Pentaquark Observed

July 14th 2015

Summary: The discovery of a class of particles known as pentaquarks, containing five valence quarks, has been made

Image: Two peaks (purple and magenta areas) corresponding to new particles are observed by fitting a model (red points) to the data (black points).

(Credit LHCb)

CERN's LHCb experiment reports observation of exotic pentaquark particles

(Based on CERN Press release)

Geneva, 14 July 2015. Today, the LHCb experiment at CERN's Large Hadron

Collider has reported the discovery of a class of particles known as

pentaquarks. The collaboration has submitted a paper reporting these

findings to the journal Physical Review Letters.

”This result shows the breadth of LHCb's physics programme.  As well as exploring the asymmetry between matter and antimatter in the Universe, our precision detector allows us to discover new ways that  particles can be bound together by the strong interaction.” says Prof. Tim Gershon of Warwick University who chaired the review committee in LHCb for this paper. “LHCb has previously shown that four quarks can be bound together into "resonances" -- short-lived particles that decay as soon as they are produced.  Now we can see that bound states of five quarks are also possible.”

Prof. Gershon points out that “The LHCb analysis is significantly different from those of previous experiments that found hints of five-quark particles ("pentaquarks") that were later disproved.  LHCb has analysed all the available information in the decay distribution to prove that the peak in the mass distribution cannot be a fake caused by other processes.”

"The pentaquark is not just any new particle," said LHCb spokesperson Guy Wilkinson. "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 fifty years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we're all made, is constituted."

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 Lambda b into three other particles, a J-psi, a proton and a charged kaon. Studying the spectrum of masses of

the J-psi; 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 LHC, 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", says LHCb physicist Tomasz Skwarnicki of Syracuse University.

"More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark."

Earlier experiments that have searched for pentaquarks have proved inconclusive. Where the LHCb experiment differs is that it has been able to look for pentaquarks from many perspectives, with all pointing to the same conclusion. It's as if the previous searches were looking for silhouettes in the dark, whereas LHCb conducted the search with the lights on, and from all angles. The next step in the analysis will be to study how the quarks are bound together within the pentaquarks.

"The quarks could be tightly bound," said LHCb physicist Liming Zhang of Tsinghua University, "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."

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 will allow progress to be made on these questions.


Prof Tim Gershon, Tim.GershonATNOSPAM


The UK participation in the international LHCb experiment is from eleven institutes.

University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.

Image: A horizontal band on this plot seen at around 19.5 GeV2 is due to these new particles.
(Credit LHCb)