Post on 08-Jan-2016
description
Decadimenti rari radiativi e leptonici del mesone B
Risultati più recenti dalle B-factory
F.Bucci
INFN-PisaCollaborazione BaBar
XV IFAE
Lecce, 23-26 Aprile 2003
Rare Decays: Physics Motivation
Bbd,VtdVts)
BXs constraints on MSSM,mb,1 BK(*) l+l- (constraints on SUSY models
BXs l+l- from B.R., BF-asymmetry, dilepton mass spectrum )
Bl+l- (multi-Higgs-doublet models, leptoquarks, R-parity violating SUSY,...) B(|Vub|fb
Sensitivity to new physics Information on non-
perturbative form-factors
None of these occur at tree level (all involve internal loops or boxes or bu annihilation) new particles can show up in the loops
Analysis Techniques
Continuum background rejection: exploit spherical decay of the B in the (4s)
system vs the jet-like qq background decay (thrust, sphericity)
tag or full reconstruction of the other B
2*2*BbeamES pEm
**beamB EEE Typical resolutions:
(mES) 2.5 MeV
(E) 25-40 MeV
E*BE*beam improve mES resolution
mE
S
E
BaBar mES Belle Mbc
Kinematic Signatures for exclusive B decays:
B
The Standard Model calculation has large theoretical (hadronization) uncertainties
B (B+
B (B0B (B0 B (B+
Measure B (BB (Bhas less theoretical uncertainty and is sensitive to |Vtd|/|Vts|
The observation of B would constitute the first evidence of the bd radiative transition
15-35% error in |Vtd|/|Vts|
extraction
Goal is to measure and compare to DMs/DMd B mixing to over-constraint the CKM triangle
d
B
Experimental Challenges: B(BB(B
K
Background: Continuum with high energy
from 0() decay or ISR Bwith K misidentified as
a B0, b s
BaBar (Belle) combine continuum rejection variables in a neural net (Fisher discriminant ) to reduce continuum
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
B
Combined BaBar limit:
Combined Belle limit:
CL) (90% 109.1))(( 6 BB
CL) (90% 100.3))(( 6 BB
Use an unbinned maximum-likelihood fit in mES , E and (for ) m
No significant signals observedB00
B++
B0
mES GeV/c2
5measurement with 500 fb-1
B/B = 20-30 % |Vtd|/|Vts|)/|Vtd|/|Vts|=15-20%
Already at level of theory uncertainty
BaBar Bprojections
Luminosity fb-1 Luminosity fb-1
BaBar 78fb-1
E GeV
500 fb-1
BXs
B(BXs has been computed in NLO with < 10%
precision : B(BXs used to constrain new physics
Photon energy spectrum computed in term of the b quark mass (mb) and a Fermi momentum parameter (1)
The photon energy spectrum and its moments are related to those in BXl 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
BXs fully inclusive
Lepton tag supresses continuum bkg by 1200 BB background reduced with veto on 0 and decays remaining continuum backgound is subtracted using
off-resonance data BB contibution estimated from MC simulation
checked with a B X0 control sample
443.023.0 10)(.)(37.0.)(36.088.3)( modelsyststatXB S B
Just measure E spectrum
2.1< E4s< 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
BXs semi inclusiveThe hadronic Xs is reconstructed in 12 final states 50% bs forMXs< 2.4 GeV/c2
• Anaysis in E, 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 E spectrum moments to extract HQET parameters
215.021.0
03.004.01
2
2
GeV/c )(mod)(02.0)(24.0
GeV/c )(mod3.1)(10.0)(08.079.4
GeV/c )(mod10.0)(07.0)(09.037.0
elsyststat
elsyststatm
elsyststat
b
MXs GeV/c2
20.7fb-1
2.1<EB<2.6 GeV from fraction of
missing final state
410)(3.1.)(8.0.)(5.03.4)( modelsyststatXB S B
Can be reduced increasing the fraction of reconstructed final
states
BXs Status
410))(35.0)(32.0)(23.047.3()( theosyststatXBB Sworld average from 2003 CKM Workshop:
Good agreement with theory
BK(*) l+l-
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*+
Susy models
SM non res
J/ K
22 GeV m22 GeV m
FB Asymmetry
K*+-
dB/m2
SM prediction
(2s)K
BK(*) l+l-
Analysis key points: Lepton and kaon ID Background suppression:
Continuum events BB semi-leptonic decays BJ/(l+l-)K decays
Both experiments suppresscontinuum with topological cutsand exclude regions in E,m(l+l-) plane consistent withJ/(l+l-)
Both BaBar and Belle measure 8 modes: K/K*, charged/neutral, e+e-/+-
Shifts in m() and in E are due to radiating or mismeasured leptons from J/l+l-
E
GeV
m(e+e-) m(+-)
GeV/c2
Nominal signal region
BK(*) l+l-
Extract signal with likelihood fit to mES and E
SM prediction: B(BK l+l-)=(0.350.13)10-6
Measurements consistent with SM prediction
Belle
60.1 fb-1 77.8 fb-1
BaBar finds only 2.8 effect in B K*l+l- upper limit
BXs l+l-
BSM(BXs l+l-)=(4.2±0.7)10-6
64.11.1 10))()(4.11.6()(
syststatllXBB S
60fb-1
Belle has also measured the inclusive B.R. with a semi-inclusive analysis
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 m2
l+l- below J/ and above ’
theoretical error 10% in ‘windows’
Bl+l-
Analysis key points: Lepton ID (e 90%, e mis-id 10-3 %,
70%, mis-id 2.5%) Continuum Suppression Define a signal box in mES and E Bkg estimated from data sidebands
54.4 fb-1
C.L. 90%at 101.2 )(C.L. 90%at 100.2)(C.L. 90%at 103.3 )(
7
7
7
eBBBB
eeBBBd+- < 10-7 from the upper limit on Bs+- set by CDF
highly suppressed in the SM (bd transition, helicity suppression ): B(Be+e-) 10-15
B(B+-) 10-10
rate changes up to two order of magnitude in models beyond the SM
B
A measurement could provide fB |Vub| (within SM)
Bmeasurement hard due to missing neutrinos
Two preliminary BaBar measurements : Semi-leptonic tagging Exclusively-reconstructed tags
Pure leptonic charged B decays in SM are cleanly computed:
BubBB
llBF Vfm
mmmGlBB
22
2
2
222
||18
)(
B(B) 7.510-5
B
Semi-Leptonic Tags BDlv X with X= ,0,nothing (e,) v(e,) v
Semi-Exclusive Tags BD0(*) Xhad
(e,) v(e,) v and
(,0,) v
Reconstruct one meson B
The remaining neutrals and tracks are defined as belonging to the signal-side
81.9 fb-
1
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
CL) (90% 101.4)( 4 BB
still far: need 5-7 ab-1
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 bdsignal might be near
Measurements of B(BXs ) are moving toward useful precision on the Espectrum
The first observation of inclusive BXs 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