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

The Higgs boson was discovered about twelve years ago in the detectors of the Large Hadron Collider. It has proven to be a particle so difficult to produce and observe that, despite the passage of time, its properties are still not known with satisfactory accuracy. Now we know a little more about its origin, thanks to the recently published achievement of an international group of theoretical physicists with the participation of the Institute of Nuclear Physics of the Polish Academy of Sciences.

The research was published in the journal Physical assessment letters.

The scientific community unanimously agrees that the greatest discovery made with the Large Hadron Collider (LHC) is the famous Higgs boson. For twelve years, physicists have tried to learn as precisely as possible about the properties of this very important elementary particle. The task is extremely difficult because of both the experimental challenges and the numerous computational hurdles.

Fortunately, significant progress has recently 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 Kraków, the RWTH Aachen University (RWTH) in Aachen and the Max Planck Institute for Physics (MPI) in Garching near Munich.

The Standard Model is a complex theoretical structure that was formulated in the 1970s to coherently describe the currently known elementary particles of matter (quarks, but also electrons, muons, tau and the associated trinity of neutrinos) and electromagnetic forces (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 in 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 giving mass to the other elementary particles. The discovery of the Higgs boson was announced in mid-2012. Since then, scientists have been trying to gather 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. This is because it gives us information about how often we can expect the particle to appear in collisions of a given 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 boson, traces of whose presence have been registered in the detectors of the LHC accelerator”, explains Dr. Rene Poncelet (IFJ PAN).

Prof. 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. It 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 the Higgs bosons can be seen from the fact that the secondary corrections calculated in the paper, although apparently small, contribute almost one fifth of the value of the sought active cross section. This compares with third-order corrections of 3% (but which reduce the computational uncertainties to only 1%).

A novelty of the work was that the effect of bottom quark masses was taken into account, leading to a small but noticeable shift of about 1%. It is worth recalling here that the LHC collides protons, i.e. particles consisting of two up quarks and one down quark. The temporary presence of quarks with larger masses inside protons, such as the beauty quark, is a consequence of the quantum nature of the strong interactions that bind quarks inside the proton.

“The values ​​of the active cross section for Higgs boson production found by our group and measured in previous beam collisions at the LHC are very similar, of course taking into account current calculation and measurement inaccuracies. It therefore seems that no precursors of new physics are visible within the mechanisms responsible for the formation of Higgs bosons that we are investigating, at least for now,” Dr. Poncelet summarizes the team’s work.

The widespread belief among scientists in the necessity of the existence of new physics stems from the fact that a number of fundamentally important questions cannot be answered with the Standard Model. Why do elementary particles have the masses that they do? Why do they form families? What makes up dark matter, 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 completely ignores gravity, which is such a common interaction.

Importantly, the latest achievement of theoretical physicists from the IFJ PAN, RWTH and MPI does not definitively rule out the presence of new physics in the phenomena associated with the birth of the Higgs boson. Much can change when data from the gradually beginning fourth research cycle of the Large Hadron Collider are analyzed.

The increasing number of observations of new particle collisions may make it possible to reduce the measurement uncertainties to such an extent that the measured range of allowed cross sections for Higgs production no longer coincides with that defined by theory. Whether this will happen, physicists will discover in a few years.

For now, the Standard Model may feel safer than ever, and this fact is slowly becoming the most surprising discovery made at the LHC.