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

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

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)  25-40 MeV

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)  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

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

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 = 20-30 %  s(|Vtd|/|Vts|)/|Vtd|/|Vts|=15-20% Already at level of theory uncertainty Luminosity fb-1 Luminosity fb-1

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

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

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 (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 2.1<EBg<2.6 GeV from fraction of missing final state Can be reduced increasing the fraction of reconstructed final states

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

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

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-)

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

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’

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

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

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

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