This thesis describes the search for neutrino-production of single photons using the off-axis near detector at 280 metres (ND280) of the T2K experiment. A photon selection is used to perform the searches using the first Fine Grained Detector (FGD1) of the ND280. The thesis also highlights the importance of systematic uncertainties in the analysis, since the selection is background dominated. After careful characterisation of the systematic uncertainties and estimation of the efficiency, it is concluded that, with the selected 39 data events and the expected background of 45 events, the limit for neutrino-induced single photons, at T2K energies, is 0:0903 x 10-38cm2/nucleon. This result can be compared with the expected limit of 0:1068 x 10-38cm2/nucleon. Using ND280's neutrino energy distribution (peaked at 600 MeV), NEUT predicts a flux-averaged cross section of 0:000239 x 10-38cm2/nucleon. A fit to the muon and electron (anti-) neutrinos selections in the ND280 was performed. The aim of this analysis is to use a data-driven method to constrain the electron (anti-) neutrinos background events at SK, the far detector and electron neutrino cross section parameters for oscillation analyses. These are fundamental inputs in the context of the searches for Charge-Parity (CP) violation in the neutrino sector. After a fit to the nominal Monte Carlo was realised, the electron neutrino and anti-neutrino cross section normalisation uncertainties are found to be 7.6% and 19.3%, repectively. Although these numbers are much higher than the assumed 3% uncertainty of all the CP violation searches performed at T2K up to now, the difference in the δCP log-likelihood is found to be acceptable as the one sigma contours are not very different and the exclusion of the δCP = 0 is roughly the same.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-08T14:31:19Z No. of bitstreams: 2 Tese - Stefano Castro Tognini - 2018.pdf: 57497993 bytes, checksum: 7f2bc280505dae763265dbf74341c9ca (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-06-08T14:44:34Z (GMT) No. of bitstreams: 2 Tese - Stefano Castro Tognini - 2018.pdf: 57497993 bytes, checksum: 7f2bc280505dae763265dbf74341c9ca (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-06-08T14:44:34Z (GMT). No. of bitstreams: 2 Tese - Stefano Castro Tognini - 2018.pdf: 57497993 bytes, checksum: 7f2bc280505dae763265dbf74341c9ca (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-04-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The interaction of a cosmic ray particle with an element of the atmosphere results in a cascade of particles, known as extensive air showers, which can be divided into three main branches, known as the hadronic, the electromagnetic, and the muonic component. As for the latter, since muons can reach high depths underground, they are generally used to study cosmic rays at different underground depths. The dynamics of extensive air showers is directly connected to the density of the atmosphere, as it defines the probability of particles to decay or interact. As muons are mainly produced from the decay of pions and kaons, and a warmer atmosphere results in higher number of meson decays, such particles are prone to suffer a sea- sonality effect that is directly correlated to the yearly seasonal variations of the atmosphere, an effect that has been verified by a large number of experiments over the past six decades. In 2015 the MINOS experiment presented an anti correlation between the effective temperature of the atmosphere and the seasonality of the muon flux for multiplicities higher than one (i.e. more than one muon track per cosmic ray event). Said anti correlation is not yet fully understood, counting with only a qualitative hypothesis as a probable mechanism. As such, the main goals of this study are to verify the MINOS anti correlation effect and extend the study to verify the seasonality of the effect as a function of different variables in order to improve the understanding of the phenomenon and possible corroborations with the known hypothesis. Two full years of the NOνA Near Detector, ranging from April 2015 to April 2017, were used as the dataset for the analysis. The anti correlation between the multiple muon flux and the effective temperature of the atmosphere is confirmed by the NOνA Near Detector, being in full agreement with the results presented by the MINOS Collaboration. The seasonal effect is also broken down by different variables: i) track separation, ii) zenith angle, iii) track angular separation, and iv) multiplicity. Different regions of these variables represent different energy ranges for the detected underground muons, their hadron parents or the primary particles that originated the cosmic ray shower, being a way to verify any particular dependency with energy. The results show that there are no clear trends in any of the studied variables, except for the multiplicity, in which the intensity of the seasonal variation increases for higher multiplicities. / A interação entre um raio cósmico e algum elemento da atmosfera é responsável por produzir uma cascata de partículas, conhecida como chuveiro atmosférico extenso. Tal cascata pode ser dividida em três principais componentes, conhecidas como hadrônica, eletromagnética e muônica. O fato de que múons são capazes de alcançar grandes profundidades no subsolo, faz com que a última componente citada seja ideal para ser utilizada por detectores subterrâneos para estudar chuveiros atmosféricos extensos. A dinâmica desses chuveiros está diretamente associada à densidade da atmosfera, a qual é um fator determinante nas probabilidades de decaimento ou interação das partículas produzidas ao longo da mesma. Em vista do fato de que muons são produzidos à partir do decaimento de píons e káons, e considerando que uma atmosfera mais quente/fria implica em um maior/menor número de decaimento desses mésons, espera-se que o fluxo de múons de raios cósmicos tenha um sazonalidade anual, um efeito confirmado por um conjunto de experimentos ao longo das últimas 6 décadas. Em 2015 o experimento MINOS apresentou uma anticorrelação entre a temperatura efetiva da atmosfera e a sazonalidade do fluxo de múons para eventos com multiplicidade maior que um (ou seja, mais de uma trajetória detectada por evento de raio cósmico). Tal correlação inversa não é completamente compreendida, contando apenas com uma hipótese qualitativa como provável mecanismo físico. Portanto, os principais objetivos deste estudo implicam em verificar este efeito sazonal invertido no Near Detector do experimento NOνA, além de extender a verificação da sazonalidade de múons múltiplos em função de diferentes variáveis, de forma a trazer uma melhor compreensão sobre o fenômeno e trazer conclusões que possam corroborar com a hipótese mencionada anteriormente. Como forma de atingir os objetivos, desenvolveu-se uma análise usando dois anos de dados do NOνA Near Detector, acumulados entre abril de 2015 e abril de 2017. A anticorrelação entre o fluxo de múons múltiplos e a temperatura efetiva da atmosfera foi confirmada, estando de acordo com os resultados apresentados pelo experimento MINOS. O efeito sazonal foi estudado de acordo com diferente variáveis: i) separação entre trajetórias, ii) ângulo zenital, iii) separação angular e iv) multiplicidade. Diferentes regiões de valores destas variáveis representam diferentes regiões de energia dos múons detectados, dos hádrons que os originaram ou dos respectivos primários. Os resultados mostram que não existem claras correlações em nenhuma das variáveis estudadas, exceto pela multiplicidade, a qual mostra que a intensidade da variação sazonal de múons múltiplos aumenta à medida que a multiplicidade do evento aumenta.

The T2K ("Tokai-to-Kamioka") Experiment is a long-baseline neutrino oscillation experiment. A beam of primarily muon neutrinos (in neutrino beam mode) or antineutrinos (in antineutrino beam mode) is produced at the J-PARC ("Japan Proton Accelerator Research Complex") facility. The near detector (ND280), located 280 m from the proton beam target, measures a large event rate of neutrino interactions in the unoscillated beam, while the far detector, Super-Kamiokande, 295 km away, searches for the signatures of neutrino oscillation. This dissertation describes the analyses of data at ND280 and Super-Kamiokande leading to T2K's first results from running in antineutrino beam mode: a measurement of muon antineutrino disappearance. The measured values of the antineutrino oscillation parameters (Normal Hierarchy) are (sin²(θ̅₂₃), |Δm̅²₃₂|) = (0.450, 2.518 x 10ˉ³ eV²/c⁴), with 90% 1D confidence intervals 0.327 < sin²(θ̅₂₃) < 0.692 and 2.03 x 10ˉ³ eV²/c⁴ < |Δm̅²₃₂| < 2.92 x 10ˉ³ eV²/c⁴. These results are consistent with past measurements of these parameters by other experiments, and with T2K's past measurements of muon neutrinos. / Graduate