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

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

3 Motivations Dominant decays of Y(4S) are to BB. …but decays to other bottomonium states or light hadrons are expected with BR~ 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 :  c , J/  (CLEO,BES). Previous measurements: BR(Y(4S)  Y(1S)     )<1.2  BR(Y(4S)  Y(2S)     )<3.9  BR(Y(4S)  Y(1S)     )=(1.0±0.2±0.4)  PRD 59, e.g. PRD 24, 2874 PLB 605, 63, PRD 73, , hep-ex/ Preliminary evidence! hep-ex/

4 The B A B AR Experiment at PEP-II Detector of Internally Reflected Cherenkov Light (DIRC) Solenoid 1.5T Electromagnetic Calorimeter (EMC) Drift Chamber (DCH) Muon Detector (IFR) Silicon Vertex Tracker (SVT) e + (3.1 GeV) e - (9 GeV) Measurement of electron and photon energies  (E)/E=1.33%E -1/4  2.1% Particle identification (PID) through Cherenkov radiation. Separation K-  >3.4  for p<3.5GeV/c Momentum measurement for charged particles + dE/dx  (p T )/P T =0.13%P T +0.45% Vertex and trajectory measurements + dE/dx Efficiency 97% z 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. e  e  CM energy ~ GeV Boost:  ~0.56

5 Analysis Overview Y(4S)  Y(1S)     and Y(2S)    , with Y(1S,2S)    ( = e  ) –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 on     Use 2S  1S and 3S  1S,2S transitions in 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. ee ee     Y

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(     ) compatible with known Y(1S,2S) mass mass resolution ~ 75 MeV/c 2 –  M=m(         )  m(     ) compatible with m(Y(mS))-m(Y(nS)) mass resolution ~ 7 MeV/c 2 –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      with photon conversion to e  e . Removed Y candidates for which: either pion positively identified as electron; m(e  e  ) < 100 MeV/c 2 ; dipion opening angle cos(   ) > 0.95 Additional requirement for     Y(  e  e  ):  (e  ) > 0.75 rad to remove Bhabhas.

8 Signal Extraction (I)     channel Select events with |m( +  )- m(Y(1S))| < 200 MeV/c 2 and |m(   )-m(Y(2S))| < 150 MeV/c 2. Unbinned extended maximum likelihood fit to  M: background: linear shape; signal: Gaussian (  )  Cauchy (width  )  and  from MC; verified on control samples; peaks of  M found to be in agreement with world averages: 4S  1S: ( ± ) GeV/c 2 4S  2S: ( ± ) GeV/c 2 Notice: not a mass measurement!

9 Signal Extraction (II) Statistical significance 4S  1S4S  2S N sig n  N sig n   167± ±157.3 eeee 74± ±112.5 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.  (     ) = 32.5 ± 3.9% (4S  1S); 24.9 ± 3.0% (4S  2S)

11 Results (I) N(4S) = (230.0 ± 2.5)  10 6 Using world average for B(nS      ) and  (Y(4S)) = (20.7 ± 3.0) MeV: PRD 72, B(4S  1S )  B(1S  ) = (2.23  0.25 stat  0.27syst)  10  6 B(4S  2S )  B(2S  ) = (1.69  0.26 stat  0.20syst)  10  6

12 Results (II) The e  e  channels (not used) give consistent results. The partial widths of Y(4S) to Y  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. Using CLEO’s recent measurement of B(Y(2S)      ): B(Y(4S)  +   Y(2S) ) = (0.83  0.16)  10  4  (Y(4S)  +   Y(2S) ) = (1.7  0.5) keV PRL 94,

13 Dipion Invariant Mass (I) Fit to  M in equal ranges of m(     ). 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. QCD multipole model Efficiency Data (efficiency corrected) m(     ) resolution ~ 5 MeV/c 2

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

15 Conclusions Reported first measurement of non BB decays of Y(4S) to Y(1S,2S) . 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± 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± Y(2S)     candidates with |  s-M(2S)|<20 MeV with expected background of 13.5±2.4