Theoretical physicists find that the Higgs boson does not appear to contain any harbingers of new physics

The Higgs boson was discovered in the detectors of the Large Hadron Collider about ten years ago. It turned out to be such a difficult particle to produce and observe that, despite the passage of time, its properties are still not known with satisfactory precision. We now know a little more about its origin, thanks to the recently published discovery of an international group of theoretical physicists with the participation of the Institute of Nuclear Physics of the Polish Academy of Sciences.

The research is published in the journal Physical Exam Letters.

The scientific world is unanimous in affirming that the greatest discovery made thanks to the Large Hadron Collider (LHC) is the famous Higgs boson. For twelve years, physicists have been trying to learn as precisely as possible about the properties of this very important elementary particle. The task is extremely difficult because of the experimental challenges and the many computational obstacles.

Fortunately, significant progress has just been made in theoretical research, thanks to a group of physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow, RWTH Aachen University (RWTH) in Aachen and the Max-Planck-Institut für Physik (MPI) in Garching near Munich.

The Standard Model is a complex theoretical framework constructed in the 1970s to coherently describe the currently known elementary particles of matter (quarks, as well as electrons, muons, tau and the associated trinity of neutrinos) and the electromagnetic (photons) and nuclear forces (gluons in the case of strong interactions, W and Z bosons in the case of weak interactions).

The icing on the cake of the creation of the Standard Model was the discovery, thanks to the LHC, of ​​the Higgs boson, a particle that plays a key role in the mechanism responsible for assigning masses to other elementary particles. The discovery of the Higgs boson was announced in mid-2012. Since then, scientists have been trying to obtain as much information as possible about this fundamentally important particle.

“For a physicist, one of the most important parameters associated with any elementary or nuclear particle is the cross section for a specific collision. Indeed, it tells us how often we can expect the particle to appear in collisions of a certain type. We focused on the theoretical determination of the cross section of the Higgs boson in gluon-gluon collisions. They are responsible for the production of about 90% of the Higgs, traces of whose presence have been recorded in the detectors of the LHC accelerator,” explains Dr. René Poncelet (IFJ PAN).

Professor Michal Czakon (RWTH), co-author of the paper, adds: “The essence of our work was the desire to take into account, when determining the active cross section for the production of Higgs bosons, certain corrections which, due to their apparently small contribution, are usually neglected, since ignoring them significantly simplifies the calculations. This is the first time that we have managed to overcome the mathematical difficulties and determine these corrections.”

The importance of the role of higher-order corrections for understanding the properties of Higgs bosons can be demonstrated by the fact that the secondary corrections calculated in the paper, apparently small, contribute to almost one-fifth of the value of the sought active cross-section. This is to be compared with third-order corrections of 3% (but which reduce the calculation uncertainties to only 1%).

A novelty of this work was to take into account the effect of the masses of the bottom quarks, which led to a slight but notable shift of about 1%. It is worth recalling here that the LHC collides protons, i.e. particles made up of two up quarks and one down quark. The temporary presence of quarks of greater mass inside the protons, such as the beauty quark, is a consequence of the quantum nature of the strong interactions that bind the quarks in the proton.

“The values ​​of the active cross section of Higgs boson production found by our group and measured during previous beam collisions at the LHC are practically the same, taking into account of course the current calculation and measurement inaccuracies. It therefore seems that no harbinger of new physics is visible in the mechanisms responsible for the formation of the Higgs boson that we are studying, at least for the moment,” summarizes Dr. Poncelet.

The widespread belief among scientists that new physics is needed is that the Standard Model fails to answer a number of fundamental questions. Why do elementary particles have such mass? Why do they form families? What is dark matter composed of, traces of which are so clearly visible in the cosmos? What is the reason for the predominance of matter over antimatter in the universe? The Standard Model also needs to be extended because it does not take into account gravity at all, such a common interaction.

It is important to note that the latest advances by theoretical physicists from IFJ PAN, RWTH and MPI do not definitively rule out the presence of new physics in the phenomena accompanying the birth of the Higgs boson. Much could change when the data from the fourth search cycle of the Large Hadron Collider, which is gradually starting up, begin to be analyzed.

The increasing number of observations of new particle collisions could reduce measurement uncertainties so that the measured range of admissible cross sections for Higgs production no longer matches that defined by theory. Physicists will know this in a few years.

For now, the Standard Model may seem safer than ever – and that fact is slowly becoming the most surprising discovery made with the LHC.