According to the standard model, the size of CP violation in the weak decay Λ → pπ- is predicted to be ∼ 10-5. This means that a very high precision is needed if we want to be able to measure it. It turns out that the precision mainly depends on three factors: if the decay already has a large parity-breaking effect, if we take more measurements, or if the Lambda baryon has a large polarization. The uncertainty is inversely proportional to the root mean square of the polarization, √⟨P2⟩, of Λ. Polarization is a measurement of the fraction of particles with parallel spin. In this project, we simulated the kinematic properties of the three-body decay of the charmed lambda baryon Λc+  → Λπ+ π0 by assuming that Λ+ c decays 100% of the times via the ρ+-resonance, i.e. Λc+ decays like Λc+ → Λρ+ , and ρ+ subsequently decays ρ+ → π+π0. The invariant masses of the daughter particles were then plotted in a so-called Dalitz plot. The fact that the ρ+-meson is a vector meson is exploited to compute the helicity amplitudes HλΛ,λρ, where λ is the helicity of respective particle, meaning the projection of the spin in the direction of the ρ+-mesons momentum. The helicity amplitudes describe the helicities of the particles involved before and after the decay. These are then used to compute the polarization of Λ. After doing the simulation, the invariant masses are plotted in the Dalitz plot and the polarization in each point is computed. The root mean square polarization of Λ was √⟨P2⟩ = 0.81 ± 0.08. This result gives a large polarization and points to this process being a good baryon polarizer. When conducting more research into the three-body decay of Λc+ as a potential source of Λ with large polarization, the contributions of other resonances should be taken into account. In light of these results, it would also be interesting to investigate if three-body decays could work as a baryon polarizer for other baryons. / CP brytning i sönderfallet Λ → pπ-, via den svaga växelverkan, kommer enligt standardmodellen att ha en storlek runt ∼ 10-5. Det betyder att en väldigt hög precision är nödvändig om man ska kunna observera denna effekt. Det visar sig att precisionen av mätningarna främst beror på tre faktorer: om sönderfallet redan har stor paritestbrytande effect, om man tar fler mätningar, eller om Lambdabaryonerna har stor polarisation. Osäkerheten är proportionerlig mot inversen av det kvadratiska medelvärdet, √⟨P2⟩ av Lambdabaryonerna. Polarisationen av en mängd partiklar är ett mått på andelen partiklar med parallellt spinn. I detta projekt kommer vi simulera de kinematiska egenskaperna hos trekroppssönderfallet av den charmade Lambdabaryonen Λc+ → Λπ+π0 genom att anta 100% sönderfall via ρ+-resonansen, dvs att Λc+ först sönderfaller som Λc+ → Λρ+, och att ρ+ sedan sönderfaller som ρ+ → π+π0. De invarianta massorna av dotterpartiklarna plottades sedan i en s.k Dalitz-plot. Vi utnyttjade att ρ+-mesonen är en vektormeson för att beräkna helisitetsamplituderna HλΛ,λρ, där λ är helisiteterna av respektive partikel, dvs projektionen av partiklarnas spinn längst ρ+-mesonens rörelsemängdsriktning. Helisitetsamplituderna beskriver helisiteten hos partiklarna före och efter sönderfallet. Dessa användes sedan för att beräkna polarisationen av Λ. Efter att simuleringen gjorts, plottades de invarianta massorna i en Dalitz-plott och polarisationen beräknades i varje punkt. Det kvadratiska medelvärdet av polarisationen hos Λ var √⟨P2⟩ = 0.81 ± 0.08. Detta är en stor polarisation som indikerar att trekroppsönderfall kan vara en bra baryonpolarisator. För vidare forskning om trekroppssönderfallet av Λc+ som en källa av Λ med stor polarisation, borde bidragen från andra resonanser också medräknas. Vidare skulle det även vara intressant att undersöka om trekroppssönderfall kan fungera som en baryonpolarisator för andra baryoner.

[pt] O Modelo Padrão (SM) da física de partículas é atualmente a melhor teoria para descrever as interações entre partículas elementares e suas propriedades. A observação da violação do CP em diversos processos fracos no setor de quarks - permitindo uma distinção absoluta entre partículas e antipartículas - é bem explicada pelo SM (através do chamado Cabibbo-Kobayashi-Maskawa ansatz). A violação do CP é uma das condições necessárias para a bariogênese e pode ser a chave para explicar a assimetria matéria-antimatéria no universo. Esta dissertação apresenta a busca por violação de CP no decaimento D+ → π−π+K+ duplamente suprimido por Cabibbo (DCS), usando dados coletados pelo LHCb de 2016-2018 de colisões pp com uma energia de centro de massa de 13 TeV, correspondendo a uma luminosidade integrada de 5,6 fb1. O objetivo desta análise é a implementação de uma técnica independente de modelo para realizar uma comparação estatística entre o espaço de fase, denominado Dalitz Plot (DP), de partícula e antipartícula no canal de decaimento, buscando diferenças locais na distribuição de eventos entre os dois DPs. Primeiro, foi feito um processo de seleção para remover contribuições de background de outros decaimentos de charme, bem como uma análise multivariada utilizando algoritmos de aprendizado de máquina para reduzir os níveis de background combinatoriais. Após a seleção, foi obtida uma amostra final 6M de candidatos a sinal, que é hoje a maior amostra já obtida para um canal de decaimento D+ DCS, permitindo uma excelente sensibilidade para busca de VCP. Essa é a primeira busca de VCP no canal de decaimento estudado. Esta análise é realizada de forma cega, o que significa que não há resultado para a região do sinal nesta primeira etapa e, para garantir que não haja assimetrias espúrias, como efeitos de produção e detecção, foram realizados testes para a região de background, para o canal de controle D+ → K−π+π+ e para a amostra simulada de Monte Carlo. / [en] The Standard Model (SM) of particle physics is currently the best theory to describe the interactions between elementary particles and their properties. The observation of CP violation in a variety of weak processes in the quark sector — allowing an absolute distinction of particles and antiparticles — is well explained by the SM (through so-called Cabibbo-Kobayashi-Maskawa ansatz). CP violation is one of the necessary conditions for baryogenesis and might be the key to explain the matter-antimatter asymmetry in the universe. This dissertation presents the search for CP violation in the doubly Cabibbo suppressed (DCS) D+ → π−π+K+ decay, using data collected by LHCb from 2016-2018 of pp collisions with a centre of mass energy of 13 TeV, corresponding to an integrated luminosity of 5.6 fb1. The goal of this analysis is the implementation of a model-independent technique to perform a statistical comparison between the phase space, called Dalitz Plot (DP), of particle and antiparticle decay channel, searching for local differences in the distribution of events between the two DPs. First, a selection process was executed to remove background contributions from other charm decays, as well as a multivariate analysis using machine learning algorithms to reduce combinatorial background levels. After the selection, a final sample of ¨6M signal candidates was obtained, which is nowadays the largest sample ever obtained for a DCS D+ decay channel, allowing an outstanding sensitivity for CPV search. This is the first CPV search in the studied decay channel. This analysis is performed blinded, meaning that there is no actual result for the signal region at this first stage and in order to guarantee that there are no nuisance asymmetries, from production and detection effects, tests were performed using the background region, the control channel D+ → K−π+π+ and the Monte Carlo simulated sample.