collision data from 2015.

The fourth chapter describes a software tool developed within the radiative decays context. This tool is aimed for a better description of the photon/neutral pion separation variable, which is built making use of the electromagnetic clusters that these particles leave in the LHCb electromagnetic calorimeter (ECAL). This tool builds a variable that separates between the two particles but discrepancies between the data and the simulation distributions are observed. Because of this, two additional tools were developed to improve the agreement between them. These tools trust that the separation variable can be binned in terms of the particle transverse momentum and pseudorapidity. The first of the tools, “efficiency table tool”, assigns the efficiency for a certain 2-dimensional bin as a weight to the simulation sample, so the distribution is corrected and matches the one for data. The “resampling tool”, however, creates a new variable from calibration histograms built from background- subtracted distributions, trusting all radiative decays to have a similar behaviour in the 2-dimensional bins. The two tools can also be applied to neutral pions selection.

The main chapter of the thesis is related to the measurement of the ratio of branching fractions for Bs decaying into phi (which decays into two kaons) and a photon and Lambda_b, decay into a Lambda bayron (which decays into a kaon and a proton) and a photon with respect to the best controlled radiative decay, Bd decaying into a K* (which decays into a kaon and a pion) and a photon, as well as the measurement of the direct CP asymmetry for the Lambda_b and Bd decays. This analysis also implies the first observation of a b-baryon radiative decay and constitutes the best measurement of the observables presented above. The analysis is performed over the whole Run 1 LHCb dataset (3 inverse fb). Special care is applied to the Lambda_b decay since it had never been done before; taking into account the different resonances that may

intervene (that cannot be distinguished since they overlap and interfere). A 2- dimensional binning (on the resonance mass and on the proton angle) is defined for the study of the efficiencies, as recommended by the theory approaches to this decay and trusting (and confirmed afterwards) the different resonance decays to give a similar efficiency for the offline selection for a given bin.

The thesis concludes with an overview of the whole work presented, given the calculated results for the physical observables and comparing them to the previously measured ones in the LHCb.

Esta tesis está dividida en varias partes. Después de una breve introducción sobre el contenido de la tesis, las dos primeras de ellas presentan el Modelo Estándard de la Física de Partículas (ME), el contexto teórico que describe la naturaleza de las partículas y sus interacciones; y el CERN, el laboratorio de física de partículas más grande del mundo. Se dedica especial énfasis a la física de las “desintegraciones radiativas”, que son aquellas en las que un fotón forma parte del estado final de la desintegración.

La tercera de las secciones de la tesis describe la toma de datos por parte del sub- detector SPD así como el proceso de recuperación del funcionamiento posterior a la observación de pérdida de eficiencia debida a la presencia de radiación en la caverna del LHCb. Este sub-detector es el encargado de proporcionar la información necesaria que permita la diferenciación entre fotones y piones neutros y por tanto muy importante en procesos radiativos.

La cuarta sección explica el desarrollo y funcionamiento de una herramienta de software destinada a una mejor descripción de la variable de separación entre fotones y piones neutros a partir de la forma de los clusters que estas partículas dejan en el calorímetro electromagnético del LHCb.

La quinta sección explica el análisis de datos recogidos por el experimento LHCb durante los años 2011 y 2012 en términos de la medida de frecuencia de desintegración de procesos radiativos. En particular, se estudia la relación entre la frecuencia de desintegración entre el proceso de desintegración de un mesón Bs decayendo a una partícula phi (que a su vez se desintegra en dos kaones) y un fotón y un barión Lambda_b decayendo a una partícula Lambda* (que a su vez se desintegra en un protón y un kaón) y un fotón respecto de la desintegración de un mesón Bd a un K* (que a su vez se desintegra en un kaón y un pión) y un fotón así como la medida de asimetría de CP-paridad para las dos últimas desintegraciones. Este análisis supone la medida más precisa para estas magnitudes hasta la fecha y la primera observación de una desintegración radiativa de un barión con un quark b.

La tesis termina con un capítulo de conclusiones donde se detallan los valores encontrados para las magnitudes físicas y el impacto del trabajo presentado en esta tesis, comparando los valores hallados con los existentes anteriormente y recordando que este trabajo supone la primera observación de una desintegración radiativa de un barión cuyo contenido de quarks incluye un quark b.