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Lattice QCD

In the quark sector, it is a difficult task to extract the fundamental parameters of the SM due to the fact that, at low energies, Quantum Chromodynamics (QCD), the theory that describes the color interaction between quarks and gluons, becomes non-perturbative. Fundamental problems such as if the lightest quark mass is different from zero, related to the problem of the absence of heavy CP violation in the SM, requiere the precise prediction of the lightest hadron masses (pions and kaons) from QCD. The mechanism of chiral symmetry breaking in QCD, which determines the hadronic spectrum at low energy, is a non-perturbative effect that requires a QCD treatment without approximations.

This mechanism could be related to the gauge symmetry breaking in the SM in the context of SM extensions with very strong interactions, such as technicolor theories. The only model-independent method to study QCD non-perturbatively is lattice QCD.

In this area members of our group have studied the QCD chiral regime via its formulation in a space-time grid. There has been great progress in the possibility of regularizing QCD without breaking the chiral symmetry explicitly, with Ginsparg-Wilson fermions. The QCD chiral regime has been studied from first principles and the connection between the fundamental theory with the effective chiral theory that describes the dynamics of pions and kaons at low energy has been established.

Physical Cosmology

The observation of dark energy demonstrates that our well established theories of particles and gravity are incomplete, if not incorrect. For this reason, even if this problem emerged within cosmology and from astrophysical observations, it will have repercussions in fundamental physics. Future, large galaxy surveys (such as BOSS, EUCLID and LSST) will cover O(10000) squared degrees on the sky. Their primary science goal is to unravel the nature of the physics responsible for the current accelerated expansion of the universe.

This acceleration likely involves new physics which could imply either a modification of our understanding of particles and fields (if the acceleration is caused by a new ingredient, the so-called "dark energy") or a change of our understanding of space and time (by modifying Einstein's General Relativity laws). The unprecedented and exquisite data provided by these surveys will make possible also other interesting science with physics (e.g. inflation, neutrino properties) and astrophysical (e.g., galaxy formation and evolution) implications. Our team, which benefits from the interaction of Cosmology, Astrophysics and Particle Physics, aims to:

  1. Identify and model the observables which will allow us to distinguish Dark Energy from Modified Gravity, paying special attention to the perturbation evolution.
  2. Study in detail the observational effects predicted by the different theoretical explanations for the current acceleration: quintessence, coupled dark energy and modified gravity, among others.
  3. Weigh neutrinos with cosmology.
  4. To maximize the physics extracted from large-scale structure and CMB data and their implications for particle physics.

Neutrinos

During the last decade, the evidence for flavour oscillations in atmospheric and solar neutrinos has been confirmed in accelerator and reactor experiments. The immediate consequence is that neutrinos are massive and mix, and therefore new physics beyond the Standard Model (SM) is called for. The interplay between particle physics and cosmology and astrophysics is inherent in the study of neutrino physics, and in our group there are contributions in both sides.

  • Neutrino phenomenology

 

Members of our group have liderated the analysis of present and future oscillation experiments (solar, atmospheric, reactor and accelerator) to determine the parameters of the lepton sector. Future experiments that may lead to the discovery of CP violation in the neutrino sector are currently being discussed. Members of our team have contributed decisevely to the analysis of the real potential of these big projects: Super-beams, Beta-beams and neutrino factories. In particular, P. Hernández is the principal organizer of the European project EUROnu "A High Intensity Neutrino Oscillation Facility in Europe", recently approved.

In the context of particle physics, members of our group have proposed and studied extensions of the SM that try to explain neutrino masses and their mixings and, if possible, address other issues such as the hierarchy problem or the dark matter of the Universe (with supersymmetry or extra dimensions). In these extensions, new TeV scale particles are predicted and could be produced at the LHC.

Neutrinos have outstanding implications in astrophysics and cosmology. In particular, members of the group liderate theoretical calculations of solar neutrino fluxes, trying to find a solution to the solar composition problem. Propagation of high energy neutrinos from their source and their connection to extragalactic cosmic rays is also studied in the group. Other members have studied the possibility to measure the neutrino flux using neutrino experiments from hypothetical annihilations of dark matter particles in the Sun. In cosmology, large scale structure formation imposes strong constraints on the scale of neutrino masses, that can be larger than the laboratory ones (Tritium beta-decay, for instance), and it is also a matter of study of the group.

  • Leptogenesis

Leptogenesis is a theoretical mechanism that tries to explain the origin of the matter-antimatter asymmetry of the Universe through heavy Majorana neutrino decays in the Early Universe, which would imply lepton number violation. As in the SM sphaleron processes preserve baryon minus lepton number, this lepton asymmetry would be converted to a baryon one. For the neutrino masses obtained from oscillation experiments, in the context of the Seesaw mechanism, the value of the ratio between the density of baryons and that of photons is obtained naturally, independently of initial conditions. Members of our group have studied the possible connection between the CP violation phases in leptogenesis and at low energies, as well as the regions of the parameter space in the context of supersymmetry that give lepton flavour violation in future charged lepton precision experiments.

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