The LHCb experiment leads to the observation of an exotic tetraquark

Ingrid Fadelli is a writer for the website Phys.org.

Physicists have discovered many elementary particles over the last 20 years. The hadrons are the largest family of particles.

There are many particles with the same properties in this family. TheQuark Model was introduced by M. Gell-Mann and G. Zweig.

There are either three quarks or two quark-antiquark pairs in hadrons. The model theorizes that hadrons with more complex structures, such as four quarks and an antiquark, may exist.

In the 70s, many studies theorize about the mechanisms behind the hadron structures. Some of the particles observed after 2003 are difficult to explain using the model, since the structures that match one of the two main types were found before that date.

The "beauty quark" is the type of particle that is the focus of the experiment. An exotic tetraquark with two charm quarks has been observed by a group of researchers.

The discovery of the heavy charm quark in 1974 led to the recognition that there were two heavy quarks and two light antiquark. Experiments suitable for the search and study of double heavy objects were only found in the 21st century.

Physicists are able to study the production of heavy and double heavy particles at the collider. The probability of the simultaneous production of two charm-anticharm quark pairs at these high energies was far from low according to the analysis done by the LHCb collaboration.

The double charm baryon Xcc ++ consisting of the two charm quarks and light U-quark was observed by the collaboration. "With this observation, it became clear that if double charm tetraquarks existed, their observation would just be a matter of time."

M.Karliner and J.Rosner were able to use the measured properties of the double charm to predict the properties of a hypothetical quadrupole. Two charm quarks, a U-antiquark and a d-antiquark would make up a quadrangle. Tcc + is the theoretical particle's name.

The particle will exhibit itself as a narrow peak in the mass distribution for the pair of charmed mesons D *+ and D 0, where D *+ and D 0 are conventional charmed mesons If the mass is 1% lower than predicted, the properties of the D *+ and D 0 mesons will be changed. It will be very difficult to observe if the mass is just 5% higher.

The exact conditions that would be suitable to observe the hypothetical Tcc + tetraquark were pointed out by M. Karliner and J.Rosner. Their predictions helped guide the recent work.

The mass spectrum of the D *+ and D 0 meson pairs was carefully studied by the collaboration. The researchers used only 4% of the data available in 2012 to study the region of D *+ and D 0 pairs.

They focused on the region that is close to the sum of the D *+ and D 0 meson mass. They observed over one hundred signal Tcc + tetraquarks that form a narrow peak very close to the sum of the D *+ and D 0 meson mass.

Belyaev said that the statistical significance of the signal is so high that it excludes statistical fluctuations. The D *+ meson consists of a charm quark and anti-d quark and D 0 meson consists of charm quark and anti-u-quark.

Several tests were performed to verify the results. The signal they observed was associated with a Tcc +tetraquark. The width of the peak and mass of the Tcc were measured.

Tcc + is the name of the exotic hadron.

The researchers conducted a follow up study on the properties of the Tcc + particle. The decay pattern is consistent with Tcc + (D *+D 0 p + )D 0 The distribution of the mass of D 0 D 0 and D + D 0 pairs is very similar to the decays Tcc + (D *+ D 0 p + )D 0 with missing p + me.

We have not yet measured the quantum numbers of the Tcc + particles directly, but we offered strong arguments in support of the total spin J and parity P of the observed particle, which are the most important quantum numbers. There were no signs that the isospin of the state is 0, in line with the predictions.

There could be at least two different internal structures of the Tcc. In a "molecular-like structure," two charm quarks are separated by a large distance, similar to the size of the atomic nucleus, a "compact structure," or a combination of the two.

The fundamental properties of the Tcc + state, such as the scattering length, effective range and pole position, are important when trying to determine a particle's internal structure. The values measured by the researchers are compatible with a structure that is a bit like a molecule.

A significant contribution to the field of high energy and particle physics can be found in the observation of the Tcc +tetraquark. It has sparked important theoretical discussions about the nature of Tcc + and related states, as well as the existence of the "compact tetraquarks."

The collaboration plans to try to determine the quantum numbers of the new state through indirect evidence.

Belyaev said that it was important to understand the production mechanism of the Tcc. There are some counterintuitive observations and more data is needed to resolve them. It will be interesting to compare the production of the Tcc + and Xcc ++ particles, but also to compare the properties, including production properties.

More information: LHCb Collaboration, Observation of an exotic narrow doubly charmed tetraquark, Nature Physics (2022). DOI: 10.1038/s41567-022-01614-y

R. Aaij and his colleagues wrote about the observation of the faintly charmed baryon cc++. There is a book titled "PhysRevLett.119.112001."

There was an observation of double charm production. journal of high energy physics The book is titled "JHEP06(2012)141."

The Discovery of the Doubly Charmed cc Baryon Implies a Stable bbu d Tetraquark was published. There is a book titled "PhysRevLett.119.202001."

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