Physics at CFTP and the Nobel Prize of 2013.
The Nobel Prize in Physics 2013 was awarded jointly to François 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.
Physics at CFTP and the Nobel Prize 2008
It is with great excitement that, at CFTP, we received the news that the
Nobel Prize for Physics for 2008 has been awarded to Yoichiro Nambu,
Makoto Kobayashi and Toshihide Maskawa.
Yoichiro Nambu got the prize "for the discovery of the mechanism of
spontaneous broken symmetry in subatomic physics". Today, this idea is
incorporated in all the modern theories of Particle Physics, namely in
the so-called Higgs mechanism, that is responsible for giving masses to
the Standard Model Particles, and predicts one particle, the Higgs boson
that soon will searched for in the LHC at CERN. Also Nambu's ideas had a
strong impact on hadronic Physics - chiral symmetry, and its spontaneous
breaking, is one essential feature of QCD, and is responsible for
essential features of the structure of hadrons, and their interaction or
couplings. This is currently one of the research interests at CFTP
(Teresa Peña, João Seixas, Pedro Bicudo ) focusing on predictions for
the meson and baryon spectra, and the nuclear interaction.
Makoto Kobayashi e
Toshihide Maskawa were awarded the prize "the proposal of a successful
mechanism for CP violation in the Standard Model", which is one of the
hot topics of research at CFTP. Their work, and our research, are also
intimately related to the Creation of Matter in the Universe, as well as
to the understanding of fermion masses and mixing, the so-called Mass
Problem, one of the most profound questions in Fundamental Physics.
The recent measurement of gamma by BaBar and Belle provided irrefutable
evidence that the CKM (Cabibbo Kobayashi and Maskawa) matrix is indeed
complex thus confirming the pioneering proposal of Kobayashi and Maskawa
in 1973. This important conclusion was also stated in works involving
collaborations of CFTP members, e.g. in New
physics and evidence for a complex CKM, Nucl. Phys. B 725 (2005)
155 by F.J. Botella, G.C. Branco, M. Nebot and M.N. Rebelo or in Yukawa
Textures, New Physics and Nondecoupling published in Phys. Rev. D
76 (2007) 033008, by G.C. Branco, M.N. Rebelo and J.I. Silva-Marcos.
Another important aspect of the origin of CP violation is the question
of whether CP is a good symmetry of the Lagrangian, only broken by the
vacuum. In the context of Supersymmetric Theories it is a great
challenge to construct models where CP is spontaneously broken and yet
the CKM matrix is complex. One of the rare examples of such a model was
constructed by CFTP members: Spontaneous
CP Violation in a SUSY Model with a complex CKM by G.C. Branco,
D.Emmanuel-Costa, J.C.Romão, published in Phys. Lett. B 639 (2006) 661.
It is also known that the strength of CP violation in the Standard Model
is not sufficient to generate the Baryon Asymmetry of the Universe
(BAU). Almost all extensions of the Standard Model including
Supersymmetry have new sources of CP violation. The members of CFTP have
been actively investigating models with possible additional sources for
CP violation and also the generation of (BAU) through leptogenesis.
These are some of the present hottest topics in Particle Physics. Click
here for an
extensive list of papers of CFTP written on CP violation.
One of the best general references on CP violation is the book written
by three CFTP members, published in the Oxford International Series of
Monographs on Physics, the same series where appeared the famous Dirac
Book on Quantum Mechanics: CP Violation, Gustavo C. Branco, Luis
Lavoura, Joao P. Silva, International Series of Monographs on Physics,
No. 103 Oxford University Press. Oxford, UK: Clarendon (1999) 511 p.
(The three CFTP members who authored this Book received The Gulbenkian
Science Prize for this work).
Read the press release from the Nobel Prize Organization
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