A basic assumption in fundamental physics is that equal amounts of matter and antimatter were created after the Big Bang. When particles and antiparticles collide, they annihilate, i.e. disappear and produce photons. Nevertheless, the universe consists mainly of matter today. To explain why all matter did not disappear, violation of CP symmetry beyond the Standard Model is required. CP symmetry means that the laws of physics are the same if particles are interchanged with antiparticles and spatial coordinates of all particles are mirrored. CP symmetry is relatively poorly tested in baryon decays. A new method to study CP symmetry in hyperon-antihyperon pairs has been developed at Uppsala University. Hyperons are baryons with one or more strange quarks. The method allows determining the decay asymmetry parameters of the hyperon and antihyperon separately if the hyperon-antihyperon pair is polarized. If any significant difference between the magnitudes of these parameters is found, the process is CP violating. The particle physics experiment BESIII in China is a suitable experiment to conduct this kind of measurements because it is a high precision experiment and has collected large data samples of hyperon-antihyperon pairs. The goal of this project was to study statistical precisions of the physics parameters that can be obtained with the new method in cases of J/ψ meson decaying into ΛΛ, Σ+Σ− and Σ0Σ0. High statistical precision is required to detect CP violation, because CP violating processes are, if they exist, expected to be rare. The main focus was to study the process e+e− → J/ψ → Σ0Σ0 → ΛγΛγ → pπ−γpπ+γ. In this process, CP symmetry can be tested in two decay processes: electromagnetic decay Σ0 → Λγ and weak decay Λ → pπ−. Only the asymmetry parameter of Λ → pπ− was studied. The study served as a validity check of the new method and ongoing analyses at BESIII. The statistical precision was studied by simulations: Monte Carlo data samples were created and then a maximum-log-likelihood fit was applied to the samples. An important component when determining the asymmetry parameters turned out to be the relative phase ∆φJ/ψ. The relative phase is one of the parameters used for determining the relation between the electric and magnetic form factors. ∆φJ/ψ is also related to the polarization of the hyperon-antihyperon pair. The study showed that the value of ∆φJ/ψ has a large impact on the uncertainties of the hyperon and antihyperon asymmetry parameters. A low value of ∆φJ/ψ resulted in high uncertainties and strong correlations between the asymmetry parameters. The formalism is different for different processes, which affects the uncertainties as well. The formalism used for the Σ0Σ0 process gives poorer parameter precision of the asymmetry parameter related to the Λ → pπ− decay than the formalism used for the ΛΛ process. Therefore, the ΛΛ process is a much more suitable process for CP studies of the Λ → pπ− decay. / Ett grundantagande inom den fundamentala fysiken är att lika stora mängder av materia och antimateria skapades efter Big Bang. När partiklar och antipartiklar kolliderar, annihilerar de, dvs försvinner och producerar fotoner. Trots detta består dagens universum huvudsakligen av materia. För att förklara varför all materia inte försvann krävs ett brott mot CP-symmetrin bakom standardmodellen. CP-symmetrin innebär att fysikens lagar är desamma om man byter partiklar mot antipartiklar och speglar partikelns rumsliga koordinater. CP-symmetri i baryonsönderfall är relativt dåligt testad. En ny metod för att studera CP-symmetrin har utvecklats vid Uppsala universitet för hyperon-antihyperon par. Hyperoner är baryoner med en eller fler särkvarkar. Metoden gör det möjligt att bestämma asymmetriparametrar hos hyperon- och antihyperonsönderfall separat om hyperonantihyperonparet är polariserat. Om en signifikant skillnad mellan värden av dessa parametrar upptäcks, är processen CP-brytande. Partikelfysikexperimentet BESIII i Kina är ett lämpligt experiment för sådana här mätningar eftersom det är ett högpresicionsexperiment och har dessutom samlat in stora mängder data av hyperon-antihyperonpar. Målet för detta projekt har varit att studera de statistiska precisioner av fysikparametrar som kan nås när man använder den nya metoden i de fall där J/ψ mesonen sönderfaller till ΛΛ, Σ+Σ− och Σ0Σ0. Hög statistisk precision behövs för att upptäcka CP-brott, eftersom CP-brytande processer, om de existerar, är relativt sällsynta. Huvudfokuset var att studera processen e+e− → J/ψ → Σ0Σ0 → ΛγΛγ → pπ−γpπ+γ. I denna process kan CP-symmetri testas för två sönderfallsprocesser: det elektromagnetiska sönderfallet Σ0 → Λγ och det svaga sönderfallet Λ → pπ−. I denna rapport studerades bara asymmetriparametrarna av Λ → pπ−. Detta arbete har fungerat som validitetskontroll av den nya metoden och pågående analyser på BESIII. Den statistiska precisionen undersöktes med simuleringar: Monte Carlo datamängder skapades och sedan en maximum-log-likelihood-anpassning av datan genomfördes. En viktig komponent i bestämningen av asymmetriparametrarna visade sig vara den relativa fasen, ∆φJ/ψ. Den relativa fasen är en av de parametrar som används för att bestämma relationen mellan de elektriska och magnetiska formfaktorer. ∆φJ/ψ är också relaterad till hyperonens hyperon-antihyperonparets polarisation. I forskningsprojektet visades att ∆φJ/ψ har en stor inverkan på osäkerheterna av hyperon- och antihyperonasymmetriparametrarna. Ett lågt värde av ∆φJ/ψ resulterade i stora osäkerheter och starka korrelationer mellan asymmetriparametrarna. Formalismen är annorlunda för olika processer, vilket också påverkar osäkerheterna. Formalismen som används för Σ0Σ0-processen ger sämre parameterprecision av asymmetriparametern kopplad till sönderfallet Λ → pπ− än formalismen som används för ΛΛ-processen. Därför är ΛΛ-processen en mycket lämpligare process för att testa CP-symmetrin i Λ → pπ− sönderfallet.

Studying the reactions where electrons and positrons collide and annihilate so that hadrons can be formed from their energy is an excellent tool when we try to improve our understanding of the standard model. Hadrons are composite quark systems held together by the strong force. By doing precise measurements of the, so called, cross section of the hadron production that was generated during the annihilation one can obtain information about the electromagnetic form factors, GE and GM, which describe the inner electromagnetic structure of hadrons. This will give us a better understanding of the strong force and the standard model.During my bachelor degree project I have been using data from the BESIII detector located at the Beijing Electron-Positron Collider (BEPC-II) in China. Uppsala university has several scientists working with the BESIII experiment. My task was to do a quality assurance of previous results for the reaction e+e-→ Lambda Lambda-Bar at a center of mass energy of 2.396 GeV. During a major part of the project I have been working with Monte Carlo data. Generating the reactions was done with two generators, ConExc and PHSP. The generators was used for different means. I have analyzed the simulated data to find a method of filtering out the background noise in order to extract a clean signal. Dr Cui Li at the hadron physics group at Uppsala university have worked with several selection criteria to extract these signals. The total efficiency of Cui Li's analysis was 14%. For my analysis I also obtained total efficiency of 14%. This gave me confidence that my analysis have been implemented in a correct fashion and that my analysis now can be transferred over to real data. It is also reassuring for Cui Li and the rest of the group that her analysis has been verified by and independently implemented selection algorithm. / Att studera vad som händer vid reaktioner där elektroner och positroner kolliderar och annihilerar så att hadroner kan bildas ur energin kan vara till stor hjälp när vi vill förstå standardmodellen och dess krafter, i synnerhet den starka kraften, som kan studeras i sådana reaktioner. Genom att utföra precisa mätningar av tvärsnitt för hadronproduktion får man fram de elektromagnetiska formfaktorerna GE och GM som beskriver hadronernas inre struktur. Hadroner är sammansatta system av kvarkar och den starka kraften binder dessa kvarkar.\\Under mitt examensarbete har jag använt mig av data från detektorn BESIII som finns vid BEPC-II (Beijing Electron-Positron Collider) i Kina. Uppsala universitet har flera forskare som jobbar med BESIII experimentet. Målet var att kvalitetssäkra den tidigare analys som gjorts för reaktionen e+e- → Lambda Lambda-Bar vid 2.396 GeV. Jag började med att göra Monte Carlo-simuleringar. Reaktionerna har genererats med två olika generatorer, ConExc och PHSP. Dessa generatorer har använts till olika ändamål. De genererade partiklarnas färd genom detektorn har sedan simulerats. Då bildas data av samma typ som dem man får från experiment. Jag har analyserat dessa simulerade data för att hitta en metod som kan filtrera bort bakgrundsstörningar samtidigt som intressanta data sparas. Kriterier utarbetade av Dr. Cui Li har använts för att skapa denna metod. Min algortim gav en total effektivitet på 14%, vilket stämmer bra med den tidigare algoritmen som Cui Li skapade, även där var effektiviteten 14%. Detta ger förtroende för min algortim och den stärker även Cui Lis resultat.

This thesis studies the rare decay η' → π+π-e+e- using Monte Carlo simulations and data from the BES-III detector in Beijing, China. The branching ratio of the reaction was measured at BES-III in 2013 using a data set of 225×106 J/Ψ events. This work lays the groundwork for an updated branching ratio measurement using the full data set of 1.3×109 J/Ψ and determines a potential CP-violating asymmetry in the angle between the decay planes of the π+π-- and e+e--pairs. A total of 2558 signal events are observed after cuts, and the asymmetry parameter is determined to be  Aφ = (1.96 ± 1.97stat.± 0.4syst. ) × 10-2. The result is consistent with zero within the uncertainty.

In this project, an algorithm has been implemented in Python to estimate the parameters describing the production and decay of a spin 1/2 baryon - antibaryon pair. This decay can give clues about a fundamental asymmetry between matter and antimatter. A model-independent formalism developed by the Uppsala hadron physics group and previously implemented in C++, has been shown to be a promising tool in the search for physics beyond the Standard Model (SM) of particle physics. The program developed in this work provides a more user-friendly alternative, and is intended to motivate further use of the formalism through a more maintainable, customizable and readable implementation. The hope is that this will expedite future research in the area of charge parity (CP)-violation and eventually lead to answers to questions such as why the universe consists of matter. A Monte-Carlo integrator is used for normalization and a Python library for function minimization. The program returns an estimation of the physics parameters including error estimation. Tests of statistical properties of the estimator, such as consistency and bias, have been performed. To speed up the implementation, the Just-In-Time compiler Numba has been employed which resulted in a speed increase of a factor 400 compared to plain Python code.

Chiral Perturbation Theory (ChPT) is a tool for studying the strong interaction at low energies. The Perturbation theory is developed around the limit where the light quarks, u,d,s are approximated to be massless. In this approximation the isospin symmetry, one of the main features of the strong interaction, is fulfilled automatically. The study of the light quark masses and isospin violation can be done with the η'→πππ and η'→ηππ decay channels by analyzing the kinematic distribution using so-called Dalitz plots. A Dalitz plot analysis of the η'→ηπ+π- decay mode is conducted by the BESIII collaboration. The unbinned maximum likelihood method is used to fit the parameters that describe the Dalitz plot distribution. In this fit a polynomial expansion of the matrix element squared is used. However, in order to study light quark masses, it is better to use a parameterization which includes the description of the final-state interaction based on a dispersion relation. Hence, it is desirable to use a representation of the Dalitz plot as a two-dimensional histogram with acceptance corrected data as input to extract the substraction constants. Therefore, the goal of this thesis is to make a consistency check between the unbinned and binned representation of the data. In this thesis Monte Carlo data of η'→ηπ+π- decay channel is generated based on the BESIII. An unbinned maximum likelihood fit is performed to find the Dalitz plot parameters repeating the BESIII analysis method. The Monte Carlo data is then used for a binned maximum likelihood and a χ2 fit. Finally, the prepared binned experimental acceptance corrected data from BESIII is used to fit the Dalitz plot parameters using the same statistical methods. The results based on the binned maximum likelihood and the χ2 methods are consistent with the fit using the unbinned maximum likelihood method applied in the original BESIII publication.