Following the updated measurement of the lepton flavour universality (LFU) ratio $$R_K$$ in $$B\rightarrow K\ell \ell $$ decays by LHCb, as well as a number of further measurements, e.g. $$R_{K^*}$$ by Belle and $$B_s \rightarrow \mu \mu $$ by ATLAS, we analyse the global status of new physics in $$b\rightarrow s$$ transitions in the weak effective theory at the b-quark scale, in the Standard Model effective theory above the electroweak scale, and in simplified models of new physics. We find that the data continues to strongly prefer a solution with new physics in semi-leptonic Wilson coefficients. A purely muonic contribution to the combination $$C_9 = -C_{10}$$, well suited to UV-complete interpretations, is now favoured with respect to a muonic contribution to $$C_9$$ only. An even better fit is obtained by allowing an additional LFU shift in $$C_9$$. Such a shift can be renormalization-group induced from four-fermion operators above the electroweak scale, in particular from semi-tauonic operators, able to account for the potential discrepancies in $$b \rightarrow c$$ transitions. This scenario is naturally realized in the simplified $$U_1$$ leptoquark model. We also analyse simplified models where a LFU effect in $$b\rightarrow s\ell \ell $$ is induced radiatively from four-quark operators and show that such a setup is on the brink of exclusion by LHC di-jet resonance searches.

/pdf/b-decay-discrepancies-after-moriond-2019-29574tu7zz.pdf

B-decay discrepancies after Moriond 2019

We perform a model-independent global fit to $b\to s\ell^+\ell^-$ observables to confirm existing New Physics (NP) patterns (or scenarios) and to identify new ones emerging from the inclusion of the updated LHCb and Belle measurements of $R_K$ and $R_{K^*}$, respectively. Our analysis, updating Refs. [1,2] and including these new data, suggests the presence of right-handed couplings encoded in the Wilson coefficients ${\cal C}_{9'\mu}$ and ${\cal C}_{10'\mu}$. It also strengthens our earlier observation that a lepton flavour universality violating (LFUV) left-handed lepton coupling (${\cal C}_{9\mu}^{\rm V}=-{\cal C}_{10\mu}^{\rm V}$), often preferred from the model building point of view, accommodates the data better if lepton-flavour universal (LFU) NP is allowed, in particular in ${\cal C}_{9}^{\rm U}$. Furthermore, this scenario with LFU NP provides a simple and model-independent connection to the $b\to c\tau \nu$ anomalies, showing a preference of $\approx 7\,\sigma$ with respect to the SM. It may also explain why fits to the whole set of $b\to s\ell^+\ell^-$ data or to the subset of LFUV data exhibit stronger preferences for different NP scenarios. Finally, motivated by $Z^\prime$ models with vector-like quarks, we propose four new scenarios with LFU and LFUV NP contributions that give a very good fit to data. We provide also an addendum collecting our updated results after including the data for the $B\to K^*\mu\mu$ angular distribution released in 2020 by the LHCb collaboration.

/pdf/emerging-patterns-of-new-physics-with-and-without-lepton-d41lgxe0lo.pdf

Emerging patterns of New Physics with and without Lepton Flavour Universal contributions.

We present a thorough investigation of the vector leptoquark hypothesis for a combined explanation of the B-physics anomalies. We analyze this hypothesis from a twofold perspective, taking into account recent results from B-physics observables and high-pT searches. First, using a simplified model, we determine the general conditions for a successful low-energy fit in presence of right-handed leptoquark couplings (neglected in previous analyses). Second, we show how these conditions, in particular a sizable 2-3 family mixing, can be achieved in a motivated ultraviolet completion. Our analysis reinforces the phenomenological success of the vector leptoquark hypothesis in addressing the anomalies, and its compatibility with motivated extensions of the Standard Model based on the idea of flavor non-universal gauge interactions. The implications of right-handed leptoquark couplings for a series of key low-energy observables, namely Bs → ττ and τ → μ lepton flavor violating processes, both in τ and in B decays, are discussed in detail. The role of the ultraviolet completion in precisely estimating other low-energy observables, most notably ΔF = 2 amplitudes, is also addressed.

/pdf/revisiting-the-vector-leptoquark-explanation-of-the-b-4sot2f2rzk.pdf

Revisiting the vector leptoquark explanation of the B-physics anomalies

We perform a model-independent global fit to $$b\rightarrow s\ell ^+\ell ^-$$ observables to confirm existing New Physics (NP) patterns (or scenarios) and to identify new ones emerging from the inclusion of the updated LHCb and Belle measurements of $$R_K$$ and $$R_{K^*}$$, respectively. Our analysis, updating Refs. Capdevila et al. (J Virto JHEP 1801:093, 2018) and Alguero et al. (J Matias Phys Rev D 99(7):075017, 2019) and including these new data, suggests the presence of right-handed couplings encoded in the Wilson coefficients $${{{\mathcal {C}}}}_{9'\mu }$$ and $${{{\mathcal {C}}}}_{10'\mu }$$. It also strengthens our earlier observation that a lepton flavour universality violating (LFUV) left-handed lepton coupling ($${{{\mathcal {C}}}}_{9\mu }^{\mathrm{V}}=-\,{{{\mathcal {C}}}}_{10\mu }^{\mathrm{V}}$$), often preferred from the model building point of view, accommodates the data better if lepton-flavour universal (LFU) NP is allowed, in particular in $${{{\mathcal {C}}}}_{9}^{\mathrm{U}}$$. Furthermore, this scenario with LFU NP provides a simple and model-independent connection to the $$b\rightarrow c\tau \nu $$ anomalies, showing a preference of $$\approx 7\,\sigma $$ with respect to the SM. It may also explain why fits to the whole set of $$b\rightarrow s\ell ^+\ell ^-$$ data or to the subset of LFUV data exhibit stronger preferences for different NP scenarios. Finally, motivated by $$Z^\prime $$ models with vector-like quarks, we propose four new scenarios with LFU and LFUV NP contributions that give a very good fit to data.

/pdf/emerging-patterns-of-new-physics-with-and-without-lepton-50cj7scn0j.pdf

Emerging patterns of New Physics with and without Lepton Flavour Universal contributions

We present a global likelihood function in the space of dimension-six Wilson coefficients in the Standard Model Effective Field Theory. The likelihood includes contributions from flavour-changing neutral current B decays, lepton flavour universality tests in charged- and neutral-current B and K decays, meson-antimeson mixing observables in the K, B, and D systems, direct CP violation in $$K\rightarrow \pi \pi $$
 , charged lepton flavour violating B, tau, and muon decays, electroweak precision tests on the Z and W poles, the anomalous magnetic moments of the electron, muon, and tau, and several other precision observables, 265 in total. The Wilson coefficients can be specified at any scale, with the one-loop running above and below the electroweak scale automatically taken care of. The implementation of the likelihood function is based on the open source tools flavio and wilson as well as the open Wilson coefficient exchange format (WCxf) and can be installed as a Python package. It can serve as a basis either for model-independent fits or for testing dynamical models, in particular models built to address the anomalies in B physics. We discuss a number of example applications, reproducing results from the EFT and model building literature.

/pdf/a-global-likelihood-for-precision-constraints-and-flavour-12yxf6b6q2.pdf

A Global Likelihood for Precision Constraints and Flavour Anomalies

In recent years experiments revealed intriguing hints for new physics (NP) in $B$ decays involving $b\ensuremath{\rightarrow}c\ensuremath{\tau}\ensuremath{\nu}$ and $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ transitions at the $4\ensuremath{\sigma}$ and $5\ensuremath{\sigma}$ level, respectively. In addition, there are slight disagreements in $b\ensuremath{\rightarrow}u\ensuremath{\tau}\ensuremath{\nu}$ and $b\ensuremath{\rightarrow}d{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ observables. While not significant on their own, they point in the same direction. Furthermore, ${V}_{us}$ extracted from $\ensuremath{\tau}$ decays shows a slight tension ($\ensuremath{\approx}2.5\ensuremath{\sigma}$) with its value determined from Cabibbo-Kobayashi-Maskawa unitarity, and an analysis of BELLE data found an excess in ${B}_{d}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$. Concerning NP explanations, the vector leptoquark SU(2) singlet is of special interest since it is the only single particle extension of the standard model which can (in principle) address all the anomalies described above. For this purpose, large couplings to $\ensuremath{\tau}$ leptons are necessary and loop effects, which we calculate herein, become important. Including them in our phenomenological analysis, we find that neither the tension in ${V}_{us}$ nor the excess in ${B}_{d}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ can be fully explained without violating bounds from $K\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\nu}\overline{\ensuremath{\nu}}$. However, one can account for $b\ensuremath{\rightarrow}c\ensuremath{\tau}\ensuremath{\nu}$ and $b\ensuremath{\rightarrow}u\ensuremath{\tau}\ensuremath{\nu}$ data finding intriguing correlations with ${B}_{q}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ and $K\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\nu}\overline{\ensuremath{\nu}}$. Furthermore, the explanation of $b\ensuremath{\rightarrow}c\ensuremath{\tau}\ensuremath{\nu}$ predicts a positive shift in ${C}_{7}$ and a negative one in ${C}_{9}$, being nicely in agreement with the global fit to $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ data. Finally, we point out that one can fully account for $b\ensuremath{\rightarrow}c\ensuremath{\tau}\ensuremath{\nu}$ and $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ without violating bounds from $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\phi}\ensuremath{\mu}$, $\mathrm{\ensuremath{\Upsilon}}\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\mu}$, or $b\ensuremath{\rightarrow}s\ensuremath{\tau}\ensuremath{\mu}$ processes.

/pdf/importance-of-loop-effects-in-explaining-the-accumulated-2zka6qsxma.pdf

Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark.

In recent years, experiments revealed intriguing hints for new physics (NP) in semi-leptonic B decays. Both in charged current processes, involving b → cτν transitions, and in the neutral currents b → sl +l −, a preference for NP compared to the standard model (SM) of more that 3σ and 5σ was found, respectively. In addition, there is the long-standing tension between the theory prediction and the measurement of the anomalous magnetic moment (AMM) of the muon (aμ) of more than 3σ. Since all these observables are related to the violation of lepton flavor universality (LFU), a common NP explanation seems not only plausible but is even desirable. In this context, leptoquarks (LQs) are especially promising since they give tree-level effects in semi-leptonic B decays, but only loop-suppressed effects in other flavor observables that agree well with their SM predictions. Furthermore, LQs can lead to a mt/mμ enhanced effect in aμ, allowing for an explanation even with (multi) TeV particles. However, a single scalar LQ representation cannot provide a common solution to all three anomalies. In this article we therefore consider a model in which we combine two scalar LQs: the SU(2)L singlet and the SU(2)L triplet. Within this model we compute all relevant 1-loop effects and perform a comprehensive phenomenological analysis, pointing out various interesting correlations among the observables. Furthermore, we identify benchmark points which are in fact able to explain all three anomalies (b → cτν, b → sl +l − and aμ), without violating bounds from other observables, and study their predictions for future measurements.

/pdf/flavor-phenomenology-of-the-leptoquark-singlet-triplet-model-22cwle4xzb.pdf

Flavor phenomenology of the leptoquark singlet-triplet model

Recently, both ATLAS and CMS measured the decay $h\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, finding a signal strength with respect to the standard model expectation of $1.2\ifmmode\pm\else\textpm\fi{}0.6$ and ${1.19}_{\ensuremath{-}0.39\ensuremath{-}0.16}^{+0.41+0.17}$, respectively. This provides, for the first time, evidence that the standard model Higgs couples to second generation fermions. This measurement is particularly interesting in the context of the intriguing hints for lepton flavor universality violation, accumulated within recent years, as new physics explanations could also be tested in the $h\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay mode. Leptoquarks are prime candidates to account for the flavor anomalies. In particular, they can provide the necessary chiral enhancement (by a factor ${m}_{t}/{m}_{\ensuremath{\mu}}$) to address ${a}_{\ensuremath{\mu}}$ with tera-electron-volt scale new physics. In this Letter we point out that such explanations of ${a}_{\ensuremath{\mu}}$ also lead to enhanced effects in $h\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and we examine the correlations between $h\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and ${a}_{\ensuremath{\mu}}$ within leptoquark models. We find that the effect in the branching ratio of $h\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ ranges from several percent up to a factor of 3, if one aims at accounting for ${a}_{\ensuremath{\mu}}$ at the $2\ensuremath{\sigma}$ level. Hence, the new ATLAS and CMS Collaboration measurements already provide important constraints on the parameter space, rule out specific ${a}_{\ensuremath{\mu}}$ explanations, and will be very important to test the flavor anomalies in the future.

/pdf/correlating-h-m-m-to-the-anomalous-magnetic-moment-of-the-522c780ef8.pdf

Correlating h→μ^{+}μ^{-} to the Anomalous Magnetic Moment of the Muon via Leptoquarks.

Leptoquarks are hypothetical new particles, which couple quarks directly to leptons. They experienced a renaissance in recent years as they are prime candidates to explain the so-called flavor anomalies, i.e. the deviations between the Standard Model predictions and measurements in b → sl+l− and b → cτν processes and in the anomalous magnetic moment of the muon. At the one-loop level these particles unavoidably generate effects in the purely leptonic processes like Z → l+l−, Z → $$ v\overline{v} $$
 , W → lν and h → l+l− and can even generate non-zero rates for lepton flavor violating processes such as l → l′γ, Z → l+l′−, h → l+l′− and l → 3l′. In this article we calculate these processes for all five representations of scalar Leptoquarks. We include their most general interaction terms with the Standard Model Higgs boson, which leads to Leptoquark mixing after the former acquires a vacuum expectation value. In our phenomenological analysis we investigate the effects in modified lepton couplings to electroweak gauge bosons, we study the correlations of the anomalous magnetic moment of the muon with h → μ+μ− and Z → μ+μ− as well as the interplay between different lepton flavor violating decays.

/pdf/scalar-leptoquarks-in-leptonic-processes-qgb9yhyktz.pdf

Scalar leptoquarks in leptonic processes

In recent years, experiments revealed intriguing hints for new physics (NP) in semi-leptonic $B$ decays. Both in charged current processes, involving $b \to c \tau \nu$ transitions, and in the neutral currents $b\to s\ell^+\ell^-$, a preference for NP compared to the standard model (SM) of more that $3\sigma$ and $5\sigma$ was found, respectively. In addition, there is the long-standing tension between the theory prediction and the measurement of the anomalous magnetic moment (AMM) of the muon ($a_\mu$) of more than $3\sigma$. Since all these observables are related to the violation of lepton flavor universality (LFU), a common NP explanation seems not only plausible but is even desirable. In this context, leptoquarks (LQs) are especially promising since they give tree-level effects in semi-leptonic $B$ decays, but only loop-suppressed effects in other flavor observables that agree well with their SM predictions. Furthermore, LQs can lead to a $m_t/m_\mu$ enhanced effect in $a_{\mu}$, allowing for an explanation even with (multi) TeV particles. However, a single scalar LQ representation cannot provide a common solution to all three anomalies. In this article we therefore consider a model in which we combine two scalar LQs: the $SU(2)_L$ singlet and the $SU(2)_L$ triplet. Within this model we compute all relevant one-loop effects and perform a comprehensive phenomenological analysis, pointing out various interesting correlations among the observables. Furthermore, we identify benchmark points which are in fact able to explain all three anomalies ($b \to c \tau \nu$, $b\to s\ell^+\ell^-$ and $a_\mu$), without violating bounds from other observables, and study their predictions for future measurements.

Francesco Saturnino

Papers

Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark.

Flavor phenomenology of the leptoquark singlet-triplet model

Correlating h→μ^{+}μ^{-} to the Anomalous Magnetic Moment of the Muon via Leptoquarks.

Scalar leptoquarks in leptonic processes

Flavor Phenomenology of the Leptoquark Singlet-Triplet Model