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