The LHCb experiment at CERN’s Large Hadron Collider (LHC) has reported the discovery of resonances in a B-hadron decay that are consistent with particles known as pentaquarks. The collaboration has submitted a paper reporting these findings to the journal Physical Review Letters.
This is an exciting development since, as the name suggests, pentaquarks are particles that are composed of five quarks rather than the typical three-quark combinations that make up protons and neutrons or two-quark combinations that make mesons. Their existence has been predicted for more than 50 years and over that time many different experiments have made claims of their existence. However, these have all eventually been refuted as more data has been collected.
LHCb (Large Hadron Collider beauty) is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today. 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. Where the LHCb result differs from previous experimental results is that they are able to use additional information about the direction of the decaying particles to better understand the system.
“The huge dataset collected by LHCb and the excellent detector performance have allowed this discovery to be made. We tried to use all known processes to explain what we saw in the data but they all fell short. Only by introducing the two pentaquark states were we able to fully describe the processes we measured,” Dr Greig Cowan, STFC research fellow in the particle physics group at the University of Edinburgh and a member of the LHCb collaboration.
Due to the fact that these resonances decay into a J/ψ meson and proton, the researchers know that these new states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark.
“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,” Prof. Franz Muheim, LHCb group leader at Edinburgh.
More studies will be needed to distinguish between these possibilities, and to see what else these new resonances can teach us. The new data that LHCb will collect in LHC run 2 will allow progress to be made on these questions.