1. 1 The theoretical understanding of Y(4260) CONG-FENG QIAO Graduate School, Chinese Academy of Sciences 20-23 SEPT 2006, DESY

2. 2 Brief Introduction  BaBar found a resonance-like structure in very recently, named Y(4260), in the process. [BaBar, 05]

3. 3 The potential model and experimental data agree quite well on charmonium sepecturm before the discovery of X(3872)

4. 4 Experimental signature; e  e    ISR     J/  J/  e  e ,     } Expect J PC =1 -- cc states     J/  states in ISR production BABAR: 233 fb -1 PRL 95, 142001 (2005)  (2S)

5. 5  Y(4260) was further confirmed in the B meson decays. [BaBar, 06]

6. 6 Indications of Y (4260) in B     J/  states Conservative approach assumes M and  from ISR analysis and fit raw mass spectrum. Obtain 3.1  significance. B±B± BABAR: 232M BB PRD 73, 011101 (2006)

7. 7  As well, CLEO Collaboration discovered the Y(4260) in direct process, and they also observed the process for the first time. [CLEO, 06]

8. 8 Confirmation of Y(4260) in CLEO-c Scan √s=3.77 - 4.26 GeV CLEO-c: 13.2 pb -1 PRL 96, 163003, 06 BABAR  +  -J/  peak cross section: (51±12) pb N=37 (11 σ) N=8 (5.1 σ)  Y(4260) →  +  - J/  confirmed (11  )  First observation of Y(4260) →  0  0 J/  (5.1  ) and first evidence for Y(4260) → K + K - J/  (3.7  ) Enhancement of cross section for e + e -   J/  @ √s=4.26 GeV

9. 9  There are more indications from CLEO & BELLE about the existence of Y(4260).  Obviously, the newly discovered Y(4260) has the same quantum number assignment as photon,.

10. 10  As far, we know the Y(4260) is a broad resonance with MeV  Since the mass of Y(4260) is within the range of conventional Charmonium state, a nature explanation is that it contains the charm-anticharm constituents.

11. 11 However, although the observed enhancement is at 4.26 GeV, quite away above the open charm threshold, there is no experimental signal for in its observed decays. This unique character intrigues many speculations on its nature.

12. 12 One interpretation takes it as a hybrid. [Zhu, 05; Kou, 05; Close et al, 05] However, the lattice calculation does not favor this assignment. [Zhang, this workshop] Maiani thinks the new resonance Y(4260) may be the first orbital excitation of a diquark- antidiquark state. [Maiani, 05] Theoretical Interpretations

13. 13 Li et al. explain the state as ρand molecular state. [Liu, Zeng, Li, 05] Yuan et al. interpret it as the ω and molecular state.[Yuan, Wang, Mo, 06] Estrada still interprets the Y(4260) as a normal member in charmonium spectra, the 4S state(mainly). [Estrada, 05]

14. 14 Baryonium interpretation Phys. Lett. B638(2006), Qiao  Theoretical investigations of baryon- antibaryon bound states date back to the Fermi-Yang model. [Fermi and Yang,1949]  The baryon-antibaryon bound states are allowed in the large-Nc limit. [Witten,1979]

15. 15  Recently, possible baryonium-state signals are observed by BES and BELLE. [BELLE, 02; BES, 03] In our understanding, it is possible that the Y(4260) might be a baryonium state, containing hidden charms, and made out of

16. 16 In the assignment, the decay process can proceed easily, as schematically shown in the Figure(a).  However, the (semi)exclusive decay is highly suppressed, as, e.g., shown in the Figure(b).

17. 17

18. 18  Since for the baryonium, there is no constituent strange quark, the decays to final states with strangeness, such as and, should be suppressed.  In the baryonium model, the important open charm decay process of the state should be to, rather.

19. 19 In the baryonium configuration of Y(4260), the constituent baryons and in principle should not be restricted to color- singlet. These two three-quark baryon-like clusters can carry color indices, which enables the binding energy to be easily as large as several hundred MeV, like in the case of charmonium, or bottomonium.

20. 20  In our case, roughly speaking, the binding energy of Y(4260) is about M(Y(4260)) - 2 M( )~ -310 MeV.  Due to the large uncertainties in experiment measurement, the above number should only be considered as an order-of-magnitude estimation.

21. 21 To discriminate baryonium interpretation from other exotic state explanations is easy experimentally. One of the main differences of our model from others is that the new observed state should have a relatively large branching ratio in the process, which is however much suppressed in most of other speculations.

22. 22 In our model, it is possible and we can even give a rough estimation of the relative rates of these two decay channels, i.e.,

23. 23 Since there are more than one hundred Y(4260) events are observed in the mode, we would expect a small number of with the present experimental statistics.

24. 24 In recently, BaBar also search for in ISR, and find no structure at 4260. (BaBar, talk given at QWG, BNL, 2006)

25. 25 Another unique feature of our model is that two body decay is generally suppressed, while the three body decay is favored since the six-quark components of the baryonium makes the later much easier.

26. 26 In our approach, the process is allowed, and with the half rate as the, that is ≈

27. 27  BaBar has also measured the  process and find no clear signal of a resonance-like structure at 4260. [BaBar, 05]  In our model, this in quite easy to understand, the hard propagator suppresses it.

28. 28 If the baryonium assignment for the resonance Y(4260) is correct, the pseudoscalar para-baryonium state,, should exist in nature.

29. 29 Conclusions To our understanding, in the study of the nature of Y(4260) state there is no definite conclusions yet!

30. 30 Especially, Baryonium model still survives in front of the present experimental data.

31. 31  One problematic thing is that the BaBar Collaboration observed a new structure in the ISR process [BaBar talk, QWG, BNL, 2006] at 4350(but not 4260).  Does this new state have the similar inner structure as Y(4260)? Not clear right now!

32. 32

33. 33 Thank You