On the trail of exotic particles

Since the quark model was first conceived 50 years ago, physicists have been searching for “exotic” hadrons – strongly interacting particles that are neither quark–antiquark pairs (mesons) nor three-quark states (baryons). Now the LHCb collaboration has published results that for the first time unambiguously demonstrate the exotic nature of one of the candidate exotic hadrons – the Z(4430). At the same time, LHCb’s measurements show that the f₀(500) and the f₀(980) states cannot be four-quark states (tetraquarks), contrary to what has long been suggested.

The first evidence for the Z(4430) came in 2008 from the Belle collaboration at KEK’s B-factory, KEKB. It appeared as a narrow peak in the ψ΄π^– mass distribution in B → ψ΄Kπ^– decays. With negative charge, the Z(4430) cannot be a charmonium state, raising the possibility that it could be a multiquark state, for example ccud.

LHCb has now analysed about 25,200 decays of the kind B⁰→ ψ΄Kπ^–, ψ΄ → μ⁺μ^– in data corresponding to an integrated luminosity of 3 fb⁻¹ of proton–proton collisions at the LHC at centre-of-mass energies of 7 and 8 TeV. The collaboration observes the Z(4430) in the ψ΄π^– mass distribution with a significance of at least 13.9σ, and determines the quantum numbers J^P to be 1⁺, by ruling out 0^–, 1^–, 2⁺ and 2^– at more than 9.7σ (LHCb collaboration 2014a). While this emphatically confirms the evidence from Belle, the LHCb analysis also establishes the resonant nature of the observed state. Its Argand diagram (figure 2) shows unambiguously that the Z(4430) really is a particle. Moreover, with a minimal quark content of ccud, it must be a tetraquark state.

In a related analysis, LHCb has also studied the decay B⁰ → J/ψπ⁺π⁻, extracting the invariant mass of the π⁺π⁻ pairs. While this clearly reveals a peak corresponding to the f₀(500) meson, there is no evidence for the f₀(980). This rules out at 8σ the production of the f₀(980) at the rate expected for tetraquarks, which would lead to a much smaller difference in the production rates for the two f₀ mesons. However, the f₀(980) is clearly visible in the corresponding π⁺π⁻ invariant mass distribution for the decay B⁰_s →J/ψπ⁺π⁻. The absence of the f₀(980) in B⁰ decays and its presence in B⁰_s  decays in addition to the presence of the f₀(500) only in the B⁰ decays is exactly what is expected if these states are normal quark–antiquark states (LHCb collaboration 2014b).