Test of Lepton Universality and Searches for Lepton Flavor Violation at BaBar Page: 4 of 6
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An extended unbinned maximum likelihood fit is performed to the distribution of the variable
x = lpiJ/EB, that is, the momentum of the primary lepton (pi) normalized to the beam energy
(EB). The signal distribution is expected to peak at x - 0.97, while the T-pair background
x distribution is smooth and approaches zero as x - 0.97, and the Bhabha and p-pair events
have instead a peaking behavior at x - 1. PDFs are chosen for each of these components,
using samples of data and of MC simulated events. The signal yield NSIG is extracted and
found consistent with the no signal-hypothesis within 1.8a- in all the signal channels. Since
no statistically significant signal is observed, the 90% CL upper limit (UL) on the branching
fraction 13 of each decay is determined, using a Bayesian technique, in which the prior likelihood
is uniform in 13 and assumes that 13 > 0.
In the UL calculation, the systematic uncertainties affecting the measurement are also taken
into account. The dominant contribution to the systematic uncertainty comes from an imperfect
knowledge of the PDF shapes. The resulting ULs4 are summarized in Table 1 and are of 0(10-6),
representing the first constraints on 3(T(nS) - e T+), while improving the sensitivity with
respect to the previous ULs 13 on B(T(nS) - P Tf).
Table 1: 90% CL ULs on the branching fractions B for signal decays T(nS) -> - .
3.2 Search for charged LFV in the decays TW - e*ry and TW -4 Wy
Another environment for LFV processes is T decay. In particular, TW 1 -2 (where l = e, p) is
a favored decay mode in several NP scenarios, with predicted branching fractions close to the
current experimental limits.
Besides being a B-factory, BABAR has been usefully employed as a T-factory as well, since
the cross sections for the production of T-pairs and BB-pairs are comparable. This analysis uses
the complete BABAR dataset, which corresponds to - 960 x 106 T decays.
The reconstructed events e+e- - T+T- show a clear topology, being well divided in two
hemispheres: the signal side, containing the l -ypair, required to have mass and energy compat-
ible with the T mass and the beam energy, respectively; and the tag side, which is expected to
contain a SM T decay, reconstructed in events where the T lepton goes to one or three charged
tracks (with undetected neutrinos). The signal side is further required to contain only one
with energy greater than 1 GeV and one track identified as an electron or a muon, separated by
an angle determined by the kinematic of the process, since ry and 1 are emitted back-to-back in
the T rest frame.
The main sources of background come from irreducible T-pair events, l+l-- events and
hadronic T decays with mis-identification of the charged 7r.
Signal decays are identified by two kinematical variables: the energy difference AE = ECU _
s/2 and the beam-energy constrained T mass (mEe). The distributions of events in mEe versus
AE are shown in Figure 2. In this plane, a region is defined as mEe E [1.55, 2.05] GeV/c2 and
AE E [-0.14,0.14] GeV, and used to extract from fits the expected fractions of background
events. The number of events in the 2a- signal ellipses (0 events for TW -4 e W and 2 events
for TW 4 p/W y) are found to be compatible with the background expectation, without evidence
for a signal. After the estimate of the systematic uncertainties, which are mainly due to the
Mode UL (10-6)
3(T(2S) -4 e TI) < 3.2
3(T(2S) - P T+) < 3.3
3(T(3S) -4 e T+) < 4.2
3(T(3S) - P T+) < 3.1
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Guido, Elisa & /Genoa U. /INFN, Genoa. Test of Lepton Universality and Searches for Lepton Flavor Violation at BaBar, article, June 28, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc842613/m1/4/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.