Decadimenti rari radiativi e leptonici del mesone B

1
Decadimenti rari radiativi e leptonici del mesone
B

• Risultati più recenti dalle B-factory
• F.Bucci
• INFN-Pisa
• Collaborazione BaBar
• XV IFAE
• Lecce, 23-26 Aprile 2003

2
Rare Decays Physics Motivation
None of these occur at tree level (all involve
internal loops or boxes or bu annihilation) ? new
particles can show up in the loops
t

• B?r(w) g (b?dg, Vtd/Vts)
• B?Xs g (constraints on MSSM, mb,l1)
• B?K() ll- (constraints on SUSY models
• B?Xs ll- from B.R., BF-asymmetry,
  dilepton mass spectrum )
• B?ll- (multi-Higgs-doublet models,
  leptoquarks, R-parity violating SUSY,...)
• B?tnt (Vub?fb)

• Sensitivity to new physics
• Information on non-perturbative form-factors

3
Analysis Techniques

• Continuum background rejection
• exploit spherical decay of the B in the U(4s)
  system vs the jet-like qq background decay
  (thrust, sphericity)
• tag or full reconstruction of the other B

mES

• Kinematic Signatures for exclusive B decays

BaBar mES ? Belle Mbc
EB??Ebeam improve mES resolution
DE
Typical resolutions s(mES) ? 2.5 MeV s(DE) ?
25-40 MeV
4
B?r(w) g

• The observation of B?r(w) g would constitute the
  
• first evidence of the b?d g radiative
  transition

• The Standard Model calculation has large
  theoretical (hadronization) uncertainties
• B (B?r g) ? 0.9-1.5 ?10-6
• B (B0?r0 g) ? B (B0? w g) ? B (B?r g) / 2
• Measure B (B?r(w) g)? B (B?K g) has less
  theoretical uncertainty and is sensitive to
  Vtd/Vts

15-35 error in Vtd/Vts extraction
Goal is to measure and compare to DMs/DMd B
mixing to over-constraint the CKM triangle
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B?r(w) g

• BaBar (Belle) combine continuum
• rejection variables in a neural net
• (Fisher discriminant ) to reduce
• continuum

• Experimental Challenges
• B(B?r(w) g) ? 1/50 B(B?K g)
• Gr ? 3 G K
• Background
• Continuum with high energy g from p0(h) decay or
  ISR
• B?K g with K misidentified as a p
• B?rp0, b ?s g

BaBar use particle ID in DIRC to reduce kaon
misidentification to 1
Belle use a kinematic veto on charged K mass as
well as particle ID information
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B?r(w) g
Use an unbinned maximum-likelihood fit in mES ,
DE and (for rg) mpp No significant signals
observed
BaBar 78fb-1
DE GeV

• Combined BaBar limit

BaBar B?r(w) g projections

• Combined Belle limit

500 fb-1

• 5s measurement with ? 500 fb-1
• sB/B 20-30 ? s(Vtd/Vts)/Vtd/Vts15-20
  
• Already at level of theory uncertainty

Luminosity fb-1
Luminosity fb-1
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B?Xs g

• B(B?Xs g) has been computed in NLO with lt 10
  precision B(B?Xs g) (3.57?0.30)?10-4 ? used
  to constrain new physics
• Photon energy spectrum computed in term of the b
  quark mass (mb) and a Fermi momentum parameter
  (l1)
• The photon energy spectrum and its moments are
  related to those in B?Xln used in extracting
  Vcb and Vub

• Two preliminary BaBar measurement reported at
  ICHEP-2002
• Fully inclusive
• Semi-inclusive

Challenge is to reduce the background while
controlling systematic and theoretical
uncertainties
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B?Xs g fully inclusive
Just measure Eg spectrum

• Lepton tag supresses continuum bkg by ?1200
• BB background reduced with veto on p0 and h
  decays
• remaining continuum backgound is subtracted using
  off-resonance data
• BB contibution estimated from MC simulation
  checked with a B? Xp0 control sample

2.1lt EgU(4s)lt 2.7 GeV as a balance between model
dependence and BB background
54.6 fb-1
Can be reduced increasing the statistics in the
control sample
Can be reduced lowering the photon energy
threshold
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B?Xs g semi inclusive

• The hadronic Xs is reconstructed in 12 final
  states ?50 b?s g for MXslt 2.4 GeV/c2

• Anaysis in DE, mES plane considering several bins
  in MXs (0.6-2.4 GeV)
• Partial rate in each bin continuum and B decay
  backgrounds are subtracted using fits to the mES
  distribution
• Fit hadronic mass spectrum (Kagan-Neubert model )
  to extract inclusive rate
• Fit Eg spectrum moments to extract HQET parameters

20.7fb-1
MXs GeV/c2
2.1ltEBglt2.6 GeV from fraction of missing final
state
Can be reduced increasing the fraction of
reconstructed final states
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B?Xs g Status
world average from 2003 CKM Workshop
Good agreement with theory
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B?K() ll-

• Proceeds via loop or box diagrams ? more
  opportunity for new heavy particles to appear
  virtually
• SM branching ratio prediction ? few 10-7
• Rate changes up to ? factor 2 in SUSY models
• Deviation in the FB-asymmetry predicted by the SM

K?mm
J/y K
K?mm-
Susy models
SM prediction
FB Asymmetry
dB/m2mm
y(2s)K
SM non res
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B?K() ll-
Both BaBar and Belle measure 8 modes K/K,
charged/neutral, ee-/mm-

• Analysis key points
• Lepton and kaon ID
• Background suppression
• Continuum events
• BB semi-leptonic decays
• B?J/y (?ll-)K decays

• Both experiments suppress
• continuum with topological cuts
• and exclude regions in DE,
• m(ll-) plane consistent with
• J/y (?ll-)

Nominal signal region
DE GeV
Shifts in m(y) and in DE are due to radiating or
mismeasured leptons from J/Y?ll-
GeV/c2
m(ee-)
m(mm-)
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B?K() ll-

• Extract signal with likelihood fit to mES and DE

Belle
BaBar finds only 2.8s effect in B? Kll- ? upper
limit
SM prediction B(B?K ll-)(0.35?0.13)?10-6 Measur
ements consistent with SM prediction
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B?Xs ll-

• Belle has also measured the inclusive B.R. with a
  semi-inclusive analysis

• BSM(B?Xs ll-)(4.20.7)?10-6

• Lepton forward-backward asymmetry
• shape better known for inclusive
• position of zero quite well-determined in
  inclusive and exclusive cases
• need first measurement
• Dilepton mass spectrum
• need separate BF measurements for m2ll- below
  J/Y and above Y
• theoretical error ?10 in windows

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B?ll-

• highly suppressed in the SM (b?d transition,
  helicity suppression )
• B(B?ee-)? 10-15
• B(B?mm-)? 10-10
• rate changes up to two order of magnitude in
  models beyond the SM

• Analysis key points
• Lepton ID (ee ? 90, pe mis-id ? 10-3 ,
• em ? 70, pm mis-id ?
  2.5)
• Continuum Suppression
• Define a signal box in mES and DE
• Bkg estimated from data sidebands

54.4 fb-1
Bd?mm- lt 10-7 from the upper limit on Bs?mm-
set by CDF
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B?tnt
Pure leptonic charged B decays in SM are cleanly
computed
t
B(B?tnt) ? 7.5?10-5

• A measurement could provide fB Vub (within
  SM)
• B?tnt measurement hard due to missing neutrinos
• Two preliminary BaBar measurements
• Semi-leptonic tagging
• Exclusively-reconstructed tags

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B?tnt
81.9 fb-1

• Reconstruct one meson B
• The remaining neutrals and tracks are defined
  as belonging to the signal-side

• Semi-Leptonic Tags
• B?Dlv X with X g,p0,nothing
• t?(e,m) v(e,m) v t

• Semi-Exclusive Tags
• B?D0() Xhad
• t?(e,m) v(e,m) v t and
• t?(p,pp0,ppp) v t

Eleft, energy in the EMC not matched with charged
tracks, is the signal-definying quantity
no evidence of signal
the combined BaBar upper limit is
still far need 5-7 ab-1

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Conclusions

• Rare B decays could exhibit physics beyond the
  SM, but no deviation found yet
• Limits on several exclusive modes have come down
  significantly
• A first b?dg signal might be near
• Measurements of B(B?Xs g) are moving toward
  useful precision on the Eg spectrum
• The first observation of inclusive B?Xs ll-
  opens up a rich new area of investigation
• Both BaBar and Belle are continually updating
  results to new data and improving analysis
  techniques