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In 2012, the famous Higgs boson was discovered by ATLAS and CMS collaborations in proton-proton collisions at the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland.1,2. Now, by writing Letters of Physics B3 and Letters of physical examination4, both collaborations report the observation of the decomposing Higgs boson into a pair of elementary particles called bottom quarks. This important step in particle physics confirms the role of the Higgs field – the quantum field associated with the Higgs boson – in the supply of particles of mass matter.
When the standard model of particle physics appeared in the 1960s, the main focus of the ad hoc Higgs field was to explain the masses of the weak vector bosons – the force vectors of the I & # 39; weak nuclear interaction. Mathematical consistency required that force carriers be massless, while the extremely short range of weak interaction was a signature of massive particles. The Higgs mechanism5–8 has addressed this issue: the masses of the weak vector bosons are not intrinsic, but are the result of interactions between these particles and the omnipresent Higgs field. It was soon realized that the elementary particles of matter called fermions could also draw their mass from interactions with the Higgs field.9,ten.
Several decades later, twelve elemental fermions are known and divided into three families. The first family includes three charged particles – the up quark, the down quark and the electron – and a neutral particle called an electron neutrino. These fermions are the basic ingredients of ordinary matter: top-down quarks are components of protons and neutrons, and electron neutrinos are emitted by some radioactive decays.
For a reason that is not yet well understood, there are two replicas of the first family. The second family includes the charming quark, the strange quark, the muon and the muon neutrino, where the charged fermions have larger masses than their counterparts of the first family. And the third includes the top quark, the bottom quark, the tau and the tau neutrino, where the charged fermions are even more massive.
After discovering the Higgs boson1,2, one of the objectives of the ATLAS and CMS collaborations was to probe the properties of the particle, such as its couplings with fermions – the strength of its interactions with fermions. In the current articles, the collaborations combine all the data recorded from 2011 to 2017 and each one affirms to have observed the decay of the Higgs boson in quarks of bottom.
In both sets of data, the decay signal is wider than the background, which results from other processes related to particle physics. The statistical significance of the signal is 5.4 and 5.6 standard deviations, respectively, for the ATLAS and CMS experiments – well above the conventional threshold of 5 standard deviations required to qualify for an observation. In addition, the overall decay efficiencies are consistent with standard model predictions with an experimental uncertainty of about 20%.
The Higgs boson disintegrates almost immediately after its production. The probability of a particular decay occurring depends on the Higgs boson couplings, which are determined by the masses of the decay products. Since bottom quarks are among the heavier fermions, decomposition into these particles is the most common, occurring about 58% of the time. But even if this decay is dominant, in proton-proton collisions, the signal is submerged by the background of the background quarks produced by the strong nuclear interaction. For this reason, the discovery of the Higgs boson in 2012 involved decays only in vector bosons: photons resulting from the electromagnetic interaction and weak vector bosons resulting from the weak interaction.
To observe the disintegration up to the bottom quarks, both collaborations had to search for sub-dominant modes of Higgs-boson production, such as the production of the Higgs boson with a weak vector boson (Fig. 1). . In-depth understanding of particle detector responses and sophisticated data analysis methods, including machine learning, were needed to accurately reconstruct the energy and momentum of weak vector bosons, mark particle jets from quarks the backgrounds and separate these backgrounds from the signal.
Figure 1 | Production of the Higgs boson with a weak vector boson.L & # 39; ATLAS3 and CMS4 Collaborations report evidence that a particle known as the Higgs boson can disintegrate into pairs of elementary particles called bottom quarks. To detect this disintegration, the collaborations looked for a particular process in which two quarks from colliding protons merge to form a weak vector boson, carrying the force of the weak nuclear interaction. The weak vector boson emits a Higgs boson that decays to lower quarks.
The results are not totally surprising, for at least two reasons. First, there has been several pieces of evidence of the disintegration of the Higgs boson into lower quarks in the past. In 2012, scientists at the Tevatron proton-antiproton collider, located near Chicago, claimed a signal with a level of 2.8 standard deviations.11. Between 2012 and 2018, ATLAS and CMS collaborations regularly reported the results of their caries research. In their last articles before the work in progress, they obtained respective proofs at the level of 3.6 and 3.8 standard deviations.12,13. These different proofs could be considered as a combined observation of decomposition.
Secondly, many other experimental results at the LHC limit what could actually be observed with respect to this degradation. For example, if the Higgs boson behaved as in the standard model, but coupled to zero at the bottom quarks, the yields of all the other decay modes would be multiplied by about 2.4, which is in contradiction with the data. Considering the situation as a whole, unless there are unexpected cancellation effects, the allowed deviations from the standard model are in the order of a few percent – below the current sensitivity of 20 percent. % of experiments conducted at the LHC.
Nevertheless, the current results are a great success and constitute a major step in particle physics. With observations earlier this year of the Higgs boson in decomposition into tau particles14 and the production of the Higgs boson with the best quarks15,16the findings directly establish interactions between the Higgs boson and the third fermion family, indicating that the Higgs field is responsible for masses of fermions.
The results are the starting point of an era of precision measurement for Higgs boson couplings to fermions. With more data from the LHC, in particular, after a few years of improvement in beam intensity, it would be necessary to obtain a precision of a few percent in the measurements. This would open the possibility of finding deviations from the standard model and discovering, for example, currently unknown particles.
Another important step would be the observation of Higgs boson couplings with the second family of fermions. The disintegration of the Higgs boson into a pair of muons is within the reach of the future upgraded LHC. However, due to the extremely high background noise in proton-proton collisions, decay to seduce quarks could probably only be demonstrated using a giant electron-positron collider, which has not yet been built. The boson Higgs is far from having revealed all its secrets.
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