CMS Collaboration observes new full-weight quark structure

For more than a decade, the CMS Collaboration, a large team of researchers from different institutes around the world, has been analyzing the compact Muon solenoid, a universal particle detector at CERN’s Large Hadron Collider (LHC). Collected data. This massive international scientific collaboration has been trying to observe a variety of elusive physical phenomena, including exotic particles and dark matter candidates.

In a recently published paper, Physical Review Letters, the CMS collaboration reports three strange full-weight quark structures. These structures appear to be part of a common family and could open new and interesting avenues for particle physics research.

“Almost all matter we encounter in our daily lives is composed of three types of particles: electrons, protons and neutrons,” co-author Professor Yi Kai of Nanjing Normal University and Tsinghua University told Phys.org. “As far as we know, the electron is fundamental, but the other two are made of triplets of quarks. The quark model of the particle was proposed in 1964, and by the early 1970s there was compelling evidence that it was correct .

The model introduced in 1964 describes quarks as tightly bound particles so closely connected that they cannot exist alone and can only be observed as bound quark triplet (qqq) or quark antiquark (qq) arrive.^–) doublet. Physicists have identified massive systems of tightly bound quarks, also known as “hadrons.”

“These quark systems are huge in number, but with the exception of protons and neutrons, their existence is very short-lived,” Professor Yi explained. “There was a hole in the 1964 quark theory. Maybe, just maybe, quartets and quintets of quarks might also form into particles called ‘strange’ hadrons. Physicists have been considering this possibility for decades. Sex, but only on the edge.

Observing strange hadrons has long seemed a challenging and elusive research goal. One reason is that existing experimental tools only allow physicists to look for exotic systems composed entirely of light (u, d, s) quarks, which are difficult to distinguish from ordinary hadrons.

“With the advent of more powerful particle colliders, systems containing heavier (c, b) quarks are becoming increasingly clear – the heavier and more quarks there are, the easier the system is to understand,” Professor Yi said . “The mass of a single charm (c) quark is about one and a half times that of a proton, the mass of a bottom (b) quark is about five times that of a proton, and the masses of individual u and d quarks are less than the mass of a proton. About 0.5 times the mass of a proton. %.

In 2003, a paper by Japan’s Belle Collaboration sparked new interest in exotic systems by revealing X(2872), which was proposed as a possible cc^–QQ^– system (i.e. a system containing two heavy quarks). This paves the way for new research, introducing other exotic hadron candidates containing charm quarks and even bottom quarks, hinting at the existence of tetraquark and pentaquark systems.

Despite these efforts, the internal structure of strange hadrons remains a mystery because the reported systems include light quarks and are therefore inherently difficult to model. Therefore, observations of systems composed only of heavy quarks could open a new window into exotic structures, allowing physicists to better understand the strong interactions between quarks.

“The problem with heavy quarks is that they are difficult to make,” Professor Yi said. “One step in this direction would be to find systems in which the u quark or the d quark is replaced by the s quark. Although the s quark is still considered a light quark, its mass is about 40 times that of the au quark. In 2009, this goal by discovering Y(4140), now called chi_c(4140), which is a candidate for cc^–SS^– Tetraquarks (i.e. the first exotic candidate without any very light (u,d) quarks).

The discovery of chi_c(4140) encouraged more research teams to search for structures composed entirely of c and d quarks. After confirming the existence of the system, CMS also began looking for systems that decay into J/psi particle pairs or Upsilon particle pairs.

“J/psi is cc^– Bound state, Upsilon a bb^– Professor Yi said: “Using data collected by LHC Run I in 2011 and 2012, CMS did find a clue.” Two J/psi-J/psi structures, but there was not enough data at the time to make a convincing conclusion. claim.

In 2019, the CMS collaboration resumed the search for fully heavy quark systems that decay into J/psi or Upsilon particle pairs, this time using data collected at the CERN Large Hadron Collider between 2016 and 2018 (run II ). However, the first particle, known as X(6900), was eventually observed by another CERN effort, the Large Hadron Collider experiment.

“The LHCb experiment was the first out of the gate, and they reported the decay of X(6900) to J/psi – J/psi in 2020,” Professor Yi said. “However CMS continued their work, and we eventually paid off by identifying three J/psi-J/psi structures: confirming X(6900) and reporting two new structures called X(6600) and X(7100 ).

As part of this latest study, the CMS team looked specifically for J/Psi meson pairs. These particles are powerful detectors of full-weight quark systems because they can be clearly identified at the Large Hadron Collider, where the environment is complex and marked by high-intensity pp collisions.

“In this study, the analysis team designed a search strategy in Run II based on information from Run I, without actually looking at the data. This approach is called ‘blind’ analysis and can be very effective in avoiding potential bias. For example, by deceiving oneself as if one has found something one thinks one should or wants to find, after the new data is finally revealed, these three structures jump out,” Professor Yi explained.

Using this blind analysis strategy, Professor Yi and his CMS collaborators were able to confirm the existence of structures previously detected by the LHCb collaboration, while also revealing two entirely new structures. The three structures appear to be part of the same family of full-weight quark systems.

“Although this may not be the only possible explanation, the model of three structural quantum mechanical interferences describes the CMS data well,” Professor Yi said. “This requires that all three have the same quantum properties and further suggests that these states are a family of excited four-quarks.”

Three full-weight quark structures reported by the CMS collaboration provide important new clues about the properties and internal structure of exotic hadrons. Specifically, they point to a new system in which physicists can apply the theory of strong interactions: “quantum chromodynamics” systems.

“CMS is now preparing to improve the measurement of these state properties,” Professor Yi added. “The new data raise the new and exciting possibility of searching for possible exotic states composed purely of heavier bottom quarks.”

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