Search

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

If dark matter couples predominantly to the axial-vector currents with heavyquarks, the leading contribution to dark-matter scattering on nuclei is eitherdue to one-loop weak corrections or due to the heavy-quark axial charges of thenucleons. We calculate the effects of Higgs and weak gauge-boson exchanges fordark matter coupling to heavy-quark axial-vector currents in an effectivetheory below the weak scale. By explicit computation, we show that theleading-logarithmic QCD corrections are important, and thus resum them to allorders using the renormalization group.

The decay KS → (μ+μ−)ℓ=0, with the final muon pair in an angular-momentum zero state, is a sensitive probe of short-distance physics. It has recently been shown how to extract this branching ratio from neutral kaon decay data. We point out that the impact of indirect CP violation on the standard-model prediction of this mode, while nominally of order |ϵK| ∼ 10−3, is enhanced by a large amplitude ratio and leads to a shift of the branching ratio Br(KS → μ+μ−)ℓ=0 by a few percent, depending on the size of a relative phase that can be extracted from data. We also update the standard-model prediction of the short-distance contribution. [ABSTRACT FROM AUTHOR]

The decay KS → (μ+μ−)ℓ=0, with the final muon pair in an angular-momentum zero state, is a sensitive probe of short-distance physics. It has recently been shown how to extract this branching ratio from neutral kaon decay data. We point out that the impact of indirect CP violation on the standard-model prediction of this mode, while nominally of order |ϵK| ∼ 10−3, is enhanced by a large amplitude ratio and leads to a shift of the branching ratio Br(KS → μ+μ−)ℓ=0 by a few percent, depending on the size of a relative phase that can be extracted from data. We also update the standard-model prediction of the short-distance contribution. [ABSTRACT FROM AUTHOR]

We discuss how LHC di-muon data collected to study Bq → μμ can be used to constrain light particles with flavour-violating couplings to b-quarks. Focussing on the case of a flavoured QCD axion, a, we compute the decay rates for Bq → μμa and the SM background process Bq → μμγ near the kinematic endpoint. These rates depend on non-perturbative Bq → γ(*) form factors with on- or off-shell photons. The off-shell form factors — relevant for generic searches for beyond-the-SM particles — are discussed in full generality and computed with QCD sum rules for the first time. This includes an extension to the low-lying resonance region using a multiple subtracted dispersion relation. With these results, we analyse available LHCb data to obtain the sensitivity on Bq → μμa at present and future runs. We find that the full LHCb dataset alone will allow to probe axion-coupling scales of the order of 106 GeV for both b → d and b → s transitions. As a spin-off application of the off-shell form factors we further analyse the case of light, Beyond the Standard Model, vectors. [ABSTRACT FROM AUTHOR]

We study the fixed point that controls the IR dynamics of QED in d = 4 − 2 ϵ dimensions. We derive the scaling dimensions of four-fermion and bilinear operators beyond leading order in the ϵ-expansion. For the four-fermion operators, this requires the computation of a two-loop mixing that was not known before. We then extrapolate these scaling dimensions to d = 3 to estimate their value at the IR fixed point of QED as function of the number of fermions N . The next-to-leading order result for the four-fermion operators corrects significantly the leading one. Our best estimate at this order indicates that they do not cross marginality for any value of N , which would imply that they cannot trigger a departure from the conformal phase. For the scaling dimensions of bilinear operators, we observe better convergence as we increase the order. In particular, the ϵ-expansion provides a convincing estimate for the dimension of the flavor-singlet scalar in the full range of N . [ABSTRACT FROM AUTHOR]

Discrete flavour groups have been studied in connection with special patterns of neutrino mixing suggested by the data, such as Tri-Bimaximal mixing (groups A, S...) or Bi-Maximal mixing (group S...) etc. We review the predictions for sin θ in a number of these models and confront them with the experimental measurements. We compare the performances of the different classes of models in this respect. We then consider, in a supersymmetric framework, the important implications of these flavour symmetries on lepton flavour violating processes, like μ → eγ and similar processes. We discuss how the existing limits constrain these models, once their parameters are adjusted so as to optimize the agreement with the measured values of the mixing angles. In the simplified CMSSM context, adopted here just for indicative purposes, the small tan β range and heavy SUSY mass scales are favoured by lepton flavour violating processes, which makes it even more difficult to reproduce the reported muon g − 2 discrepancy. [ABSTRACT FROM AUTHOR]

The parameter εK describes CP violation in the neutral kaon system and is one of the most sensitive probes of new physics. The large uncertainties related to the charm-quark contribution to εK have so far prevented a reliable standard-model prediction. We show that Cabibbo-Kobayashi-Maskawa unitarity enforces a unique form of the |ΔS=2| weak effective Lagrangian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level. We present the updated standard-model prediction εK=2.16(6)(8)(15)×10-3, where the errors in parentheses correspond to QCD short-distance, long-distance, and parametric uncertainties, respectively. [ABSTRACT FROM AUTHOR]

We combine our new results for the O(αem) and O(αs²) corrections to Bs,d → ℓ+ℓ, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the Bs meson, our calculation gives ̅B(Bs→ μ+μ-) = (3.65 ± 0.23) × 10-9. [ABSTRACT FROM AUTHOR]

We study quantum electrodynamics in d=3 coupled to Nf flavors of fermions. The theory flows to an IR fixed point for Nf larger than some critical number Ncf. For Nf ≤ Ncf, chiral-symmetry breaking is believed to take place. In analogy with the Wilson-Fisher description of the critical O(N) models in d=3, we make use of the existence of a fixed point in d=4−2ε to study the three-dimensional coNformal theory. We compute, in perturbation theory, the IR dimensions of fermion bilinear and quadrilinear operators. For small Nf, a quadrilinear operator can become relevant in the IR and destabilize the fixed point. Therefore, the epsilon expansion can be used to estimate Ncf. An interesting novelty compared to the O(N) models is that the theory in d=3 has an enhanced symmetry due to the structure of 3D spinors. We identify the operators in d=4−2ε that correspond to the additional conserved currents at d=3 and compute their iNfrared dimensions. [ABSTRACT FROM AUTHOR]

The parameter ε_{K} describes CP violation in the neutral kaon system and is one of the most sensitive probes of new physics. The large uncertainties related to the charm-quark contribution to ε_{K} have so far prevented a reliable standard-model prediction. We show that Cabibbo-Kobayashi-Maskawa unitarity enforces a unique form of the |ΔS=2| weak effective Lagrangian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level. We present the updated standard-model prediction ε_{K}=2.16(6)(8)(15)×10^{-3}, where the errors in parentheses correspond to QCD short-distance, long-distance, and parametric uncertainties, respectively.

We combine our new results for the O(αem) and O(αs2) corrections to Bs,d→ℓ+ℓ-, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the Bs meson, our calculation gives B¯(Bs→μ+μ-)=(3.65±0.23)×10-9.