Measurement of the B0 ->π−ℓνand B ->η(′)ℓνBranching Fractions, the B0 ->π−ℓνand B ->ηℓνForm-Factor Shapes, and Determination of |Vub| Page: 4 of 18
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4
We report the results of a study of the exclusive charmless semileptonic decays, B+ - f+v
and B0 -> 7r-f+v, undertaken with approximately 464 million BB pairs collected at the T(4S)
resonance with the IABAR detector. The analysis uses events in which the signal B decays are
reconstructed with a loose neutrino reconstruction technique. We obtain partial branching fractions
for B+ -* V+v and B0 -> 7r-f+v decays in three and 12 bins of q2, respectively, from which
we extract the f+(q2) form-factor shapes and the total branching fractions B(B+ -* qf+v)
(0.36+ 0.05stat + 0.04syst) x 10-4 and B(B0 -> 7r-f+v) = (1.42 + 0.05stat + 0.07syst) x 10-4. We
also measure B(B+ + q'v) = (0.24 + 0.08stat +0.03syst) x 10-4. We obtain values for the
magnitude of the CKM matrix element V7bl using three different QCD calculations.
PACS numbers: 13.20.He, 12.15.Hh, 12.38.Qk, 14.40.NdI. INTRODUCTION
A precise measurement of the CKM matrix [1] ele-
ment Vub j will constrain the description of weak inter-
actions and CP violation in the Standard Model. The
rate for exclusive charmless semileptonic decays involv-
ing a scalar meson is proportional to Vbf+(q2)12, where
the form factor f+(q2) depends on q2, the square of
the momentum transferred to the lepton-neutrino pair.
Values of f+(q2) are given by unquenched Lattice QCD
(LQCD) calculations [2, 3], reliable only at large q2 (> 16
GeV), and by Light Cone Sum Rules (LCSR) calcula-
tions [4, 5], based on approximations only valid at small
q2 (C 16 GeV2). The value of IVueb can thus be deter-
mined by the measurement of partial branching fractions
of charmless semileptonic B decays. Extraction of the
f+(q2) form-factor shapes from exclusive decays [6] such
as B0 -> -f+v [7] and B+ -> (')f+v may be used to
test theoretical calculations [8]. The values of the branch-
ing fractions (BF) of the B+ -> y(')+v decays will also
improve our knowledge of the composition of charmless
semileptonic decays and help constrain the size of the
gluonic singlet contribution to the form factors for these
decays [5].
In this paper, we present measurements of the par-
tial BFs AB(B+ _> yf+v, q2) and AB(B0 -> 7--f+, q2)
in three and 12 bins of q2, respectively, as well as the
total BFs for all three decay modes. Values of the
total BFs were previously reported in Refs. [7, 9 12].
We use the values of AB(q2) for the B0 -> --f+v
mode with form-factor calculations [2 4] to obtain val-
ues of Vubl. Values of Vuebj have previously been ex-
tracted from B0 - 7--f+v measurements by CLEO [9],
BABAR [7, 10, 13] and Belle [11]. A very recent measure-
ment by BABAR [14] will be discussed in Section VII .
*Now at Temple University, Philadelphia, Pennsylvania 19122,
USA
tAlso with UniversitA di Perugia, Dipartimento di Fisica, Perugia,
Italy
tAlso with UniversitA di Roma La Sapienza, I-00185 Roma, Italy
5Now at University of South Alabama, Mobile, Alabama 36688,
USA
'Also with UniversitA di Sassari, Sassari, ItalyII. DATA SAMPLE AND SIMULATION
We use a sample of 464 million BB pairs correspond-
ing to an integrated luminosity of 422.6 fb-1 collected
at the T(4S) resonance with the BABAR detector [15] at
the PEP-II asymmetric-energy e+e- storage rings and a
sample of 44 fb-I1 collected approximately 40 MeV be-
low the T(4S) resonance (denoted "off-resonance data").
Detailed Monte Carlo (MC) simulations are used to op-
timize the signal selections, to estimate the signal ef-
ficiencies, and to obtain the shapes of the signal and
background distributions. MC samples are generated for
T (4S) -> BB events, e+e->-- vru/dd/ss/cc/T+T- (con-
tinuum) events, and dedicated BB samples containing
B0 -> --f+v and B+ -> y('){+v signal decays. The sig-
nal MC events are produced with the FLATQ2 genera-
tor [16] and are reweighted to reproduce the f+(q2, a, CB)
Becirevic-Kaidalov (BK) parametrization [17], where the
values of the shape and normalization parameters, a and
CR, are taken from Ref. [7]. The BABAR detector's ac-
ceptance and response are simulated using the GEANT4
package [15].
III. EVENT RECONSTRUCTION AND
CANDIDATE SELECTION
We reconstruct the B0 -* --f+v and B+ nW f+v
decays. The y meson is reconstructed in the y -y-y and
n -> ,+7-r decay channels (combined BF of 62%) while
the n' is reconstructed in the n' -> yq+j-- channel, fol-
lowed by the y - ry decay (product BF of 17.5%) [18].
The n' -> p%- decay channel suffers from large back-
grounds and we do not consider it. We carry out an
untagged analysis with a loose neutrino reconstruction
technique [7], thereby obtaining a large candidate sam-
ple.
Event reconstruction with the BABAR detector is de-
scribed in detail elsewhere [15]. Electrons (muons) are
identified by their characteristic shower signatures in
the electromagnetic calorimeter (muon detector), while
charged hadrons are identified using the Cherenkov de-
tector and dE/dx measurements in the drift chamber.
The average electron (muon) reconstruction efficiency
is 93% (70%), while its misidentification probability is
< 0.2% (< 1.5%). The neutrino four-momentum, P~
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del Amo Sanchez, P.; Lees, J.P.; Poireau, V.; Prencipe, E.; Tisserand, V.; /Annecy, LAPP et al. Measurement of the B0 ->π−ℓνand B ->η(′)ℓνBranching Fractions, the B0 ->π−ℓνand B ->ηℓνForm-Factor Shapes, and Determination of |Vub|, article, August 12, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc833019/m1/4/: accessed December 12, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.