Recent BESII Results Representing BESII Collaboration Weiguo Li Institute of High Energy Physics, Beijing , P.R. China NSTAR Workshop Bonn, Sep. 6, 2006

Outline  Introduction  Light Hadron Spectroscopy from J/  Decays  Study of Excited Baryon States  Selected BF Measurements  Summary

BESII Detector VC:  xy = 100  m TOF:  T = 180 ps  counter:  r  = 3 cm MDC:  xy = 250  m BSC:  E/  E= 22 %  z = 5.5 cm  dE/dx = 8.4 %   = 7.9 mr B field: 0.4 T  p/p=1.8  (1+p 2 )  z = 2.3 cm Dead time/event: 〈 10 ms

Data from BESII BESII finished data taking April 2004

 Scalars: ,  clearly observed  X(1835) in J/    ’      The  threshold enhancement in J/     New observation of a broad resonance in J/   K + K -  0   (1760) in J/     Light Hadron Spectroscopy from J/  Decays

The  pole in at BESII M(  ) M(  +  -  0 ) M(  +  - ) 00    Averaged pole position: MeV Phys. Lett. B 598 (2004) 149

κ Phys. Lett. B 633 (2006) 681

Observation of X(1835) in at BESII The  +  -  mass spectrum for  decaying into  +  -  and   Statistical Significance 7.7  PRL 95, (2005)

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)

Re-fit to J/    pp including FSI Include FSI curve from A.Sirbirtsev et al. ( Phys.Rev.D71:054010, 2005 ) in the fit (I=0) M =  6.7 MeV  < 153 In good agreement with X(1835)

Candidate of 0 -+ pp Bound State  X(1835) could be the same structure as pp mass threshold enhancement X(1860)/X(1830).  It could be a pp bound state since it dominantly decays to pp when its mass is above pp mass threshold.  Its spin-parity should be 0 -+ : this would be an important test. There is already an  (1760) nearby, so that  X(1835) is very likely to be an unconventional 0 -+ meson.

 This result cannot be explained by pure FSI effect, since FSI is a universal effect.  This indicates that X(1860) has a production property similar to  ’ meson. This strong threshold enhancement is NOT observed in at CLEO No enhancement near threshold Phys.Rev.D73, (2006)

 FSI interpretation of the narrow and strong ppbar threshold enhancement is disfavored.  This again indicates that X(1860) has a production property similar to  ’ meson. No strong threshold enhancement is observed in at BESII No significant narrow strong enhancement near threshold (2.0  if fitted with X(1860)) Phys.Rev.Lett.99, (2007)

J/   pp Clear  signal is seen, B(J/   pp) =( 9.8  0.3  1.4 )  10 –4 B(J/   X(1860))B(X(1860)  pp) < 1.5  10 95% C.L.

 This again disfavors FSI and indicates that X(1860) has a production property similar to  ’ meson.  This also indicates X(1860) may have strong coupling to gluons as  ’ meson. This strong threshold enhancement is NOT observed in at BESII

Summary of new studies on pp mass threshold structure X(1860)  Pure FSI effect cannot explain X(1860) structure.  X(1860) has production properties similar to  ’ meson  Why a baryonium (candidate) has a production properties similar to  ’ meson?  ’ excitation? Why an  ’ excitation dominantly decays into ppbar above ppbar mass threshold?

 We studied DOZI process: J/    +  +    +  -  0 K + K -    recoiling against   mass threshold enhancement Phys. Rev. Lett. 96, (2006)

A clear mass threshold enhancement is observed Acceptance Side-bands

 The radiative decay of J/  has been observed in the 58M J/  data.  A significant structure of  has been found near the mass threshold.  PWA shows the structure favors 0 ++, M =,  =105  20  28 MeV, and the corresponding branch fraction = (2.61  0.27  0.65)x  It could be a multiquark/hybrid/glueball state.  Its relation with f 0 (1710), f 0 (1790)?

New observation of a broad 1 – resonance in J/   K + K -  0 00 background  0 sideband ? K*(892) K * (1410) X(1580) PID and kinematical fit can significantly reduce the dominant background from J/    +  -  0. Phys. Rev. Lett. 97 (2006)

 Four decay modes are included :  Amplitudes are defined by Covariant tensor formalism B.S. Zhou and D.V. Bugg, Eur. Phys. J. A16, 537(2003)  BW with energy-dependent width J.H. Kuhn, A. Satamaria, Z. Phys. C48, 445 (1990). Partial Wave Analysis of J/   K + K -  0 events Phys. Rev. Lett. 97 (2006)

 Parity conservations in J/   K + K -  0 requires that spin-parity of K + K - should be 1 --,3 --,…  PWA fit with and phase space (PS) gives:  ( can be ruled out by much worse likelihood )  X pole position   big destructive interference among and PS Partial Wave Analysis of J/   K + K -  0 events

Broad X cannot be fit with known mesons or their interference  It is unlikely to be  (1450), because: The parameters of the X is incompatible with  (1450).  (1450) has very small fraction to KK. From PDG:  It cannot be fit with the interference of  (770),  (1900) and  (2150): The log-likelihood value worsens by 85 (  2 =170).

How to understand broad X(1580)?  Search of a similar structure in J/   K S K  will help to determine its isospin.  X(1580) could have different nature from conventional mesons: There are already many 1- - mesons nearby. Width is much broader than other mesons. Broad width is expected for a multi-quark state.

M(  +  -  0 )(GeV/c 2 )    sideband  signal after best candidate selection (best  masses)  signal with multiple entries Phys. Rev. D 73 (2006) Meson

Eff. curve Phase Space Side-band M(  )

 (1760) f 0 (1710) f 0 (1790) f 0 (1810) PWA analysis M(  ) Total f2(1910) f2(160) BG Using observed mass and width for f 0 (1810) in J/    f 0 (1710) f 2 (1910) f 2 (1640) BG  (1760) > 10  f 0 M(  ) (GeV/c 2 )

 The existence of  (1760) is confirmed  Its mass and width were first correctly measured with PWA. Phys. Rev. D 73 (2006)

Study of Excited Baryon States Motivation  Probe the internal structure of light quark baryons  Search for missing baryons predicted by quark model  Obtain a better understanding of the strong interaction force in the non-perturbative regime

Experimental Advantages processesbranching ratios(10 -3 )N* decays 2.0       0.3 relatively large branching ratios 58 M J/  and 14 M  (2S) at BES II

Pure isospin 1/2 Feynman diagram of the production of For and, and systems are limited to be pure isospin 1/2.

N* in. Phys. Rev. Lett. 97, (2006)

N* in +c.c. L=0 limits it to be 3/2+ or 1/2+ N*(1440) N*(1520) N*(1535) N*(1650) N*(1675) N*(1680) ?

 About 10 K events are selected  N*(1440) is observed directly in the mass spectrum  Possible new N* resonance  L=0 limits it to be 3/2+ or 1/2+  Detailed information need PWA N* in +c.c.

Phys. Rev. D74, (2006)  Large accumulation below 1.5 GeV/c 2 ， which may be due to N(1440), N(1520), N(1535) etc.  The cluster above 2 GeV/c 2 is partly due to the reflection of N(1440) etc., and partly may due to high mass N*.  No clear N(2065) peak, but we can not rule out its existence.

N* in. M  00  M 2 (p  0 )

N* in.

Resonances used in the PWA

N* in.  data  Fit results Fit results agree with data reasonably, especially in the low mass region.

N* in. Optimized masses and widths, other resonances are fixed.

N* in.  More than 10 K events were selected, and detailed PWA was performed  N(2065) is needed in the fit, and J PC favor 3/2+  Only N(1710) is needed in the M = 1.7 GeV region  The masses and widths of some other N* have also been given

 A faint accumulation of events around 2065 MeV/c 2  The enhancement between 1.4 and 1.7 GeV/c 2  Possible N*(1535) in the p  invariant mass Phys. Rev. D71, (2005) 00 

Phys. Rev. Lett. 93, (2004) For a S-wave BW fit: M = 2075  12  5 MeV Γ = 90  35  9 MeV

 Two clear peaks at 1520, 1690 MeV/c 2 in pK mass  N* in K  mass  PWA is being performed N*(1535)? N*(1650)?  *(1520)  *(1690) PS, eff. corrected (Arbitrary normalization)

 A strong enhancement is observed near the mass threshold of M K  at BES II.  Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure N x *has: Mass 1500~1650MeV Width 70~110MeV J P favors 1/2 - The most important is: It has large BR(J/ψ  pN X *) BR(N X *  KΛ)  2 X 10 -4, suggesting N X * has strong coupling to KΛ.

A ΛK resonance predicted by chiral SU(3) quark model  Based on a coupled- channel study of ΛK and ΣK states in the chiral SU(3) quark model, the phase shift shows the existence of a ΛK resonance between ΛK and ΣK mass threshold. ( F. Huang, Z.Y. Zhang et al. Phys. Rev. C71: , 2005 ) E cm – ( M Λ +M K ) (MeV)

 The KΛ mass threshold enhancement N X (1610) could be a KΛ bound/resonant state.  Whether N X (1610) is N(1535) needs further study.

 KSKS

 A enhancement near  S threshold is evident  N* and  * found in the  S and n  S spectrum N* **

x Angular distribution: 2

Baryonic decays Rich structure (N*s), statistics (~850 evts in each mode). Still low for a partial wave analysis. PRD74, (2006)

Test I-spin symmetry (prediction: 1:2:2)

 ’(J/  )   +  0 /   Is it really B(  0 )>B(  )? arXiv: [hep-ex] Submitted to PRD First measurement! background from J/   ΣΛπ signal

J/      with     0 p,   p  + Clear  is seen, B( J /      + c.c. ) = 1.52±0.08±0.16)  10 -3

BEPC II  Upgrade of BEPC  Double-ring collider, 93 bunches  High luminosity Design Goal

Magnet: 1 T Super conducting MDC: small cell & He gas  xy =130  m  p /p = dE/dx=6% TOF:  T = 100 ps Barrel 110 ps Endcap Muon ID: 9 layer RPC EMCAL: CsI crystal  E/E = GeV  z = 0.6 cm/  E Trigger: Tracks & Showers Pipelined; Latency = 6.4  s Data Acquisition: Event rate = 3 kHz Thruput ~ 50 MB/s BESIII Detector The detector is hermetic for neutral and charged particle with excellent resolution, PID adequate, and large coverage.

ResonanceEnergy(GeV)Peak Lum. (10 33 cm -2 s -1 ) Physics Cross Section (nb) Nevents/yr J/   10 9   10 6  (2S)  10 9 D 0 D 0 bar  10 6 D+D-D+D  10 6 DsDs  10 6 DsDs  10 6 Average Lum: L = 0.5×Peak Lum.; data taking time: T = 10 7 s/year Yearly Event Production Huge J/  and  (2S) samples at BESIII

 Two rings are commissioned with normal quadrupole magnets, with 100 mA in each beam of 20 bunches, with a luminosity close to  Two periods providing synchrotron Radiation users with a max. current of 180 mA at 2.5 GeV.  SCQ and Detector SC magnet have been tested and fields were measured.  Most detectors are assembled, will be fully assembled end of 2007, and cosmic ray for 2 months, to the beam line end of March  Commissioning machine/detector together next summer.  Some test runs, luminosity to reach 3  end of 2008.

Summary New states are observed in the hadron spectroscopy:  , , X(1835) in J/   ’    , X(1580) in J/  KK , enhancement in J/  ,  (1760) in J/   ;  Excited baryons, N* observed in J/  pn   ; J/  pp  0 ; J/  nK S  ; Compared with  (2S) decay; Some N*, as N*(1535), N*(2065) better measured.  Some BF involving baryons are measured;  BEPCII/BESIII should collect data in 2008.

Thanks

 (2s) data samples   ’ decays  Leptonic decays  Radiative decays  Hadronic decays   cJ decays  VV  PPP 3M 14M 4M outline 26M Also 6.42 pb -1 of data at Ecm=3.65 GeV for background study.

Discussion on KΛ mass threshold enhancement N X (1610)  N X (1610) has strong coupling to KΛ: From (S&D-wave decay) and is a P-wave decay, we can estimate From BESII, The phase space of N X to KΛ is very small, so such a big BR shows N X has very strong coupling to KΛ, indicating it has a big hidden ssbar component. (5-quark system)

Non-observation of N X in suggests an evidence of new baryon :  It is unlikely to be N*(1535). If N X were N*(1535), it should be observed in process, since: From PDG, for the N* in the mass range 1535~1750 MeV, N*(1535) has the largest, and from previous estimation, N X would also have almost the largest BR to KΛ.  Also, the EM transition rate of N X to proton is very low.