The muon g-2 discrepancy and hints about new physics
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Data
2022-09-30
Autores
Aguiar, Rodrigo Teixeira
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
Neste trabalho foi apresentado a estruturação do modelo padrão e analisados alguns problemas que este modelo enfrenta atualmente, selecionando para discussão a questão do muon g-2. Uma detalhada avaliação do muon g-2 é feita, incluindo todas ferramentas necessárias para se obter os resultados numéricos, a conta a um loop e a prova da invariancia de gauge feitas explicitamente. Feito isso, foi mostrado qual o problema com o muon g-2 e foi dada uma explicação detalhada do experimento que confronta a previsão teórica. Na segunda parte uma possível solução do problema foi testada: a inclusão de um vertice com violação de sabor e duas partículas pesadas novas, um fermion e um escalar. Usando um modelo hipotético foi observado que modelos que contém este tipo de vértice e partículas poderia incluir nas previsões valores da ordem de magnitude necessária, mesmo que a escala de energia destas particulas esteja fora dos limites experimentais. A relação entre as massas do novo fermion sobre a do novo escalar sendo acima de 0.1 poderiam resultar em valores para o muon g-2 da ordem da discrepancia entre teoria e experimento atuais para escalares com massa na escala de TeV. Tendo isso em mente, um modelo concreto como exemplo (um modelo do tipo 3-3-1, retirado da literatura) foi analisado, mas o mesmo efeito não foi observado. Por causa das interações quirais dos escalares dessa teoria, as contribuições para o g-2 são reduzidas por volta de 10^{-3} (m/M_H). De modo geral, este resultado parece mais viável, devido as interações conhecidas serem normalmente quirais, e portanto adições de escalares com interações do tipo do modelo padrão parecem ser, em geral, maneiras não naturais de resolver a discrepância do g-2
In this work, the structuring of the standard model was presented and some issues that this model currently faces were analyzed, selecting for discussion the question of the muon g-2. A detailed evaluation of this factor was made, including all the necessary tools to obtain the result, the computations up to one loop, and the proof of the gauge invariance for the same. After that, it was shown what is the issue with the muon g-2 and provided a detailed explanation of the experiment that confronts the theoretical prediction. \par In the second part, a possible solution to the problem was tested: the inclusion of a flavor-violating vertex and two new heavy particles fields, a fermion, and a scalar. Using at first a toy model it was observed that models that contain this type of vertex and particles could include, in the predictions, values at the right order of magnitude as the SM discrepancy, even if the mass scale of these particles is above the experimental exclusion. The ratio between the masses of the new fermion over the new scalar being above 0.1 could result in values for the muon g-2 at the order of the current discrepancy between theory and experiment for scalar masses in the TeV scale. Having that in mind, a more concrete example (a 3-3-1 type model, taken from the literature) was analysed, but the same effect was not observed. Due to the chiral interactions of the scalar fields in this theory the contributions to the muon g-2 are lowered by around 10^{-3} (m/M_H). Overall, this result seems to be more reliable than the toy model, since the know interactions are chiral, and additions of scalars with SM-like interactions seem to be, in general, unnatural ways of fixing the g-2 discrepancy.
In this work, the structuring of the standard model was presented and some issues that this model currently faces were analyzed, selecting for discussion the question of the muon g-2. A detailed evaluation of this factor was made, including all the necessary tools to obtain the result, the computations up to one loop, and the proof of the gauge invariance for the same. After that, it was shown what is the issue with the muon g-2 and provided a detailed explanation of the experiment that confronts the theoretical prediction. \par In the second part, a possible solution to the problem was tested: the inclusion of a flavor-violating vertex and two new heavy particles fields, a fermion, and a scalar. Using at first a toy model it was observed that models that contain this type of vertex and particles could include, in the predictions, values at the right order of magnitude as the SM discrepancy, even if the mass scale of these particles is above the experimental exclusion. The ratio between the masses of the new fermion over the new scalar being above 0.1 could result in values for the muon g-2 at the order of the current discrepancy between theory and experiment for scalar masses in the TeV scale. Having that in mind, a more concrete example (a 3-3-1 type model, taken from the literature) was analysed, but the same effect was not observed. Due to the chiral interactions of the scalar fields in this theory the contributions to the muon g-2 are lowered by around 10^{-3} (m/M_H). Overall, this result seems to be more reliable than the toy model, since the know interactions are chiral, and additions of scalars with SM-like interactions seem to be, in general, unnatural ways of fixing the g-2 discrepancy.
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Palavras-chave
Particulas (Fisica nuclear), Modelo padrão (Física nuclear), Muons