Test of Lepton Universality and Searches for Lepton Flavor Violation at BaBar Page: 3 of 6
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2500 D 3000
- 2500 2
1500 - 50
9 91 92 93 94 95 96 944 9445 945 9455 946 9465 947 9475 948
M (GeV/c2) M (GeV/c2)
S1406 - C
944 9445 9 45 9455 940 9465 9 4 9415 941
Figure 1: 1-D fit projections for M+,- (top left) and for M,00 (top right) in the D, sample, and for M..,
(bottom) in the DT sample. In each plot the dashed line represents the background shape, while the solid line is
the sum of signal and background contributions to the fit, and the points are the data.
Assuming values for Xd, F(gb(1S)) and M06(ls) as previously stated 7, the present measurement
excludes an A0 with mass lower than 9 GeV/c2 at 90% of confidence level (CL).
3 Searches for charged lepton flavor violation
Lepton flavor violation (LFV) can occur via neutrino oscillation, but this has never been observed
in charged processes because the tree-level contributions are suppressed to rates not achievable
by the current experimental sensitivity. In many extensions of the SM, enhancements of these
rates are possible, up to a detectable level, with expected branching fractions of 0(10-6-10-8).
An observation of LFV in charged decays would be a clear signature of NP, and improved limits
on the branching fractions of such processes further constrain the theoretical models proposed.
BABAR can search for charged LFV in several typologies of decays, both of the T resonances
and of the T leptons.
3.1 Search for charged LFV in narrow T decays
This analysis searches for the charged LFV decays T(nS) - lr-TF, with l = e, p and n = 2, 3,
using the - 1.2 x 108 T(3S) and - 1.0 x 108 T(2S) resonances collected by BABAR.
The signature of the signal events consists of exactly two oppositely charged particles: a
primary lepton, identified as an electron or a muon, with momentum close to the beam energy,
and a secondary charged lepton or pion from the T decay (along with other neutral particles
not reconstructed). In order to suppress background events, if the T decays leptonically, the
primary lepton and the T-daughter are required to have different flavors. Thus, for each value of
n, four signal channels are defined, consisting of leptonic and hadronic T decay modes, and with
an electron or a muon as primary lepton. The main sources of background come from T-pair
production, as well as from Bhabha and p-pair events.
The event selection consists of several requirements, related to the particle identification and
to the kinematics of the T-daughter. The final selection efficiencies, estimated on samples of MC
simulated events, vary in the range (4-6)% depending on the decay mode considered.
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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/m1/3/: accessed July 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.