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