Presentation on theme: "Measurement of non BB Decays of Y(4S) to Y(1S) and Y(2S) Silvano Tosi Università & INFN Genova."— Presentation transcript:
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, 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 : c , J/ (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 PRD 59,052003 e.g. PRD 24, 2874 PLB 605, 63, PRD 73, 012002, hep-ex/0509030 Preliminary evidence! hep-ex/0512034
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 ~ 10.58 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. ee ee 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: (1.1185 ± 0.0009) GeV/c 2 4S 2S: (0.5571 ± 0.0010) GeV/c 2 Notice: not a mass measurement!
9 Signal Extraction (II) Statistical significance 4S 1S4S 2S N sig n N sig n 167±1910.097±157.3 eeee 74±223.626±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)
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, 012001
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.
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