High-dimensional neutrino masses

For Majorana neutrino masses the lowest dimensional operator possible is the Weinberg operator at $d=5$. Here we discuss the possibility that neutrino masses originate from higher dimensional operators. Specifically, we consider all tree-level decompositions of the $d=9$, $d=11$ and $d=13$ neutrino mass operators. With renormalizable interactions only, we find 18 topologies and 66 diagrams for $d=9$, and 92 topologies plus 504 diagrams at the $d=11$ level. At $d=13$ there are already 576 topologies and 4199 diagrams. However, among all these there are only very few genuine neutrino mass models: At $d=(9,11,13)$ we find only (2,2,2) genuine diagrams and a total of (2,2,6) models. Here, a model is considered genuine at level $d$ if it automatically forbids lower order neutrino masses {\em without} the use of additional symmetries. We also briefly discuss how neutrino masses and angles can be easily fitted in these high-dimensional models.Comment: Coincides with published version in JHE

Testing the low scale seesaw and leptogenesis

Heavy neutrinos with masses below the electroweak scale can simultaneously generate the light neutrino masses via the seesaw mechanism and the baryon asymmetry of the universe via leptogenesis. The requirement to explain these phenomena imposes constraints on the mass spectrum of the heavy neutrinos, their flavour mixing pattern and their $CP$ properties. We first combine bounds from different experiments in the past to map the viable parameter regions in which the minimal low scale seesaw model can explain the observed neutrino oscillations, while being consistent with the negative results of past searches for physics beyond the Standard Model. We then study which additional predictions for the properties of the heavy neutrinos can be made based on the requirement to explain the observed baryon asymmetry of the universe. Finally, we comment on the perspectives to find traces of heavy neutrinos in future experimental searches at the LHC, NA62, BELLE II, T2K, SHiP or a future high energy collider, such as ILC, CEPC or FCC-ee. If any heavy neutral leptons are discovered in the future, our results can be used to assess whether these particles are indeed the common origin of the light neutrino masses and the baryon asymmetry of the universe. If the magnitude of their couplings to all Standard Model flavours can be measured individually, and if the Dirac phase in the lepton mixing matrix is determined in neutrino oscillation experiments, then all model parameters can in principle be determined from this data. This makes the low scale seesaw a fully testable model of neutrino masses and baryogenesis.Comment: We corrected errors in the experimental sensitivities and in the discussion of the full testability of the model. We also added and updated plots and references. 37 pages plus appendix, 12 figure

Phenomenology of Quasi-Dirac neutrinos and a study of high-dimensional neutrino masses

Con la observación de las oscilaciones de neutrinos sabemos que los neutrinos son partículas con masa y esto implica física más allá del Modelo Estándar. Las oscilaciones de neutrino surgen de una mezcla entre el sabor y los autoestados de masa, las dos bases están relacionadas por una transformación unitaria, llamada matriz de mezcla. Cuando un neutrino se propaga en el espacio, oscila y la probabilidad de oscilación del neutrino depende de la energía del neutrino, la distancia recorrida, el cuadrado de la diferencia de los dos estados de masa y los elementos de la matriz de mezcla. Las masas de neutrinos pueden generarse introduciendo campos de neutrinos dextrógiros en el contenido de partículas, de modo que se emparejen con el levógiro para producir términos de masa de Dirac. Por otro lado, existe otra posibilidad que requiere un solo estado de quiralidad, aunque el número leptónico (una simetría accidental en el Modelo Estándar) se rompe. Si el número leptónico ya no se conserva, un neutrino es su antiparticula y es una partícula de Majorana. Si los neutrinos son partículas de Majorana, se vuelve natural explicar la pequeñez de las masas de neutrinos en comparación con las masas de los fermiones cargados. En este contexto, el operador de orden más bajo que genera masas de neutrinos de Majorana después de la ruptura de la simetría electrodébil es unicamente el operador de Weinberg de dimensión 5. Solo hay tres formas de generar el operador $d=5$ a nivel árbol. Estos se conocen como mecanismo see-saw de tipo I, tipo II, tipo III. Sin embargo, este mecanismo no es fenomenológicamente comprobable debido a la escala de energía muy alta. Como vemos, todavía hay muchas preguntas fundamentales a las que no tenemos respuestas: ¿Cuál es el valor de la escala de masa de neutrinos? ¿Los neutrinos son partículas de Majorana o de Dirac? ¿Cuál es la jerarquía de masas? ¿Por qué las masas de neutrinos son tan pequeñas respecto de los otros fermiones? Motiva ..

Quasi-Dirac neutrino oscillations at DUNE and JUNO

Quasi-Dirac neutrinos are obtained when the Lagrangian density of a neutrino mass model contains both Dirac and Majorana mass terms, and the Majorana terms are sufficiently small. This type of neutrino introduces new mixing angles and mass splittings into the Hamiltonian, which will modify the standard neutrino oscillation probabilities. In this paper, we focus on the case where the new mass splittings are too small to be measured, but new angles and phases are present. We perform a sensitivity study for this scenario for the upcoming experiments DUNE and JUNO, finding that they will improve current bounds on the relevant parameters. Finally, we also explore the discovery potential of both experiments, assuming that neutrinos are indeed quasi-Dirac particles.Peer Reviewe