The Science
The protons and neutrons that form atomic nuclei are composed of three up or down quarks. Up and down quarks are a million times less massive than another flavor of quark – the charm quark. When researchers accelerate protons, this can excite and antiquarks (quarks' antiparticles), causing them to manifest as new particles. These particles are mesons, made of a quark and an antiquark, and baryons, made of three quarks. However, compact exotic particles with four or five quarks, known as tetraquarks and pentaquarks, can also appear in these experiments. Recent calculations provide insights into the structure of reported four-quark states that contain charm quarks, assuming those tetraquarks are produced within a single proton.
The Impact
Scientists have discovered many four-quark states that contain charm quarks, but only some of these states may be tetraquarks. Researchers believe that tetraquarks are smaller and more compact than protons. This makes tetraquarks distinct from the larger hypothesized four-quark configurations that are similar to atomic or molecular structures. The calculations in this new research show that four-quark states with two charm quarks form relatively small molecules made of two spatially separated mesons. However, four-quark configurations with one charm quark are more like the tetraquark in the figure above, but they are not as tightly packed as researchers thought. The calculations also predict that the planned Electron-Ion Collider may easily find four-quark states produced within a proton.
Summary
In this project, researchers studied the mass and kinematic distributions of charmed four-quark states in a model in which charm quark-antiquark pairs (or intrinsic charm) can appear as quantum fluctuations within the proton's structure. If the constituent quarks and gluons in the proton have relatively small transverse momenta with respect to each other, there is good agreement with the masses of the measured four-quark states. The mass distributions also suggest that states with two charm quarks have a meson pair structure with a rather small separation between the mesons, smaller than the molecule-like state in the figure. On the other hand, states with a single charm quark are compatible with the tetraquark in the figure but are not as tightly bound.
The research assumes that the kinematic distributions are independent of the structure. Because the charm quarks are comoving with the light quarks of the proton, these four-quark states are boosted to high longitudinal momentum with large transverse momentum. The predicted cross sections are small but could dominate production in regions where production by perturbative quantum chromodynamics is reduced. The intrinsic charm contribution to tetraquark production is negligible in proton-proton collisions at 13 TeV but could be important at 5 TeV. At lower energies, such as in the fixed-target LHCb SMOG device at CERN or at the future U.S. Electron-Ion Collider, tetraquarks from intrinsic charm might easily be detected.
Funding
This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics and the Laboratory Directed Research and Development program at Lawrence Livermore National Laboratory.