indico.cern.ch · 2018. 11. 15. · alessandro rossi - ckm 2012 10/02/2012 2 • invisible decay...
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Alessandro Rossi, INFN Perugia on behalf of BABAR collaboration
10/02/2012 Alessandro Rossi - CKM 2012 2
• Invisible decay products include neutrinos as well as exotic particles
• The SM B0→νν decay is suppressed by a factor (mν/mB)2 while the BF(B0→ννγ) is of the order of 10-9
• In SUSY model BR(B0→Invisible) is enhanced up to 10-7-10-6 due to neutrino+neutralino production in the final state
]-1
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Delivered LuminosityRecorded LuminosityRecorded Luminosity Y(4s)Recorded Luminosity Y(3s)Recorded Luminosity Y(2s)Off Peak
BaBarRun 1-7
PEP II Delivered Luminosity: 553.48/fbBaBar Recorded Luminosity: 531.43/fb
BaBar Recorded Y(4s): 432.89/fbBaBar Recorded Y(3s): 30.23/fbBaBar Recorded Y(2s): 14.45/fbOff Peak Luminosity: 53.85/fb
BaBarRun 1-7
PEP II Delivered Luminosity: 553.48/fbBaBar Recorded Luminosity: 531.43/fb
BaBar Recorded Y(4s): 432.89/fbBaBar Recorded Y(3s): 30.23/fbBaBar Recorded Y(2s): 14.45/fbOff Peak Luminosity: 53.85/fb
As of 2008/04/11 00:00
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• PEP-II @ SLAC • e+e- asymmetric energy collider • Center of mass energy 10.58 GeV
• Υ(4S) production • Runs also at Υ(2S)and Υ(3S)
threshold
10/02/2012 Alessandro Rossi - CKM 2012 3
• “On-peak” data [@Υ(4S)] • 424fb-1
• BB Pairs: ~471M
• “Off-peak” data • 40 MeV below Υ(4S) threshold • 45fb-1
• Update of a previous BABAR result with an Integrated Luminosity of 88.5fb-1
• New analysis strategy • Semileptonic Recoil Technique:
o reconstruct events in which a B0 decays to D( ) l+ ( “tag side”), then look for consistency with an invisible(+γ) decay of the other neutral B (“signal side”)
10/02/2012 Alessandro Rossi - CKM 2012 4
B0 → D-lν D- → K+π-π-
D- → KSπ-
D-(*)
l+
νν
νB0 →D*-lν
D*-→D0π-, D-π0
D0 → K+π-, K+π-π0, K+π-π-π+
• Preselection cuts based on : o Nothing reconstructed on signal side or only one
photon o No extra charged tracks o Missing momentum inside detector sensitive
volume
• : polar angle of the missing momentum
o Kinematical constraints on D meson decay products on tag side
o Events with one photon with energy > 1.2GeV are reconstructed as +γ
10/02/2012 Alessandro Rossi - CKM 2012 5
• After the preselection a Neural Network is implemented
• Samples: o Signal: Signal MC with D(*)lν generated on tag side (~5x106 events) o Background: BB MC (x3 data lum.) + Off-peak data (all normalized to On-
peak luminosity)
• Variables o B0 Invisible
• Dtag : 9 variables • D*
tag : 6 variables
o B0 Invisible+γ • Dtag : 6 variables • D*
tag : 4 variables
10/02/2012 Alessandro Rossi - CKM 2012 6
NN output-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6
Arb
itrar
y U
nits
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Background
Signal MC
• •
• Unbinned Maximum Likelihood fit to Eextra
• Fit strategy: o Offpeak can not be used, very low statistics o Background shape from MD and M Onpeak data sidebands
• Side bands MD : 15 MeV<|MD-MDPDG|<60MeV
• Side band M : 148 MeV< M<155 MeV o In this way the measured upper limit will not depend on the
Eextra shape of the Offpeak data
o Signal shape from MC simulation
10/02/2012 Alessandro Rossi - CKM 2012 7
• Minimum neutral energy threshold 30 MeV o Eextra distribution not continuous
• Two different likelihood terms form Eextra>30MeV (N1) and Eextra<30MeV (N0)
• fsig and fbkg fraction of events with Eextra<30MeV (fixed)
• Psig and Pbkg PDF distributions (fixed)
10/02/2012 Alessandro Rossi - CKM 2012 8
• B0 Invisible signal yield compatible with zero if systematic errors are considered
• B0 Invisible+γ signal yield compatible with zero within 0.6σ
10/02/2012 Alessandro Rossi - CKM 2012 9
(GeV)extraE
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Signal
Data
• Signal Efficency: o Tag Efficency
• From : BB MC + Offpeak data DoubleTag samples o Cut on D mass ( M)
• Change Signal Region window o Preselection
• Smearing of Signal variables o Neural Network
• Smearing of Signal Input variables o Neutral Energy Resolution
• Single Photon from 0 decay
• B Counting o systematic error of 0.6%
• Signal Yield: o Background Parametrization o Signal Parametrization o Eextra Shape
10/02/2012 Alessandro Rossi - CKM 2012 10
• Bayesian evaluation of the ULs:
o BR(B Invisible)<2.4x10-5 (at 90% C.L)
o BR(B Invisible+γ)<1.7x10-5 (at 90% C.L)
10/02/2012 Alessandro Rossi - CKM 2012 11
10/02/2012 Alessandro Rossi - CKM 2012 12
This analysis: • B Invisible
o Signal Yield: -22±9(stat)±16(sys)
o Signal Efficiency: (1.78±0.02(stat)±0.14(syst))x10-3
• B Invisible+γ o Signal Yield: -3.1±5.2(stat)±6.9(sys)
o Signal Efficiency: (1.60±0.02(stat)±0.15(syst))x10-3
Upper Limits
o B to Inv. : 2.4x10-5 o B to Inv.+ : 1.7x10-5
o Improved by a factor 9 and 4 w.r.t previous BABAR results
o Still far (1 order of magnitude) from NP sensitivity • New generation High Luminosity b-factories
Published Sep 27, Phys. Rev. D 86, 051105(R) (2012)
10/02/2012 Alessandro Rossi - CKM 2012 13
(GeV)extra0E
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• Onpeak data MD ( M) sidebands • 2nd order polynomial plus a bin for Eextra=0
10/02/2012 Alessandro Rossi - CKM 2012 14
(GeV)extra0E
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• MC simulation • RooKeysPDF (not analytic
function) plus a bin for Eextra=0 o Superposition of gaussian kernels one for
each point with a 1/N weight
10/02/2012 Alessandro Rossi - CKM 2012 15
• Flat prior probability is assumed for positive values of branching fraction
• Gaussian likelihood is adopted for signal yield o Width is fixed to the sum in quadrature of the statistical and systematic
yield errors
• Efficiency and normalization modeled with Gaussian PDF
• Posterior PDF extracted using Bayes’ theorem o Random BR, number of BB pairs and efficiency o BR weighted with the probability that the Number of signal event
calculated is coming from the Gaussian likelihood
10/02/2012 Alessandro Rossi - CKM 2012 16
• Same method to evaluate the UL • Same statistical and systematic errors • Hypothesis Nsig=0 for both channel • UL at 90% C.L.
o 3.8x10-5 wrt 2.4x10-5
o 1.9x10-5 wrt 1.9x10-5
10/02/2012 Alessandro Rossi - CKM 2012 17
• “Unphysical” decay B+ Invisible(γ) as control sample • Same selection applies as for B0 Invisible(γ) • Results compatible with zero within 1.1σ (stat. only)
10/02/2012 Alessandro Rossi - CKM 2012 18
(GeV)extra0E
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γ
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