Searches for the Decays B0 to l+- tau-+ and B+ to l+ nu(L=e,mu) using Hadronic Tag Reconstruction Page: 4 of 8
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in any of the decay modes. Branching fraction upper limits of B(B+ _ e+v) < 5.2 x 10-6,
B(B+ _> +v) < 5.6 x 10-6, B(B0 -> e+-) < 2.8 x 10-- and B(B0 -pT+r-) < 2.2 x 10-5, are
obtained at 90% confidence level.
PACS numbers: 13.25.Hw, 12.15.Hh, 11.30.ErIn this paper, we present searches for the decays B+ ->
{+v and the lepton flavor violating decays B0 -> i T,
where = e, p [1], using a technique in which the ac-
companying B in the event is exclusively reconstructed.
This method has not previously been used for searches
for these modes and, although statistically limited with
the present BABAR data sample, shows promise for future
studies at, for example, a high luminosity Super B fac-
tory [2]. While the former decay modes are allowed in
the Standard Model (SM) and the latter are not, both
are potentially sensitive to New Physics (NP) effects,
such as contributions by neutral and charged non-SM
Higgs [3, 4].
Searches for rare B decays with neutrinos in the final
state are challenging due to the limited availability of
kinematic constraints. However, purely leptonic B de-
cays involving an electron or a muon have a clear ex-
perimental signature in the form of a high momentum
lepton. Combined with clean theoretical predictions due
to the lack of QCD contributions in the final state, such
leptonic B decays present an ideal place to test the SM
against NP models.
In the SM, B+ j+v decays proceed via an annihila-
tion of b and a quarks into a virtual W+ boson. In the
SM the branching fraction for this type of decay is given
by [5]:L3(B+ -> QtY ) = m 1 (1
(1)
where GF is the Fermi coupling constant, ml is the
lepton mass and mB, rB and fB are the mass, life-
time and decay constant for the B meson. Vubl is the
Cabibbo-Kobayashi-Maskawa matrix element which de-
scribes the transition from b to a quarks [6]. Within the
SM, a determination of any one of the leptonic branch-
ing fractions represents a determination of the product
Vub" -fB, which can be directly compared with determina-
tions from lattice calculations [6], B-mixing and semilep-
tonic decay measurements [7, 8]. As seen in Eq.(1), the
decay rates are proportional to mj2, resulting in SM pre-
dictions for the p and e modes which are suppressed by
factors on the order of 250 and 107, respectively, com-
pared with the T mode. Taking the branching fraction
B(B+ _ T+v7) (1.31 0.48) x 10-4 from the combina-
tion of recent BABAR and BELLE results [9, 10] implies
BSM(B+ , +vp) ~ 5.2 x 10-7 and BSM(B+ _ e+ve) ~
1.2 x 10-1. New Physics contributions to these pro-
cesses can enhance or suppress the decay rates compared
to the SM, and may either preserve or violate the rela-tive rates of the three leptonic modes depending on the
particular NP model [3, 11]. Thus, the e and p modes be-
come particularily interesting in light of recent evidence
for the B+ _ + v7 decay mode. Currently, the most
stringent published limits on B+ , P+v are from the
BELLE collaboration with B(B+ - e+v) < 9.8 x 10-
and 3(B+ -* p+v) < 1.7 x 10-6 [12]. Earlier stud-
ies by CLEO and BABAR collaborations are also avail-
able [13, 14].
The lepton-flavor-violating (LFV) leptonic B decays,
such as B0 -> {+ -, are forbidden in the SM in the ab-
sence of non-zero neutrino masses, but can occur via one-
loop diagrams if neutrino oscillations are included. The
rates of such processes, however, would be substantially
below current or anticipated future experimental sensi-
tivities. On the other hand, many models of physics be-
yond the SM, in particular supersymmetric seesaw mod-
els [4], predict dramatically higher rates for these de-
cays. In the case of Higgs-mediated LFV processes, cou-
plings to heavier leptons are favored, making B0 -> + q-
particularily interesting. In the general flavor-universal
MSSM, the branching fractions allowed for B0 - +T-
are ~ 2 x 10-10 [4]. Such decays could be within the
reach of a Super B factory with a data sample of 10 to
50 ab-1. The current best experimental limits on the
branching fractions for these two decays are B(B0 -
e+-) < 1.1 x 10-4 and 3(B0 -> p+-) < 3.8 x 10-,
set by the CLEO collaboration with 10 fb-i of data [15].
The searches described in this work are based on a
data sample of approximately 378 million BB pairs, cor-
responding to an integrated luminosity of 342 fb-i col-
lected at the T(4S) resonance by the BABAR detector
at the PEP-II asymmetric e+e- storage ring. Recon-
structing the accompanying B meson in specific hadronic
modes prior to the signal selection allows the missing mo-
mentum vector of the neutrino(s) to be fully determined.
The resulting increase in the energy resolution and the
ability to infer the signal B meson rest frame provide the
extra kinematic handles that permit signal events to be
cleanly distinguished from the background. Previous B
factory searches for B+ { +v and B0 -> + w- have
used an inclusive method in which the accompanying B
is not explicitly reconstructed. This results in a signifi-
cantly higher efficiency, but also a substantially increased
background compared with the exclusive reconstruction
method presented here. With the current level of lu-
minosity, the inclusive method provides more stringent
limits. However, due to the very low background achiev-
able with the exclusive method, the two methods have
about the same sensitivity for a statistically significantm2 w T
B
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Aubert, Bernard; Bona, M.; Karyotakis, Y.; Lees, J. P.; Poireau, V.; Prudent, X. et al. Searches for the Decays B0 to l+- tau-+ and B+ to l+ nu(L=e,mu) using Hadronic Tag Reconstruction, article, January 30, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc898996/m1/4/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.