CERN’s LHCb experiment observes exotic pentaquark particles

14 July 2015

The LHCb experiment at CERN has reported the discovery of a class of particles known as pentaquarks which have eluded physicists’ searches for over 50 years. The collaboration has submitted a paper reporting these findings to the journal Physical Review Letters.

Our understanding of the structure of matter was revolutionized in 1964 when American physicist, Murray Gell-Mann, proposed that a category of particles known as baryons, which includes protons and neutrons, are comprised of three fractionally charged objects called quarks, and that another category, mesons, are formed of quark-antiquark pairs. Gell-Mann was awarded the Nobel Prize in physics for this work in 1969. 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.

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. The pentaquarks must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark.
Mass of J/ψ p combinations from Λb → J/ψpK-decays: The Pc(4380)+ and Pc(4450)+ contributions are indicated in purple and black distributions, respectively
How the quarks are bound within the pentaquark is an open question. They could be tightly bound, or loosely bound in a meson-baryon ‘molecule’. More studies will be needed to distinguish between these possibilities, and to see what else pentaquarks can teach us. The new data that LHCb will collect in LHC run 2 (2015-2018) will allow progress to be made on these questions.

Oxford is a founding member of LHCb, and Oxford physicists play a central role in the experiment. Neville Harnew was responsible for overseeing the design and construction of the Ring Imaging Cherenkov (RICH) detectors, which are essential for selecting the proton candidates in the analysis, and Malcolm John has critical responsibilities for the operation of the Vertex Locator (VELO), the high precision silicon strip detector which selects the decay vertices of the baryon and pentaquark decays. Guy Wilkinson is currently Spokesperson (PI) of the experiment.

For more information, the preprint is here.