The LHCb collaboration, one of the four large experiments operating at CERN’s Large Hadron Collider (LHC), has taken an important step towards closing this gap. Combining the data collected during the LHC’s first and second operational runs, the team observed quantum correlations between pairs of charm and anti-charm hadrons originating from a single tetraquark state.
Particles such as tetraquarks are not elementary particles, but composite states made of several more fundamental constituents, called quarks and gluons. The latter hold quarks together, mediating the strong force between them. Tetraquarks are predicted by the theory of strong interactions, Quantum Chromodynamics (QCD), and have been extensively searched for in high-energy particle physics experiments.
This latest LHCb analysis reveals how these exceptional tetraquark states are formed and decay. Quantum correlations between pairs of charm and anti-charm hadrons provide information on where these particles are produced inside the LHCb detector and offer insight into the production dynamics of tetraquarks.
The research team investigated all possible combinations of pairs of charm (c) and anti-charm (c‾) hadrons. Most of the pairs, including those originating from the same tetraquark state, show a preference for being produced centrally in the detector. This is expected for most of the hadronic production mechanisms occurring in high-energy collisions. However, quantum correlations are observed for pairs of charm and anti-charm hadrons stemming from the same tetraquark state. In this case, the correlations indicate that the production point is displaced towards the side where charged particles (the proton’s valence quarks) of the incoming protons are located. This hints at a possible production mechanism for tetraquarks in which the gluon emitted from the incoming proton or antiproton (referred to as the “pomeron”) fluctuates into the tetraquark state that subsequently decays into the pair of hadrons.
This LHCb analysis also provides insight into how the tetraquark state subsequently decays into the pair of charm and anti-charm hadrons. The observations indicate that the tetraquark state converts into pairs of charm and anti-charm quarks, which then rearrange to form the final hadrons.
The results of this study provide important information about the production and decay of the observed tetraquark state and offer complementary insights to other LHCb measurements of such particles. The quantum effects observed for the first time in this work could also help in the future to distinguish tetraquarks from other multi-quark states.
The LHCb collaboration is looking forward to collecting more data at the LHC in the future, which will allow them to further investigate the properties of tetraquarks and other exotic particles.
