86 research outputs found
Charged-Lepton Mixing and Lepton Flavor Violation
We present a model for calculating charged-lepton mixing matrices. These
matrices are an essential ingredient for predicting lepton flavor-violating
rates in the lepton number nonuniversal models recently proposed to explain
anomalies in B-meson decays. The model is based on work on "constrained flavor
breaking" by Appelquist, Bai and Piai relating the charged-lepton mass matrix,
M_l, to those for the up and down-type quarks, M_{u,d}. We use our recent model
of lepton nonuniversality to illustrate the magnitudes of flavor-violating
B-decay rates that might be expected. Decays with mu tau final states generally
have the highest rates by far.Comment: 13 pages, 1 figure. v3: predicted rates involving tau-leptons
corrected for phase space; added textual comparison of our predictions with
existing literature; other minor textual corrections. Matches journal versio
BR(Bs to mu+ mu-) as an electroweak precision test
Using an effective-theory approach, we analyze the impact of BR(Bs to mu+
mu-) in constraining new-physics models that predict modifications of the
Z-boson couplings to down-type quarks. Under motivated assumptions about the
flavor structure of the effective theory, we show that the bounds presently
derived from BR(Bs to mu+ mu-) on the effective Z-boson couplings are
comparable (in the case of minimal flavor violation) or significantly more
stringent (in the case of generic partial compositeness) with respect to those
derived from observables at the Z peak.Comment: 8 pages, 1 figure. v3: minor textual modifications; minor update in
the numerics; no change in results. Matches journal version (PLB
MT2-reconstructed invisible momenta as spin analizers, and an application to top polarization
Full event reconstruction is known to be challenging in cases with more than
one undetected final-state particle, such as pair production of two states each
decaying semi-invisibly. On the other hand, full event reconstruction would
allow to access angular distributions sensitive to the spin fractions of the
decaying particles, thereby dissecting their production mechanism. We explore
this possibility in the case of Standard-Model t-tbar production followed by a
leptonic decay of both W bosons, implying two undetected final-state neutrinos.
We estimate the t and tbar momentum vectors event by event using information
extracted from the kinematic variable MT2. The faithfulness of the estimated
momenta to the true momenta is then tested in observables sensitive to top
polarization and t-tbar spin correlations. Our method thereby provides a novel
approach towards the evaluation of these observables, and towards testing
t-tbar production beyond the level of the total cross section. While our
discussion is confined to t-tbar production as a benchmark, the method is
applicable to any process whose decay topology allows to construct MT2.Comment: 28 pages, 14 figures, 3 tables. Changes in v2: new paragraph on ISR
(end of sec. 2.2); new sec. 3.3 on the method's comparison with existing
ones; typos fixed; reference added. Matches journal version (JHEP
More Lepton Flavor Violating Observables for LHCb's Run 2
The R_K measurement by LHCb suggests non-standard lepton non-universality
(LNU) to occur in b -> s l+ l- transitions, with effects in muons rather than
electrons. A number of other measurements of b -> s l+ l- transitions by LHCb
and B-factories display disagreement with the SM predictions and, remarkably,
these discrepancies are consistent in magnitude and sign with the R_K effect.
Non-standard LNU suggests non-standard lepton flavor violation (LFV) as well,
for example in B -> K l l' and B_s -> l l'. There are good reasons to expect
that the new effects may be larger for generations closer to the third one. In
this case, the B_s -> mu e decay may be the most difficult to reach
experimentally. We propose and study in detail the radiative counterpart of
this decay, namely B_s -> mu e gamma, whereby the chiral-suppression factor is
replaced by a factor of order alpha/pi. A measurement of this mode would be
sensitive to the same physics as the purely leptonic LFV decay and, depending
on experimental efficiencies, it may be more accessible. A realistic
expectation is a factor of two improvement in statistics for either of the
B_{d,s} modes.Comment: 14 pages, 2 figures. v4: some typos corrected. Such corrections are
not implemented in the journal versio
Lepton Flavor Violation in B Decays?
The LHCb Collaboration's measurement of R_K = B(B+ -> K+ mu+ mu-)/B(B+ -> K+
e+e-) lies 2.6 sigma below the Standard Model prediction. Several groups
suggest this deficit to result from new lepton non-universal interactions of
muons. But non-universal leptonic interactions imply lepton flavor violation in
B decays at rates much larger than are expected in the Standard Model. A simple
model shows that these rates could lie just below current limits. An
interesting consequence of our model, that B(B_s -> mu+ mu-)_{exp}/B(B_s -> mu+
mu-)_{SM} = R_K = 0.75, is compatible with recent measurements of these rates.
We stress the importance of searches for lepton flavor violations, especially
for B -> K mu e, K mu tau and B_s -> mu e, mu tau.Comment: 8 pages. Discussion of P'5 added; some discussion sharpened;
discussion of decays with tau's modified; references adde
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