Working Group 4:

Extraction of Vub and Vcb from B decays

Theorist Conveners: Thomas Mannel, Matthias Neubert

Experimentalist Conveners: Klaus Schubert , Michael Witherell

This group will study the extraction of Vub and Vcb from B decays. We will review the present status, both experimental and theoretical, and study how well one should be able to do with BABAR data.

1. Determination of Vcb

  • Exclusive decays
  • Experimental issues
  • Separation of D l nu, D* l nu, and D** l nu final states
  • Measuring the differential decay rates close to zero recoil
  • Interaction with form factor measurements
  • Improved Monte Carlo generators
    •
  • Theoretical issues
  • HQET and corrections to the heavy quark limit
  • Models for exclusive decays
    •
  • Inclusive decays
  • Experimental issues
  • Measuring the lepton momentum spectrum from B -> X_c l nu
  • Using dilepton events to measure the primary lepton spectrum
  • Separate semileptonic rates for charged vs neutral B mesons
    •
  • Theoretical issues
  • The heavy quark expansion
  • Inclusive models
  • Models based on summing exclusive modes, or hybrid models
  • Uncertainties in quark masses and other parameters
    •

    • 2. Determination of Vub

  • Exclusive decays
  • Experimental issues
  • Measuring branching ratios for B -> pi l nu, B -> rho(omega) l nu with large acceptance
  • Neutrino reconstruction technique, requiring events with no missing hadrons
  • Improved Monte Carlo generators
    •
  • Theoretical issues
  • Models for B -> pi l nu, B -> rho(omega) l nu
  • Relating B decays to D decays using HQET
    •
  • Inclusive decays
  • Experimental issues in measuring B -> X_u l nu at the endpoint of the lepton spectrum
  • Suppressing the continuum background which extends beyond the B -> X_c l nu endpoint
  • Measuring the q^2 dependence of the decay rate to discriminate between models
    •
  • Theoretical issues in using models to convert the rate in the endpoint region into a value of Vub
  • Inclusive models vs sum of exclusive models
  • Shape functions from different models
  • QCD corrections
  • Using the energy spectrum in B ->X_s gamma to reduce model uncertainties
    •
    • References:
    1. an extensive review of semileptonic decays of heavy hadrons by Pat Burchat and Jeff Richman, Rev.Mod.Phys.67:893-976,1995.
    2. HQET and applications:
      N.Isgur and M.B.Wise, Phys.Lett.B 232:113,1989 and 237:527,1990.
      Reviews include
      M.Neubert, Phys.Rep. 245: 259,1994, and
      T.Mannel, p.21 in Proceedings of the 5th International Symposium on Heavy Flavour Physics, Edition Frontieres, 1993.
    3. the ACCMM model: G.Altarelli, N.Cabibbo, G.Corbò, L.Maiani, G.Martinelli, Nucl.Phys. B208:365,1982.
    4. models for exclusive decays:
      M.Wirbel, B.Stech, M.Bauer, Z.Phys. C29:637 (1985).
      N.Isgur, D.Scora, B.Grinstein, M.B.Wise, Phys.Rev. D39:799 (1989) and
      B.Grinstein, M.B.Wise, N.Isgur, Phys.Rev.Lett. 56:298 (1986).
      Nathan Isgur, Phys.Rev.D54:5896-5898 (1996).
      J.G.Körner, G.A.Schuler, Z.Phys. C38:511 (1988).
      J.G.Körner, G.A.Schuler, Phys.Lett. B226:185 (1989).
      M.A. Shifman, M.B. Voloshin, Sov.J.Nucl.Phys.47:511 (1988).
    5. CLEO papers on semileptonic decays:
    6. Measurement of the B Semileptonic Branching Fraction
      with Lepton Tags (dilepton analysis)
      Phys.Rev.Lett.76:1570-1574,1996; or from Cornell.
    7. Measurement of Charmless Semileptonic Decays of B Mesons
      (inclusive measurement of Vub)
      Phys.Rev.Lett.71:4111-4115,1993; or Postscript.
    8. Measurement of the Form Factors for Bbar0 -> D*+ l- nubar
      Phys.Rev.Lett.76:3898-3902,1996; or from Cornell.
    9. Measurement of the B -> D*l nu Branching Fractions
      and Vcb
      CLNS 94/1285, CLEO 94-13, Postscript (3.4 MB) - DVI
    10. Measurement of B(B0-bar -> D+ l- nu-bar) and
      Extraction of |Vcb| (preliminary result submitted to ICHEP-96)
      CLEO-CONF 96-3, Postscript
    11. A Search for Exclusive b->u Semileptonic Decays of B Mesons
      (original search for B->rho l nu in endpoint region)
      Phys.Rev.Lett.70:2681-2685,1993.
    12. First Measurement of the B -> pi l nu and B -> rho(omega) l nu Branching Fraction
      This uses the "neutrino reconstruction" technique.
      CLNS 96/1419, CLEO 96-9 available from Cornell.
      13.