Advancing Precision Measurement of |Vub| in B→pℓn Decays

Measurement of |Vub|with B→ pℓn Masahiro Morii Harvard University Laboratory for Particle Physics and Cosmology

Motivation • |Vub| determines the left side of the UT • Precise |Vub| and sin2b strong constraint on the UT • Uncertainty on |Vub| is dominated by theory errors • Measurements with different methods important • InclusiveB→ Xuℓn • Use difference in kinematics to separate uℓnfrom cℓn • Theory (OPE, SCET) must predict signal spectra • Current theory error ~5% of |Vub| • ExclusiveB→ pℓn,rℓn, wℓn, … • Better S/B, esp. if we’ve tagged one B • Theory (LCSR, LQCD, etc.) must predict form factors • Theory error hard to quantify M. Morii, Harvard

B → pℓn Form Factors • Two (among many) types of calculations • Light-Cone Sum Rules • Latest Ball/Zwicky (PRD71:014015) quote 10-13% error at q2 = 0 • Not valid above q2 ~ 14 GeV2 • Lattice QCD • Older calculations were “quenched”  extra 15% error • Unquenched calculations from HPQCD (PRD73:074502) and Fermilab (hep-lat/0409116) quote ~11% systematic error at high q2 • Not valid below q2 ~ 15 GeV2 • Theory errors on |Vub| comparable to the inclusive approach • We must measure partial rates in q2 bins M. Morii, Harvard

Previous Measurements • Untagged measurements have better statistics • Background and cross-feed (from rℓn) higher • Tagged-B measurements have better S/B • Statistics limited  Binning in q2 requires large statistics • Semileptonic recoil Balance between efficiency and purity M. Morii, Harvard

Analysis Flow a.k.a. Y Data sample contains232 M BB events Event preselection ℓ D(*) v Find D(*)ℓv tag(s) Reconstruct D and D*Combine with lepton Find pℓv candidate(s) Recoil of the tag containsp + ℓand little else tag B Pick the best candidate signal B Allow one candidate/event Extract signal yield Fit cos2fB distirubtion v Divide by efficiency p ℓ Double-tag sample determines edata/eMC Branching fraction M. Morii, Harvard

D(*)Reconstrction • Reconstruct D mesons • Reconstruct D*+ mesons We use mD sidebands to subtract combinatoric background, assuming linear distribution Twice as wide as the other channels Little statistics in this channel M. Morii, Harvard

B→ D(*)ℓn Tag • Combine a D(*) candidate with a lepton candidate with p* > 0.8 GeV in the CMS • Calculate • For correct tags, qBY = anglebetween B and D(*)ℓ momenta • Signal should peak in−1 < cosqBY< +1 • Background is broad on-peak datab → uℓv MCB0B0 MCB+B− MCoff-peak data M. Morii, Harvard

B→ pℓnSignal • Look for a lepton and a pion in the recoil side • Lepton p* > 0.8 GeV • Pion with opposite charge • Nothing else left in the event • No tracks in the drift chamber • No cluster in the calorimeter • Calculate • Signal between ±1 on-peak datasignal MCb → uℓv MCB0B0 MCB+B− MCoff-peak data M. Morii, Harvard

Signal Kinematics • Tag B and recoil B are back-to-back • Combine kinematical information into a single variable • cos2fB < 1 for correctly-reconstructed signal events Angle between the B momentum and the plane defined by the D(*)ℓ and pℓ momenta M. Morii, Harvard

on-peak datasignal MCBB-bar MCcombinatoric background cos2fB • Use cos2fB distribution to distinguish signal from background • Background distributions are nearly flat • Tested using sideband control samples • Perform unbinned maximum likelihood fitto extract signal yields in 3 bins of q2 q2 < 8 GeV2 8 < q2 < 16 GeV2 q2 > 16 GeV2 M. Morii, Harvard

Double-Tag Sample • Events with two non-overlapping tags • Number of double-tags  (Tag efficiency)2 • Selection of double-tag events reproducethe signal selection as closely as possible • Not perfect – e.g., the number of remainingneutral clusters depend on both sides • Compare data and MC • Error includes statistics, backgroundnormalization, Ncluster cut dependence, etc. on-peak datasignalincorrect tagsbackground M. Morii, Harvard

B(B0→ p–ℓ+n) • We measure the partial and total BFs (in 10-4) M. Morii, Harvard

Systematic Errors • Main systematics are: • Tagging efficiency • cos2fB distribution of BB background •  → rℓn and other Xuℓn background in high-q2 bin • Monte Carlo statistics • Still small comparedwith the stat. error • Some of the errors are intentionally conservative M. Morii, Harvard

Combining Analyses • We combine results of the analyses by 3 groups B0 semileptonic tag B+ semileptonic tag B0 hadronic tag B+ hadronic tag M. Morii, Harvard

How We Compare • Competitive and statistics-limited result • Paper has been submitted to Phys. Rev. Lett. • Next steps: • Update with 211  400 fb-1 data • Include other light hadrons (′) M. Morii, Harvard

Extraction of |Vub| • We use four calculations of the FF and find • c.f. HFAG average of inclusive measurements is where PRD71:014015 PRD73:074502 hep-lat/0409116 NPB619:565 M. Morii, Harvard

Summary • |Vub| is a critical piece of the CKM “puzzle” • Harvard group makes strong contribution in this area • We pursue two analyses based on complementary theoretical approaches • We measured B(B0→ p–ℓ+n) in the recoil of B0 → D(*)+ℓ–n and extracted |Vub| • Result (hep-ex/0607089) has been presented at ICHEP 2006 and submitted to Phys. Rev. Lett. using FF from a LQCD calculation M. Morii, Harvard

Advancing Precision Measurement of |Vub| in B→pℓn Decays