Selected Results and Prognostications on V cb & V ub : A B Factory Perspective

1. Selected Results and Prognostications on Vcb & Vub : A B Factory Perspective Vivek Sharma University of California San Diego vsharma@ucsd.edu Ringberg Phenomenology Workshop on Heavy Flavors : Rottach-Egern, Germany

2. The Two Approaches in Vxb • This talk is an “appetizer” not a review. See recent CKM workshop page for complete results • http://ckm-workshop.web.cern.ch/ckm-workshop/ckm-workshops/Default2003.htm

3. Inclusive Semileptonic Decay Rate Babar (also CLEO) Basic foundation of all semileptonic studies. Modern measurements agree

4. Experimentally favored (S/N) w.r.t BDl nu • Experimental Challenges: • Slow pion tracking ( helical path of decreasing radius) • Charm branching fractions • Knowledge of higher mass Dnpi states: • excited D**, Non-res. D(*) npi l nu ?? • Form Factors (mostly for  measurement) Siliconvertex tracker CLEO has been the most experienced player on this topic  new results with ¼ total data

5. A complete set of measurements from CLEO: CLEO ~6.5% Measurement

6. Branching Ratio of B  D* l nu disagreement between CLEO & Babar/Belle in value of the branching ratio Each experiment have much more data left to analyze Remove stat. fluctuation as source of disagreement, focus on finding systematic biases “Repeat n>1 more time and very carefully”

7. 3% Stat |Vcb |World Average (CKM’03 preliminary) Updated prelim average using F(1) = 0.91  0.04 |Vcb|=[ 42.6 ± 0.6(stat) ±1.0(syst) ± 2.1(theory) ] 10-3

8. Seeking Consistency ?

9. (II) Moments in B Decay: Elegant Measurements from CLEO Cleo Moment Analyses Use HQE/OPE to predict SL rate & Moments of inclusive B decay spectra Photon energy spectrum in B Xs g Hadronic mass spectrum in BXc  n Lepton energy spectrum in BXc  n

10. = 0.39+0.03stat+0.06sys+0.12thGeV • l1=-0.25+0.02stat+0.05sys+0.14thGeV2 1s Theoretical Ellipse Cleo Moment Analyses: Consistent picture Lepton energy spectrum in BXc n Hadronic mass spectrum in BXc n Photon energy spectrum in B Xsg Remarkable ! 3% measure

11. Comparing Exclusive & Inclusive Vcb Measurements

12. Babar Hadronic Moment Analysis: ICHEP Preliminary BABARPreliminary CLEO • Strong dependence of moments on p*min • For p*min=1.5 GeV/c and =0.35 ± 0.13 GeV [1] (reliance on b  sg spectrum) 1= - 0.17 ± 0.06 ±0.07 GeV2 CLEO [1] 1= - 0.226 ± 0.07  0.08 GeV2 ? OPE (Falk, Luke) L, l1 free param. <MX2-MDspin2> No non-resonant states (MC) OPE (Falk,Luke)  = 0.35 GeV But these parameters do not describe P* dependence of the moments! l1(0.9 GeV/c) – l1(1.5 GeV/c) = 0.22±0.04±0.05 GeV2 p*min [GeV/c] NB: Data points highly correlated [1] CLEOPRL 87, 251808 (2001)

13. B D**  B D** l nu factorization ? D** D** Belle D*+ D+  Non-reso Can Hadronic B Decays Help Understand Nature of High Mass Dn states in SL Decay? What can one Learn from such results ?

14. Desperately Seeking Vub !! An important measurement in shaping - Real estate Constraint from Vub

15. MB= Desperately Seeking Vub ! : By Exclusive Reconstruction • Recent measurements from Babar, Belle, Cleo talk about this • SL decays  missing   need to measure 4-momentum in absence of  signature in detector  very demanding and very important for Vub ! • Exploit Hermiticity of detector (if you have few holes and have excellent particle reconstruction capability • CLEO is best for this purpose (95% hermetic) , then Belle, then Babar

16. New results comparing data q2 with models

17. Wins the most improved Vubmeasurement award Measurement statistics limited

18. Vub From Measurement of Exclusive Final States Difficult to combine results from several experiments due to diff ranges of modeling, FF variations…dust needs to settle here (HFAG)

19. Note : superficially all measurement look too consistent ! Probably because of (large) common Theory systematic error ? (4) |Vub| From Inclusive Measurements CKM Workshop 2003

20. Desperately Seeking Vub : From Inclusive Measurements interpretation Luke @ CKM

21. The Perfect Detector for B Semileptonic Measurements Has No Holes ! (B Decay “bomb” goes off in all directions) B1 B2 This was an animation Need A Spherical Detector ! (4S)

22. (4S) Detectors: Characteristic Features Machine optics

23. Babar Not Hermetic Due to Intrusion of accelerator optics near Interaction Region (dipoles)

24. Belle is Perhaps Better but not like CLEO

25. Missing Momentum Resolution : Hermiticity Issues 85 MeV  >> 160 MeV could be very costly in SL measurements Need an alternate solution that pays in the long run

26. Beginning of an Experimental Paradigm Shift in Vxb Measurements at B Factories • Hermiticity, so vital for SL measurements is not the best feature of Belle/ Babar detectors due to intrusion of machine into detector • Necessary holes ! • But B-factory detectors recording ever increasing samples of B-Bbar pairs (> 100 Million BBbar recorded already 1000 Million) • At the price of modest efficiency (4%), can fully reconstruct one B decay into all hadronic final states (Breco) and examine the other(recoiling) B decay • This “Recoil side” studies perfectly suited for many Vxb studies • Much “cleaner” and more powerful than neutrino reconstruction a la CLEO • I will show you (with example) that this is the most promising way for the future Vub and other Semileptonic measurements

27. In this (rare) case all particles were sprayed within fiducial volume of detector The Perfect (4S) Event: Example of Recoil Side Analysis Replace this with your Favourite Vxb mode

28. Advantages in Recoil Side Measurements • Full reconstruction of B1 “perfect” knowledge of B pm • Turn around and examine the recoiling B2 with this info • Pmissing knowledge much better than in “neutrino reconstruction) • Most backgrounds in Vxb measurements “disappear” • Udsc background (continnum) • Leptons from other B (bclnu, b cs l nu) • Combinatorial (1/2 event accounted for) • The Y(4s) decay so much better understood, • Various charge correlation (D not a Dbar) allows background rejection • Can fit entire event for event hypothesis in question • Price to pay: Efficiency (but have ever growing data) • Sometimes one can “have the cake and eat it too”

29. l n Si B Cui  l duality n b u Example: Babar’s Inclusive bu l nu Measurement Theoretically relatively “simple” in principle but • parton level calculation has to be extended to account for hadronization effects and Fermi motion (b quark mass) • calculation of decay rate relies on OPE for which the convergence depends on full acceptance and is impacted by non-perturbative effects Exptal measurement challenging because • Rejection of large BXcln background • (~60 times higher BR) • Extrapolation to full phase space introduces theoretical uncertainties

30. Xu n l p D* Xu B B Y(4s) Y(4s) Breco Brecoil D* Y(4S) B Candidate Mass l p n Recoil Side Study : Technique Reco side Recoil side missing mass squared • MX reconstruction • Kinematic constraints to • improve MX resolution

31. Fully Reconstructed B Sample • Initial B Reco efficiency is 0.4% • About 4000 B/fb-1 • 1300 B0 • 2700 B- • Analysis optimized to provide maximum • Number of reconstructed B without consideration of “recoil side” physics • Use basic requirements of “recoil side “ • Physics to clean up signal, e.g. additional • Lepton • High energy photon S/B~0.3 Require Lepton (p*>1.0GeV) S/B~2.5

32. Steps in Measurement: B -> Xu l nu Signal Generation Integral of the hybrid model has to be compatible with the non-resonant one (duality hypothesis) Purely non-resonant model based on the De Fazio-Neubert model Hybrid model: Babar MC Resonant (PDG +ISWG2) + non-resonant

33. reconstructed reconstructed generated generated Analysis Method: If it walks like a duck, talks like a duck …it is a duck !...(well, most of the time) • Brecoil selection and reconstruction ofthe X systemB Xu l n: • One and only one lepton with p*> 1 GeV/c • Correlation between lepton charge and Breco flavor (B0 mixing is corrected) • Cut on the missing mass:Mmiss2 < 0.5GeV2, • charge conservation:Qtot=0 • Partially reconstructed neutrinoto reject B0 D*l nevents • kinematic fit (2-C):improve hadronic mass resolution • SeparateBXuln in signal enriched and depleted: • signal enriched:veto events with K± and KS • used to perform the measurement • signal depleted :oneor moreK±or KS in theevent • used as control sample • Systematic error in measurement reduced by measuring ratio Ru/sl=B(B Xu l n )/B(B Xl n )

34. Kinematic Fit To Entire (4S)  BBbar Event Breco Recoil Well known energy and momentum of the incoming Electron and positron Pe- Pe+ (EPEPII , PPEPII) well known! Lepton: 3 measured quantities Reco. B: 4 measured quantities • Energy and Momentum Conservation • Ebreco + EX+ El+E- EPEPII = 0 • Pbreco + PX+ Pl+P- PPEPII = 0 • 4 Constraints + equal mass constraint M(Breco)=M(X,l,)  1 Constraint Neutrino: 3 unknown quantities 5 Constraints –3 unknown quantities = Over constrained system (x 2)

35. MX Correlation: Generated Vs Reconstructed Linear Correlation Unbiased Mass Reconstruction Mass Resolution ~ 300 MeV

36. Effect of Tight Recoil Side (Vub) Selection S/B after recoil side selection S/B when only lepton required quality cuts kinematic constraint Kaon rejection lepton requirement Recoil analysis S/B S/B S/B ~ 1.7 S/B ~ 0.05 Unbiased MX reconstruction and s(MX)~300 MeV.

37. Extraction of B(buln) • Fit on the signal enhanced sample • Three components to fit the MX distribution: buln , bcln , & Hadronic background • Signal efficiency (eselueMxu), Breco efficiency ratio (etu/etsl) and lepton efficiency ratio (elu/elsl) from MC Then multiply by B(BXln) measured by BaBar

38. MX (bu l nu) Selection & Background Rejection Mx cut optimized by minizing the total error: Statistical +Branching ratio uncertainty +Other experimental systematics +Systematics from theory (mb & a) • Optimal point is 1.63 GeV  Cut lowered to a safer 1.55 GeV cut (negligible change in the total error) total error statistical error BR syst. error detector syst. error theory syst. error

39. Fit to the MX distribution Background Subtracted spectrum Resulting MX Spectrum (MC)

40. Breakdown By Category & Stability In Event Selection

41. Theoretical Uncertainty Due to MX Cut • b quark is not at rest in the B meson (Fermi motion) • Fermi motion depends on non-perturbative parameters (mb and a) • Uncertainties on mband a affect the shape of MX spectrum • MX spectrum reweighted • (De Fazio et al JHEP 9906,017) taking into account uncertainties on l1 and L (from CLEO moments analysis PRL87:251808,2001)

42. Theoretical Uncertainty on bu l nu rate • Two effects: • Systematic error due to efficiency of the MX cut • (MX <1.55 GeV) changes since the MX spectrum changes • Systematic error due to selection efficiency (since the efficiency depends mostly on MX itself) • The combination of these two effects (of the same order of magnitude, ~9% each) gives: • s (theory) = 17.5% eMxu

43. Total Systematic Uncertainty in Rate Measurement Statistical error (data+MC) 13.7% Detector simulation errors + Fit systematics 9.8% bcln and D decays modeling + buln decays modeling 6.0% Fermi motion 17.5%

44. |Vub| Result : Preliminary Measure the charmless semileptonic branching ratio And extract Vub Interesting check of theory uncertainty: result very stable if apply a cut on the invariant mass of the lepton-neutrino system (q2) (Bauer et at. hep-ph/0111387) Ru/sl q2 (GeV2)

45. This Result on |Vub| Inclusive |Vub| measurements Precision in this measurement alone is better than the LEP average

46. Future Prognostications (conservative) Redoing the same analysis (no improvement) in 500fb-1 data , the errors should scale as: stat err. exp. syst theo. syst total NOW 6.8% 6% 10.5% 13.5% 500fb-1 < 2.7% < 3% 10.5%?? 11.2% Measurement will be dominated by theoretical uncertainty if nothing improves But… errors on mband a should go down in future Expect the total error can go well below 10% ? Expected systematic error due to shape function? [decrease it with info from Radiative Penguin measurements?]

47. A combination of cuts on q2 and MX reduces theoretical error (Bauer et al. hep-ph/0111387) With 80fb-1 this 2D technique is not suited (additional 40% efficiency due to q2 cut) With 500 fb-1 can lead to better precision With a combination of MX <1.7GeV q2> 8.0 GeV2 Theory error < 9 % ? Future Direction with More Data: q2 vs MX analysis

48. Future: other possible checks & approaches (?) • Try combination of variables. Ciuchini et al. ph/0204140 This approach (uses buln and bsg) in not dependent on shape function. Since resonances in bsg have to be removed, efficiency will go down by >50%. In 500fb-1stheo(Vub) ~ 5% ?? • Can we measure mb directly on our data-sample? Kowalewski et al. (ex/0205038) claim one can, using With the current data-sample (80fb-1) s(mb)~120MeV • in 500fb-1s(mb)~50MeV  stheo(Vub) ~ 6% ?? • Too aggressive expectation ?

49. Reconstructing Exclusive B0 p- l+n , B+ r0 l+nOn the Recoil Side ~500fb-1 B0p-ln(*) B+r0ln (*) 18 Fitted MX Fitted MX 16 14 12 10 8 6 6 4 2 (*)cuts not yet optimized

50. Sensitivity With 500 fb-1 ? B0p-ln B+r0ln B+wln B+p0ln