LHCb discovers new pentaquark states

26 March 2019

In 2015 the LHCb collaboration, of which Oxford is a founding member, reported the discovery of pentaquark states, an exotic form of subatomic matter that had eluded discovery since its prediction over 50 years previously. Today (26/3/19), using its full data set from Runs 1 and 2 of the LHC, the collaboration has announced a major and surprising update to this analysis.

The protons and neutrons which constitute the atomic nucleus are each made of three quarks. Mesons, such as the pion or J/ψ particle, are made of a quark and antiquark. Pentaquarks, in contrast, contain four quarks and an antiquark. The original analysis, performed with Run-1 data alone, uncovered a clear peak in the invariant-mass spectrum of b-hadrons decaying to a J/ψ meson and a proton. A sophisticated analysis of all the kinematical information in these decays led to the interpretation that this peak was a pentaquark resonance, dubbed the Pc(4450). Moreover, the analysis also required the presence of another wider state, not so apparent from the mass spectrum, named the Pc(4380). These states can be seen in the left of the figure, which is from this analysis. The resulting publication in Physical Review Letters generated an enormous amount of interest, and has so far accumulated almost 700 citations.

pentaquark.png

The updated measurement benefits from an order of magnitude increase in signal decays, obtained from the Run-2 data and improved analysis techniques. The increased resolution that these data provide has led to surprising discoveries. Firstly, a new state, the Pc(4312), is clearly seen at lower masses (in retrospect a hint of this resonance can be discerned in the smaller data set, but with low statistical significance). Furthermore, the original Pc(4450) resonance is now resolved into two individual states, named the Pc(4440) and the Pc(4457). All three new states can be seen in the right of the figure. What is striking is that all three states are very narrow, which is invaluable information in discriminating between different explanations of the how the quarks are arranged within the pentaquarks. These discoveries are certain to lead to intensive discussion within the theory community, and a better understanding of how the strong interaction binds quarks within hadrons.

Oxford physicists are responsible for the Ring Imaging Cherenkov detectors that allow these pentaquark resonances to be reconstructed, and have played a central role in the unfolding story of these fascinating states. Appearing soon after the discovery of CP violation in charm (see story of 21 March), it seems certain that many exciting surprises are still to emerge from the Run 1 and Run 2 data sets of the LHC.

Links:
LHCb public page: http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Pentaq
CERN press release: https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark
Conference presentation: http://moriond.in2p3.fr/QCD/2019/TuesdayMorning/Skwarnicki.pptx (powerpoint)