We present a comprehensive review of physics effects generated by leptoquarks (LQs), i.e., hypothetical particles that can turn quarks into leptons and vice versa, of either scalar or vector nature. These considerations include discussion of possible completions of the Standard Model that contain LQ fields. The main focus of the review is on those LQ scenarios that are not problematic with regard to proton stability. We accordingly concentrate on the phenomenology of light leptoquarks that is relevant for precision experiments and particle colliders. Important constraints on LQ interactions with matter are derived from precision low-energy observables such as electric dipole moments, (g − 2) of charged leptons, atomic parity violation, neutral meson mixing, Kaon, B, and D meson decays, etc. We provide a general analysis of indirect constraints on the strength of LQ interactions with the quarks and leptons to make statements that are as model independent as possible. We address complementary constraints that originate from electroweak precision measurements, top, and Higgs physics. The Higgs physics analysis we present covers not only the most recent but also expected results from the Large Hadron Collider (LHC). We finally discuss direct LQ searches. Current experimental situation is summarized and self-consistency of assumptions that go into existing accelerator-based searches is discussed. A progress in making next-to-leading order predictions for both pair and single LQ productions at colliders is also outlined.

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B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B ! " decay, where the large mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B ! D ðÃÞ " rates. Because of the rich spin structure of the final state, the decay mode B ! D à " offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b ! c (semi)leptonic transitions. We explore such contributions to the B ! D à " decay amplitudes by determining the differential decay rate, longitudinal D à polarization fraction, D à À opening angle asymmetry and the helicity asymmetry. We identify the size of possible new physics contributions to these observables constrained by the present B ! D ðÃÞ " rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the helicity asymmetry can still deviate by as much as 80%.

Present measurements of b→cτν and b→uτν transitions differ from the standard model predictions of lepton flavor universality by almost 4σ. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super-)B factories are presented.

We propose that three recent anomalies in B meson decays, R D ( * ) , RK , and P 5 , might be explained by a single vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering t → bτ + ν data, lepton flavor universality tests in the kaon sector, bounds on B → K ( * )ν ν, bound on the lepton flavor violating decay B → Kµτ , and measurements of b → cµ −ν decays. The presence of such vector leptoquark could be exposed in precise measurements of t → bτ ν and B → K ( * )ν ν decays. The model also predicts approximate equality of lepton flavor universality ratios RK * , RK , and suppressed branching fraction of Bs → µ + µ − . * Electronic address:svjetlana.fajfer@ijs.si † Electronic address:nejc.kosnik@ijs.si arXiv:1511.06024v4 [hep-ph]

We show that a model with a scalar leptoquark of hypercharge Y = 1/6 which includes the light right-handed neutrinos, can successfully describe both of the B-physics anomalies, R expWe discuss the corresponding low energy effective theory and, after using the known experimental data as constraints, we show that the model is viable and that it offers several predictions which can be tested experimentally.

The LHCb bounds on the branching ratio of the rare decay D 0 → μ + μ − and the constraints on the branching ratio of D + → π + μ + μ − in the nonresonant regions enable us to improve constraints on new physics contributions. Using the effective Lagrangian approach we determine the sizes of the Wilson coefficients allowed by the existing LHCb bounds on rare charm decays. Then we discuss contributions to rare charm meson decay observables in several models of new physics: a model with an additional spin-1 weak triplet, leptoquark models, Two Higgs doublets model of type III, and a Z model. Here we complement the discussion by D 0 -D 0 oscillations data. Among the considered models, only leptoquarks can significantly modify the Wilson coefficients. Assuming that the differential decay width for D + → π + μ + μ − receives a NP contribution, while the differential decay width for D + → π + e + e − is Standard Model-like, we find that lepton flavor universality can be violated and might be observed at high dilepton invariant mass.

We address the B-physics anomalies within a two scalar leptoquark model. The low-energy flavor structure of our set-up originates from two SU (5) operators that relate Yukawa couplings of the two leptoquarks. The proposed scenario has a UV completion, can accommodate all measured lepton flavor universality ratios in B-meson decays, is consistent with related flavor observables, and is compatible with direct searches at the LHC. We provide prospects for future discoveries of the two light leptoquarks at the LHC and predict several yet-to-be-measured flavor observables. * Electronic address:damir.becirevic@th.u-psud.fr †