Over the past years, experiments accumulated intriguing hints for new physics (NP) in flavor observables, namely in the anomalous magnetic moment of the muon (a µ ), in R(D ( * ) ) = Br(B → D ( * ) τ ν)/Br(B → D ( * ) ν) and in b → sµ + µ − transitions, which are all at the 3 − 4 σ level. In this article we point out that one can explain the R(D ( * ) ) anomaly using two scalar leptoquarks (LQs) with the same mass and coupling to fermions related via a discrete symmetry: an SU(2) L singlet and an SU(2) L triplet, both with hypercharge Y = −2/3. In this way, potentially dangerous contributions to b → sνν are avoided and non-CKM suppressed effects in R(D ( * ) ) can be generated. This allows for smaller overall couplings to fermions weakening the direct LHC bounds. In our model, R(D ( * ) ) is directly correlated to b → sτ + τ − transitions where an enhancement by orders of magnitude compared to the standard model (SM) is predicted, such that these decay modes are in the reach of LHCb and BELLE II. Furthermore, one can also naturally explain the b → sµ + µ − anomalies (including R(K)) by a C 9 = −C 10 like contribution without spoiling µ − e universality in charged current decays. In this case sizable effects in b → sτ µ transitions are predicted which are again well within the experimental reach. One can even address the longstanding anomaly in a µ , generating a sizable decay rate for τ → µγ. However, we find that out of the three anomalies R(D ( * ) ), b → sµ + µ − and a µ only two (but any two) can be explained simultaneously. We point out that a very similar phenomenology can be achieved using a vector leptoquark SU(2) L singlet with hypercharge 2/3. In this case, no tuning between couplings is necessary, but the model is non-renormalizable.

The LHCb Collaboration reported anomalies in B→K^{*}μ^{+}μ^{-}, B_{s}→ϕμ^{+}μ^{-}, and R(K)=B→Kμ^{+}μ^{-}/B→Ke^{+}e^{-}. Furthermore, BABAR, BELLE, and LHCb Collaborations found hints for the violation of lepton-flavor universality violation in R(D^{(*)})=B→D^{(*)}τν/B→D^{(*)}ℓν. In this Letter we reexamine these decays and their correlations to B→K^{(*)}νν[over ¯] using gauge invariant dim-6 operators. For the numerical analysis we focus on scenarios in which new physics couples, in the interaction eigenbasis, to third generation quarks and lepton only. We conclude that such a setup can explain the b→sμ^{+}μ^{-} data simultaneously with R(D^{(*)}) for small mixing angles in the lepton sector (of the order of π/16) and very small mixing angles in the quark sector (smaller than V_{cb}). In these regions of parameter space, B→K^{(*)}τμ and B_{s}→τμ can be order 10^{-6}. Possible UV completions are briefly discussed.

No abstract

We perform a systematic study of the d = 5 Weinberg operator at the one-loop level. We identify three different categories of neutrino mass generation: (1) finite irreducible diagrams; (2) finite extensions of the usual seesaw mechanisms at one-loop and (3) divergent loop realizations of the seesaws. All radiative one-loop neutrino mass models must fall into one of these classes. Case (1) gives the leading contribution to neutrino mass naturally and a classic example of this class is the Zee model. We demonstrate that in order to prevent that a tree level contribution dominates in case (2), Majorana fermions running in the loop and an additional Z 2 symmetry are needed for a genuinely leading one-loop contribution. In the type-II loop extensions, the Yukawa coupling will be generated at one loop, whereas the type-I/III extensions can be interpreted as loop-induced inverse or linear seesaw mechanisms. For the divergent diagrams in category (3), the tree level contribution cannot be avoided and is in fact needed as counter term to absorb the divergence.

No abstract

We discuss the measurement of new physics in long baseline neutrino oscillation experiments. Through neutrino oscillations, the probability to detect new physics effects such as flavor violation is enhanced by interference with the weak interaction. We carefully explain the situations in which interference can take place. Assuming a neutrino factory and an upgraded conventional beam, we estimate the feasibility to observe new physics numerically and point out that we can search new interactions using some channels, for example, → , in these experiments. We also discuss several models which induce effective interactions interfering with the weak interaction, and show that some new physics effects are large enough to be observed in an oscillation enhanced way.

We discuss the systematic decomposition of the dimension nine neutrinoless double beta decay operator, focusing on mechanisms with potentially small contributions to neutrino mass, while being accessible at the LHC. We first provide a (d = 9 tree-level) complete list of diagrams for neutrinoless double beta decay. From this list one can easily recover all previously discussed contributions to the neutrinoless double beta decay process, such as the celebrated mass mechanism or "exotics", such as contributions from left-right symmetric models, R-parity violating supersymmetry and leptoquarks. More interestingly, however, we identify a number of new possibilities which have not been discussed in the literature previously. Contact to earlier works based on a general Lorentz-invariant parametrisation of the neutrinoless double beta decay rate is made, which allows, in principle, to derive limits on all possible contributions. We furthermore discuss possible signals at the LHC for mediators leading to the short-range part of the amplitude with one specific example. The study of such contributions would gain particular importance if there were a tension between different measurements of neutrino mass such as coming from neutrinoless double beta decay and cosmology or single beta decay.

We study the optimization of a neutrino factory with respect to nonstandard neutral current neutrino interactions, and compare the results to those obtained without nonstandard interactions. We discuss the muon energy, baselines, and oscillation channels as degrees of freedom. Our conclusions are based on both analytical calculations and on a full numerical simulation of the neutrino factory setup proposed by the international design study (IDS-NF). We consider all possible nonstandard parameters, and include their complex phases. We identify the impact of the different parameters on the golden, silver, and disappearance channels. We come to the conclusion that, even in the presence of nonstandard interactions, the performance of the neutrino factory hardly profits from a silver channel detector, unless the muon energy is significantly increased compared to the IDS-NF setup. Apart from the dispensable silver channel detector, we demonstrate that the IDS-NF setup is close to optimal even if nonstandard interactions are considered. We find that one very long baseline is a key component in the search for nonstandard interactions, in particular, for |\epsilon^m_{\mu\tau}| and |\epsilon^m_{\tau\tau}|