Recent empirical developments in the study and understanding of XYZ states

Exotic hadronic states beyond the conventional quark model (called charmoniumlike/bottomoniumlike states or XYZ particles) have been searched for and many candidates were proposed including glueballs, hybrids, multi-quark states, hadron molecules, etc. Dramatic progress was made in the study of the exotic states after the running of the two B-factories, i.e., Belle at KEK and BaBar at SLAC. In my review report, I present the most recent results on the study of the XYZ states from the BESIII, Belle, BaBar, LHCb, CMS experiments, etc., including (1) X states: the observation of the X(3872) in e+e− → γX(3872) at around 4.26 GeV; searches for the Xb state; (2) Y states including the updated results for the Y(4008), Y(4260), Y(4360), Y(4660), etc; (3) Z states including the observations of the Z(4430), Z1(4050), Z2(4250), Zc(3900), Zc(4020), Zc(4200); the evidence for the Zc(4050)± → π±ψ(2S ); search for the Zcs in e+e− → K+K−J/ψ.


Introduction
It has been prospering for dozen years that lots of the "XYZ" particles mainly were observed by Belle, BaBar, and BESIII Collaborations.Most of them above the open charm threshold can not be described well by quark potential models, which decay into the final states containing a charmonium and light hadrons, but not open charm pairs with a detectable rate as expected [1,2].Their underlying exotic properties have been stimulating significant interests in theoretical studies, and indicate several possible popular interpretations such as tetraquarks, molecules, hybrids, hadrocharmonia, or glueballs [1,2].Here, I present the most recent results on the study of the "XYZ" states from the BESIII, Belle, BaBar, LHCb, CMS experiments, etc.
It is very natural to search for a similar state with J PC = 1 ++ , called X b , in the bottomonium system.The search for X b supplies important information about the discrimination of a compact multiquark configuration and a loosely bound hadronic molecule configuration for the X(3872).The existence of the X b is predicted in both the tetraquark model [20] and those involving a molecular interpretation [21][22][23].The CMS Collaboration ever searched for the X b decaying to π + π − Υ(1S ) based on a sample of pp collisions at √ s = 8 TeV, corresponding to an integrated luminosity of 20.7fb −1 [24].Figure 1(b) shows the π + π − Υ(1S ) invariant mass distribution.Except the clear Υ(2S ) signal, no evidence for an X b signal was observed.However, unlike the X(3872), whose decays exhibit large isospin violation, the X b would decay preferably into π + π − π 0 Υ(1S ) rather than π + π − Υ(1S ) if it exists [22,25].So Belle did a search for an X b signal decaying to ωΥ(1S ) in e + e − → γX b at a centerof-mass energy of 10.867 GeV [26].

The Y states
The Y(4260) state was first observed by the BaBar Collaboration in the initial-state-radiation (ISR) process e + e − → γ ISR π + π − J/ψ [27] and then confirmed by the CLEO [28] and Belle experiments [29] using the same technique.After the initial observations of the Y(4260) [27][28][29], CLEO collected 13.2 pb −1 of e + e − data at √ s = 4.26 GeV and investigated 16 possible Y(4260) decay modes with charmonium or light hadrons in the final state [30].An ISR analysis by the Belle experiment with 548 fb −1 of data showed a significant Y(4260) signal as well as a broad excess of π + π − J/ψ event production near 4 GeV -the so-called Y(4008) [31].
Later, the BaBar Collaboration reported an updated ISR analysis with 454 fb −1 of data and a modified approach for the background description [32]; the Y(4260) state was observed with improved significance, but the Y(4008) structure was not confirmed.Instead, they attributed the structure below EPJ Web of Conferences 01016-p.2In an analysis of the e + e − → γ ISR π + π − ψ(2S ) process, BaBar found a structure near 4.32 GeV/c 2 (called the Y( 4360)) [34], while Belle observed two resonant structures at 4.36 and 4.66 GeV/c 2 , denoted as the Y(4360) and Y(4660) [35].Recently, BaBar updated their results on e + e − → γ ISR π + π − ψ(2S ) analysis with its full data sample and confirmed the existence of the Y(4660) state [36].Figure 3(a) shows the invariant mass distribution of π + π − ψ(2S ) from the latest BaBar's measurement [36] together with that from previous Belle's measurement [35], where the Y( 4360  BESIII accumulated large data samples between 4.0 and 4.6 GeV for the study of the charmonium and charmoniumlike states.For the cross sections measurements to some processes, some results seem very interesting.For examples: (1) BESIII measured e + e − → π + π − h c cross sections [38] at centerof-mass energies between 3.90 and 4.42 GeV.The measured cross sections of π + π − h c are of the same order of magnitude as those of the e + e − → π + π − J/ψ, but with a different line shape.There is a broad structure at high energy with a possible local maximum at around 4.23 GeV.(2) Based on data samples collected at 9 center-of-mass energies from 4.21 to 4.42 GeV, BESIII searched for the production of e + e − → ωχ c0 [39], where χ c0 was reconstructed with π + π − and K + K − decay modes.Figure 4(a) shows the Born cross sections of e + e − → ωχ c0 , which were fitted with a Y(4260) resonance or a phase space term.Assuming the ωχ c0 signals come from a single resonance, the fitted mass and width of the resonance are (4230 ± 8 ± 6) MeV/c 2 and (38 ± 12 ± 2) MeV, respectively, and the statistical significance is more than 9σ.The position of this resonance is consistent with the Y(4220) state observed in the cross section of e + e − → π + π − h c [40].It also indicates that the Y(4260) signals observed in e + e − → π + π − J/ψ [27,33] may have fine structures, and the lower mass structure at about 4230 MeV/c 2 has a sizable coupling to the ωχ c0 channel as predicted in Ref. [41].(3) BESIII searched for production of the ψ 2 state, called X(3823), via the process e + e − → π + π − ψ 2 [42], where the ψ 2 candidates were reconstructed in their γχ c1 and γχ c2 decay modes.The measured energydependent cross sections of e + e − → π + π − X(3823) are shown in Fig. 4(b), which were fitted with a Y(4360) shape or a ψ(4415) shape.Both fits can describe the data well due to the large statistical errors.For the above described processes, in the future at BelleII, they need to be rechecked to see if there are new vector charmoniumike states or new decay modes of the discovered Y states.
In 2013, Belle and BESIII investigated the same Y(4260) → π + π − J/ψ decays at the same time to search for the intermediate resonance [33,51].Figure 5 shows the distributions of M max (π ± J/ψ), the maximum of M(π + J/ψ) and M(π − J/ψ), from Belle and BESIII measurements.Unbinned maximum likelihood fits are applied to the distributions of M max (π ± J/ψ), where the signal shape is parameterized as an S-wave Breit-Wigner function convolved with a Gaussian with a mass resolution fixed at the MC simulated value.The measured masses are (3899.0±3.6±4.9)MeV/c 2 and (3894.5±6.6±4.5)MeV/c 2 and the measured widths are (46 ± 10 ± 20) MeV and (63 ± 24 ± 26) MeV/c 2 from BESIII and Belle, respectively.They are consistent with each other within the errors.The signal significance is greater than 5σ in both of the measurements.This state is close to the D D * mass threshold.Recently, the neutral Z c (3900) was found in e + e − → π 0 π 0 J/ψ [52], which established the isospin of Z c (3900) to be 1.The Z c (3900) mass is ∼ 20 MeV/c 2 above the D D * mass threshold, which is suggestive of a virtual D D * molecule-like structure.So it is very nature to check the (D D * ) invariant mass distribution to see if there is a threshold enhancement.BESIII reported a study of the process e + e − → π + (D D * ) − at Ecm=4.26 GeV using a 525pb −1 data sample.As expected, a distinct charged structure is observed in the (D D * ) invariant mass distribution, called the Z c (3885).The mass and width of the Z c (3885) are 2σ and 1σ, respectively, below those of the Z c (3900).The angular distribution of the πZ c (3885) system favors a J P = 1 + quantum number assignment [53].PDG2014 has taken the Z c (3900) and Z c (3885) as the same state [54].
Replacing the J/ψ with the h c , BESIII studied e + e − → π + π − h c at 13 energies, and found a distinct structure at 4.02 GeV/c 2 , referred to as Z c (4020), in the π ± h c mass spectrum.A fit to the π ± h c invariant mass spectrum results in a mass of (4022.9±0.8±2.7)MeV/c 2 and a width of (7.9±2.7±2.6)MeV [38].Similarly, BESIII also studied the process e + e − → (D * D * ) ± π ∓ at a center-of-mass energy of 4.26 GeV using a 827pb −1 data sample.A structure near the (D * D * ) ± threshold, denote as the Z c (4025) ± , was observed [55].The Z c (4025) and Z c (4020) should be from the same state.Without any surprise, the neutral Z c (4020) was found in e + e − → π 0 π 0 h c and the measured Born cross sections are about half of those of e + e − → π + π − h c [56].

+99
−149 To find more production modes for the Z c (3900), Belle did an amplitude analysis of B0 → K − J/ψπ + decays [48].Unfortunately, no significant signal of the Z c (3900) was found.But a new charged charmoniumlike state Z c (4200) + decaying to J/ψπ + was observed with a significance of 6.2σ.The mass and width of the Z c (4200) + are 4196 +31+17 −29−13 MeV/c 2 and 370 +70+70 −70−132 MeV, respectively; the preferred assignment of the quantum numbers is J P = 1 + [48].At BelleII, this state needs to be cross checked and confirmed.
Belle also tried to search for a strange partner of the Z c (3900) ± , called the Z cs , in J/ψK ± system in the process e + e − → K + K − J/ψ [57].No obvious structures were observed in the J/ψK ± system.At BelleII, with much larger statistic, such searches should be continued.
Up to now, we have found a series of charged Z states.Table 1 gives the summary of the charged Z states discovered recently.As these Z state have strong couplings to charmonium and are charged, they cannot be conventional cc states.There have been a number of different interpretations, including tetraquark states, hadronic molecules, hadron-charmonium states and so on.These observations indicate one kind of the exotic states has been observed.The nature of these states have been discussed in many proposals, but no solid conclusion can be drawn.Searches for new decay modes and measuring their quantum numbers may provide further information that is useful for understanding the nature of them.There are also many open questions.For examples, may we observe more excited Z c states ?Do the strange partners of the Z c states exist ?

Summary
There have been great progress in the study of the XYZ states, especially Belle and BESIII are still producing more results.With more discoveries, we found we have more questions to answer.For examples: • The X(3872) and X(3823) signals were observed by BESIII via e + e − → γX(3872) and π + π − X(3823) processes.Are the signals from resonances decays or continuum productions ?May other similar X states could be observed in similar processes ?
• Although the Y(4260) has been well established, there is a possible local maximum at around 4. 23 GeV in e + e − → π + π − J/ψ cross sections.Is the Y(4260) a single resonance?Is the Y(4008) a real structure?
• In the updated e + e − → π + π − ψ(2S ) analysis by Belle, there are a few events accumulating at around 4.26 GeV.With the Y(4260) signal included, the signal significance is less than 3σ.Does the Y(4260) decay to π + π − ψ(2S ) ?Are there any other decay modes ?
• There is a broad structure at high energy in the e + e − → π + π − h c cross sections with a possible local maximum at around 4.23 GeV.How many structures are there ?
• Many charged Z c states have been observed.Meanwhile, in the invariant mass distributions of the charm meson pairs, like (D D * ) ± , (D * D * ) ± , some similar Z c states were observed.Are they the same states ?What are the correlations between them ?Are there any Z cs states ?Can the Z c states decay into light hadrons ?
In the future at BelleII with a 50ab −1 data sample, most of the above questions can be answered.The physics behind them should be clear at that time.For some channels, we can even do partial wave analysis or amplitude analysis.

DOI: 10
.1051/ C Owned by the authors, published by EDP Sciences,

Figure 2 .
Figure 2. Invariant mass distributions of π + π − J/ψ from BaBar and Belle measurements.Points with error bars are data, and the shaded histograms are the normalized J/ψ mass sidebands.The solid curves show the total best fits.

Figure 3 .
Figure 3. Invariant mass distributions of π + π − ψ(2S ) from BaBar and Belle measurements.Points with error bars are data.Two peaks are clear corresponding to the Y(4360) and Y(4660) resonances.

Figure 5 .
Figure 5. Unbinned maximum likelihood fits to the distributions of the M max (πJ/ψ) from Belle and BESIII experimental data.The solid curves are the best fits, the dashed histograms represent the results of phase space distribution and the shaded histograms are J/ψ sidebands.

Table 1 .
Summary of the charged Z states discovered recently.The masses M (MeV/c 2 ) and widths Γ (MeV) are weighted averages of measurements with uncertainties added in quadrature.In the J PC column, question marks indicate the educated guess or no information.