Physicists from CERN’s LHCb experiment announced today the observation of a pentaquark, a hadron consisting of five quarks.
Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. Image credit: CERN.
Our understanding of the structure of matter was revolutionized in 1964 when the American physicist Murray Gell-Mann proposed that particles known as baryons are comprised of three fractionally charged objects called quarks, and that another category of particles, 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.
“The pentaquark is not just any new particle. It represents a way to aggregate quarks, namely the fundamental constituents of ordinary protons and neutrons, in a pattern that has never been observed before in over 50 years of experimental searches,” said Dr Guy Wilkinson, LHCb spokesperson.
“Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”
The LHCb physicists 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.
“Benefitting from the large data set provided by the Large Hadron Collider, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states,” said Dr Tomasz Skwarnicki of Syracuse University, LHCb scientist and co-author on a paper submitted to the journal Physical Review Letters (arXiv.org preprint).
“More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark.”
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R. Aaij et al. 2015. Observation of J/ψp resonances consistent with pentaquark states in Λ0b→J/ψK−p decays. Phys. Rev. Lett., submitted for publication; arXiv: 1507.03414
