An IPhT physicist and two collaborators have proposed a theoretical model to explain the low mass of the Higgs boson and the cosmological constant – both of which are considered inconsistent with current physics. This model is expected to be tested soon at CERN.

The standard model (SM) of particle physics can account for many phenomena with excellent precision. However, it cannot be used to understand two very fundamental quantities: the cosmological constant and the mass of the Higgs boson.

The first one – consistent with a curvature of space – is related to the expansion of the Universe and thus determines its large-scale energy density. Based on the standard model and on symmetry principles that have become classics of modern physics, the theorists estimate that the cosmological constant should be 10¹²⁰ times greater than the value deduced from astrophysical measurements, which would amount to fitting the entire Universe into a volume much smaller than that of an atom! Similarly, they estimate that the mass of the Higgs boson should be several orders of magnitude greater than what is measured from proton collisions at the LHC (Large Hadron Collider) at CERN.

These small values both seem to result from very improbable "fine tuning". As if multiple, unrelated and gigantic contributions finely "neutralized" each other!

Until now, scientists have developed without any real success:

• so-called dynamic explanations that use new structures, particles or symmetries (such as "supersymmetry");
• statistical interpretations based on the hypothesis of parallel worlds (the multiverse explanation).

It should be noted that the lack of discovery of any "superparticles" at the LHC increases the tension between experiment and theory regarding the Higgs mass.

Three theorists, including one from the IPhT, have proposed a new explanation based on an original relationship between the Higgs boson and the cosmological constant. They identified a new class of mechanisms for producing fine tuning via the Higgs mass. Only certain specific values of the Higgs mass are able to "select" extremely low values of the cosmological constant and still leave enough room for the Universe to exist. Furthermore, this scenario opens up new possibilities for the creation of dark matter.
 
The researchers' reasoning is all the more interesting as it produces predictions that can be verified at the "High-Luminosity" LHC, which will soon come into operation. Rare decays of new Higgs bosons (Atlas and CMS) could help to decide whether or not to support this model. To be continued!