D D Shape. Speaker: Yi FANG for BESIII Collaboration. The 7th International Workshop on Charm Physics May 18-22, 2015 Detroit, Michigan

Transcription

1 D D Shape Speaker: Yi FANG for BESIII Collaboration The 7th International Workshop on Charm Physics May 18-22, 2015 Detroit, Michigan Y. Fang (IHEP) D D Shape Charm / 16

2 Outline 1 Introduction 2 Analysis 3 Summary Y. Fang (IHEP) D D Shape Charm / 16

3 Introduction Introduction Since the discovery of ψ(3770), it is a long-standing puzzle in understanding of ψ(3770) production and decays Includes interference between resonant BES-II - e + e hadrons PLB 652, 238 (2007) and nonresonat DD production ±0.4±0.3 - e + e hadrons PLB 660, 315 (2008) ±1.9 Belle B D D K PRL 100, (2008) ±5.0±4.0 BABAR - e + e γdd PRD 76, 11105(R) (2007) ±1.9±0.9 KEDR PDG 2014 B DDK PRD 77, (2008) - e + e hadrons PLB 711, 292 (2012) ±2.4± ± (MeV) M ψ(3770) Discrepant results of ψ(3770) parameters are observed Model? Interference? Analyze D D shape around ψ(3770) at BESIII with higher statistic Y. Fang (IHEP) D D Shape Charm / 16

4 Introduction BESIII Experiment BEPCII Collider symmetric e + e collider, double-rings, 2.0 GeV < E CM < 4.6 GeV BESIII Detector Y. Fang (IHEP) D D Shape Charm / 16

5 Introduction Data Sets ψ(3770) Scan Data 70 pb 1, 3.74 < E CM < 3.89 GeV Luminosity (L) is determined using large-angle Bhabha scattering events 8 6 ) -1 L (pb E CM (GeV) Monte Carlo Simulation 1 ψ(3770) D 0 D0, D + D 2 e + e q q, τ + τ, γ ISR J/ψ, γ ISR ψ(2s) Y. Fang (IHEP) D D Shape Charm / 16

6 Reconstruction of D Mesons The cross sections for e + e D D are measured using single tag method Tag modes (charge conjunction is implied): D 0 K π + D + K π + π + D + KS 0π+ π 0 D 0 K π + π 0 D + K π + π + π 0 D + KS 0π+ D 0 K π + π + π D + K + K π + D + KS 0π+ π + π Define variables: E = E tag E beam m BC = E 2 beam p tag 2 Select D candidate with E closest to 0 for each mode Y. Fang (IHEP) D D Shape Charm / 16

7 Extraction of Signal Yields Signal yield (N tag ) is extracted from a maximum-likelihood fit to the 2D distribution in E vs. m BC Signal and background PDFs are formed from MC simulation Float normalization of signal and background Data Signal - - Background Fit Y. Fang (IHEP) D D Shape Charm / 16

8 Changes in m BC Shapes due to ISR Effect Two peaks seen in higher beam energy m BC distribution Data Signal - - Background Fit Left Peak Events from Born Level contribution Right Peak Events from ISR contribution Y. Fang (IHEP) D D Shape Charm / 16

9 Cross Section Reconstruction Efficiency Tag efficiency (ɛ i ) for each mode is determined from MC simulation via ɛ i = Ni found /N generated i and then weighted by the branching fraction of mode i (B i ) to obtain the average efficiency ɛ = i ɛ i B i ɛ 0 tag = (11.3 ± 0.2)%, ɛ + tag = (9.8 ± 0.1)% (the branching fractions of sub-resonance decays are included) Calculation of Cross Section σ RC D D (E i) = N tag(e i ) 2ɛ tag L(E i ) Y. Fang (IHEP) D D Shape Charm / 16

10 Fits to Cross Sections Fit the measured D 0 D 0 and D + D cross sections simultaneously using the theoretical cross sections σ RC D D (W ) = z D D(W 1 x)σ D D(W 1 x)f(x, W 2 )dx z D D Factor describing Coulomb interaction F(x, s) Probability to lose a fraction of s in initial state radiation σ D D (W ) = π2 α 3W 2 β3 D F D(W ) 2, F D (W ) = F R D (W )eiφ R + F NR D (W ) Use Breit-Wigner formula for resonant component 6W (Γ ee /α 2 )(Γ FD R (W ) = D D(W )/β 3 D ) M 2 W 2, Γ imγ (W ) D D (W ) = Γ (W ) (1 B nd D ) Analyze two models for non-resonant component Exponential Model - FD NR(W ) = F NR exp( qd 2 /α2 NR ) Vector Dominance Model (VDM) - FD NR ψ(2s) (W ) = FD (W ) + F 0 Y. Fang (IHEP) D D Shape Charm / 16

11 Fits to Cross Sections Single Breit-Wigner Shape Single Breit-Wigner formula is unable to describe data Y. Fang (IHEP) D D Shape Charm / 16

12 Fits to Cross Sections Exponential Model Y. Fang (IHEP) D D Shape Charm / 16

13 Fits to Cross Sections Vector Dominance Model Y. Fang (IHEP) D D Shape Charm / 16

14 Preliminary Results and Comparisons ψ(3770) D D Use Γee = Γee ψ(3770) B(ψ(3770) D D) Remains constant from fit independent of branching fraction Preliminary results of ψ(3770) parameters (errors are only statistical) Preliminary results of ψ(3770) parameters are consistent with those measured at KEDR Y. Fang (IHEP) D D Shape Charm / 16

15 Systematics Expect statistics-limited result due to scan data size Systematics evaluation still in progress Current main sources (ranked by contribution to total) 1 Meson radii used for ψ(2s) and ψ(3770) 2 Charged tracking 3 Neutral tracking 4 Luminosity Negligible effect seen when altering B(ψ(3770) D D) All parameters remain constant except Γ ψ(3770) ee Scales inversely to input branching fraction Y. Fang (IHEP) D D Shape Charm / 16

16 Summary Summary Cross sections for e + e D D in the vicinity of the ψ(3770) are studied at BESIII Able to well fit the D D line shape near ψ(3770) with interference based models Both exponential model and vector dominance model provide quality description of data Upcoming aspects 1 Finalize estimation of systematic uncertainty 2 Compare to alternate models Y. Fang (IHEP) D D Shape Charm / 16