Measurement of Observed Cross Sections for e + e hadrons non-d D. Charmonium Group Meeting

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1 Measurement of Observed Cross Sections for e + e hadrons non-d D YI FANG, GANG RONG Institute of High Energy Physics, Beijing 0049, People s Republic of China March 22, 207 Charmonium Group Meeting Y. Fang (IHEP) e + e hadrons March 22, 207 / 27

2 Outline Introduction 2 Measurement of Cross Sections Event Selection Background Subtraction Efficiency Cross Sections 3 Analysis of Cross Sections Expected Cross Section Results 4 Conclusion Y. Fang (IHEP) e + e hadrons March 22, / 27

3 Introduction The BES-II previously observed an anomalous line shape of the cross section for e + e hadrons [Phys. Rev. Lett., 2004 (2008)]. This anomalous line shape more likely indicates that there are new structure(s) in addition to the ψ(3770) around GeV or some dynamics affect ψ(3770) and D D production and decays. S. Dubynskiy and M. B. Voloshin interpret this anomalous line shape of cross section as a diresonance, which may arise from existence of both a charmonium state and a molecular D D threshold resonance [Phys. Rev. D 78, 604 (2008)]. However, this observation of the line shape need to be confirmed at other experiment. Experimental study of the process of e + e annihilating into hadrons in this energy range with a larger data set at the BESIII experiment will provide some important information to elucidate this anomalous line-shape. Y. Fang (IHEP) e + e hadrons March 22, / 27

4 Data and MC Samples Data sets: 70 pb energy scan data taken in range from 3.64 to 3.87 GeV MC samples: Process J/ψ hadrons ψ(3686) hadrons D 0 D 0, D + D ψ(3770) non-d D e + e q q e + e e + e, µ + µ, γγ e + e τ + τ e + e e + e f f Generator KKMC+BESEVTGEN KKMC+BESEVTGEN KKMC+BESEVTGEN KKMC+BESEVTGEN KKMC BABAYAGA KKMC BESTWOGAM For inclusive decays of the J/ψ, ψ(3686) and ψ(3770), the known decay modes are generated by EVTGEN with branching fractions taken from PDG, while the remaining unknown decay modes are modeled by LUNDCHARM Software environment: BOSS p0 Y. Fang (IHEP) e + e hadrons March 22, / 27

5 Event Selection At least three good charged tracks V r <.0 cm, V z < 5.0 cm, cos θ < 0.93 To suppress the Bhabha and dimuon backgrounds, require the ratio of Fox-Wolfram moments H 2 /H 0 < 0.85 Fraction of Events / e e hadrons e e e e e e µ µ H 2 /H Fraction of Events / e e hadrons e e e e l l e e e e hadrons miss p /E z vis To suppress the two-photon and beam-gas events, require the ratio pz miss /E vis < 0.30 p miss z : the z component of the missing momentum of the event E vis : the visible energy of the event Y. Fang (IHEP) e + e hadrons March 22, / 27

6 Event Selection In order to separate some beam-gas associated background events and cosmic-ray events, which are suspected to be produced at random Z positions in the beam pipe, we can examine the averaged Z position (Z aver) of the charged tracks. The numbers of observed hadronic events are extracted from maximum likelihood fits to the Z aver distributions. Events / ( 0.2 cm ) (cm) Z aver s = GeV N obs N obs N obs N obs ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 86 Y. Fang (IHEP) e + e hadrons March 22, / 27

7 Background Subtraction Background sources QED processes: e + e e + e, µ + µ, τ + τ, γγ Two-photon interactions: e + e e + e e + e, e + e µ + µ, e + e τ + τ e + e e + e uū, e + e d d, e + e s s The number of background events ( ) N bkg = L σ bkg, i η i i L: luminosity σ bkg : theoretical cross section of background process η: contamination rate Y. Fang (IHEP) e + e hadrons March 22, / 27

8 Background Subtraction QED Processes The cross sections of e + e e + e, µ + µ, γγ are taken from the outputs of MC generators. To calculate the e + e τ + τ cross section, the effects of ISR, FSR, Coulomb interaction, ψ(3686) production and interference are considered [Phys. Rev. D 74, 2003 (2006)]. Cross Section (nb) e + e e + e - - e + e τ + τ - - e + e µ + µ - e + e γγ η (%) e + e e + e - - e + e τ + τ - - e + e µ + µ - e + e γγ The events selection criteria applied to data is also applied to the MC simulated background events. The result of fitting a polynomial to these contamination rates is used for further calculations Y. Fang (IHEP) e + e hadrons March 22, / 27

9 Background Subtraction Two-Photon Processes Cross Section (nb) η (%) e + e e + e e + e - - e + e e + e uu e + e µ e e µ - e + e e + e τ + τ - - e + e e + e dd - - e + e e + e ss The cross sections of e + e e + e f f (f = e, µ, τ, u, d, s) are obtained from MC simulation. The events selection criteria applied to data is also applied to the MC simulated background events. The result of fitting a polynomial to these contamination rates is used for further calculations. Y. Fang (IHEP) e + e hadrons March 22, / 27

10 The D D Contributions The number of e + e D D events is given by N D D = Lσ D Dε D D. Here σ D D is measured using singly tagged D events. D 0 K π + D + K π + π + To reduce the statistic, the values expected from the fit are used. (nb) DD σ e e D D e + e D D (%) DD D D + - D D The efficiencies ε D D are determined from MC simulation. The results of fitting a polynomial to these efficiencies are used for further calculations. Y. Fang (IHEP) e + e hadrons March 22, 207 / 27

11 Efficiencies The efficiency for hadronic events is obtained by analyzing the MC simulated signal events. We first determine the detection efficiency for each category of hadronic events. To reduce MC statistic, the results of fitting a polynomial to these efficiencies are used for further calculations. Efficiency (%) Cross Section (nb) 3 2 J/ ψ ψ(3686) ψ(3770) ndd qq 40 J/ ψ ψ(3686) ψ(3770) ndd qq We then weight these efficiencies according to their cross sections to obtain the overall efficiency. Y. Fang (IHEP) e + e hadrons March 22, 207 / 27

12 Efficiencies (%) had-ndd The detection efficiencies for e + e hadrons non-d D as a function of the CM energy Y. Fang (IHEP) e + e hadrons March 22, / 27

13 Cross Sections (nb) σ obs had-ndd The observed cross section for e + e hadrons non-d D is given by 22 σ obs had-nd D (E cm) = N obs(e cm ) N bkg (E cm ). L(E cm ) ε(e cm ) 26 Analysis of this observed cross 24 section needs the expectedobserved cross sections for this final state, which can be obtained with BW function and ISR sampling function Y. Fang (IHEP) e + e hadrons March 22, / 27

14 Systematic Uncertainty Source Systematic uncertainty (%) Charged track multiplicity.4 R 2 cut 0.5 p miss z /E vis cut 0.9 Fit to the V z distribution 0.8 Luminosity.0 Sum in quadrature 2.2 Y. Fang (IHEP) e + e hadrons March 22, / 27

15 Expected Cross Section The cross section of e + e hadrons nd D can be expressed as where s E 2 cm. For resonances, one has σ exp (s) = σ exp (s) + σexp (s) + σexp J/ψ ψ(3686) Rs (3770) (s) + σ q q(s), σ exp R (s) = σ 0 (s ( x))f(x, s )G(s, s)dxds, where s s( x), x is the fraction of the radiative energy to the beam energy, F(x, s ) is the sampling function, G(s, s) is a Gaussian function describing the distribution of the e + e collision energy. We use a Breit-Wigner function to calculate the dressed cross section: σ 0 (s) = 2πΓ ee ΓB had (s M 2 ) 2 + (MΓ) 2, where Γ ee and Γ are respectively the leptonic width and total width of the resonance, M is the mass and B had denotes the decay branching fraction. The cross section for light hadron production is parameterized as σ q q (s) = f q q σ B µ + µ (s), where σ B µ + µ (s) is the Born cross section of e + e µ + µ and f q q is a free parameter. Y. Fang (IHEP) e + e hadrons March 22, / 27

16 Fitting Procedure We use the least squares method to fit the experimental data. The following function is minimized: χ 2 = i ( σ obs (E cm,i ) σ exp (E cm,i ) (σ obs (E cm,i )) )2, where i is the energy point number, (σ obs (E cm,i )) is the sadistical uncertainty of the measured cross section and σ exp (E cm,i ) is the predicted cross section. The resonance parameters of J/ψ and ψ(3686) are fixed at PDG206 values in the fit. The energy spread is fixed at σ E =.37 MeV. For the R s (3770) resonance(s), we use one or two Breit-Wigner amplitude(s) to fit the observed hadronic cross sections. Y. Fang (IHEP) e + e hadrons March 22, / 27

17 Fit Results Scenario Assuming that there is only ψ(3770) (nb) had-ndd σ obs M (MeV) (fixed) Γ (MeV) 27.2 (fixed) Γ ee B(ψ(3770) nd D) (ev) ± 3.9 f q q ± χ 2 /N dof 64.6/ Y. Fang (IHEP) e + e hadrons March 22, / 27

18 Fit Results Scenario (nb) 3 had-ndd σ obs The observed cross sections after subtracting contributions from J/ψ, ψ(3686) and continuum light hadron production Y. Fang (IHEP) e + e hadrons March 22, / 27

19 Fit Results Scenario 2 2 Assuming that there are two structures around GeV (nb) had-ndd σ obs M (MeV) Γ (MeV) Γ ee B(R nd D) (ev) M 2 (MeV) Γ 2 (MeV) Γ ee 2 B(R 2 nd D) (ev) f q q χ 2 /N dof 24.6/30 The signal significance of the diresonance is 5.3σ (from analyzing the observed cross sections). Y. Fang (IHEP) e + e hadrons March 22, / 27

20 Fit Results Scenario 2 (nb) 3 had-ndd σ obs The observed cross sections after subtracting contributions from J/ψ, ψ(3686) and continuum light hadron production Y. Fang (IHEP) e + e hadrons March 22, / 27

21 Systematic Uncertainties We first vary the measured cross sections, parameters of J/ψ, ψ(3686), and energy spread by ±σ, then repeat the fit procedure. The shifts of the parameters are taken as systematic uncertainties. Source M Γ tot Γ ee B R nd D M 2 Γ tot 2 Γ ee 2 B R 2 nd D [MeV] [MeV] [ev] [MeV] [MeV] [ev] σ had obs measurement M J/ψ Γ tot J/ψ Γ ee J/ψ M ψ(3686) Γ tot ψ(3686) Γ ee ψ(3686) Energy spread Sum in quadrature f q q Y. Fang (IHEP) e + e hadrons March 22, / 27

22 Comparison (nb) had-ndd σ obs Parameter This work BES-II M (MeV) ± ±.8 ± 0.5 Γ tot (MeV) ± ± 32. ± 0. M 2 (MeV) ± ±.3 ± 0.5 Γ tot 2 (MeV) ± ± 3. ± 0. This analysis confirms (at 5.3σ) the BES-II observation of Di-Structure R s (3770) in the range from 3.7 to 3.87 GeV. Y. Fang (IHEP) e + e hadrons March 22, / 27

23 Conclusion The cross sections of e + e hadrons nd D are measured in the energy range from 3.64 to 3.87 GeV at the BESIII experiment. We observed an enhancement of non-d D hadron production in the range from 3.74 to 3.80 GeV, which confirms the anomalous line shape of cross sections for e + e hadrons observed at the BES-II experiment. To well describe the line-shape of these observed cross sections, one more BW amplitude additional to the ψ(3770) resonance is needed. By analyzing these observed cross sections, We obtain the parameters of the Di-structures: M (MeV) ± 0.2 Γ (MeV) ± 0.5 Γ ee B(R non-d D) (ev) ±.5 M 2 (MeV) ± 0. Γ 2 (MeV) ±.4 Γ ee 2 B(R 2 non-d D) (ev) ± 4.2 Y. Fang (IHEP) e + e hadrons March 22, / 27

24 Backup Slides Y. Fang (IHEP) e + e hadrons March 22, / 27

25 Fox-Wolfram Moments For a collection of N particles with momenta p i, the k-th order Fox- Wolfram moments H k is defined as H k = N p i p j P k (cos θ ij ), i,j where θ ij is the angle between p i and p j, and P k is the k-th order Legendre polynomial. Y. Fang (IHEP) e + e hadrons March 22, / 27

26 Systematic Uncertainty Multiplicity Requirement The systematic uncertainty due to the dependence on the charged track multiplicity is estimated by varying the requirement from N ch 3 to N ch 2. Additional cuts on N ch = 2 events: The charged track must not be identified as an electron or a muon: E EMC /p < 0.7 and depth MUC < 30 cm The visible energy of event: E vis > 0.3 E cm The relative changes of σ obs are less than.4%, which is taken as had-nd D the systematic uncertainty. 2 H 2 /H 0 cut To estimate the systematic uncertainty due to the H 2 /H 0 cut, we vary this cut from its nominal level of 0.85 down to The relative changes of σ obs are less than 0.5%, which is taken as had-nd D the systematic uncertainty. 3 pz miss /E vis cut To estimate the systematic uncertainty due to the cut on pz miss /E vis, we vary this cut from its nominal level of 0.30 up to The relative changes of σ obs are less than 0.9%, which is taken as had-nd D the systematic uncertainty. Y. Fang (IHEP) e + e hadrons March 22, / 27

27 Systematic Uncertainty 4 Fit to V z distributions To estimate the uncertainties due to the fits to the V z distributions, we refit the V z distributions by varying bin size, fit range ( [ 8, 8] cm, [ 2, 2] cm ), background PDF (first, third, fourth order polynomial), and signal PDF (a core Gaussian plus two exponential tails, double-gaussian). Item Systematic uncertainty (%) Bin size 0.0 Range 0.2 Background PDF 0.7 Signal PDF 0.4 Sum in quadrature Luminosity.0% [Chin. Phys. C 37, 2300 (203)] Y. Fang (IHEP) e + e hadrons March 22, / 27