In which canal was the pentaquark discovered?

Does the pentaquark consist of two parts?

Three plus two: Pentaquarks are exotic particles that consist of five instead of three quarks. Now, for the first time, new data from the LHC particle accelerator at CERN is providing information on how these short-lived exotic species are structured. Accordingly, the five quarks apparently form two closely related subunits. One consists of three quarks similar to the atomic building blocks proton and neutron. The other is a meson, a pairing of quark and antiquark.

Quarks are the basic building blocks of matter. These elementary particles tend to combine to form pairs of two or three: Baryons, including the neutrons and protons in the atomic nucleus, each consist of three quarks. Mesons, unstable decay products of heavier particles, consist of a quark and an antiquark. In recent years, however, physicists have discovered other exotic combinations of quarks - particles made up of four, five and even six quarks.

What is the structure of the pentaquark?

The big question about these exotic quark combinations, however, is how they are built up internally: Are all quarks connected to one another equally? Or do they form subunits, which in turn are coupled to one another like atoms in a molecule? Quantum chromodynamics - the theory that describes the binding of quarks with the help of gluons - theoretically allows both, as the researchers from the LHCb collaboration explain.

In search of clues to the substructure of the pentaquarks, the physicists again evaluated all the data from the first run of the Large Hadron Collider (LHC) at CERN and specifically searched for the signals of the pentaquarks in the results of the LHCb detector. In total, they found 246,000 particle decays typical of pentaquarks - nine times more than before, as they report.

Two peaks instead of one

The exciting thing, however, is that the previously rather indistinct signal of a pentaquark at 4,450 megaelectron volts (MeV) was better resolved in the new data - and turned out to be a double peak. “The structure at 4,450 MeV shows up as two narrow peaks at 4,440 and 4,457 MeV,” the researchers report. “The significance of this double-peak structure is 5.4 sigma.” From a value of five sigma, a result in particle physics is considered to be proven.

“This significance is large enough to refute the previous interpretation as a single peak at 4,450 MeV,” the physicists state. In their opinion, the pentaquark could actually consist of two subunits. One would then be a baryon made up of a down and two up quarks, while the other would be a meson made up of a charm and an anticharm quark, as the researchers report.

“Molecule” made up of a baryon and a meson?

"Our data thus provide the strongest experimental evidence to date that there are bound states of a baryon and a meson," explain the physicists of the LHCb collaboration. Physicists have been suspecting since the 1950s that there could be such molecular-like couplings between these two types of particles. But they have not yet been proven.

Even the LHC data cannot yet provide the final proof, because the results also allow some alternative explanations - even if these are in some cases rather unlikely. Nevertheless, further experiments and results are now necessary, emphasize the physicists. (Physical Review Letters, 2019; doi: 10.1103 / PhysRevLett.122.222001)

Source: American Physical Society

17th June 2019

- Nadja Podbregar