New results from experiments at the Large Hadron Collider continue to explain how elementary particles acquire their mass.
The Standard Model was formulated in the 60s of the last century with the goal of describing all elementary particles known at that time and the types of interactions between them in one “language.” Several decades passed from the emergence of the Standard Model to its complete experimental confirmation. With the development of particle accelerators, detectors and methods for processing the resulting data, physicists discovered many unknown particles predicted by the model at that time. Such experiments have confirmed the correctness of the Standard Model, although physicists are very carefully looking for phenomena that would not fit into it and explain what is still completely unclear to us, for example, dark matter.
CMS detector at CERN. Photo: CERN View full size ‹ ›
However, even within the Standard Model there is scope for new experiments. The mechanism of particle mass formation, due to technical limitations, has long been a purely theoretical concept. The big breakthrough in this field came just recently with the experimental discovery of the Higgs boson, the last particle predicted by the Standard Model. With it, a new area for study opened up. For example, the so-called Higgs field explains the mass of weak vector bosons – carriers of the weak nuclear force, whose spin is equal to one (W- and Z-bosons). The fact is that the mathematical consistency of the model requires zero mass for the carriers of nuclear interaction, but at the same time, the extremely short distance at which this force manifests itself suggests the participation of massive particles. The Higgs mechanism resolves this contradiction: according to it, the masses of weak vector bosons are not intrinsic characteristics of particles, they appear as a result of interaction with the omnipresent Higgs field. So, it is possible that a similar mechanism should explain the mass of fermions – particles with half-integer spin. To do this, we need to experimentally observe the interaction of the Higgs boson with fermions.
Today we know of 12 elementary fermions, which are divided into three “generations”. The first generation consists of up and down quarks, an electron and an electron neutrino. These fermions are the basic ingredients of matter: protons and neutrons are made up of up and down quarks, and electron neutrinos are emitted in some radioactive decays. For a reason that is still not entirely clear, the first generation of fermions has two “heavier” copies. The second generation includes charm and strange quarks, the muon and the muon neutrino. Charged particles of the second generation weigh significantly more. The third generation consists of b- and t-quarks, tau lepton and tau neutrino, and the mass of charged fermions in this generation is even greater than in the previous ones.
If the mass of fermions actually appears when they interact with the Higgs field, then the difference in the mass of fermions from each group should be reflected in the strength of their interaction with the Higgs boson. The collaboration of the ATLAS and CMS experiments began studying exactly this after it confirmed the existence of the Higgs boson. Recent evidence shows that the Higgs boson decays into two b quarks. This observation confirms the role of the Higgs field in the formation of the mass of the third generation of fermions.
The observation of the decay of the Higgs boson into two b quarks is the result of processing data collected over the past 6 years. Both experiments saw a signal of this decay with a statistical significance of 5.4 and 5.6 σ (σ is the square deviation, a statistical significance of 5 σ is accepted as a condition for recognizing the result as valid in particle physics). The frequency with which this decay is observed is consistent with the prediction of the Standard Model, although the measurement uncertainty is about 20%.
The Higgs boson is formed when high-energy particles collide and decays almost immediately into different particles. The probability of each type of decay depends on the strength of interaction between the Higgs boson and the particles into which it decays, and this force in turn is determined by their mass. Among elementary particles, b-quarks are among the heaviest, so the decay scenario involving them is the most likely: it should occur in 58% of cases. It is likely that b-quarks can also be formed during strong nuclear interaction, which is also possible during proton-proton collisions. Their background masks the decay of the Higgs boson, which is why for its first experimental observation, announced in 2012, physicists looked only for decays involving photons, the carriers of the electromagnetic force, and the weak vector bosons we mentioned earlier.
So, to see the decay into down quarks, the researchers turned to rarer scenarios for the formation of the Higgs boson, in particular, to processes in which they are formed together with weak vector bosons. On the technical side, this requires very careful processing of the data from the particle detector. Advanced data processing techniques include machine learning. It requires reconstructing the energies and momenta of weak vector bosons, tagging particle beams produced by down quark decays, and accurately simulating the background created by other decays to isolate the signal. Unfortunately, the current accuracy of experiments does not allow us to detect deviations from the Standard Model.
Nevertheless, the latest results are an important achievement in particle physics. They directly confirm the interaction of the Higgs boson with the third generation of fermions. More recently, experimental data were published showing that the Higgs boson decays into tau particles, and is also formed together with t-quarks. Together, these results mean that the Higgs field does indeed form third-generation fermion particle masses.
These experiments open a new series of high-precision measurements of the interaction of the Higgs boson with fermions. New data from the Large Hadron Collider, especially after its beam power is increased, will significantly improve the measurements. With measurement uncertainty an order of magnitude smaller than the current one, physicists will continue to test the Standard Model. Another important goal of the ATLAS and CMS experiments is to study the interaction of the Higgs boson with the second generation of fermions. The decay of the Higgs boson into a pair of muons should finally be observable after a collider upgrade. Unfortunately, due to the strong background, decay into charm quarks during proton-proton collisions is possible only in a giant electron-positron collider, which does not yet exist. So the Higgs boson will occupy our minds for a long time.
Based on materials from Nature.