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Decadimenti rari radiativi e leptonici del mesone B

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. B  r(w) g ( bd g , | V td |/| V ts | ) B X s g ( constraints on MSSM , m b , l 1 )

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Decadimenti rari radiativi e leptonici del mesone B

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  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. Br(w) g (bdg, |Vtd|/|Vts|) BXsg (constraints on MSSM,mb,l1) BK(*) l+l- (constraints on SUSY models BXs l+l- from B.R., BF-asymmetry, dileptonmass 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 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+

  3. 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 Kinematic Signatures for exclusive B decays: Analysis Techniques mES BaBar mES  Belle Mbc E*BE*beam improve mES resolution DE Typical resolutions: s(mES)  2.5 MeV s(DE)  25-40 MeV

  4. 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)haslesstheoretical uncertainty and is sensitive to|Vtd|/|Vts| The observation of Br(w) g would constitute the first evidence of the bd gradiative transition d Br(w) g 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. Experimental Challenges: B(Br(w) g)  1/50 B(BK* g) Gr  3 G K* Background: Continuum with high energygfromp0(h)decay or ISR BK* gwith K misidentified as ap Brp0, b s g BaBar (Belle) combine continuum rejection variables in a neural net (Fisher discriminant )to reduce continuum Br(w) 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. Combined BaBar limit: Combined Belle limit: B0r0g B+r+g B0wg mES GeV/c2 Br(w) g Use an unbinned maximum-likelihood fit in mES ,DE and (forrg) mpp No significant signals observed BaBar 78fb-1 DE GeV BaBar Br(w) g projections 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

  7. B(BXsg)has been computed in NLO with < 10% precision : B(BXsg) = (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 inBXlnused in extracting|Vcb|and|Vub| Two preliminary BaBar measurement reported at ICHEP-2002: Fully inclusive Semi-inclusive BXsg Challenge is to reduce the background while controlling systematic and theoretical uncertainties

  8. Lepton tag supresses continuum bkg by 1200 BB background reduced with veto on p0 and hdecays remaining continuum backgound is subtracted using off-resonance data BB contibution estimated from MC simulation checked with a B Xp0 control sample BXsg fully inclusive Just measure Eg spectrum 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. The hadronic Xs is reconstructed in 12 final states 50% bs g forMXs< 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 BXsg semi inclusive 20.7fb-1 MXs GeV/c2 2.1<EBg<2.6GeV from fraction of missing final state Can be reduced increasing the fraction of reconstructed final states

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

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

  12. Analysis key points: Lepton and kaon ID Background suppression: Continuum events BB semi-leptonic decays BJ/y (l+l-)K decays Both experimentssuppress continuum with topological cuts and exclude regions in DE, m(l+l-) plane consistent with J/y (l+l-) BK(*) l+l- Both BaBar and Belle measure 8 modes: K/K*, charged/neutral,e+e-/m+m- Nominal signal region DE GeV 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. Extract signal with likelihood fit to mES and DE 77.8 fb-1 60.1 fb-1 BK(*) l+l- Belle 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 SMprediction

  14. 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 BXs l+l- • 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. Analysis key points: Lepton ID (ee 90%, pemis-id  10-3 %, em 70%, pmmis-id  2.5%) Continuum Suppression Define a signal box in mES and DE Bkg estimated from data sidebands 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 totwo order of magnitude in models beyond the SM Bl+l- 54.4 fb-1 Bdm+m- < 10-7from the upper limit on Bsm+m- set byCDF

  16. Btnt Pure leptonic charged B decays in SM are cleanly computed: • A measurement could provide fB |Vub|(within SM) • Btnt measurement hard due tomissing neutrinos • Two preliminary BaBar measurements : • Semi-leptonic tagging • Exclusively-reconstructed tags t+ B(Btnt)  7.510-5

  17. 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 Btnt 81.9 fb-1 • Reconstruct one meson B • The remaining neutrals and tracks are defined as belonging to the signal-side 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dgsignal might be near • Measurements of B(BXsg)are moving toward useful precision on the Eg spectrum • The first observationof inclusiveB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

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