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Measurement of non BB Decays of Y(4S) to Y(1S)  +   and Y(2S)  +  

Measurement of non BB Decays of Y(4S) to Y(1S)  +   and Y(2S)  +  . Silvano Tosi Universit à & INFN Genova. Contents. Motivations. The B A B AR experiment at SLAC. Event selection. Validation and systematic studies. Measurement of partial widths and dipion masses.

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Measurement of non BB Decays of Y(4S) to Y(1S)  +   and Y(2S)  +  

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  1. Measurement of non BB Decays of Y(4S) to Y(1S)+ and Y(2S)+ Silvano Tosi Università & INFN Genova

  2. Contents • Motivations. • The BABAR experiment at SLAC. • Event selection. • Validation and systematic studies. • Measurement of partial widths and dipion masses. Published in PRL 96, 232001

  3. Motivations • Dominant decays of Y(4S) are to BB. • …but decays to other bottomonium states or light hadrons are expected with BR~10-4. • Comparison of partial widths and dipion spectra with QCD multipole expansion. • Works successfully for (2S)J/, Y(mS)Y(nS) (m>n). • But doesn’t work for dipion spectrum in Y(3S)Y(1S). Other effects (mixing, coupled-channels …) ? • Non DD decays of (3770) recently observed with BR~10-3: ccg, J/ypp (CLEO,BES). • Previous measurements: BR(Y(4S)Y(1S)+)<1.210-4 BR(Y(4S)Y(2S)+)<3.910-4 BR(Y(4S)Y(1S)+)=(1.0±0.2±0.4) 10-4 e.g. PRD 24, 2874 PLB 605, 63, PRD 73, 012002, hep-ex/0509030 PRD 59,052003 Preliminary evidence!hep-ex/0512034

  4. The BABAR Experiment at PEP-II Electromagnetic Calorimeter (EMC) Solenoid 1.5T Measurement of electron and photon energies (E)/E=1.33%E-1/42.1% Muon Detector (IFR) z e+ (3.1 GeV) e+e- CM energy ~ 10.58 GeV Boost: bg~0.56 e- (9 GeV) Detector of Internally Reflected Cherenkov Light (DIRC) Silicon Vertex Tracker (SVT) • Data sample: • Here used 211 fb-1 taken at the Y(4S) peak and 22 fb-1 taken 40 MeV below. • So far integrated ~350 fb-1. Vertex and trajectory measurements + dE/dx Efficiency 97% Particle identification (PID) through Cherenkov radiation. Separation K- >3.4 for p<3.5GeV/c Drift Chamber (DCH) Momentum measurement for charged particles + dE/dx (pT)/PT=0.13%PT+0.45%

  5. m+ p+ e+ Y p- e- m- Analysis Overview • Y(4S)Y(1S)+and Y(2S)+, with Y(1S,2S)  + (= e, m ) • BR(Y(1S)  + )~2.4%; BR(Y(2S)  + )~1.3% • Smaller sensitivity of e-channels: larger background, trigger-level inefficiency (pre-scaling of Bhabhas)  focus onm+m • Use 2S 1S and 3S1S,2S transitionsin ISR events as control samples • Validation of simulation and event selection; • Cross-check of event yields; • Validation of m(+) distributions and systematic studies. • Simulated signal events include Y(1S,2S) polarizations, used phase-space for dipion transitions. • Signal regions in data not looked at until selection finalized • Sidebands used to understand backgrounds.

  6. Event Selection • Signal signature: • events with 4 charged tracks from a common vertex and with net charge zero. • two oppositely-charged tracks identified as muons in EMC and IFR • CM momenta of muons greater than 4 GeV/c • transverse momenta of pions greater than 100 MeV/c • m(m+m ) compatible with known Y(1S,2S) mass • mass resolution ~ 75 MeV/c2 • DM=m(+ m+m ) - m(m+m ) compatible with m(Y(mS))-m(Y(nS)) • mass resolution ~ 7 MeV/c2 • CM momentum (p*) consistent with 0 for Y(4S). • Same criteria (except p*) for the Y(2S) and Y(3S) ISR control samples.

  7. Additional Selections • Major remaining background is from m+m-g with photon conversion to e+e-. Removed Y candidates for which: • either pion positively identified as electron; • m(e+e-) < 100 MeV/c2; • dipion opening angle cos(qp+p-) > 0.95 • Additional requirement for +Y( e+e-): • q (e-)> 0.75 rad to remove Bhabhas.

  8. Signal Extraction (I) • Select events with |m(+-)-m(Y(1S))| < 200 MeV/c2 and |m(+-)-m(Y(2S))| < 150 MeV/c2. • Unbinned extended maximum likelihood fit to DM: • background: linear shape; • signal: Gaussian (s)  Cauchy (width G) • s andG from MC; verified on control samples; m+m- channel • peaks of DM found to be in agreement with world averages: 4S1S: (1.1185 ± 0.0009) GeV/c2 4S2S: (0.5571 ± 0.0010) GeV/c2 Notice: not a mass measurement!

  9. Signal Extraction (II) Statistical significance • Signal yields are consistent with expectations for Y(3S) and Y(2S) control samples. • No Y(4S) signal observed in off-peak data.

  10. Selection Efficiency • Evaluated on MC. • Largest systematics: • unknown dipion invariant mass: • by comparing acceptance for phase-space to what obtained with QCD multipole model  10% • uncertainty in tracking efficiency: 1.3% per track; • selection cuts: 4.3% (from ISR control samples); • muon-ID: 1.4% (from ISR control samples); • signal and background parameterizations: negligible; • choice of fit ranges: negligible. e (m+m-) = 32.5 ± 3.9% (4S  1S); 24.9 ± 3.0% (4S  2S)

  11. Results (I) N(4S) = (230.0 ± 2.5) 106 B(4S1S ) B(1Smm) = (2.23  0.25 stat  0.27syst)10-6 B(4S2S ) B(2Smm) = (1.69  0.26 stat  0.20syst)10-6 PRD 72, 032005 • Using world average for B(nS  m+m-) and (Y(4S)) = (20.7 ± 3.0) MeV:

  12. Results (II) PRL 94, 012001 • Using CLEO’s recent measurement of B(Y(2S)  m+m-): B(Y(4S)+-Y(2S) ) = (0.830.16)10-4 G(Y(4S)+-Y(2S) ) = (1.70.5) keV • The e+e- channels (not used) give consistent results. • The partial widths of Y(4S) to Ypp are comparable to other dipion transitions among the bottomonium states (few keV). • The branching fraction B(Y(4S)Y(1S)+) is in agreement with Belle’s preliminary result and with CLEO’s upper limit.

  13. Dipion Invariant Mass (I) QCD multipole model Efficiency Data (efficiency corrected) m(p+p-) resolution ~ 5 MeV/c2 • Fit to DM in equal ranges of m(p+p-). • Divide the number of signal events in each bin by the corresponding selection efficiency. • The 4S1S transition is reasonably compatible with the QCD multipole expansion model. • The 4S2S transition is not in agreement.

  14. Dipion Invariant Mass (II) • Something special when Dn=2? Belle 4S-1S Belle 2S-1S Belle 3S-1S CLEO 2S-1S CLEO 3S-1S preliminary preliminary Dn=1 Dn=2

  15. Conclusions • Reported first measurement of non BB decays of Y(4S) to Y(1S,2S)pp. • Branching ratios and partial widths compatible with expectations from other Y(nS) states and previous results. • Dipion spectrum for 4S2S incompatible with QCD multipole expansion model. • Published in Phys. Rev. Lett.

  16. Backup Slides

  17. Check on Off-peak Data • On off-peak data (40 MeV below the 4S peak): • di-muon: • 19 Y(1S)+ candidates with |s-M(1S)|<20 MeV with expected background of 18.1±2.8 • 14 Y(2S)+ candidates with |s-M(2S)|<20 MeV with expected background of 13.1±2.4 • di-electron: • 50 Y(1S)+ candidates with |s-M(1S)|<20 MeV with expected background of 63.3±5.2 • 14 Y(2S)+ candidates with |s-M(2S)|<20 MeV with expected background of 13.5±2.4

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