Leptonic CP Violation and its Origin

Abstract

At the end of the 20th century, experiments studying neutrinos coming from the Sun and cosmic rays hitting the atmosphere revealed that these particles can change their flavour, behaving in a way that the Standard Model (SM) explicitly forbids. The quest for understanding the properties of neutrinos, which may reveal the next underlying structure of Nature, has since then led thousands of scientists. The scenario when this thesis was initiated was driven by the latest experimental surprise that neutrinos provided: the initial hints towards their strong violation of the particle-antiparticle CP symmetry. Characterising the statistical significance, robustness and physical origin of that hint is the main goal that this work pursuits.

For that, there is a rich programme of present and future long baseline (LBL) accelerator neutrino experiments. At the beginning of the development of this thesis, the LBL accelerator neutrino experiment NOvA released its first data. To obtain a global picture, this work combines it with the results of other relevant neutrino experiments. The status of leptonic mixing and CP violation is quantitatively assessed as LBL accelerator neutrino experiments kept releasing data.

As the unknowns start getting clarified, the data points towards maximal CP violation. This hint is driven by an excess of electron neutrino appearance events in the LBL accelerator experiment T2K. In the three massive neutrino paradigm and with the other leptonic mixing parameters accurately measured by different experiments, the excess can only be accommodated by large CP violation.

Nevertheless, three massive neutrinos is just a minimal extension of the SM: other new physics could be present, masking the results as direct leptonic CP violation has not yet been conclusively observed. This thesis confronts with data the scenario that is less bounded by other experiments: neutral current Non-Standard Interactions (NSI) among neutrinos and matter. Physically, they are generated by new interactions among neutrinos and matter mediated by potentially light particles.

Due to the large parameter space involved, first just CP-conserving NSI (i.e., their moduli) are explored. Current bounds are evaluated, as well as the synergies and complementarity among different experiments. Thanks to the experiments working with various neutrino energies and travelled distances, the determination of leptonic mixing parameters is found to be quite robust. Thus, it is possible to move on and evaluate the current sensitivity to leptonic CP violation assuming the most generic CP-violating NSI are present. CP violation induced by neutrino masses and leptonic mixing is found to be quite robust, due to the large amount of neutrino flavour transition data collected along three decades.

Nevertheless, the next generation LBL accelerator experiments are aimed at precision measurements that could be more severely affected. Luckily, in the last years the COHERENT experiment has provided independent constraints on NSIs. This experiment measures neutral current coherent neutrino-nucleus elastic scattering, a low momentum transfer process quite sensitive to NSI induced by potentially light mediators.

In the final chapter of the thesis, the data from the COHERENT experiment is analysed and integrated into the global analyses from previous chapters. Particular attention is paid to how the results depend on the assumptions about the experiment background, nuclear structure, and detector response. Combining COHERENT data with flavour transition experiments unveils its incipient role in increasing the robustness of their interpretation.

These first results could be greatly improved by increasing the statistics of the signal and/or by performing the measurements with different nuclei sensitive to different NSI models. For that, the European Spallation Source is an ideal future facility. It will produce a neutrino beam one order of magnitude more intense than the one used at COHERENT, and as it is still under construction there is potential space for various modern detectors. Its prospects for bounding NSI are also explored.

In summary, this thesis deals with the current experimental hint for large CP violation in the leptonic sector. It first quantifies its global significance, and then moves on to checking its robustness against the framework in which the experimental data is interpreted. For that, complementary experiments on neutrino-nucleus coherent scattering play, and will keep on playing in the future, a significant role. Thus, a global approach is taken to rigorously assess whether cutting-edge leptonic flavour measurements are pointing towards a new strong violation of a symmetry of Nature.

A finales del siglo XX, una serie de experimentos que estudiaban neutrinos provenientes del Sol o de rayos cósmicos que colisionaban contra la atmósfera terrestre mostraron que estas partículas pueden cambiar su sabor, comportándose de una manera que el Modelo Estándar prohíbe explícitamente. El camino hacia la comprensión de las propiedades de los neutrinos, que podría revelar la siguiente estructura subyacente de la naturaleza, ha guiado desde entonces a miles de científicos. El escenario al inicio de esta tesis estaba impulsado por la última sorpresa experimental de los neutrinos: los primeros indicios que apuntan hacia su fuerte violación de la simetría materia-antimateria o CP.

El objetivo de este trabajo es abordar el problema desde una perspectiva global para evaluar rigurosamente si las medidas punteras de física de sabor leptónico están apuntando hacia una nueva violación fuerte de una simetría de la naturaleza.

Para ello, se combinan los resultados de todos los experimentos de neutrinos relevantes. Se evalúa cuantitativamente el estatus de la mezcla leptónica y de la violación de CP, que los datos apuntan a que podría ser máxima. Este indicio está dominado por un exceso de neutrinos electrónicos en el experimento de neutrinos con acelerador a larga distancia T2K. Dentro del paradigma de tres neutrinos masivos, y con el resto de parámetros de mezcla leptónica medidos con precisión en varios experimentos, el exceso solamente se puede acomodar mediante una violación de CP grande.

A pesar de esto, tres neutrinos masivos es solamente una extensión mínima del Modelo Estándar: podría haber otra nueva física enmascarando los resultados, ya que la violación de CP leptónica aún no se ha medido de manera directa y concluyente. Por ello, esta tesis confronta la nueva física que podría afectar a los experimentos de transiciones de sabor de neutrinos con datos experimentales. La violación de CP inducida por las masas de los neutrinos y por la mezcla leptónica resulta ser bastante robusta. Tal y como se explora en este trabajo, los experimentos complementarios de interacción coherente entre neutrinos y núcleos juegan, y continuarán jugando en el futuro, un papel importante a este respecto.