Recent Results on Light Hadron Spectroscopy at BESIII by hcj

VIEWS: 32 PAGES: 20

									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
        29th May . -3rd June., 2011, Blois, France
Outline

• 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 a0-f0 mixing via J/ψ → ϕf0
  → ϕa0 and χc1→π0a0→π0f0
                    Haolai TIAN, Blois, France        2
   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;




       0      0.1     0.2      0.3
                     Mpp-2mp (GeV)                       M(η’ππ) (GeV/c2)
                            Haolai TIAN, Blois, France
The BEPCII/BESIII Project


            225M J/ψ                Double-ring collider
                                    Designed Luminosity: 1×1033 cm-2s-1
                                    Record Luminosity : ~6.49×1032 cm-2s-1
                       106M ψ’                         @3770GeV
                                    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, France                   4
   pp mass threshold enhancement
                 3-body phase space
                                                      Theoretical speculations:
              X(1860)                   BESII         •Pure FSI effect (disfavored by fit)
                                                      •Baryonium: pp bound state
                                                      •…
                                                      No similar structure is observed in
                                                      relative channels
       0          0.1             0.2           0.3
acceptance                        Mpp-2mp (GeV)
             Υ(1S)→γpp @CLEO                          J/ψ→ωpp @BESII           ψ’→γpp @BESII




           PRD 73 (2006) 032001                         EPJ C53 (2008) 15
                                                                            PRL 99 (2007) 011802



                                           Haolai TIAN, Blois, France                              5
pp mass spectrum in ψ’ → π+π-J/ψ, J/ψ → γpp



            3-body phase space




                                              BESIII results:
                                              •M = 1861+6-13+7-26 MeV/c2
                                              •Γ < 38 MeV/c2 at 90% C.L.

                                              BESII results:
                                              •M = 1859+3-10+5-25 MeV/c2
       Chinese Physics C, 34 421 (2010)       •Γ < 30 MeV/c2 at 90% C.L.
                                              Consistence within the error
                                Haolai TIAN, Blois, France                   6
pp mass spectrum in J/ψ → γpp

              BES III
              Preliminary




                                 BESII results:
              BES III            •M = 1859+3-10+5-25 MeV/c2
              Preliminary        •Γ < 30 MeV/c2 at 90% C.L.
                                 BESIII preliminary results:
                                 •M = 1861± 0.8 MeV/c2
                                 •Γ < 8 MeV/c2 at 90% C.L.
                                 Consistence within the error
                                 PWA is on going.
                     Haolai TIAN, Blois, France                 7
pp mass spectrum in ψ’ radiative decay



    ψ’ → γpp                                  ψ’ → γpp




                BES III                                           BESII
                Preliminary

                                                   PRL 99 (2007) 011802

                                       No significant narrow strong
  No obvious narrow
                                       enhancement near threshold
  threshold enhancement                (~2 if fitted with X(1860))



                     Haolai TIAN, Blois, France                           8
Observation of X(1835) in J/ψ → γη’ππ

                                        BESII result
                                        (PRL95, 262001 (2005) )
                                        •M = 1833.7±6.1(stat)±2.7(syst) MeV/c2
                                        •Γ = 67.7±20.3(stat)±7.7(syst) MeV/c2
                                        •Significance: ~7.7σ




Theoretical interpretations: pseuodscaler glueball / radial excitation of η’ ?
•A 0-+ glueball is expected to have similar decay properties of ηc, which
decays primarily via η’ππ.
•LQCD predicts that the 0-+ glueball lies around 2.3GeV.
       The confirmation of X(1835) is necessary with higher statistics
       data sample and better detector like BESIII.

                              Haolai TIAN, Blois, France                    9
Mass spectrum of η’π+π-
                                                    




                                                Combination for  to +- and 
  • X(1835) and ηc 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, France                                 10
  Fit of the combined mass spectrum

     Fitting with four resonances
     Three background components:
      1.   η’ mass sideband
      2.   J/ψ → π0η’π+π- with reweighting
      3.   “PS background”


                                             The angular distribution of the events
                                             in X(1835) ∝ 1+cos2(Ѳ) ,which agrees
                                             with JPC=0-+
                                       Red line: estimated contribution of 1. + 2.
                                       Black line: total background




PRL 106, 072002 (2011)
                                 Haolai TIAN, Blois, France                           11
Summary of J/ψ → γη’π+π-

• The existence of X(1835) is confirmed at BESIII, with a much higher
  statistical significance ( >20σ ).

• Two additional resonances, X(2120) and X(2370), are observed with
  significance larger than 7.2σ 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, France                      12
Analysis of J/ψ → ωηππ

                            M(ηππ), with a0(980)           M(ηππ), veto a0(980)

           a0(980)           f1(1285)               ?



                 a2(1320)                                    ’

                                        (1405)
   BES III                                  BES III                    BES III
   Preliminary                              Preliminary                Preliminary




• In addition to the well-known η’, f1(1285) and η(1405), an unknown
  structure (denoted as X(1870)) around 1.87GeV/c2 is observed.
• The f1(1285), η(1405) and X(1870) decay primarily via a0(980)π mode.



                              Haolai TIAN, Blois, France                        13
Mass spectrum fitting results

                                    Signal peak: acceptance weighted BW⊗Gauss
                                    Background: 4th-order polynomial


                                     Fitted result of X(1870):
                                     •M = (1877.3±6.3) MeV/c2
                    BES III          •Γ = (57±12) MeV/c2
                    Preliminary      •Significance: 7.1σ



• 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
  η2(1870) (Γ =225±14 MeV/c2)or a new resonance.



                             Haolai TIAN, Blois, France                   14
Studying a0(980)-f0(980) mixing

• Interpretation of a0/f0: qq, four quarks, KK molecule, hybrids, …
• Study of the mixing of a0 and f0 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).

                 ϕ                                                 π0
      J/ψ                                                 χc1                       π+
                                 η


                  f0 → a0                                           a0 → f0
                                 π0                                                 π-

    J.Wu, Q.Zhao, B.Zou PRD75 114012,                    J.Wu, B.Zou PRD78 074017

    C. Hanhart etc. PRD76 074028, etc.



                                  Haolai TIAN, Blois, France                             15
a0(980)-f0(980) mixing: f0 → a0 transition
          PRD83 032003(2011)

         J/ψ → ϕf0 → ϕa0 → ϕηπ0
                                        N(mixing)=25.8  8.6
              Significance: 3.4σ
                                           (<39.7 @90% C.L)

                                        Br(J/   f 0   a0   )
                                                                         0




                                                                                             6
                                        =(3.1  1.1(stat .)  0.4(sys.)  1.4(para.))  10
                                                        6
                                          (<5.4  10 @ 90% C.L.)

                                        Mixing intensity:

                                                 Br(J/   f 0   a0   )
                                                                                      0


                                         fa =
                                                   Br(J/   f 0   )
                                                                            [BESII]
 …. Mixing signal
 --- a0(980) contribution from           (0.60  0.20 ( stat .)  0.12( sys.)  0.26( para.))%
     J/ψ → γ*/K*K → ϕa0(980)
 --- Background polynomial                (<1.1% @ 90% C.L.)


                               Haolai TIAN, Blois, France                                    16
a0(980)-f0(980) mixing: a0 → f0 transition
         PRD83 032003(2011)

  ψ→γχc1, χc1→a0π0→ f0π0→ π+π-π0         N(mixing)=6.4  3.2
               Significance: 1.9σ
                                            (<13.0 @90% C.L)

                                         Br(    c1 )Br( c1  a0
                                                                            0                   0           0
                                                                                 f0                      )
                                                                                                            7
                                         =(2.7  1.4(stat .)  0.7( sys.)  0.3( para.))  10
                                                           7
                                           (<6.0  10 @ 90% C.L.)
                                         Mixing intensity:

                                                  Br( c 1  a0  f 0     )
                                                                0       0                         0


                                          af =
  …. Mixing signal                                  Br( c 1   a0    )
                                                                    0       0    0       [PDG]



  --- f0(980) contribution from
                                          (0.31  0.16( stat .)  0.14( sys.)  0.03( para.))%
      from other process
  --- Background polynomial                (<1.0% @ 90% C.L.)


                              Haolai TIAN, Blois, France                                                    17
Comparison with different models

           PRD83 032003(2011)



                                                  a0-f0 mixing:
                                                   a sensitive probe




  Shaded region: BESIII measurements
  Red Line: BESIII upper limits
  Dots: various predictions

                            Haolai TIAN, Blois, France                 18
Summary

• 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 → wX(1870) →
  wηπ+π-.

• The transition of a0 ↔ f0 is studied and the mixing
  intensity is extracted.
                        Haolai TIAN, Blois, France      19
Thank you!

								
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