Measurement of form-factor-independent observables in the decay B0→K(*0)μ(+)μ(-)

Short- vs. long-distance physics in B → K ( ∗ ) ℓ + ℓ - : a data-driven analysis

We analyze data on B → K μ + μ - and B → K ∗ μ + μ - decays in the whole dilepton invariant mass spectrum with the aim of disentangling short- vs. long-distance contributions. The sizable long-distance amplitudes from c c ¯ narrow resonances are taken into account by employing a dispersive approach. For each availableq 2 = m μ μ 2 bin and each helicity amplitude an independent determination of the Wilson coefficient C 9 , describing b → s ℓ + ℓ - transitions at short distances, is obtained. The consistency of the C 9 values thus obtained provides an a posteriori check of the absence of additional, sizable, long-distance contributions. The systematic difference of these values from the Standard Model expectation supports the hypothesis of a non-standard b → s μ + μ - amplitude of short-distance origin.

Amplitude Analysis of the B^{0}→K^{*0}μ^{+}μ^{-} Decay

An amplitude analysis of the B^{0}→K^{*0}μ^{+}μ^{-} decay is presented using a dataset corresponding to an integrated luminosity of 4.7  fb^{-1} of pp collision data collected with the LHCb experiment. For the first time, the coefficients associated to short-distance physics effects, sensitive to processes beyond the standard model, are extracted directly from the data through a q^{2}-unbinned amplitude analysis, where q^{2} is the μ^{+}μ^{-} invariant mass squared. Long-distance contributions, which originate from nonfactorizable QCD processes, are systematically investigated, and the most accurate assessment to date of their impact on the physical observables is obtained. The pattern of measured corrections to the short-distance couplings is found to be consistent with previous analyses of b- to s-quark transitions, with the largest discrepancy from the standard model predictions found to be at the level of 1.8 standard deviations. The global significance of the observed differences in the decay is 1.4 standard deviations.

Round table on Standard Model Anomalies

This contribution to “The XVth Quark confinement and the Hadron spectrum conference" covers a description, both theoretical and experimental, of the present status of a set of very different anomalies. The discussion ranges from the long standing b → sℓℓ anomalies, (g − 2) and the new MW anomaly.

Search for B^{+}→K^{+}νν[over ¯] Decays Using an Inclusive Tagging Method at Belle II

A search for the flavor-changing neutral-current decay B^{+}→K^{+}νν[over ¯] is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of 63  fb^{-1} collected at the ϒ(4S) resonance and a sample of 9  fb^{-1} collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the B^{+}→K^{+}νν[over ¯] decay, but also the inclusive properties of the other B meson in the ϒ(4S)→BB[over ¯] event, to suppress the background from other B meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of B^{+}→K^{+}νν[over ¯] of 4.1×10^{-5} is set at the 90% confidence level.

Minimal Explanation of Flavor Anomalies: B-Meson Decays, Muon Magnetic Moment, and the Cabibbo Angle

Significant deviations from the standard model are observed in semileptonic charged and neutral-current B decays, the muon magnetic moment, and the extraction of the Cabibbo angle. We propose that these deviations point towards a coherent pattern of new physics effects induced by two scalar mediators, a leptoquark S_{1} and a charged singlet ϕ^{+}. While S_{1} can provide solutions to charged-current B decays and the muon magnetic moment, and ϕ^{+} can accommodate the Cabibbo-angle anomaly independently, their one-loop level synergy can also address neutral-current B decays. This framework provides the most minimal explanation to the above-mentioned anomalies, while being consistent with all other phenomenological constraints.

Angular Analysis of the B^{+}→K^{*+}μ^{+}μ^{-} Decay

We present an angular analysis of the B^{+}→K^{*+}(→K_{S}^{0}π^{+})μ^{+}μ^{-} decay using 9  fb^{-1} of pp collision data collected with the LHCb experiment. For the first time, the full set of CP-averaged angular observables is measured in intervals of the dimuon invariant mass squared. Local deviations from standard model predictions are observed, similar to those in previous LHCb analyses of the isospin-partner B^{0}→K^{*0}μ^{+}μ^{-} decay. The global tension is dependent on which effective couplings are considered and on the choice of theory nuisance parameters.

Heavy Quark Expansion of Λb→Λ*(1520) Form Factors beyond Leading Order

I review the parametrisation of the full set of Λb→Λ*(1520) form factors in the framework of Heavy Quark Expansion, including next-to-leading-order O(αs) and, for the first time, next-to-leading-power O(1/mb) corrections. The unknown hadronic parameters are obtained by performing a fit to recent lattice QCD calculations. I investigate the compatibility of the Heavy Quark Expansion and the current lattice data, finding tension between these two approaches in the case of tensor and pseudo-tensor form factors, whose origin could come from an underestimation of the current lattice QCD uncertainties and higher order terms in the Heavy Quark Expansion.

Flavour Anomalies in a U(1) SUSY Extension of the SM

Flavour anomalies have attracted a lot of attention over recent years as they provide unique hints for possible New Physics. Here, we consider a supersymmetric (SUSY) extension of the Standard Model (SM) with an additional anomaly-free gauge U(1) group. The key feature of our model is the particular choice of non-universal charges to the gauge boson Z′, which not only allows a relaxation of the flavour discrepancies but, contrary to previous studies, can reproduce the SM mixing matrices both in the quark and lepton sectors. We pay special attention to the latter and explicitly enumerate all parameters relevant for our calculation in the low-energy effective theory. We find regions in the parameter space that satisfy experimental constraints on meson mixing and LHC Z′ searches and can alleviate the flavour anomalies. In addition, we also discuss the predictions for lepton-flavour violating decays B+→K+μτ and B+→K+eτ.

Gravitational Imprints of Flavor Hierarchies

The mass hierarchy among the three generations of quarks and charged leptons is one of the greatest mysteries in particle physics. In various flavor models, the origin of this phenomenon is attributed to a series of hierarchical spontaneous symmetry breakings, most of which are beyond the reach of particle colliders. We point out that the observation of a multipeaked stochastic gravitational wave signal from a series of cosmological phase transitions could well be a unique probe of the mechanism behind flavor hierarchies. To illustrate this point, we show how near future ground- and space-based gravitational wave observatories could detect up to three peaks in the recently proposed PS^{3} model.

Searches for physics beyond the standard model with the M T 2 variable in hadronic final states with and without disappearing tracks in proton-proton collisions at s = 13 Te

Two related searches for phenomena beyond the standard model (BSM) are performed using events with hadronic jets and significant transverse momentum imbalance. The results are based on a sample of proton-proton collisions at a center-of-mass energy of 13 Te , collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 137fb - 1 . The first search is inclusive, based on signal regions defined by the hadronic energy in the event, the jet multiplicity, the number of jets identified as originating from bottom quarks, and the value of the kinematic variable M T 2 for events with at least two jets. For events with exactly one jet, the transverse momentum of the jet is used instead. The second search looks in addition for disappearing tracks produced by BSM long-lived charged particles that decay within the volume of the tracking detector. No excess event yield is observed above the predicted standard model background. This is used to constrain a range of BSM models that predict the following: the pair production of gluinos and squarks in the context of supersymmetry models conserving R-parity, with or without intermediate long-lived charginos produced in the decay chain; the resonant production of a colored scalar state decaying to a massive Dirac fermion and a quark; or the pair production of scalar and vector leptoquarks each decaying to a neutrino and a top, bottom, or light-flavor quark. In most of the cases, the results obtained are the most stringent constraints to date.

Anomaly-free models for flavour anomalies

We explore the constraints imposed by the cancellation of triangle anomalies on models in which the flavour anomalies reported by LHCb and other experiments are due to an extra U(1)' gauge boson Z' . We assume universal and rational U(1)' charges for the first two generations of left-handed quarks and of right-handed up-type quarks but allow different charges for their third-generation counterparts. If the right-handed charges vanish, cancellation of the triangle anomalies requires all the quark U(1)' charges to vanish, if there are either no exotic fermions or there is only one Standard Model singlet dark matter (DM) fermion. There are non-trivial anomaly-free models with more than one such 'dark' fermion, or with a single DM fermion if right-handed up-type quarks have non-zero U(1)' charges. In some of the latter models the U(1)' couplings of the first- and second-generation quarks all vanish, weakening the LHC Z ' constraint, and in some other models the DM particle has purely axial couplings, weakening the direct DM scattering constraint. We also consider models in which anomalies are cancelled via extra vector-like leptons, showing how the prospective LHC Z' constraint may be weakened because theZ ' → μ + μ - branching ratio is suppressed relative to other decay modes.

The Belle II experiment: status and physics program

The Belle II experiment at the SuperKEKB asymmetric-energy e+e- collider in Japan aims to search for new physics in the flavour transitions in the quark and lepton sectors. The SuperKEKB accelerator will operate at the target instantaneous luminosity of 8 × 1035 s-1cm2. It requires a substantial upgrade of the detector subsystems which are expected to record 50 ab-1 of data. Such a huge data sample in clean background environment allows for probing signatures of new physics through suppressed flavour physics reactions and cross checks for deviations from the Standard Model measured at the LHCb experiment. Physics data taking at the Belle II experiment successfully started in April 2018.

On flavourful Easter eggs for New Physics hunger and lepton flavour universality violation

Within the standard approach of effective field theory of weak interactions for Δ B = 1 transitions, we look for possibly unexpected subtle New Physics effects, here dubbed "flavourful Easter eggs". We perform a Bayesian global fit using the publicly available HEPfit package, taking into account state-of-the-art experimental information concerning these processes, including the suggestive measurements from LHCb of R K and R K ∗ , the latter available only very recently. We parametrise New Physics contributions to b → s transitions in terms of shifts of Wilson coefficients of the electromagnetic dipole and semileptonic operators, assuming CP-conserving effects, but allowing in general for violation of lepton flavour universality. We show how optimistic/conservative hadronic estimates can impact quantitatively the size of New Physics extracted from the fit. With a conservative approach to hadronic uncertainties we find nonzero New Physics contributions to Wilson coefficients at the level of ∼ 3 σ , depending on the model chosen. Furthermore, given the interplay between hadronic contributions and New Physics effects in the leptonic vector current, a scenario with nonstandard leptonic axial currents is comparable to the more widely advocated one with New Physics in the leptonic vector current.

Lepton-Flavor-Dependent Angular Analysis of B→K^{*}ℓ^{+}ℓ^{-}

We present a measurement of angular observables and a test of lepton flavor universality in the B→K^{*}ℓ^{+}ℓ^{-} decay, where ℓ is either e or μ. The analysis is performed on a data sample corresponding to an integrated luminosity of 711  fb^{-1} containing 772×10^{6} BB[over ¯] pairs, collected at the ϒ(4S) resonance with the Belle detector at the asymmetric-energy e^{+}e^{-} collider KEKB. The result is consistent with standard model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of 2.6σ.

Minimal Leptoquark Explanation for the R_{D^{(*)}}, R_{K}, and (g-2)_{μ} Anomalies

We show that by adding a single new scalar particle to the standard model, a TeV-scale leptoquark with the quantum numbers of a right-handed down quark, one can explain in a natural way three of the most striking anomalies of particle physics: the violation of lepton universality in B[over ¯]→K[over ¯]ℓ^{+}ℓ^{-} decays, the enhanced B[over ¯]→D^{(*)}τν[over ¯] decay rates, and the anomalous magnetic moment of the muon. Constraints from other precision measurements in the flavor sector can be satisfied without fine-tuning. Our model predicts enhanced B[over ¯]→K[over ¯]^{(*)}νν[over ¯] decay rates and a new-physics contribution to B_{s}-B[over ¯]_{s} mixing close to the current central fit value.

Weak Decays of Excited B Mesons

We investigate the decays of the excited (bq[over ¯]) mesons as probes of the short-distance structure of the weak ΔB=1 transitions. These states are unstable under the electromagnetic or strong interactions, although their widths are typically suppressed by phase space. Compared to the pseudoscalar B meson, the purely leptonic decays of the vector B^{*} are not chirally suppressed and are sensitive to different combinations of the underlying weak effective operators. An interesting example is B_{s}^{*}→ℓ^{+}ℓ^{-}, which has a rate that can be accurately predicted in the standard model. The branching fraction is B∼10^{-11}, irrespective of the lepton flavor and where the main uncertainty stems from the unmeasured and theoretically not well known B_{s}^{*} width. We discuss the prospects for producing this decay mode at the LHC and explore the possibility of measuring the B_{s}^{*}→ℓℓ amplitude, instead, through scattering experiments at the B_{s}^{*} resonance peak.

Future flavour physics experiments

The current status of flavour physics and the prospects for present and future experiments will be reviewed. Measurements in B-physics, in which sensitive probes of new physics are the CKM angle γ, the Bs mixing phase ϕs, and the branching ratios of the rare decays B(s)0→μ+μ- , will be highlighted. Topics in charm and kaon physics, in which the measurements of ACP and the branching ratios of the rare decays K→πνν¯ are key measurements, will be discussed. Finally the complementarity of the future heavy flavour experiments, the LHCb upgrade and Belle-II, will be summarised.

Effective Field Theory Approach to b→sℓℓ^{(')}, B→K^{(*)}νν[over ¯] and B→D^{(*)}τν with Third Generation Couplings

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.

Constraints on tensor and scalar couplings from [Formula: see text] and [Formula: see text]

The angular distribution of [Formula: see text] ([Formula: see text]) depends on two parameters, the lepton forward-backward asymmetry, [Formula: see text], and the flat term, [Formula: see text]. Both are strongly suppressed in the standard model and constitute sensitive probes of tensor and scalar contributions. We use the latest experimental results for [Formula: see text] in combination with the branching ratio of [Formula: see text] to derive the strongest model-independent bounds on tensor and scalar effective couplings to date. The measurement of [Formula: see text] provides a complementary constraint to that of the branching ratio of [Formula: see text] and allows us - for the first time - to constrain all complex-valued (pseudo-)scalar couplings and their chirality-flipped counterparts in one fit. Based on Bayesian fits of various scenarios, we find that our bounds even become tighter when vector couplings are allowed to deviate from the standard model and that specific combinations of angular observables in [Formula: see text] are still allowed to be up to two orders of magnitude larger than in the standard model, which would place them in the region of LHCb's sensitivity.

Loophole in K→πνν Search and New Weak Leptonic Forces

Weakly interacting K→πX(0) emission with mX(0)≅mπ(0) is out of sight of the current K+→π+ νν study, but it can be sensed by the KL→π(0)νν search. This evades the usual Grossman-Nir bound of B(KL→π(0)νν)<1.4×10(-9); thus, the KOTO experiment is already starting to probe new physics. An intriguing possibility is the Z' gauge boson of a weak leptonic force that couples to Lμ-Lτ (the difference between the muon and tauon numbers), which may explain the long-standing "muon g-2" anomaly, but is constrained by νμN→νμNμ+ μ- scattering to mZ'≲400  MeV. An explicit model for K→πZ' is given, which illustrates the link between rare kaon and B→Kμ+ μ-, K(*)νν decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson ϕ responsible for light mZ' generation via ϕ→Z'Z'→2(μ+ μ-).

Explaining h→μ(±)τ(∓), B→K*μ(+)μ(-), and B→Kμ(+)μ(-)/B→Ke(+)e(-) in a Two-Higgs-Doublet Model with Gauged L(μ)-L(τ)

The LHC has observed, so far, 3 deviations from the Standard Model (SM) predictions in flavor observables: LHCb reported anomalies in B→K*μ(+)μ(-) and R(K)=B→Kμ(+)μ(-)/B→Ke(+)e(-), while CMS found an excess in h→μτ. We show, for the first time, how these deviations from the SM can be explained within a single well-motivated model: a two-Higgs-doublet model with gauged L(μ)-L(τ) symmetry. We find that, despite the constraints from τ→μμμ and B(s)-B¯(s) mixing, one can explain h→μτ, B→K*μ(+)μ(-) and R(K) simultaneously, obtaining interesting correlations among the observables.

Lepton flavor violation in B decays?

The LHCb Collaboration's measurement of R_{K}=B(B^{+}→K^{+}μ^{+}μ^{-})/B(B^{+}→K^{+}e^{+}e^{-}) lies 2.6σ below the Standard Model prediction. Several groups suggest this deficit to result from new lepton nonuniversal interactions of muons. But nonuniversal 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}→μ^{+}μ^{-})_{exp}/B(B_{s}→μ^{+}μ^{-})_{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→Kμe, Kμτ, and B_{s}→μe, μτ.

Towards the identification of new physics through quark flavour violating processes

We outline a systematic strategy that should help in this decade to identify new physics (NP) beyond the standard model (SM) by means of quark flavour violating processes, and thereby extend the picture of short distance physics down to scales as short as 10(-20) m and even shorter distance scales corresponding to energies of 100 TeV. Rather than using all of the possible flavour-violating observables that will be measured in the coming years at the LHC, SuperKEKB and in Kaon physics dedicated experiments at CERN, J-PARC and Fermilab, we concentrate on those observables that are theoretically clean and very sensitive to NP. Assuming that the data on the selected observables will be very precise, we stress the importance of correlations between these observables as well as of future precise calculations of non-perturbative parameters by means of lattice QCD simulations with dynamical fermions. Our strategy consists of twelve steps, which we will discuss in detail while illustrating the possible outcomes with the help of the SM, models with constrained minimal flavour violation (CMFV), MFV at large and models with tree-level flavour changing neutral currents mediated by neutral gauge bosons and scalars. We will also briefly summarize the status of a number of concrete models. We propose DNA charts that exhibit correlations between flavour observables in different NP scenarios. Models with new left-handed and/or right-handed currents and non-MFV interactions can be distinguished transparently in this manner. We emphasize the important role of the stringent CMFV relations between various observables as standard candles of flavour physics. The pattern of deviations from these relations may help in identifying the correct NP scenario. The success of this program will be very much facilitated through direct signals of NP at the LHC, even if the LHC will not be able to probe the physics at scales shorter than 4 × 10(-20) m. We also emphasize the importance of lepton flavour violation, electric dipole moments, and (g - 2)e, μ in these studies.

Comprehensive Bayesian analysis of rare (semi)leptonic and radiative [Formula: see text] decays

The available data on [Formula: see text] decays are in good agreement with the Standard Model when permitting subleading power corrections of about [Formula: see text] at large hadronic recoil. Constraining new-physics effects in [Formula: see text], [Formula: see text], [Formula: see text], the data still demand the same size of power corrections as in the Standard Model. In the presence of chirality-flipped operators, all but one of the power corrections reduce substantially. The Bayes factors are in favor of the Standard Model. Using new lattice inputs for [Formula: see text] form factors and under our minimal prior assumption for the power corrections, the favor shifts toward models with chirality-flipped operators. We use the data to further constrain the hadronic form factors in [Formula: see text] and [Formula: see text] transitions.