Mass Threshold Structure and Final State Interaction Shan JIN Institute of High Energy Physics (IHEP) NSTAR09, Beijing April 19, 2009

Outline  Experimental results on mass threshold structure from various experiments  Discussions on I =0 FSI interpretation  Discussions on I =1 FSI interpretation  Resonance interpretation  Conclusion

Review of experimental results on mass threshold structure from various experiments

Observation of an anomalous enhancement near the threshold of mass spectrum at BES II M=1859 MeV/c 2  < 30 MeV/c 2 (90% CL) J/    pp M(pp)-2m p (GeV) body phase space acceptance  2 /dof=56/56 acceptance weighted BW  10  25 BES II Phys. Rev. Lett. 91, (2003)

Features of the enhancement near the threshold of mass spectrum at BES II J/    pp M(pp)-2m p (GeV) BES II  Peak position: ~ 0 MeV above threshold  “Width”: ~ 60M eV  Strong (“Height”): (S+B)/B ~ 2 The above features may help us easily to judge whether it is observed in other processes.

This narrow threshold enhancement is NOT observed in B decays  The structure in B decays is obviously different from the BES observation: Belle BES II The structure in B decays is much wider and is not really at threshold. It can be explained by fragmentation mechanism. Threshold enhancement in J/  decays is obviously much more narrow and just at threshold, and it cannot be explained by fragmentation mechanism.

This narrow threshold enhancement is NOT observed in at CLEO No enhancement near threshold It is suppressed vs 2% rule, while follows.

This narrow threshold enhancement is “NOT” observed in at BESII No significant narrow strong enhancement near threshold (~2  if fitted with X(1860)) It is suppressed vs 12% rule, while follows.

This narrow threshold enhancement is NOT observed in at BESII No narrow strong enhancement near threshold

Summary on the experimental results  The strong and narrow mass threshold has only been observed in J/ψ radiative decays, not in any other place so far.  Any model trying to interpret X(1860) should also answer why it is not observed in other places, especially in ψ’ and Y(1S) radiative decays as well as in process.

Discussions on I =0 FSI interpretation

FSI Factors Most reliable full FSI factors are from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) ， which fit elastic cross section near threshold quite well. elastic cross section near threshold I=1 S-wave I=0 S-wave P-wave

Pure I =0 FSI disfavored (I) I =0 S-wave FSI CANNOT fit the BES data. FSI * PS * eff + bck FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0) Julich Model

 If X(1860) were from pure I =0 FSI, it would be very difficult to explain why such a structure is not observed in ψ’ and  (1S) radiative decays as well as process.  The mass distribution near threshold in ψ’ and  (1S) radiative decays as well as process may be consistent with pure FSI contribution. Large statistics is needed to draw firm conclusions. Pure I =0 FSI disfavored ( II )

 p FSI in ?  The ‘uniform’ Dalitz plot does NOT support strong  p FSI.

Comments on Julich fit on data  Differences between Julich fit ( arXiv: [hep- ph] ) and BESII fit: Julich BESII PS * eff + bckFSI * PS Contribution from efficiency is very important It is WRONG just to compare data with PS

Contribution from background (bck) is also very important  From 3  +  -  0 mass spectrum, non- ω background contribute about half events into the mass spectrum.  

 MA’s fit ( arXiv: [hep-ph] ) Comments on MA’s fit on data FSI * PS Correctly fitting BESII data with MA’s FSI factor FSI * PS * eff + bck Contributions from efficiency and bck are very important

Lesson and Warning from Julich and MA’s fit One should NEVER try to fit experimental data without knowing details of the experiments ( including detection efficiency, background, etc. )

 Therefore, X(1860) CANNOT be explained by pure I =0 FSI.

Discussions on I =1 FSI interpretation

Pure I =1 FSI interpretation disfavored (I)  Although I =1 FSI can explain the non-observation in, it is hard to explain why X(1860) is NOT observed in  ’ and  (1S) radiative decays.

Pure Ⅰ =1 FSI interpretation disfavored ( II )  Naive estimation on Ⅰ =1 contribution in : If is dominated by Ⅰ =1 process from  However, we have So, it is very hard to interpret X(1860) as FSI Ⅰ =1

Pure Ⅰ =1 FSI interpretation disfavored ( Ⅲ ) Pure Ⅰ =1 S-wave FSI is disfavored by more than 4 . M = 1773  21 MeV  = 0  191 MeV FSI * PS * eff + bck FSI * BW * PS * eff + bck X(1860)=FSI Ⅰ =1 *  (1800)?

Ⅰ =1 process strongly suppressed in J/  radiative decays (theoretically)  Ⅰ = 0 ： ~  s 4  Ⅰ = 1 ： ~  s 6  Ⅰ =1 one order lower than Ⅰ =0

Experimental results support these theoretical picture  Most Ⅰ = 0 states have been observed in J/  radiative decays with big production rate ( especially for 0 -+ mesons ) such as ,  ’,  (1440),  (1760), f 2 (1270), f 2 (1525), f 0 (1500), f 0 (1710).  The only observed Ⅰ =1 meson in J/  radiative decays is  0 with very low production rate 4*10 – 5, e.g., no evidence for  (1800) in J/    3  process. X(1860) is unlikely to be from  (1800).

No evidences for a 2 (1320) and  (1800) at BES Ⅱ BESII Preliminary

 All above evidences show that X(1860) cannot be interpret as any Ⅰ =1 process (neither pure Ⅰ =1 FSI, nor FSI Ⅰ =1 *  (1800)).

Resonance Interpretation

Pure FSI is strongly disfavored. However, we do not exclude the contribution from I=0 FSI.

Re-fit to including FSI Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit ( I =0) M =  6.7 MeV  = 0  93 MeV FSI * BW * PS * eff + bck

Observation of X(1835) in The  +  -  mass spectrum for  decaying into  +  -  and   Statistical Significance 7.7 

Mass spectrum fitting 7.7  The  +  -  mass spectrum for  decaying into  +  -  and  

 In the resonance interpretation, how do we understand why it is not observed in ψ’ and  (1S) radiative decays as well as process? It can be easily/naturally understood as X(1860) has production properties as η’ meson.

 This result cannot be explained by pure FSI effect, since FSI is a universal effect. FSI interpretation of the narrow and strong threshold enhancement is disfavored.  This indicates that X(1860) has a production property similar to  ’ meson. c.f. : This narrow threshold enhancement is NOT observed in at CLEO No enhancement near threshold

 FSI interpretation of the narrow and strong threshold enhancement is disfavored.  This again indicates that X(1860) has a production property similar to  ’ meson. c.f. : This narrow threshold enhancement is “NOT” observed in at BESII No significant narrow strong enhancement near threshold (2.4  if fitted with X(1860))

 This again indicates that X(1860) has a production property similar to  ’ meson. c.f. :  This also indicates X(1860) may have strong coupling to gluons as  ’ meson. This narrow threshold enhancement is NOT observed in at BESII No narrow strong enhancement near threshold

Then, what is X(1860)?

X(1860) has large BR to ppbar  We (BES) measured:  From Crystal Ball result, we etimate:  So we would have: (This would be the largest BR to among all known mesons) Considering that decaying into is only from the tail of X(1860) and the phase space is very small, such a BR indicates X(1860) has large coupling to !

Summary of the properties of the strong ppbar mass threshold enhancement X(1860)  So far, it is only observed in J/  radiative decays: It has production properties similar to  ’ meson. It could have strong coupling to gluons as  ’ meson.  It could have the largest decay BR to among all PDG particles: It has strong coupling to.

(1) bound state?  Naturally explain the large coupling to.  But why it has similar production properties as η’ meson?

(2)  ’ excitation?  This can easily explain why X(1860) has similar production properties as  ’ meson.  But very difficult to explain large decay BR.

(3) Mixture of both?  Likely to be a mixture of bound state and η’ excitation, which can naturally explain all observed properties.  In this case, we need two 0 -+ state: one is X(1835), where is the other one? Conjectured picture: η(1760) mixed with X(1835) then I would predict that process is also suppressed.  we can check this at BESIII.

Summary  (Pure) FSI cannot explain X(1860) structure which is only observed in J/  radiative decays.  I =1 process is strongly suppressed in J/  radiative decays so that X(1860) cannot be from any I =1 process.  Any model trying to interpret X(1860) should also answer why it is not observed in  ' and Y(1S) radiative decays as well as in process.  X(1860) has very large coupling to and it has similar production properties to η'. It might be a mixture of bound state and η' excitation.

谢 谢！ Thank You!

Crystal Ball results on inclusive photon spectrum of J/psi decays

 With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:  much weaker than what BES observed ! NO strong dynamical threshold enhancement in cross sections (at LEAR) |M| 2 BES Both arbitrary normalization

 With threshold kinematic contributions removed, there are very smooth threshold enhancements in elastic “matrix element” and very small enhancement in annihilation “matrix element”:  much weaker than what BES observed ! NO strong dynamical threshold enhancement in cross sections (at LEAR) |M| 2 BES Both arbitrary normalization

Pure I =0 OPE FSI disfavored Theoretical calculation (Zou and Chiang, PRD (2003)) shows: The enhancement caused by one-pion-exchange (OPE) FSI is too small to explain the BES structure. (The enhancement caused by Coulomb interaction is even smaller than one-pion-exchange FSI). BES one-pion-exchange FSI |M| 2 Both arbitrary normalization BES Both arbitrary normalization Coulomb interaction

Any inconsistency? NO!  For example: with M res = 1859 MeV, Γ = 30 MeV, J=0, BR(ppbar) ~ 10%, an estimation based on: At E cm = 2m p + 6 MeV ( i.e., p Lab = 150 MeV ), in elastic process, the resonant cross section is ~ 0.6 mb : much smaller than the continuum cross section ~ 94  20 mb.  D ifficult to observe it in cross sections experimentally.

FSI Factors Most reliable full FSI factors are from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) ， which fit ppbar elastic cross section near threshold quite well. ppbar elastic cross section near threshold I=1 S-wave I=0 S-wave P-wave

 J/  decays do not suffer large t-channel “background” as ppbar collision. >> In ppbar collision, the background is much lager (I)

A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas, Phys.Rev.D71:054010, 2005 P-wave I=0 S-wave I=1 S-wave In ppbar elastic scattering, I=1 S-wave dominant, while in J/  radiative decays I=0 S-wave dominant. ppbar elastic cross section near threshold In ppbar collision, the background is much lager (II)

So, the mechanism in ppbar collision is quite different from J/  decays and the background is much smaller in J/  decays It would be very difficult to observe an I=0 S-wave ppbar bound state in ppbar collisions if it exists. J/  decays (in e+e- collider) have much cleaner environment: “J P, I ” filter

Any special suppression of 0 -+ wave in and ?  No theoretical rule and experimental evidence.  Experimentally, we have:

 J/ψ  γ+X isospin =1 decay modes are suppressed by at least one order vs. I=0 modes in J/ψ radiative decays, supporting theoretical estimations.  Upper limits:

The large BR to ppbar suggest it could be an unconventional meson  For a conventional qqbar meson, the BRs decaying into baryons are usually at least one order lower than decaying into mesons. There are many examples in PDG. E.g.  So the large BR to ppbar (with limited phase space from the tail of X(1860)) seems very hard to be explained by a conventional qqbar meson.