collaboration* reported on new measurements of
decays of B mesons into final states containing a tau lepton, τ,
third-generation cousin of the electron and muon. The decay branching
fractions are higher than predicted by the Standard Model with a 3.4σ
level of significance.
provide an interesting probe of possible new physics at high mass scales.
While the related decays
(where l represents an electron or
muon and D(*) is a
D or D* meson (which contain a charm quark) have long been used to measure
properties of the Standard Model, the decays
the large mass of the tau implies an affinity to Higgs bosons or possibly
non-Standard Model processes. Within the Standard Model, the
Higgs boson is related to the mechanism by which the quarks, leptons and
force-carrying gauge bosons acquire mass. Consequently, the Higgs is predicted
to couple more strongly to heavier particles than to lighter ones. In
extensions of the Standard Model, such as Supersymmetry (SUSY), there can be
multiple electrically neutral and charged Higgs bosons, but the property of
preferentially coupling to more massive particles remains in these models.
the initial B meson and the final-state D(*)
and tau all
contain massive quarks or leptons, making it a very sensitive probe of
possible non-Standard Model
Higgs effects. However, because the tau rapidly decays into a variety
of lighter particles, including neutrinos which pass undetected
through particle detectors, the process
is difficult to detect.
measures the ratios of branching fractions
and R(D*) =
B(B→D*τντ)/B(B→D*lνl) to be R(D)=0.440±0.058±0.042 and
R(D*) = 0.332±0.024±0.018, which exceed the Standard Model expectations by
2.0 and 2.7 standard deviations, respectively. Taken together, and assuming
the Standard Model, the probability of obtaining these results or results
further from the Standard Model is 0.069%, which for a one-dimensional
Gaussian-distributed observable is equivalent to a significance of 3.4 standard
deviations. It is also worth noting that this excess cannot be explained
in a minimal extension of the Standard Model known as the Type II Two Higgs Doublet Model. By measuring the
ratios R(D) and R(D*), many systematic uncertainties cancel, including much of
the uncertainty associated with theoretical input, including notoriously
difficult to calculate non-perturbative QCD contributions. Restricting the
analysis to leptonic decays of the tau further minimizes experimental
systematic errors, as the same types of particles are detected in both the
numerator and denominator of the ratios R(D) and R(D*).
The results are based on the full
experiment data set and are
significantly more sensitive than previously published studies of these
recorded more than 470 million B meson pairs, produced in
the process e+e−→Upsilon(4S) at the SLAC
B-factory between 1999 and 2008. To deal with the fact that neutrinos
from the tau decays in
are not detected,
and reconstructed all of the decay products associated with the second
in each Upsilon(4S) decay. This enabled the presence of the undetected
neutrinos to be inferred from conservation of energy and momentum. Advanced multivariate
techniques were used to distinguish signal events from possible backgrounds
from other B decays or non-Upsilon(4S) events.
in each of four individual decay modes, involving the four
combinations of charged or neutral B and D(*) mesons.
They reported the first
observations of the
mode with a significance of 6.8σ.
describing these results is being submitted to
Physical Review Letters, and the results were
presented at the Flavor Physics and CP (FPCP) conference in Hefei, China
on 24 May 2012.
This work is supported by DOE and NSF (USA), NSERC (Canada), CEA and CNRS-IN2P3
(France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands),
NFR (Norway), MES (Russia), MICIIN (Spain) and STFC (United Kingdom).
Individuals have received support from the Marie Curie EIF (European Union)
and the A. P. Sloan Foundation (USA).