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Phenomenology of the Standard Model

Astroparticle physics and Cosmology

Beyond the Standard Model theoretical questions

The Brout-Englert-Higgs mechanism and boson

Phenomenology of the Standard Model

The Standard Model of particle physics describes the properties of the fundamental particles and their interactions. The theory has been successfully tested in a great variety of experiments, such as at the high-energy LEP and LHC colliders at CERN. In 2012 the ATLAS and CMS LHC experiment announced the discovery of the last fundamental particle that was still to be discovered in the Standard Model: the Brout-Englert-Higgs mechanism and boson, 48 years after it was first hypothesized in Brussels and Edinburgh.

Astroparticle physics and Cosmology

The Standard Model leaves a number of observational facts unexplained. Most of them stem from observations of the "infinitely large" Universe. The explanation of these enigmas must most probably be found at the "infinitely small" level of new particles and interactions, beyond the Standard Model. Thus, addressing these astroparticle physics issues allows a better understanding of both the Universe and the fundamental laws of nature. This concerns in particular:

- The presence of a large amount of Dark Matter in the Universe. What is this mysterious matter made of? It could be explained by the existence of new particles beyond the Standard Model, whose nature and properties have yet to be established better, or by the existence of Primordial Black Holes, whose formation in the early Universe and presence today would also have to be established. Dark Matter constitutes an interdisciplinary topic in many senses, at the crossroads of cosmology and gravitation, particle physics, galaxy astrophysics, stellar astrophysics (for instance through its potential effects on neutron stars), and also involves various aspects of nuclear physics, atomic physics, and condensed matter physics (as relevant for experiments aiming to observe its effects directly in Earth-based detectors).

- The small but non-vanishing neutrino masses, established since 1998 through observations of oscillations of neutrinos of astrophysical origin, as well as from Earth-based experiments. These observations contradict the Standard Model which predict the neutrinos to be massless. The best mechanism beyond the Standard Model that can account for them is the seesaw mechanism. More generally neutrinos are feebly interacting particles that can travel through the Universe over very large distances without being affected. Thus the observation of neutrinos coming from the cosmos provides a lot of information on the properties of the Early Universe, of astrophysical sources, as well as many constraints on theories beyond the Standard Model.

- The matter-antimatter baryon asymmetry observed in the Universe. Such an asymmetry can be explained in theories beyond the Standard Model, such as through the leptogenesis mechanism, closely related to the seesaw origin of the neutrino masses.

- The fact that the Universe is observed to be homogenous isotropic and without curvature to a high degree of accuracy at large distance scales. This can be explained from an exponential period of expansion, called cosmic inflation, that occurred shortly after the Big Bang. This implies the existence of a new scalar particle called the "inflaton". During this period the primordial density fluctuations are believed to have been created, providing the seeds for the formation of galaxies.

- The presence of a large amount of dark energy in the Universe. This also can point towards the existence of new scalar fields.

Astroparticle studies also concern the physics physics of cosmic rays (such as observed by the Telescope Array), that provide important information on astrophysical objects that are at their origin.

The last ~20 years have witnessed the rise of precision cosmology, in particular through the accurate observations of the cosmic microwave background and of the distribution of galaxies in the Universe (Euclid galaxy surveys, Lyman alpha forest,...). These observations are important for the enigmas above, as well as for constraining many beyond the Standard Model scenarios that could affect them. Important additional observational newcomers (i.e. new windows on the cosmos), are the cosmic 21 cm radiation and the gravitational waves, first observed in 2015. The future observations of gravitational waves in wider ranges of frequencies and with greater sensitivity will provide lots of information on the astrophysical phenomena or early universe phenomena that could have been at their origin (Einstein Telescope, LISA). Gravity is the dominant force on large scales in the Universe. It is also the least known force of nature, both at very large and very small scales.

Beyond the Standard Model theoretical questions

The Standard Model also leaves unanswered a number of theoretical questions, that also points towards the existence of new physics beyond the Standard Model. Important questions include :

- what is the origin of the flavor structures (of masses and mixings) that are observed for the various leptons and quarks?

- what is the status of CP-violation in the strongly coupled sector of the Standard Model?

- what is the solution of the energy scale hierarchy problem?

These questions can find a solution or partial solutions in theories beyond the Standard Model, such as supersymmetric theories, grand-unified theories, axion theories,.... Interestingly, some of these theories, that can be tested on Earth through a large variety of experiments (LHC collider, underground experiments,...) also offer clear possibilities of explanations for the above astroparticle physics enigmas.

The Brout-Englert-Higgs mechanism and boson

F. Englert and R. Brout, the founders of the Theoretical Physics group, have obtained several prestigious prizes for their research: