Light Hadron Spectroscopy at BESIII Haolai TIAN (On behalf of the BESIII Collaboration) Institute of High Energy Physics, Beijing 23rd Rencontre de Blois Particle Physics and Cosmology 29 th May. -3 rd June., 2011, Blois, France

Outline Haolai TIAN, Blois, France2 pp mass threshold enhancement study in the radiative decays of J/ ψ and ψ’ Confirmation of X(1835) and observation of two new resonances in J/ ψ → γη’π + π - Observation of X(1870) in J/ ψ → ωηπ + π - Direct measurements of a 0 -f 0 mixing via J/ ψ → ϕf 0 → ϕa 0 and χ c1 →π 0 a 0 →π 0 f 0

Remarks of BESII on light hadron spectroscopy The multi-quark states, glueballs and hybrids have been experimentally searched for a very long time. During the last few decades, a lot of unexpected experimental evidence for hadrons are reported:  pp threshold enhancement observed in J/ ψ → γpp;  X(1835) observed in J/ ψ → γX, X → η’ππ; M( η’ππ ) (GeV/c 2 ) Haolai TIAN, Blois, France M pp -2m p (GeV)

The BEPCII/BESIII Project Double-ring collider Designed Luminosity: 1×10 33 cm -2 s -1 Record Luminosity : ~6.49×10 32 cm -2 s The construction started in 2004: Apr. 14, 2009: ~106 M ψ’ events 42.3pb -1 at 3.65GeV July 28, 2009: ~225 M J/ψ events We have opportunities to confirm the existence of pp mass threshold enhancement and X(1835) at BESIII … and to search new observations ! Haolai TIAN, Blois, France4 225M J/ψ 106M ψ’

pp mass threshold enhancement PRD 73 (2006) PRL 99 (2007) EPJ C53 (2008) Haolai TIAN, Blois, France5 M pp -2m p (GeV) BESII X(1860) 3-body phase space acceptance Theoretical speculations: Pure FSI effect (disfavored by fit) Baryonium: pp bound state … No similar structure is observed in relative channels

pp mass spectrum in ψ’ → π + π - J/ψ, J/ψ → γpp 3-body phase space BESIII results: M = MeV/c 2 Γ < 38 MeV/c 2 at 90% C.L. BESII results: M = MeV/c 2 Γ < 30 MeV/c 2 at 90% C.L. Consistence within the error Haolai TIAN, Blois, France6 Chinese Physics C, (2010)

pp mass spectrum in J/ψ → γpp BES III Preliminary BES III Preliminary Haolai TIAN, Blois, France7 BESII results: M = MeV/c 2 Γ < 30 MeV/c 2 at 90% C.L. BESIII preliminary results: M = 1861 ± 0.8 MeV/c 2 Γ < 8 MeV/c 2 at 90% C.L. Consistence within the error PWA is on going.

pp mass spectrum in ψ’ radiative decay PRL 99 (2007) BES III Preliminary BESII ψ’ → γpp Haolai TIAN, Blois, France8 No significant narrow strong enhancement near threshold (~2  if fitted with X(1860)) No obvious narrow threshold enhancement

Observation of X(1835) in J/ ψ → γη’ππ Theoretical interpretations: pseuodscaler glueball / radial excitation of η’ ? A 0 -+ glueball is expected to have similar decay properties of η c, which decays primarily via η’ππ. LQCD predicts that the 0 -+ glueball lies around 2.3GeV. BESII result (PRL95, (2005) ) M = ±6.1 (stat) ±2.7 (syst) MeV/c 2 Γ = 67.7 ±20.3(stat)±7.7(syst) MeV/c 2 Significance: ~7.7 σ The confirmation of X(1835) is necessary with higher statistics data sample and better detector like BESIII. Haolai TIAN, Blois, France9

Mass spectrum of η’π + π - X(1835) and η c are evident. Two additional structures are observed at ~2.1 and 2.3 GeV. There exists a small peak at 1.5GeV. Haolai TIAN, Blois, France10 Combination for  to  +  -  and 

Red line: estimated contribution of Black line: total background Fit of the combined mass spectrum  Fitting with four resonances  Three background components: 1.η ’ mass sideband 2.J/ ψ → π 0 η’π + π - with reweighting 3.“PS background” Haolai TIAN, Blois, France11 PRL 106, (2011) The angular distribution of the events in X(1835) ∝ 1+cos 2 (Ѳ),which agrees with J PC =0 -+

Summary of J/ ψ → γη’π + π - The existence of X(1835) is confirmed at BESIII, with a much higher statistical significance ( >20 σ ). Two additional resonances, X(2120) and X(2370), are observed with significance larger than 7.2 σ and 6.4σ, respectively. PWA is needed not only to determine the spin-parities of the above three resonances, but also to make more precise measurement of masses, widths and BRs by taking possible interference into account. The study of the decay pattern is of crucial importance, i.e. in relative channels (ηππ, …) and with other decay mode (ω, ϕ, …) Haolai TIAN, Blois, France12

Analysis of J/ ψ → ωηππ a 0 ( 980 ) ’’ f 1 ( 1285 )  ( 1405 ) ? a 2 ( 1320 ) M( ηππ ), with a 0 (980)M( ηππ ), veto a 0 (980) BES III Preliminary BES III Preliminary BES III Preliminary In addition to the well-known η’, f 1 (1285) and η(1405), an unknown structure (denoted as X(1870)) around 1.87GeV/c 2 is observed. The f 1 (1285), η(1405) and X(1870) decay primarily via a 0 (980)π mode. Haolai TIAN, Blois, France13

Mass spectrum fitting results BES III Preliminary Signal peak: acceptance weighted BW ⊗ Gauss Background: 4 th -order polynomial The fit is performed under the assumption that the interference between the resonances and background can be ignored. Current results cannot settle down whether X(1870) is actually η 2 (1870) ( Γ =225 ±14 MeV/c 2 )or a new resonance. Fitted result of X(1870): M = (1877.3±6.3) MeV/c 2 Γ = (57 ±12) MeV/c 2 Significance: 7.1 σ Haolai TIAN, Blois, France14

Studying a 0 (980)-f 0 (980) mixing Interpretation of a 0 /f 0 : qq, four quarks, KK molecule, hybrids, … Study of the mixing of a 0 and f 0 will shed new light on the enigmatic light scalars. No firm experimental determination yet. Signature of the mixing signal: a narrow peak (8MeV) between the charged and neutral kaon thresholds (987~995 MeV). J.Wu, Q.Zhao, B.Zou PRD , C. Hanhart etc. PRD , etc. J.Wu, B.Zou PRD J/ ψ ϕ f 0 → a 0 η π0π0 χ c1 π0π0 a 0 → f 0 π-π- π+π+ Haolai TIAN, Blois, France15

a 0 (980)-f 0 (980) mixing: f 0 → a 0 transition …. Mixing signal --- a 0 (980) contribution from J/ ψ → γ */K*K → ϕ a 0 (980) --- Background polynomial Significance: 3.4 σ J/ ψ → ϕf 0 → ϕa 0 → ϕηπ 0 Haolai TIAN, Blois, France16 PRD (2011)

a 0 (980)-f 0 (980) mixing: a 0 → f 0 transition …. Mixing signal --- f 0 (980) contribution from from other process --- Background polynomial Significance: 1.9 σ ψ→γχ c1, χ c1 →a 0 π 0 → f 0 π 0 → π + π - π 0 Haolai TIAN, Blois, France17 PRD (2011)

Comparison with different models Shaded region: BESIII measurements Red Line: BESIII upper limits Dots: various predictions a 0 -f 0 mixing: a sensitive probe Haolai TIAN, Blois, France18 PRD (2011)

Summary Haolai TIAN, Blois, France19 pp mass threshold enhancement and X(1835) are confirmed at BESIII. In addition to X(1835), two new resonances, X(2120) and X(2370) are observed in the channels of J/ ψ → γη’π + π -. A new process is observed in J/ ψ →  X(1870) →  ηπ + π -. The transition of a 0 ↔ f 0 is studied and the mixing intensity is extracted.

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