Charmonium Spectroscopy at BESIII

Transcription

1 Charmonium Spectroscopy at BESIII Hu Liu (on behalf of the BESIII Collaboration) Institute of High Energy Physics, Beijing, China 8 th Slide 1

2 Outline Brief Introduction of BEPCII & BESIII Measurement of h c at BESIII Precision measurement of the η c properties The first observation of the M1 transition ψ γη c (2S) Summary Slide 2

3 BEPCII & BESIII Large data samples Updated detectors E cm = 2~4.6GeV L peak = /(cm 2. s) BESIII is excellent! Abundant potential physics with BESIII. Slide 3

4 BESIII data samples Energy points J/ψ ψ 3.65 GeV ψ(3770) 4.01 GeV (ψ(4040)) luminosity 81 pb pb pb fb fb -1 Number of resonant events 225 million 106 million A unique facility and Open many physics opportunity! Slide 4

5 Charmonium Spectrum below open charm threshold 106M ψ(2s) at BESIII π 0 Slide 5

6 Measurement of h c at BESIII Slide 6

7 h c ( 1 P 1 ) singlet 1P wave state Spin singlet P wave (S=0, L=1). Hyperfine mass splitting is an indication of spin-spin interaction: M hf (1P) = M(h c ) 1/9(M(χ c0 )+3M(χ c1 )+5M(χ c2 )). E835 found evidence for h c in pp h c γη c. CLEO-c observed h c in ψ π 0 h c, h c γη c M hf (1P) = 0.08±0.18±0.12MeV/c 2. Slide 7

8 Measurement of h c (inclusive) E1-tagged BESIII Collaboration: PRL104, , (2010) Select inclusive π 0 in ψ' decays (ψ π 0 h c ) Select E1-photon in h c γη c (E1 tagged) or not (E1 untagged/semi-inclusive) E1-untagged E1-tagged selection gives M(h c )= ±0.13 ±0.18MeV ( M hf (1P)=0.10±0.13±0.18MeV/c 2 ) Γ(h c )=0.73±0.45 ±0.28MeV (<1.44MeV at 90% CL) Br(ψ ψ π 0 h c ) Br(h c γη c )= (4.58±0.40±0.50) 10-4 N(h c ) = 10353±1097 E1-untagged together with tagged selection gives the first measurement Br( ψ π 0 h c ) =(8.4±1.3±1.0) 10-4 Br( h c γ η c ) =(54.3±6.7±5.2)% Slide 8

9 Measurement of the h c (exclusive) Summed π 0 recoil mass Simultaneous fit to π 0 recoiling mass M(h c ) = ±0.11±0.15 MeV Γ(h c ) = 0.70±0.28±0.25 MeV N = 832±35 χ 2 /d.o.f. = 32/46 BESIII preliminary 16 decay modes are studied: ψ π 0 h c,h c γη c,η c X i X i : ppbar, 4π, 4K, 2π2K, ppbarππ, 6π, 2K4π, KKπ 0, ppbarπ 0, KsKπ, KsK3π, ππη, KKη, 4πη, 2π2π 0, 4π2π 0 Consistent with BESIII inclusive results PRL104,132002(2010) CLEOc exlusive results M(h c )= ±0.27±0.14 MeV/c 2 N = 136±14 PRL101, (2008) Slide 9

10 Precision measurement of the η c Mass and Width Slide 10

11 ψ γη γx c i The lowest lying S-wave spin singlet charmonium η c was discovered in 1980 by MarkII. Earlier experiments using J/ψ radiative transition gives M(η c )~2978.0MeV/c 2, Γ(η c )~10MeV. Recent studies using the two-photon processes gives M(η c )=2983.1±1.0 MeV/c 2, Γ(η c )=31.3±1.9 MeV. The most recent study from CLEO-c pointed out the distortion of the η c line shape in ψ decays. Measurement of the η c properties at BESIII Data sample: 106M ψ events, 44pb -1 continuum data at 3.65 GeV Decay modes X i : KsKπ, K + K π 0, ηπ + π, KsK3π, K + K π + πi H π 0 E, P 3(π + π ), where Ks π + π, η γγ, π 0 γγ Slide 11

12 Backgrounds for ψ γη c γx i ψ π 0 X i With the optimized selection, the mass spectra for π 0 X i events are measured in data and scaled according to the full simulation to estimate the contribution in γη c candidates. Non-resonant contribution ψ γx i exactly the same final states, can not be removed. Rare backgrounds Production rate or efficiency is very low, estimated based on the inclusive MC. Continuum events Estimated by using the 44pb -1 data taken at 3.65GeV. Slide 12

13 η c Slide 13

14 η c Slide 14

15 Mass spectrum Fitting PDF = Slide 15

16 The simultaneous Fit The η c mass, width and interference phase φ are constrained to be the same. KsKπ K + K π 0 π + π η KsK3π 2K2ππ 0 6π Simultaneous fit with modified Breit-Wigner (hindered I H E M1) P with considering interference between η c and non-η c decays Slide 16

17 Mass and Width of η c BESIII preliminary mass = ±0.5 stat ±0.6 syst MeV/c 2 width = 30.5±1.0 stat ±0.9 syst MeV Interference is necessary! The world average in PDG2010 was using earlier results. Slide 17

18 The first observation of the M1 transition ψ γη c (2S) Slide 18

19 η c (2S) (never( confirmed in M1 transition) First observation by Crystal Ball in 1982 (M=3.592, Br=0.2%- 1.3% from ψ γx, never confirmed by other experiments.) experimental challenge : search for real photons ~50MeV Published results about η c (2S) observation: Combined with the results based on two-photon processes from BaBar and Belle reported at ICHEP 2010, the world average Γ(η c (2S))=12±3 MeV. Decay mode studied: ψ γη c (2S) γkskπ (K + K - p 0 etc. in progress). Better chance with ~106M y data at BESIII. Slide 19

20 Mass fitting η c (2S) signal: Γ(η c (2S)) fixed to 12MeV (world average) BG from π 0 KsKπ: M1 transition χ cj : MC shape a Gaussian Measurement + scaling with MC simulation For convergence Fixed to the linear Extrapolation from σ(χ cj ) BG from ψ KsKπ(γ FSR ) & continuum (KsKπ(γ ISR )): Novosibirsk function With radiation Without radiation Ratio of the two is fixed in the final mass fitting Slide 20

21 Mass spectrum Fitting χ 2 /ndf=0.9 BESIII preliminary N(η c (2S)) = 50.6±9.7. Pure statistical significance more than 6σ. Significance with systematic variations is still more than 5σ. Slide 21

22 Preliminary measurements from ψ γη c (2S) γkskπ M(η c (2S))=3638.5±2.3 stat ±1.0 sys (MeV/c 2 ) Br(ψ γη c (2S) γkskπ)=(2.98±0.57 stat ±0.48 sys ) 10-6 Br(η c (2S) KK KKπ)=(1.9±0.4±1.1)% from BaBar Br(ψ γη c (2S))=(4.7±0.9 stat ±3.0 sys ) 10-4 CLEO-c: < (@90% CL) (PRD81,052002(2010)) Potential model: ( ) 10 4 (PRL89,162002(2002)) Slide 22

23 Future ~0.5 fb -1 at ψ(4040) taken already -Search for XYZ states -Hadronic transitions of ψ(4040) -Radiative transitions of ψ(4040) ~0.7 B-1.0 B ψ(2s) events are expected in 2012 Slide 23

24 Summary High luminosity by BEPCII and the good performance of BESIII give us better chance to study the chamonium spectroscopy. Study of h c at BESIII (inclusive & exclusive) gives the measurements of mass, width of h c as well as Br(ψ π 0 h c, h c γη c ). Precise measurement of the properties of η c done at BESIII. The observed distorted η c line shape could be described successfully by including an interfering non-resonant amplitude. Searching for η c (2S) in the M1 transition ψ γη c (2S). More results will come out soon at BESIII. Slide 24

25 International Workshop on Heavy Quarkonium 2011 GSI, Darmstadt, Germany Slide 25

26 Backups Slide 26

27 PDF= Mass spectrum fitting S: signal function (BW with mass width floated) Non: non-resonant γx i PDF (all assumed to 0 + ) BKG: the sum of other backgrounds π 0 X i + other rare ψ decays + continuum, fixed in the fitting φ: interference phase between η c decay and non-resonant contribution Fit results for individual modes: Interference Constant fitting gives χ 2 /ndf=5.142/5 Slide 27

28 Results of the fits for different modes KsKπ K + K π 0 π + π η KsK3π 2K2ππ 0 6π Slide 28

29 η the c, lightest charmonium state mass width PDG (2010) asymmetric symmetric LINESHAPE Kai Zhu OCPA7 29

30 η c lineshape from ψ π 0 h c, h c γη c Sum of 16 of η C decay modes Background subtracted The η C lineshape is not distorted in the h C γη C Detail analysis of η c parameters is ongoing! Symmetric lineshape in γγ production Asymmetric lineshape in I J/ψ H E Pdecay Slide 30

31 Mass spectrum representation The 4C kinematic fitting used to select the γkskπ candidates (χ 2 4C <50) Still some KsKπ BG events contribute the γkskπ candidates with a fake photon. The invariant mass from 4C-kinematic fits make the BG ψ KsKπ contaminates the η c (2S) mass region (3.6~3.66GeV). The mass from 3C-kinematic fits (the measured energy of the photon is free) is little biased by the fake photon. Difference small between 4C and 3C for signal events Inc. MC 3C 4C 3C η c (2S) MC 4C So the 3C fit mass used to determine the yields and parameters Slide 31

32 More consistency checks Difference between the BG estimation and mass fitting DN=12±14 Branching ratios for ψ γχ cj γkskπ From this analysis (stat. err. only) The distributions of the selected photons M KsKp (3.6, 3.66) GeV/c 2 : Τ γ E γ θ γ E 3x3 /E 5x5 Slide 32