Review of semileptonic b clν decays

Transcription

1 Review of semileptonic b clν decays Elisabetta Barberio University of Melbourne Seattle 2005

2 semileptonic spectra b rate shape V cb shape m b,m c µ 2 G, µ 2 π difficulty: go from the measured shape to the true shape shape in B rest frame, QED corrections, detector resolution, accessible phase space, etc

3 Measurements BR : Branching Fraction for B X c n E : lepton energy spectrum in B X c n M X : Hadronic mass spectrum in B X c n E has better experimental precision M x however, is more sensitive to non perturbative terms

4 dilepton sample Select Υ(4S) events with high energy lepton Measure spectrum of any other lepton in the event l sig e - B sig e sig B tag e + ν e tag

5 lepton energy spectrum Same sign (mostly from D decays) BABAR Opp. sign (mostly from B decays) BABAR 47.4 fb -1 hep-ex/

6 The full reconstructed sample Select Υ(4S) decays with fully reconstructed hadronic B decays B tag π e - B tag D * B recoil e + M x ν l

7 Fully reconstuced samples QuickTime and a TIFF (LZW) decompressor are needed to see this picture. QuickTime and a TIFF (LZW) decompressor are needed to see this picture. Belle QuickTime and a TIFF (LZW) decompressor are needed to see this picture. QuickTime and a TIFF (LZW) decompressor are needed to see this picture. hep-ex/

8 Hadronic recoil mass B X c l ν B D l ν B D* l ν B D** l ν B D π l ν Measure moments of recoil mass of the hadron system, M X DELPHI/CDF D and D* fixed determine the contribution to moments from high mass components ( M 2 BABAR, BELLE & CLEO Measure full spectrum X M 2 D ) M 2 X

9 Belle: hadronic moments Belle 140 fb -1 Belle P l > 0.9GeV preliminary P l > 1.5GeV hep-ex/008139

10 First and second lepton moments theory: hep-ph/ Kinetic running mass

11 First and second hadronic moments BaBar measure also the 3rd and 4th moment DELPHI the third theory: hep-ph/ Kinetic running mass

12 Where we are Different experiments are now providing many measurements: they are consistent with each other the constraint on non-perturbative parameters has reduced the uncertainty on V cb to 2% We can decrease the energy cut-off and/or decrease some errors but the main limitation is from the theory Including..radiative corrections.

13 Radiative Corrections in B decays photons are emitted by any charged particle in the B decay chain to determine the moments of the lepton spectrum radiative corrections have to be included in the data analysis and/or in the experimental error. b W l u d d

14 Moments of electron energy spectrum M n =M n ( B Xeν ) M n ( B Xeν(γ) ) E cut (GeV) M 1 /M 1 M 2 /M 2 B + D 0 eν B 0 D + eν B + D* 0 eν B 0 D* + eν

15 QED radiative corrections in B X c lν to go from the measured shape to the true shape: Radiative corrections affect lepton reconstruction and identification efficiency Bremsstrahlung makes the spectrum softer, e are more affected than µ: lepton energy cut-off is affected Calculations to extract non-perturbative parameters from the moments do not contain radiative corrections (radiative corrections to the inclusive spectrum)

16 Electron Energy Spectrum Before brem. corr. After Detector Bremsstrahlung

17 Radiative Corrections in B decays Analytical calculations of radiative corrections are not available: process-independent approximate approaches are used by the experiments All experiments uses the Monte Carlo approach of PHOTOS (Leading Log b(α) approximation) PHOTOS gives pure QED radiative corrections: not including hadronic structure effects, QCD and weak corrections, Coulomb threshold corrections

18 What is Photos Photos is a MC algorithm to simulate single/double photon QED radiative correction in decays using leading log approximation. The new version has some non-leading log term included. Photos estimates the size of QED bremsstrahlung and provides final state with their full topology A photon cut-off energy divides real from virtual corrections: the cut-off depends by the detector ability to distinguish a charge particle from its brem photon (e.g calorimeter granularity)

19 Radiative Corrections in B decays From the experimental side: a) Is PHOTOS adequate? b) What error do we assign to missing higher orders? And missing effects? c) Can we use data to assess radiative corrections? d) Can we design an analysis less sensitive to radiative corrections? Some of the answers are related to the size of the corrections and effects.

20 B + D 0 eν and B + D * eν Large effect at low energy, but still few % effect over the cut-off! Ratio PHOTOS/no PHOTOS B + D 0 lν B + D * lν E* cut E* cut E* e (GeV) E* e (GeV)

21 Checks and systematics error B D 0 eν(γ) E.Richter-Was, Phys. Lett. B 303 (1993), 163 Photos reproduces LLb(α) ~1% agreement with respect to exact b(α) calculations. Larger disagreement for E e <~0.6GeV (B rest frame)

22 Checks and systematics error B D 0 eν(γ) E.Richter-Was, Phys. Lett. B 303 (1993), 163 E.S.Ginsberg, Phys. Rev. 142, (1966), 1035 D.Atwood and J.W.Marciano, Phys. Rev. D 41 (1990), 1736

23 error to radiative corrections B X c lν What error is associated to PHOTOS approximation? Need an estimation for the experimental part and one for the V cb etc extractions Different experiments have different approaches: Babar: look at photons not in Photos CLEO: compares Photos with Atwood&Marciano approximation and assigns as error the difference of the two Belle: try to make an analysis less sensitive to radiative correction+bremsstrahlung and working more on Photos

24 Radiative corrections conclusions Radiative corrections cannot be neglected in semileptonic B decays. Photos is the tool for central values, but we need comparisons with exact calculations and real data to be able to assign the correct error! b c transition: the situation is not so bad but more work is needed for a convincing error b u transition: here the task is more difficult, there is need of either exact calculations and/or data to assess the error