1. Decadimenti rari radiativi e leptonici del mesone B
Risultati più recenti dalle B-factory
F.Bucci
INFN-Pisa
Collaborazione BaBar
XV IFAE
Lecce, 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(*) l+l- (constraints on SUSY models
BXs l+l from B.R., BF-asymmetry, dilepton mass spectrum )
Bl+l (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
E*BE*beam
improve mES resolution DE
Typical resolutions:
s(mES)  2.5 MeV
s(DE)  MeV

4. Br(w) g B (B+r+ g)  0.9-1.5 10-6
The observation of Br(w) g would constitute the
first evidence of the bd g radiative transition d
The Standard Model calculation has large theoretical (hadronization) uncertainties
B (B+r+ g)  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

5. Br(w) g B(Br(w) g)  1/50 B(BK* g) Experimental Challenges:
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

6. No significant signals observed
Br(w) g
Use an unbinned maximum-likelihood fit in mES , DE and (for rg) mpp
No significant signals observed
BaBar 78fb-1
B0r0g
B+r+g
B0wg
mES GeV/c2
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 = %  s(|Vtd|/|Vts|)/|Vtd|/|Vts|=15-20%
Already at level of theory uncertainty
Luminosity fb-1
Luminosity fb-1

7. BXs g
B(BXs g) has been computed in NLO with < 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

8. 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
2.1< EgU(4s)< 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

9. BXs g semi inclusive
The hadronic Xs is reconstructed in 12 final states 50% bs g for MXs< 2.4 GeV/c2
Anaysis in DE, mES plane considering several bins in MXs ( 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
2.1<EBg<2.6 GeV from fraction of missing final state
Can be reduced increasing the fraction of reconstructed final states

10. Good agreement with theory
BXs g Status
world average from 2003 CKM Workshop:
Good agreement with theory

11. BK(*) l+l- Susy models
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*m+m
K*m+m-
J/y K
Susy models
SM prediction
FB Asymmetry
dB/m2mm
y(2s)K
SM non res

12. BK(*) l+l-
Both BaBar and Belle measure 8 modes: K/K*, charged/neutral, e+e-/m+m-
Analysis key points:
Lepton and kaon ID
Background suppression:
Continuum events
BB semi-leptonic decays
BJ/y (l+l-)K decays
Both experiments suppress
continuum with topological cuts
and exclude regions in DE,
m(l+l-) plane consistent with
J/y (l+l-)
DE GeV
Nominal signal region
Shifts in m(y) and in DE are due to radiating or mismeasured leptons from J/Yl+l-
GeV/c2
m(e+e-)
m(m+m-)

13. BK(*) l+l- Extract signal with likelihood fit to mES and DE Belle
60.1 fb-1
77.8 fb-1
BaBar finds only 2.8s effect in B K*l+l- 
upper limit
SM prediction: B(BK l+l-)=(0.350.13)10-6
Measurements consistent with SM prediction

14. BXs l+l- BSM(BXs l+l-)=(4.2±0.7)10-6
Belle has also measured the inclusive B.R. with a semi-inclusive analysis
60fb-1
BSM(BXs l+l-)=(4.2±0.7)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 m2l+l- below J/Y and above Y’
theoretical error 10% in ‘windows’

15. Bl+l- B(Be+e-) 10-15 B(Bm+m-) 10-10 Analysis key points:
highly suppressed in the SM (bd transition, helicity suppression ):
B(Be+e-) 10-15
B(Bm+m-) 10-10
rate changes up to two order of magnitude in models beyond the SM
54.4 fb-1
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
Bdm+m- < 10-7 from the upper limit on Bsm+m- set by CDF

16. Pure leptonic charged B decays in SM are cleanly computed:
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

17. the combined BaBar upper limit is
Btnt
Reconstruct one meson B
The remaining neutrals and tracks are defined as belonging to the signal-side
81.9 fb-1
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

18. 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 l+l- opens up a rich new area of investigation
Both BaBar and Belle are continually updating results to new data and improving analysis techniques