Observation of non-BBar decays of (4S)p+p- (1S, 2S) Bryan Fulsom For the BaBar Collaboration APS-DPF 2006 + JPS 2006 October 31, 2006.
Presentation Outline Motivation for search Event reconstruction and discriminating variables Fit method Validation on ISR (initial state radiation) data Calculation of systematics Results and interpretation Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Motivation (4S) decays nearly 100% to BBar Predict decay to other bottomonium states with branching fraction O(10-4) Hadronic transitions in other heavy quarkonia are well known: ie: y(2S)p+p- J/y, (mS) p+p- (nS) for m=2, 3 Theory from QCD multipole expansion makes predictions for the partial widths and dipion spectrum but fails to describe dipion spectrum in 3S1S Experimental results for (4S) p+p- (1S) CLEO: B((4S) p+p- (1S)) < 1.2 x 10-4 B((4S) p+p- (2S)) < 3.9 x 10-4 Belle (conference result): B((4S) p+p- (1S)) = (1.1+/-0.2+/-0.4) x 10-4 BaBar: Observation of both decay modes! Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Bottomonium Spectrum Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Analysis Outline Search for (mS) p+p- (nS), reconstructing (nS)l+l- Kinematic quantities of interest: Invariant mass M(l+l-), equal to M(nS) DM=M(p+p-l+l-) – M(l+l-), equal to M(mS) – M(nS) Fit to DM in regions of M(l+l-) to extract number of signal events Use Monte Carlo to simulate signal events of (mS) p+p- (nS) Includes ISR events with m = 2, 3 ISR (2S) p+p- (1S) and (3S) p+p- (1S, 2S) events from data for validation of MC, event yields, dipion distribution, and determining systematics DM 20MeV M(m+m-) 200 MeV Background events taken from data sidebands: DM 60 MeV M(m+m-) 300 MeV Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Event Reconstruction BaBar dataset: 211 fb-1 “on-peak”, 230M (4S) events 22 fb-1 “off-peak”, approximately 40 MeV below (4S) resonance Reconstruct (mS) p+p- (nS), (nS) l+l- Require at least four charged tracks originating from same vertex Discard candidates with small opening angle between charged tracks 9.0<M(l+l-)<10.5 GeV, 9.0<M(p+p- l+l-)<11.2 GeV In cases of multiple candidates, retain best reconstructed vertex c2 probability Focus mainly on (nS)m+m- (nS)e+e- has poor efficiency and higher backgrounds Dominant background is m+m-g Lesser contributions from e+e-g and ISR 4p Select cuts to discriminate between signal and background events, avoid any bias in angular and dipion invariant mass distributions Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Discriminating Variables cosqp+p-: Opening angle between the two pions Strongly correlated to dipion invariant mass Loose cut to avoid biasing M(p+p-) Mee: Invariant mass of pion candidates assuming electron hypothesis Correlated to cosqp+p-, hence M(p+p-) Loose cut to avoid bias Nveto: Number of pion candidates vetoed as electrons Pions that also pass PID as electrons Strictly require pion candidates to not be electrons p*expected: Centre of mass momentum of (mS) candidate Defined as: For ISR events, use Expected to have a value of 0 for signal events Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Summary of Cuts Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Data Distribution Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Data Distribution Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Data Distribution Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Fit Method Perform 1D unbinned extended maximum likelihood fit to DM Fit region: |M(m+m-) – (nS)| < 200 MeV/c2 (1S), < 150 MeV/c2 (2S) DM 40 MeV Parameterize the signal PDF on signal MC Attempt Gaussian, double-Gaussian, Breit-Wigner, Voigtian (B-W & Gaussian) Voigtian has best c2/d.o.f. for (4S)p+p- (1S, 2S) and ISR MC modes Use linear background, allow Nsig and DMsig to float Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Validation (ISR & Off-Peak) ISR event yield from data compares well with expected from known BFs and MC efficiency Ratio of (3S) (2S) / (3S) (1S) compatible with CLEO Ratio of ISR yield for e+e- / m+m- modes equal to 1 within error ISR dipion distribution matches previous measurements (see plots) Perform fit to off-peak data Number of (4S) events consistent with zero Size of background from on-peak fit consistent with yield in off-peak Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Systematics Number of (4S) events / luminosity BaBar B counting uncertainty 1.2%, (4S) should be equivalent Tracking Reconstruction efficiency uncertainty in BaBar estimated at 1.3% per track Muon PID Compare efficiency of particle ID selector in MC sample to ISR data control sample, find 0.7% systematic error per muon Acceptance (Uncertainty in angular distribution) Unknown dipion invariant mass distribution Estimate by comparing yield assuming phase space distribution (in MC) to yield using m(p+p-) dependent efficiency from multipole expansion As large as 10% based on ISR control samples Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Systematics (continued) Event Selection Vary the cut selection and measure the stability of the yield Release cut on p*, release all cuts but p*, widen dimuon mass window, impose helicity and angular cuts Test on ISR control sample, take maximum discrepancy in yield for each test in quadrature (total of ~4.3%) Signal Parametrization Allow PDF parameters to float individually, or try other parameterizations Compare yield with background subtracted “cut and count” number of events in 20 MeV window Negligible effect Background Parametrization Compare yield using linear versus quadratic function for background Systematic less than 2% Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Summary of Systematics Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Signal Fit Results Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Results Significance determined by: Efficiency is percentage of events returned by fit from total generated in MC Branching fraction derived from: Significant observation in m+m- mode, e+e- less compelling Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Results (continued) Use experimental values of B((nS)m+m-) to calculate B((4S)p+p- (nS)) Experimental values for B((nS)m+m-) from PDG G derived from BaBar measurement of G((4S)) Branching fraction for 4S1S compatible with preliminary result from Belle Partial widths are comparable to other bottomonium dipion transitions, O(keV) Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Dipion Invariant Mass Spectrum Shape of 4S1S distribution close to expected from theory 4S2S transition has a low M(p+p-) enhancement...unexpected! Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Summary Search for non-BBbar decays of (4S) Observe bottomonium decays (4S) p+p- (nS), (nS)m+m-, where n = 1, 2 Significance: 10.0s for 4S1S, 7.3s for 4S2S Measure product of branching fractions: Branching ratios and partial widths are compatible with theoretical predictions and previous experimental limits and results in the bottomonium system Dipion spectrum for 4S2S is incompatible with QCD multipole expansion Results published in: Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
Backup Slides Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.
The BaBar Detector Observation of non-BBar decays of U(4S)p+p-U(1S, 2S) Bryan Fulsom, DPF 2006, October 31, 2006.