The LHCb experiment leads to the observation of an exotic tetraquark

Over the course of the 20th century, physicists have discovered numerous elementary particles. The largest family of these particles are the so-called hadrons, subatomic particles that take part in strong interactions.

This broad family of particles contains numerous sub-sets of particles with similar properties. In 1964, M. Gell-Mann and G. Zweig introduced a renowned theory known as the “Quark Model,” which clearly outlined the internal structure of hadrons.

The Quark Model suggests that hadrons consist of either three quarks (baryons) or quark-antiquark pairs (mesons). While many uncovered hadrons fall into one of these two categories, the model also hypothesizes the existence of hadrons with more complex structures, such as pentaquarks (i.e., four quarks and an antiquark) and tetraquarks (i.e., two quark-antiquark pairs).

Many studies in the 1970s theorized about the possible mechanisms underpinning the formation of these complex hadron structures. All the hadrons uncovered up until 2003 had structures that match one of the two main types described by the Quark Model, yet some of the particles observed after that date are difficult to explain using the model.

The LHCb experiment is a detector at the CERN Large Hadron Collider primarily aimed at unveiling differences between matter and antimatter by studying a specific type of particle, known as the “beauty quark.” The LHCb Collaboration, the large group of researchers involved in the experiment, has 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/ψ mesons in 1974, often called as ‘November revolution’) and even heavier beauty quark in 1977, led to the recognition that tetraquarks consisting 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 for such ‘double heavy’ objects only appeared in the 21st century, with the start of the Large Hadron Collider at CERN.”

At the LHC collider, physicists can study collisions between protons at very high energies, which promote the production of numerous heavy and double heavy particles. In 2011 and 2012, the LHCb collaboration analyzed a tiny 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 collider could enable the observation of double heavy objects.

“With more data, in 2017 the LHCb collaboration reported an observation of the double charm baryon Ξ_cc⁺⁺ consisting of the two charm quarks and light u-quark,” Belyaev explained. “With this observation it became clear that if double charm tetraquarks exist, their observation would just a matter of the time.”

Following the LHCb’s observation of the double charm baryon Ξ_cc⁺⁺ , M.Karliner and J.Rosner were able to use its measured properties to precisely predict the properties that a hypothetical tetraquark would have. Such a tetraquark would consist of two charm quarks, a u-antiquark and a d-antiquark. The theoretical particle was named T_cc⁺.

“The predicted properties of the T_cc⁺ tetraquark imply that the particle will exhibit itself as a narrow peak in the mass distribution for the pair of charmed mesons D^*+ and D⁰, where D^*+ and D⁰ are conventional 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 T_cc⁺ tetraquark is very close to the sum of masses of the D^*+ and D⁰ mesons, which also means that if the mass will be just 1% lower than the predicted value, the properties of the T_cc⁺ will be very different and will not be visible in the D^*+ and D⁰ mass spectrum. If the mass will be just 5% higher, the peak will be wide (or even very wide) and it will be very difficult, almost impossible, to observe experimentally.”

Essentially, the work by M. Karliner and J. Rosner pin-pointed the exact conditions that would be suitable to observe the hypothetical T_cc⁺ tetraquark. Their predictions were ultimately what guided the recent work by the LHCb collaboration.

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 collider 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 the relatively large masses of D^*+ and D⁰ pairs.

In their new analysis, they specifically focused on the region of masses that is closer to the sum of the D^*+ and D⁰ meson masses. In this region, they observed over one hundred signal T_cc⁺ tetraquarks that form a strikingly narrow peak very close to the sum of the D^*+ and D⁰ meson masses with an overwhelming statistical significance.

“The statistical significance we observed is so high that it totally excludes that the observed signal is a statistical fluctuation,” Belyaev explained. “Since the D^*+ meson consists of a charm quark and anti-d quark, and D⁰ meson consists of charm quark and anti-u-quark, it fixes the minimal quark content of that the observed as two charm quarks, anti-d-quar and anti-u-quark.”

The LHCb collaboration then performed numerous tests to validate their results. All these tests confirmed that the signal they observed was associated with a T_cc⁺ tetraquark. Finally, they measured the mass of the T_cc⁺ 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 largest for the particles that decays due to strong interactions and the longest for all exotic hadrons found so far,” Belyaev said. “In some sense, T_cc⁺ is Methuselah of the exotic hadrons.”

The researchers have recently conducted a follow-up study, featured in Nature Communications, further exploring the properties of the T_cc⁺ particle. In this paper, they showed that the decay pattern is consistent with T_cc⁺→(D^*+→D⁰π⁺)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 T_cc⁺→(D^*+→D⁰π⁺)D⁰ with missing π⁺ meson and T_cc⁺→(D^*+→D⁺π⁰/γ)D⁰ with missing π⁰/γ.

“We have not yet measured the quantum numbers of the T_cc⁺ particles directly, but we offered strong arguments in support for the total spin J and parity P of the observed particle, that are the most important quantum numbers, are J^P=1⁺, in perfect agreement with expectations,” Belyaev said. “To probe another important quantum number, isospin, we have studied mass spectra for the D⁰D⁰, D⁺D⁰, D⁺D⁺, D⁺D^*+ pairs, searching for possible contributions from the hypothetical isospin partners. They found no signs suggesting that the isospin of the newly observed T_cc⁺ state is 0, in agreement with the predictions.”

The T_cc⁺ tetraquark observed by the LHCb collaboration could have at least two different internal structures. For instance, it could have a “molecular-like 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 the two.

In their recent follow-up paper, the team used a sophisticated model to determine what this structure could be and measured the fundamental properties of the T_cc⁺ state, including the scattering length, effective range and pole position, which are important when trying to determine a particle’s internal structure. The values measured by the researchers are compatible with a molecular-like structure, yet this is yet to be confirmed.

The LHCb collaboration’s observation of the T_cc⁺ tetraquark is a significant contribution to the field of high energy and particle physics. In fact, it has already sparked important theoretical discussions about nature of T_cc⁺, related molecular-like states, such as the enigmatic X(3872), and the general issue with the existence of the “compact tetraquarks.”

In its future studies, the collaboration plans to attempt to directly determine the quantum numbers of the new state, as so far they only attained strong, but indirect evidence of them.

“It is very important to understand the production mechanism of the T_cc⁺ state in proton-proton collision,” Belyaev added. “Currently we have some counterintuitive observations—some distributions, like transverse momentum and track multiplicity are really puzzling and more data is needed for resolution. It will be very interesting to compare the production of the T_cc⁺ and Ξ_cc⁺⁺ particles—here a certain level of similarity is expected, but also to compare the properties, including production properties, of the T_cc⁺⁺ particle and an enigmatic X(3872) particle.”

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