Scientists at CERN’s Large Hadron Collider has reported the discovery of a new class of particles known as pentaquarks. They have submitted a paper reporting these findings to the journal Physical Review Letters.
CERN, the European Organization for Nuclear Research, is the world’s leading laboratory for particle physics. It has its headquarters in Geneva.
The understanding of the structure of matter was revolutionized in 1964 when American physicist, Murray Gell-Mann, proposed that the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are called exotic hadrons. It has taken 50 years, however, for measurements to be performed that unambiguously demonstrate the existence of these exotics.
In April 2014 the LHCb collaboration published results of measurements which demonstrated that the Z(4430)+ particle is composed of four quarks (ccdu).
But now the collaboration has announced the observation of a pentaquark (five quarks), that is a hadron consisting of five quarks. This quark model also allows the existence of other quark composite states, such as pentaquarks composed of four quarks and an antiquark. Until now, however, no conclusive evidence for pentaquarks had been seen.
Scientists will now try to understand the “internal mechanism” of quark interactions inside pentaquarks. The two possibilities are illustrated in the figure. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The quarks could be tightly bound, or they could also be loosely bound in meson-baryon molecule, in which color-neutral meson and baryon feel a residual strong force similar to the one that binds nucleons together within nuclei.
LHCb researchers looked for pentaquark states by examining the decay of a baryon known as Λb (Lambda b) into three other particles, a J/ψ– (J-psi), a proton and a charged kaon. Studying the spectrum of masses of the J/ψ and the proton revealed that intermediate states were sometimes involved in their production. These have been named Pc(4450)+ and Pc(4380)+, the former being clearly visible as a peak in the data, with the latter being required to describe the data fully.
“More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark.”
It is noted that the previous searches were looking for silhouettes in the dark, whereas LHCb conducted the search with the lights on, and from all angles. The next step in the analysis will be to study how the quarks are bound together within the pentaquarks.
“The quarks could be tightly bound or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei.”
