LHCb observes an exotic particle outside the quark model

The black points at the left image above show the ψ’π- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. Credit. LHCb
I fisici che lavorano all’esperimento LHCb hanno pubblicato un articolo in merito ad una serie di misure di una particella esotica denominata Z(4430). Secondo il modello standard che descrive i quark, le particelle che sono soggette all’interazione forte, cioè gli adroni, sono formate sia da coppie quark-antiquark (mesoni) o da tre quark (barioni). Da quasi 50 anni, gli scienziati stanno cercando di identificare queste particelle, chiamate adroni esotici, che potrebbero non essere classificate secondo gli schemi tradizionali. Sono stati proposti numerosi candidati ma fino ad oggi non c’è stata alcuna evidenza sperimentale che confermasse con certezza la loro esistenza.

The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψπ mass distribution in the B → ψ decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers JP = 1+ are favored over the other spin assignments. There are many so called charmonium cc* neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence. The LHCb Collaboration has reported today an analysis of about 25 200 B0 → ψ, ψ → μ+μ decays observed in 3 fb−1 of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together.

The significance of the Z(4430) signal is overwhelming, at least 13.9σ, confirming the existence of this state.

The Z(4430) quantum numbers are determined to be JP = 1+ by ruling out 0, 1, 2+ and 2 assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle. The minimal quark content of the Z(4430) state is cc*du*. It is therefore a four quark state or a two-quark plus two-antiquark state.

LHCb: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model
The Conversation: Quirky quark combination creates exotic new particle

Quantum Diaries: Major harvest of four-leaf clover
arXiv:  Observation of the resonant character of the Z(4430)^- state