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