Note on the newly observedY(4220)resonance

Abstract

BES III Collaboration has recently observed a vector resonance in the χc0ω channel, at a mass of about 4220 MeV, named Y (4220). Hints of a similar structure appear in the hcπ + π − channel. We find that the two observations are likely due to the same state, which we identify with one of the expected diquark-antidiquark resonances with orbital quantum number L = 1. This assignment fulfills heavy quark spin conservation. The measured branching ratio of the Y (4220) into χc0ω and hcπ + π − is compatible with the prediction for such a tetraquark state. PACS numbers: 14.40.Rt, 12.39.Jh, 13.25.Gv In a very recent paper, the BES III Collaboration reports the e + e − → χ cJ ω (J = 0, 1, 2) production cross section as a function of √ s [1]. Hints of a resonant structure are present in the χ c0 ω channel at ∼ 30 MeV above threshold (i.e. at about 4220 MeV), whereas no evident structure appears in the χ c1,2 ω channels. Some theoretical interpretations for this peak have been proposed [2]. BES Collaboration also reported the measurement of e + e − → h c π + π − production cross section as a function of √ s [3]. Hints of structures not compatible with the Y (4260) have been found [4, 5]: in particular a narrow peak at ∼ 4220 MeV. Heavy quark spin symmetry prevents any ordinary charmonium from decaying into both χ c and h c . Violations of this symmetry have already been observed in the bottomonium sector, and are explained in [6][7][8]. In the charmonium mass region, many exotic charmonium-like states have been identified according to the diquark-antidiquark model [9] (for a review, see [10]). In particular, the latest model [11] predicts a tetraquark state, named Y 3 , with quantum numbers J P C = 1 −− , and mass and decay modes compatible with a Y (4220) resonance. The wave function of this tetraquark state contains both heavy quark spin states, so it can naturally decay into both χ c0 ω and h c π + π − with no violation of the heavy quark spin. Since the Breit-Wigner parameters of the peaks measured in the two channels χ c0 ω and h c π + π − are very similar, we test the hypothesis that the two observed structures may coincide.We fit data with two different models (I and II in the following) similar to those considered in Refs. [1,5]. In the h c π + π − invariant mass distribution, we add to the BES dataset the experimental point σ hcπ + π − (4.17 GeV) = (15.6 ± 4.2) pb 1 by , with statistical and systematic errors added in quadrature. For the BES data, we take into account only statistical errors, since the systematic ones are common to all points and are not expected to modify the shape of the distribution.Following model-I, we fit the h c π + π − and χ c0 ω data with the sum of a Breit-Wigner corrected for the energy dependence given by PCAC, and a pure phase-space background. To test our hypothesis, the mass and the width of the resonance are constrained to be the same in both channels. Thus, the fitting functions are:where m 0 and Γ are the mass and width of the resonance, m is the invariant mass of the system, B...

Multiquark resonances

“…The h c π + π − spectrum has been fitted first by Yuan [115,116], which found a significant signal for a Y(4220), while the higher mass region could be described either by a Y(4290) state, or by a phase-space background. The very recent energy scan by BES III [112] [114]. The possibility that, instead, the state in χ c0 ω is the ordinary ψ(4160) has been proposed in [381].…”

“…For the identification of the Y 3 state as the structures seen in e + e − → h c ππ, χ c0 ω, see [114].…”

Resonant parameters of the Y(4220)

“…This structure is in good agreement with the Y (4220) observed in e + e − → π + π − h c [11], and combined fits assuming the structures at 4.22 GeV/c 2 are the same have been tried by the authors of Refs. [14,15].…”

“…As such a state is not observed in other open charm final states [23], these four final states are probably the dominant decay modes of the Y (4220). By applying constraints to the resonant parameters in a simultaneous fit to the cross sections of these four processes, we may obtain the best knowledge on the Y (4220), including resonant parameters (mass, width, coupling to lepton pair, and decay branching fractions), and thus a better understanding of its nature [1], especially whether it is an exotic state, such as a tetraquark state in the diquark-antidiquark model [24], a vector molecular state of DD 1 (2420) [25], a mixture of two hadrocharmonium states [26], an ωχ cJ molecule [27,28], or a charmonium-hybrid state [29].…”

Overview of $XYZ$ states and tetraquarks

“…This structure is in good agreement with the Y (4220) observed in e + e − → π + π − h c [55], and combined fits assuming the structures at 4.22 GeV/c 2 are the same have been tried by the authors of Refs. [57,58].…”