Test of Lepton Universality and Searches for Lepton Flavor Violation at BaBar Page: 2 of 6
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gin, MT is the T(1S) mass and M is the lepton mass. In the SM, one expects the quantity
RT (T(1S)) = i>+ to be very close to one (in particular, Rg(T(1S)) 0.992 8
In the next-to-minimal extension of the SM 9, deviations of RT from the SM expectation
may arise due to a light CP-odd Higgs boson, A0. Present data 10 do not exclude the existence
of such a boson with a mass below 10 GeV/c2. A0 may mediate the following processes 7:
T(1S) -> A02 -> l+-- or T(1S) A nb(1S)2,nb(1S) -> A -+ i-. (2)
If the photon remained undetected, the lepton pair would be ascribed to the T(1S) and
the proportionality of the coupling of the Higgs to the lepton mass would lead to an apparent
violation of lepton universality. The deviation of RT from the expected SM value depends on
Xd = cos 0A tan 0 (where 0A measures the coupling of the T(1S) to the A0, and tan 0 is the
ratio of the vacuum expectation values of the two Higgs doublets) and on the mass difference
between A0 and nb(1S). Assuming Xd = 12, F(nb(1S)) = 5 MeV, and the measured M%(ls) 11
the deviation of RyN(T(1S)) may be as large as - 4%, depending on the A0 mass 7.
A measurement of this ratio has already been performed, with the result RT (T(1S)) =
1.02 0.02(stat.) 0.05(syst.) 12
This analysis focuses on the measurement of RyN(T(1S)) in the decays T(3S) -- T(1S)7+7-
with T(1S) -- l+- and l = p,,T of the - 1.2 x 108 T(3S) collected by BABAR. Only T decays to
a single charged particle (plus neutrinos) are considered, resulting in final states of exactly four
detected particles for both the p+-- and T+T- samples.
The event selection is optimized using Monte Carlo (MC) simulated events. Different selec-
tion criteria are used for the T(1S) -- [+ - decays (Dr) and the T(1S) -- T+T- decays (Dr),
because in the latter the presence of neutrinos in the final state leads to a larger contamination
from the background (mainly non-leptonic T(1S) decays and ce+ --3 T+T- events). The final
selection efficiency for the reconstructed decay chains, estimated from a sample of MC simulated
events, are E - 45% and Eq- - 17% for the p+-- and the T+T- final states, respectively.
An extended unbinned maximum likelihood fit, applied simultaneously to the two disjoint
datasets D and DT, is used to extract RT = -_ i where NigN, (N8agT) indicates
the number of signal events in the D (Dr) sample. For the D sample, a 2-dimensional
probability density function (PDF) is used, based on the invariant dimuon mass M +N- and
M. _ _, the invariant mass of the system recoiling against the 7r-pair, defined as: M. Y_ =
s + MA - 2. - fi E*,, where fi is the e+c- CM energy and EX* indicates the 7r-pair energy
calculated in the CM frame. For the DT sample, a 1-dimensional PDF is used, based on M _.
The functional forms of the PDFs describing the signal components are modeled from a dedicated
sub-sample consisting approximately of one tenth of the D sample, then discarded from the
final result in order to avoid any bias. The data collected below the T(3S) resonance are used to
model the background shapes. The result of the simultaneous fit is RT = 1.006 0.013, where
the quoted error is statistical only. Figure 1 shows the projections of the fit results for the three
Several systematic errors cancel in the ratio. The main systematic uncertainties are related
to the differences between data and simulation in the efficiency of event selection, the muon
identification, and the trigger and background filters. There is also a systematic uncertainty on
the signal and background yields due to the imperfect knowledge of the PDFs used in the fit.
The total systematic uncertainty, obtained by summing in quadrature all the contributions, is
estimated to be 2.2%. Including all the systematic corrections, the ratio RT is found to be 3:
R1T-(T(1S)) = 1.005 0.013(stat.) 0.022(syst.).
No significant deviation of the ratio RT from the SM expectation is observed. This result im-
proves both the statistical and systematic precision with respect to the previous measurement 12
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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/m1/2/: accessed May 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.