Physics at CFTP and the Nobel Prize of 2013.

The Nobel Prize in Physics 2013 was awarded jointly to Fran├žois Englert and Peter W. Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles". In 2012, experiments at CERN, involving various scientists from IST's Department of Physics and LIP, found the scalar particle predicted by the theoretical model. In the next few years the fundamental question is whether there is only one scalar particle, as predicted in the simplest models, or whether there are several scalars. CFTP members have been studied multi-scalar models for the past decades, making it a world leader in this area. Thus, CFTP will be at the heart of the exciting decade ahead, where theory and experiment cooperate to understand the new results from CERN's LHC accelerator.

Particles and interactions: All matter we observe is made up of three families of four particles each, subject to three types of interactions (plus gravity). There is a well determined theoretical framework describing this picture, involving special relativity and quantum mechanics. In it, both matter and interactions are described by fields and particles. Particles like the electron also have wave properties; interactions like electromagnetism are mediated by particles (the photon). The fundamental difference is that matter particles have spin (internal angular momentum) 1/2, while interaction particles have spin 1. This picture works so well that it is known as the "Standard Model".

The need for scalar particles: Over many decades, physicists found that assuming certain symmetries (certain regularities) allowed them to guess what the interactions should look like. And these results were consistent with experiment, except for a nagging problem: all particles should have zero mass, in blatant contradiction to what we all know to be true. In 1964, Englert and Brout, and, independently, Higgs found a solution to this problem. Particles could have mass and interactions could have the needed symmetries if there were a scalar field whose job was to give all particles their mass. The outcome of this mechanism is a new particle of spin 0 (a so-called scalar, also known as the Higgs boson).

The discovery: After almost half a century, a scalar particle was finally found at CERN's LHC in 2012, thus vindicating the theoretical work of 1964. This is what prompted the Swedish Academy of Sciences to award the Nobel Prize to the theoretical work.

Open questions: In the Standard Model, the number of particles mediating each interaction is determined by the mathematical properties of the symmetries involved in that interaction. In contrast, the number of matter particles is not known a priori; it was determined experimentally at a previous CERN experiment that there must be three families of matter particles (up to a certain energy). Now that at least one scalar particle has been found, we must determine experimentally how many there are.

Work at CFTP: The idea that there could be more than one Higgs boson has occupied the mind of CFTP's members for many decades. One early example is "Weinberg-Salam Model with Two Higgs Doublets and the Delta I=1/2 Rule" published by G. C. Branco in 1977. CFTP members have given leading contributions to all aspects of multi-Higgs models, including its relation to CP violation, the flavour problem, their detectability, its implications in a supersymmetric setting, etc... The 2012 review "Theory and phenomenology of two-Higgs-doublet models" co-authored by 4 CFTP members, has collected over 200 citation in less than two years. This positions CFTP at the forefront of the upcoming world effort to understand the scalar sector. More importantly, CFTP has a tradition of including early in their careers both Master and PhD students in this research. If you are looking for an exciting career in Theoretical Physics, come in a visit with us.