Many elementary particles have been discovered by physicists over the past 20 years. These subatomic particles, known as hadrons or strong interactions, are the largest group of elementary particles.
There are many sub-sets of particles that share similar properties within this broad family. M. Gell-Mann, G. Zweig and others introduced the "Quark Model" theory in 1964. This model clearly outlined the internal structure for hadrons.
According to the Quark Model, hadrons can be composed of three quarks (baryons), or three quark-antiquark pair (mesons). Many uncovered hadrons fit into one of these categories. However, the model also suggests the existence of more complex structures such as pentaquarks, which are four quarks and one antiquark, and tetraquarks, which are two quark-antiquark pair.
In the 1970s, many studies looked into the mechanisms that led to the formation of complex hadron structures. The structures of all the hadrons discovered up to 2003 were similar to the Quark Model's main types. However, some particles found after that date cannot be explained using the model.
The LHCb experiment, a detector located at the CERN Large Hadron Collider, primarily aims to reveal differences between matter and nonmatter through the study of a particular type of particle known as the "beauty quiark." The large team of researchers who participated in the experiment, the LHCb Collaboration has just observed an exotic tetraquark that had an unusual structure and contained two charm quarks.
Vanya Belyaev was one of the researchers involved in the study. She stated that she discovered the heavy charm quark in 1974. This observation of J/y mesons in 1973, commonly called the 'November revolution', and a heavier beauty quark the following year led to the recognition of the possibility of tetraquarks composed of two heavy quarks, and two light antiquarks, having interesting and unique properties. However, the Large Hadron Collider at CERN was the first to provide experimental facilities that could be used for the study and search for these 'double-heavy' objects.
The LHC collider allows physicists to study collisions between protons at extremely high energies. This promotes the production of many heavy and double-heavy particles. The LHCb Collaboration analyzed only a small portion of the LHC data in 2011 and 2012. It found that there was a high probability of producing two charm-anticharm pairs of quarks at these high energies. This suggests that the collider might be able to observe double heavy objects.
Belyaev explained that 2017 was the year when the LHCb Collaboration reported an observation of the double-charm baryon Xcc++, consisting of two charm quarks as well as a light uquark. This observation made it clear that double charm tetraquarks are possible. Their observation is only a matter time.
M.Karliner, J.Rosner and the LHCb observed the double charm baryon Xcc++ and were able to predict with precision the properties of a hypothetical tetraquark. A tetraquark like this would have two charm quarks: a U-antiquark, and a D-antiquark. Tcc+ was the name of the theoretical particle.
The predicted properties of Tcc+ tetraquark indicate that the particle will show itself as a narrow peak within the mass distribution of the pair of charmed mensons D*+ (where D*+ is the conventional charmed mansons comprising (charm queark and antid-quark) and(charm quark, anti-u-quark), respectively. It is interesting to notice that the Tcc+ predicted mass is very close to that of the D*+ mesons. This means that even if the mass is 1% less than that predicted, the Tcc+ properties will not be visible in D*+ or D0 mass spectrums. The peak will be much wider (or very wide) if the mass is just 5% lower. It will be difficult to observe experimentally.
M. Karliner's and J. Rosner's work essentially pinpointed the conditions necessary to observe the Tcc+ tetraquark. These predictions were the basis for the LHCb collaboration's recent work.
The collaboration studied the mass spectrums of D*+ and D0 meson pairs. They used a dataset that consists of all data from the LHC collider between 2011 and 2018. The researchers used only 4 percent of the data from the LHC collider to analyze the region of large masses of D*+, D0, and D0 pairs in their 2012 analysis.
Their new analysis focused specifically on the area of mass that is closest to the sum D*+/D0 meson masses. They found over 100 signal Tcc+ Tetraquarks in this region. This area forms a strikingly narrow peak that is very close to the sum D*+/D0 meson masses. It has a high statistical significance.
Belyaev stated that "the statistical significance that we observed was so high that it completely excludes that the observed sign is a statistical fluctuation." "D*+ meson is composed of anti-d quark charm quark, and D0 meson is charm quark anti-uquark. This fixes the minimum quark content that was observed as two charm quearks, anti d-quar, and anti u-quark.
To validate their findings, the LHCb collaboration performed many tests. All of these tests confirmed that the signal was associated with a Tetraquark. They measured the mass and peak widths of the Tcc+ Tetraquark.
Belyaev stated that quantum mechanics dictates that the width of the peak corresponds to the inverse life of the particle. He also said that this width corresponds with a very long lifespan, one of the longest for particles that decay due to strong interactions, and the longest for exotic hadrons so far." In a sense, Tcc+ can be described as Methuselah for exotic hadrons.
Researchers recently completed a follow up study that was featured in Nature Communications. This further explored the properties of Tcc+ particles. They showed that the decay pattern was consistent with Tcc++-(D*+–D0p+]D0. They also checked the distribution of the mass of D0D0 and D+D0 pairs and found that the enhancements in these spectra are very well consistent with the decays Tcc+-(D*+-D0p+)D0 with missing p+ meson andTcc+-(D*+-D+p0/g)D0 with missing p0/g.
Belyaev stated that although we haven't yet measured the quantum number of Tcc+ particles, we provided strong arguments for JP=1+ and parity J of the observed particle. These are the most important quantum quantities, and are in perfect agreement with our expectations. We have examined mass spectra of the D0D0 and D+D0 pairs. This was to search for potential contributions from hypothetical isospin partner. The predictions were confirmed by the researchers, and they found no evidence that the Tcc+ isospin is 0.
Two possible internal structures could be found in the Tcc+ Tetraquark, as observed by LHCb. It could be a "molecular-like" structure, where two charm quearks are separated by a long distance that is comparable to the size the atomic nucleus. Or it could be a compact structure, where the distance between them is significantly smaller.
The team developed a sophisticated model to identify the structure and measured fundamental properties of Tcc+. This is important for determining a particle's internal structure. Although the values were compatible with a molecular-like structural, this has yet to be confirmed.
The LHCb collaboration's observation on the Tcc+ Tetraquark is an important contribution to the field high energy and particle Physics. It has sparked critical theoretical discussions about the nature of Tcc+ and related molecular-like state states such as the mysterious X(3872) and the general issue surrounding the existence of "compact tetraquarks".
The collaboration will attempt to determine the quantum numbers of this new state through its future studies. They have not yet been able to establish strong but indirect evidence.
Belyaev said that it is important to understand the production mechanism for the Tcc+ state of proton-proton collision. "Currently, we have some counterintuitive observation--some distributions like track multiplicity and transverse momentum are really puzzling, and more data is required for resolution. It will also be interesting to compare production properties of the Tcc++ and Xcc++ particles, as well as the properties of the Tcc++ and the mysterious X(3872).
R. Aaij et al, Observation of the Doubly Charmed Baryon Kscc++, Physical Review Letters (2017).DOI: 10.1103/PhysRevLett.119.112001
Double charm production in open charm at pp collisions at 7 TeV. Journal of High Energy Physics (2012). DOI: 10.1007/JHEP06(2012)141
Marek Karliner et al, Discovery of the Doubly Charmed Kscc Baryon Implies a Stable bbu-d- Tetraquark, Physical Review Letters (2017).DOI: 10.1103/PhysRevLett.119.202001