Four quarks at a time.  Surprising observations at the Large Hadron Collider

According to the information provided by scientists, in the data collected during one of the collisions that were carried out as part of the particle accelerator’s second cycle, the process of simultaneous formation of four superquarks was noted.

What is the up quark?

Simply put, it is one of the six quarks (aside from the up quarks, we also have the down, charm, strange, true and beautiful quarks). Along with leptons, quarks are the fundamental elementary particles described by the Standard Model.

Most importantly, the top quark is the heaviest particle in the Standard Model. This, in turn, means that it is strongly associated with the Higgs boson, which gives the particle its mass. It is not surprising, then, that top quarks are well-suited for researching the limits of the Standard Model, but also for researching what physics might exist beyond.

Four quarks? What’s hard about that?

Scientists have known for a long time that the Standard Model allows four up quarks to form simultaneously (two up quarks and an antiquark pair). Unfortunately, the formation of such a “group” is 70,000 times smaller than the formation of such a pair. It is not surprising, then, that noting the formation of four quarks simultaneously has eluded physicists, despite numerous observations. Yes, scientists have found evidence of its formation before, but they’ve never been able to directly observe the process.

As announced during the conference, the challenge facing the researchers was not only the rarity of the process of creating four quarks simultaneously, but also, and perhaps above all, the difficulty of recording such an event. In particle physics, scientists analyze the properties of final decay particles to reconstruct the sequence that led to their formation.

In the case of formation of up quarks, each of them decays into a W boson and a down quark. The W boson then decays into a lepton and a neutrino or a quark-antiquark pair. This, in turn, means that if we want to find traces of the simultaneous formation of four top quarks, we have to look for many different signatures, which can contain anywhere from 0 to 4 charged leptons and up to 12 puffs emitted by quarks.

Source: CERN

For this reason, scientists belonging to both teams used algorithms created on the basis of machine learning methods in data analysis, which are able to search for events that could be related to the process of formation of four quarks. By examining the effects of individual events, the algorithms first pick out from background events any top quarks, and then search among them for events in which four quarks could have formed. So it was also in this case.

Most importantly, the results of the experiment, whether conducted by the ATLAS or CMS team, meet all requirements of what is called statistical significance. This means that the results obtained by the teams did not happen by chance or are not the result of random fluctuations, or at least the probability of that happening is small. Statistical significance was 6.1 sigma for ATLAS observations and 5.5 sigma for CMS. Meanwhile, the required statistical significance is 5 sigma. So this is the first observation of this process in history.

Now scientists are preparing to search for similar events during the third cycle of the Large Hadron Collider.

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