Measurement of non BB Decays of Y(4S) to Y(1S) + and Y(2S) +
description
Transcript of 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
Università & INFN Genova
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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
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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))<1.210-4
BR(Y(4S)Y(2S))<3.910-4
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
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The BABAR 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-1/42.1%
Particle identification (PID) through Cherenkov radiation.
Separation K- >3.4 for p<3.5GeV/c
Momentum measurement for charged particles + dE/dx(pT)/PT=0.13%PT+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.
ee CM energy ~ 10.58 GeV
Boost: ~0.56
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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 3S1S,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
Y
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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/c2
– 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.
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Additional Selections
• Major remaining background is from with photon conversion to ee. Removed Y candidates for which:
• either pion positively identified as electron;
• m(ee) < 100 MeV/c2;
• dipion opening angle cos() > 0.95
• Additional requirement for Y( ee):
• (e) > 0.75 rad to remove Bhabhas.
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Signal Extraction (I) channel• Select events with |m(+)-
m(Y(1S))| < 200 MeV/c2 and |m()-m(Y(2S))| < 150 MeV/c2.
• 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: 4S1S: (1.1185 ± 0.0009) GeV/c2 4S2S: (0.5571 ± 0.0010) GeV/c2 Notice: not a mass measurement!
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Signal Extraction (II)
Statistical significance
4S 1S 4S 2S
Nsig n Nsig n
167±19 10.0 97±15 7.3
ee 74±22 3.6 26±11 2.5
)0(/)(log2 LNLn sig
• Signal yields are consistent with expectations for Y(3S) and Y(2S) control samples.
• No Y(4S) signal observed in off-peak data.
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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)
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Results (I)
)4(
)()()4(SN
NnSYBnSYSYB sig
N(4S) = (230.0 ± 2.5) 106
• Using world average for B(nS ) and (Y(4S)) = (20.7 ± 3.0) MeV:
410)15.090.0()1()4( SYSYB
410)32.029.1()2()4( SYSYB
keV)4.08.1()1()4( SYSY
keV)8.07.2()2()4( SYSY
PRD 72, 032005
B(4S1S ) B(1S ) = (2.23 0.25 stat 0.27syst)106
B(4S2S ) B(2S ) = (1.69 0.26 stat 0.20syst)106
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Results (II)
• The ee 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) ) = (0.830.16)104
(Y(4S)+Y(2S) ) = (1.70.5) keV
PRL 94, 012001
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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 4S1S transition is reasonably compatible with the QCD multipole expansion model.
• The 4S2S transition is not in agreement.
QCD multipole model
Efficiency
Data (efficiency corrected)
m() resolution ~ 5 MeV/c2
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Dipion Invariant Mass (II)
• Something special when n=2?
n=1 n=2
Belle 4S-1SBelle
2S-1S
CLEO 2S-1S
preliminary
CLEO 3S-1S preliminary
Belle 3S-1S
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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 4S2S incompatible with QCD multipole expansion model.
• Published in Phys. Rev. Lett.
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Backup Slides
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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