Lecture Notes on Quantum Field Theory I

El documento ha sido corregido por el Servicio de Política Linguística de la Universidad de Valencia.Material docente preparado para la asignatura Teoría Cuántica de Campos en el Master de Física Avanzada.These lectures notes are based on the material covered in the course on Quantum Field Theory I in the Master in Advanced Physics at the University of Valencia, delivered in the years 2017-2021

N_eff in low-scale seesaw models versus the lightest neutrino mass

We evaluate the contribution to N_eff of the extra sterile states in low-scale type I seesaw models (with three extra sterile states). We explore the full parameter space and find that at least two of the heavy states always reach thermalization in the early Universe, while the third one might not thermalize provided the lightest neutrino mass is below O(10(-3) eV). Constraints from cosmology therefore severely restrict the spectra of heavy states in the range 1 eV-100 MeV. The implications for neutrinoless double beta decay are also discussed

Low-scale seesaw models versus N-eff

We consider the contribution of the extra sterile states in generic low-scale seesaw models to extra radiation, parametrized by N-eff. We find that the value of Neff is roughly independent of the seesaw scale within a wide range. We explore the full parameter space in the case of two extra sterile states and find that these models are strongly constrained by cosmological data for any value of the seesaw scale below O(100 MeV)

Leptogenesis in GeV-scale seesaw models

We revisit the production of leptonic asymmetries in minimal extensions of the Standard Model that can explain neutrino masses, involving extra singlets with Majorana masses in the GeV scale. We study the quantum kinetic equations both analytically, via a perturbative expansion up to third order in the mixing angles, and numerically. The analytical solution allows us to identify the relevant CP invariants, and simplifies the exploration of the parameter space. We find that sizeable lepton asymmetries are compatible with non-degenerate neutrino masses and measurable active-sterile mixings

Superbeam studies at CERN

A conventional low-energy neutrino beam of great intensity could be produced by the Super Proton Linac at CERN as a first stage of a Neutrino Factory. Water Cherenkov and liquid scintillator detectors are studied as possible candidates for a neutrino oscillation experiment which could improve our current knowledge of the atmospheric parameters Δmatm2, θ23 and measure or severely constrain θ13. It is also shown that a very large water detector could eventually observe leptonic CP violation

Semi-leptonic decays of heavy flavours on a fine grained lattice

We present the results of a numerical calculation of semi-leptonic form factors relevant for heavy flavour meson decays into light mesons, at β=6.4 on a 243×60 lattice, using the Wilson action in the quenched approximation. We obtain f+K(0)=0.65±0.18, V(0)=0.95±0.34, A1(0)=0.63±0.14 and A2(0)=0.45±0.33. We also obtain A1(q2max)=0.62±0.09, V(0)/A1(0)=1.5±0.28 and A2(0)/A1(0)=0.7±0.4. The results for f+K(0), V(0) and A1(0) are consistent with the experimental data and with previous lattice determinations with larger lattice spacings. In the case of A2(0) the errors are too large to draw any firm conclusion. We have also extrapolated the form factors to the B meson, showing a behaviour compatible with the predictions by the heavy quark effective theory (HQET). Within large uncertainties, our results suggest that A2/A1 increases with the heavy quark mass. We also get very rough estimates for the partial decay widths B→πlνl)=|Vub|2(12±8)1012s−1 and Γ(B→ρlνl)=|Vub|2(13±12)1012s−1, which can be used to give upper bounds on the rates

Búsqueda de efectos no-estandar en la interacción de tres bosones gauge vectoriales

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica, 31-05-199

New bounds on axion-like particles from MicroBooNE

Neutrino experiments lie at the edge of the intensity frontier and therefore can be exploited to search for new light particles weakly coupled to the visible sector. In this work we derive new constraints on axion-like particles (ALPs) using data from the MicroBooNE experiment, from a search for ee pairs pointing in the direction of the NuMI absorber. In particular, we consider the addition of higher-dimensional effective operators coupling the ALP to the electroweak gauge bosons. These would induce K → πa from kaon decay at rest in the NuMI absorber, as well as ALP decays into pairs of leptons or photons. We discuss in detail and compare various results obtained for the decay width K → πa in previous literature. For the operator involving the Higgs, MicroBooNE already sets competitive bounds (comparable to those of NA62) for ALP masses between 100 and 200 MeV. We also compute the expected sensitivities from the full NuMI dataset recorded at MicroBooNE. Our results show that a search for a a → γγ signal may be able to improve over current constraints from beam-dump experiments on the operator involving the ALP coupling to the W

Probing the chiral regime of Nf=2 QCD with mixed actions

17 páginas, 15 figuras, 9 tablas.-- El Pdf es la versión pre-print del artículo: arXiv:1008.1870v2We report on our first experiences with a mixed action setup with overlap valence quarks and nonperturbatively O(a) improved Wilson sea quarks. For the latter we employ CLS Nf=2 configurations with light sea-quark masses at small lattice spacings. Exact chiral symmetry allows to consider very light valence quarks and explore the matching to (partially-quenched) Chiral Perturbation Theory (ChPT) in a mixed ϵ/p-regime. We compute the topological susceptibility and the low-lying spectrum of the massless Neuberger-Dirac operator for three values of the sea-quark mass, and compare the sea-quark mass dependence to NLO ChPT in the mixed regime. This provides two different determinations of the chiral condensate, as well as information about some NLOlow-energy couplings. Our results allow to test the consistency of the mixed-regime approach to ChPT, as well as of the mixed action framework.F.B. and C.P. acknowledge nancial support from the FPU grant AP2005-5201 and the Ramón y Cajal Programme, respectively. This work was partially supported by the Spanish Ministry for Education and Science projects FPA2006-05807, FPA2007-60323, FPA2008-01732, FPA2009-08785, HA2008-0057 and CSD2007-00042; the Generalitat Valenciana (PROMETEO/2009/116); the Comunidad Autónoma de Madrid (HEPHACOS P-ESP-00346 and HEPHACOS S2009/ESP-1473); and the European projects FLAVIAnet (MRTN-CT-2006-035482) and STRONGnet (PITN-GA- 2009-238353).Peer reviewe

Mixed action computations on fine dynamical lattices

7 pages, 2 figures.-- En: The XXVII International Symposium on Lattice Field Theory July 26-31, 2009 Peking University, Beijing, ChinaWe report on our first experiences in simulating Neuberger valence fermions on CLS Nf = 2 configurations with light sea quark masses and small lattice spacings. Valence quark masses are considered that allow to explore the matching to (partially quenched) chiral perturbation theory both in the E- and p-regimes. The setup is discussed, and first results are presented for spectral observables.F.B. and C.P. acknowledge financial support from the FPU grant AP2005-5201 and the Ramón y Cajal Programme, respectively. This work was partially supported by the Spanish Ministry for Education and Science projects FPA2006-05807, FPA2006-60323, FPA2008-01732, HA2008- 0057 and CSD2007-00042; the Generalitat Valenciana (PROMETEO/2009/116); the Comunidad Autónoma de Madrid (HEPHACOS P-ESP-00346); and the European project FLAVIAnet (MRTNCT- 2006-035482).Peer reviewe