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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

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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

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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

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•  “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”)

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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

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•  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

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NN output-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

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Signal MC

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•  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

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•  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)

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•  B0 Invisible signal yield compatible with zero if systematic errors are considered

•  B0 Invisible+γ signal yield compatible with zero within 0.6σ

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•  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

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•  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)

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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)

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•  Onpeak data MD ( M) sidebands •  2nd order polynomial plus a bin for Eextra=0

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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

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•  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

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•  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

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•  “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)

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