Physicists from the LHCb Collaboration at CERN’s Large Hadron Collider (LHC) have observed four new exotic particles: Zcs(4000)+, Zcs(4220)+, X(4685), and X(4630). The new results provide grist for the mill of theorists seeking to explain the nature of tetraquark binding mechanisms.
Illustration of a tetraquark. Imager credit: CERN / CC BY-SA.
The new observations take the tally of new hadronic states — including several pentaquarks, rare and excited mesons and baryons — discovered at LHC to 59.
“Hadrons discovered in the 1950-60s, the pioneering years in particle physics history, were called elementary particles till their structure was finally understood in the framework of quark model,” the LHCb physicists said.
“The quark combinations can be in their lowest-energy quantum mechanical state: the ground state.”
“Like electrons in atoms, the quarks can be rearranged into excited states with different values of angular momentum and quark spin orientation.”
“Following the particle physics tradition all these quantum states are called hadrons and, more generally, particles.”
Though quantum chromodynamics naturally allows the existence of states beyond conventional two- and three-quark mesons and baryons, detailed mechanisms responsible for binding multi-quark states are still largely mysterious.
Tetraquarks, for example, could be tightly bound pairs of diquarks or loosely bound meson-meson molecules — or even both, depending on the production process.
“Who would have guessed we’d find so many exotic hadrons?” said Dr. Patrick Koppenburg, former LHCb physics coordinator.
“I hope that they bring us to a better modeling of the strong interaction, which is very much needed to understand, for instance, the anomalies we see in B-meson decays.”
The new exotic states — Zcs(4000)+, Zcs(4220)+, X(4685), and X(4630) — were observed in an almost pure sample of 24,000 B+→J/ψφK+ decays, which, as a three-body decay, may be visualised using a Dalitz plot.
“Horizontal and vertical bands indicate the temporary production of tetraquark resonances which subsequently decay to a J/ψ meson and a K+ meson or a J/ψ meson and a φ meson, respectively,” the physicists said.
“The most prominent vertical bands correspond to the cc̄ss̄ tetraquarks X(4140), X(4274), X(4500) and X(4700), which were first observed in June 2016.”
“The collaboration has now resolved two new horizontal bands corresponding to the cc̄us̄ states Zcs(4000)+ and Zcs(4220)+, and two additional vertical bands corresponding to the cc̄ss̄ states X(4685) and X(4630).”
The results have already triggered theoretical head scratching.
The BESIII collaboration at the Beijing Electron-Positron Collider II recently discovered the first candidate for a charged hidden-charm tetraquark with strangeness, tentatively dubbed Zcs(3985).
“It is unclear whether the new Zcs(4000)+ tetraquark can be identified with this state,” the researchers said.
“Though their masses are consistent, the width of the BESIII particle is 10 times smaller.”
“These states may have very different inner structures,” said Dr. Liming Zhang, member of the LHCb Collaboration.
“The one seen by BESIII is a narrow and longer-lived particle, and is easier to understand with a nuclear-like hadronic molecular picture, where two hadrons interact via a residual strong force.”
“The one we observed is much broader, which would make it more natural to interpret as a compact multiquark candidate.”
The results have been submitted for publication in the journal Physical Review Letters.
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R. Aaij et al. (LHCb Collaboration). 2021. Observation of new resonances decaying to J/ψK+ and J/ψϕ. Phys. Rev. Lett, submitted for publication; arXiv: 2103.01803
This article is based on text provided by CERN.
