Measurement of Vub with B pln

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Measurement of Vubwith B ? pln

• Masahiro Morii
• Harvard University
• Laboratory for Particle Physics and Cosmology

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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
• Inclusive B ? Xuln
• Use difference in kinematics
  to separate uln from cln
• Theory (OPE, SCET) must predict signal spectra
• Current theory error 5 of Vub
• Exclusive B ? pln, rln, wln,
• Better S/B, esp. if weve tagged one B
• Theory (LCSR, LQCD, etc.) must predict form
  factors
• Theory error hard to quantify

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B ? pln Form Factors

• Two (among many) types of calculations
• Light-Cone Sum Rules
• Latest Ball/Zwicky (PRD71014015) 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 (PRD73074502)
  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

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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

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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 DCombine with lepton
Find plv candidate(s)
Recoil of the tag containsp l and 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
Double-tag sample determines edata/eMC
l
Branching fraction
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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
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B ? D()ln Tag

• Combine a D() candidate with a lepton candidate
  with p gt 0.8 GeV in the CMS
• Calculate
• For correct tags, qBY anglebetween B and D()l
  momenta
• Signal should peak in-1 lt cosqBY lt 1
• Background is broad

on-peak datab ? ulv MCB0B0 MCBB- MCoff-peak
data
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B ? pln Signal

• Look for a lepton and a pion in the recoil side
• Lepton p gt 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 MCBB-
MCoff-peak data
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Signal Kinematics

• Tag B and recoil B are back-to-back
• Combine kinematical information into a single
  variable
• cos2fB lt 1 for correctly-reconstructed signal
  events

Angle between the B momentum and the plane
defined by the D()l and pl momenta
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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 lt 8 GeV2
8 lt q2 lt 16 GeV2
q2 gt 16 GeV2
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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, backgroundnormalizatio
  n, Ncluster cut dependence, etc.

on-peak datasignalincorrect tagsbackground
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B(B0 ? pln)

• We measure the partial and total BFs (in 10-4)

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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

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Combining Analyses

• We combine results of the analyses by 3 groups

B0 semileptonic tag B semileptonic tag B0
hadronic tag B hadronic tag
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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 (??????????')

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Extraction of Vub
where

• We use four calculations of the FF and find
• c.f. HFAG average of inclusive measurements is

PRD71014015
PRD73074502
hep-lat/0409116
NPB619565
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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 ? pln) in the recoil of B0 ?
  D()ln 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