During the 20th century, physicists have discovered many elementary particles. The largest family of these particles are the so-called hadrons, subatomic particles that participate in strong interactions.
This broad family of particles contains numerous subsets of particles with similar properties. In 1964, M. Gell-Mann and G. Zweig introduced a famous theory known as the “Quark Model”, which clearly outlined the internal structure of hadrons.
The Quark model suggests that hadrons are made up of either three quarks (baryons) or quark-antiquark pairs (mesons). While many unmasked hadrons fall into one of these two categories, the model also assumes the existence of hadrons with more complex structures, such as pentaquarks (ie four quarks and an antiquark) and tetraquarks (ie two quark-antiquark pairs).
Many studies in the 1970s theorized about the possible mechanisms underlying the formation of these complex hadron structures. All hadrons discovered up to 2003 had structures matching one of the two main types described by the Quark Model, however some of the particles observed after that date are difficult to explain using the model.
The LHCb experiment is a detector at CERN’s Large Hadron Collider that primarily aims to reveal differences between matter and antimatter by studying a specific type of particle known as a “beauty quark.” The LHCb Collaboration, the large group of researchers involved in the experiment, recently observed an exotic tetraquark with an unusual structure, containing two charm quarks.
“The discovery of the heavy charm quark in 1974 (observation of J/y mesons in 1974, often called the ‘November Revolution’) and even heavier beauty quark in 1977, led to the recognition that the four quarks consisted of two heavy quarks and two light antiquarks could have interesting and unusual properties,” Vanya Belyaev, one of the researchers who carried out the study, told Phys.org. “However, experimental facilities suitable for the search and study of such ‘double heavy’ objects appeared only in the 21st century, with the launch of the Large Hadron Collider at CERN.”
At the LHC, physicists can study collisions between protons at very high energies, which promote the production of numerous heavy and doubly heavy particles. In 2011 and 2012, the LHCb collaboration analyzed a small fraction of the data collected at the LHC and found that the probability of the simultaneous production of two charm-anticharm quark pairs at these high energies was far from low, suggesting that the accelerator could trigger the observation of double heavy objects.
“With more data, in 2017 the LHCb collaboration reported an observation of the Xcc++ doubly charmed baryon consisting of the two charm quarks and the light u-quark,” Belyaev explained. “With this observation it became clear that if doubly charmed tetraquarks exist, their observation would only be a matter of time.”
After the LHCb observation of the Xcc++ doubly charmed baryon, M.Karliner and J.Rosner were able to use its measured properties to accurately predict the properties a hypothetical four-quark would have. Such a quark would consist of two charm quarks, a u-antiquark and a d-antiquark. The theoretical particle was named Tcc+.
“The predicted properties of the Tcc+ tetraquark suggest that the particle will appear as a narrow peak in the mass distribution for the pair of charmed mesons D*+ and D0, where D*+ and D0 are conventionally charmed mesons consisting of (charm quark and anti-d-quark) and (charm quark and anti-u-quark),” Belyaev said. “It is interesting to note that the predicted mass of the Tcc+ tetraquark is very close to the sum of the masses of the D*+ and D0 mesons, which also means that if the mass is just 1% lower than the predicted value, the properties of Tcc+ will be very different and will not be visible in the D*+ and D0 mass spectrum. If the mass is just 5% higher, the peak will be broad (or even very broad) and will be very difficult, almost impossible, to observe experimentally.”
Essentially, the work of M. Karliner and J. Rosner pointed out the exact conditions that would be suitable for observing the hypothetical Tcc+ four-quark. Their predictions were ultimately what guided the recent work of the LHCb collaboration.
Credit: The LHCb collaboration, CERN.
In their study, the collaboration carefully studied the mass spectrum of the D*+ and D0 meson pairs, using a dataset containing all the data accumulated at the LHC accelerator from 2011 to 2018. In their previous analysis, conducted in 2012 , the researchers used only 4% of the data available today to study the region of relatively large masses of D*+ and D0 pairs.
In their new analysis, they focused specifically on the range of masses closest to the sum of the masses of the D*+ and D0 mesons. In this region, they observed over a hundred signal Tcc+ tetraquarks that form a strikingly narrow peak very close to the sum of the masses of the D*+ and D0 mesons with overwhelming statistical significance.
“The statistical significance we observed is so high that it completely rules out that the observed signal is a statistical variation,” Belyaev explained. “Since the D*+ meson consists of a charm quark and an anti-d quark, and the D0 meson consists of a charm quark and an anti-u quark, it determines the minimum quark content of what is observed as two charm quarks, anti-d – Quar and anti-u-quark.”
The LHCb collaboration then carried out numerous tests to validate their results. All these tests confirmed that the signal they observed was associated with a Tcc+ tetraquark. Finally, they measured the mass of the Tcc+ tetraquark and the width of its peak.
“According to the laws of quantum mechanics, the width of the peak is related to the inverse lifetime of the particle, and we found that the width corresponds to a very long lifetime, one of the longest for particles decaying due to strong interactions and the longest for all the exotic hadrons found so far,” Belyaev said. “In a sense, Tcc+ is the Methuselah of exotic hadrons.”
Researchers recently conducted a follow-up study, presented in Nature communications, further investigating the properties of the Tcc+ particle. In this paper, they show that the decay scheme is consistent with Tcc+→(D*+→D0p+)D0. They also checked the mass distribution of the 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 a missing p+ meson and Tcc+→(D* + →D+p0/g)D0 I miss p0/g.
“We have not yet directly measured the quantum numbers of the Tcc+ particles, but we have provided strong arguments to support that the total spin J and parity P of the observed particle, which are the most important quantum numbers, are JP=1+, in perfect agreement with expectations,” Belyaev said. “To investigate another important quantum number, isospin, we studied mass spectra for the D0D0, D+D0, D+D+, D+D*+ pairs, looking for possible contributions from the putative isospin partners. They found no signs to suggest that the isospin of the newly observed Tcc+ state is 0, consistent with predictions.”
The Tcc+ tetraquark observed by the LHCb collaboration could have at least two different internal structures. For example, it could have a “molecular structure”, where two charm quarks are separated by a large distance, comparable to the size of the atomic nucleus, a “compact structure”, where the distance between the two charm quarks is significantly smaller, or a combination of two.
In the recent follow-up paper, the team used a sophisticated model to determine what this structure might be and measured the fundamental properties of the Tcc+ state, including its scattering length, effective range and pole position, which are important when trying to to determine internal structure of the particle. The values measured by the researchers are compatible with a molecular structure, however this has yet to be confirmed.
The observation of the Tcc+ tetraquark by the LHCb collaboration is a major contribution to the field of high energy and particle physics. In fact, it has already sparked major theoretical debates about the nature of Tcc+, related molecular states, such as the enigmatic X(3872), and the general issue with the existence of ‘compact four-quarks’.
In their future studies, the collaboration plans to attempt to directly determine the quantum numbers of the new state, as they have so far only achieved strong, but indirect proof of it.
“It is very important to understand the production mechanism of the Tcc+ state in a proton-proton collision,” added Belyaev. “At the moment we have some counterintuitive observations – some distributions such as transverse momentum and orbital multiplicity are really confusing and need more data to analyze. It will be very interesting to compare the production of Tcc+ and Xcc++ particles – here a certain level of similarity is expected , but also to compare the properties, including production properties, of the Tcc++ particle and an enigmatic X(3872) particle.
LHCb discovers three new exotic particles: the pentaquark and the first pair of tetraquark…
