arXiv subjects

Javier Virto

Publications and source records attributed to Javier Virto.

At least 37 records · Page 2Linked to original sources

arXiv2020

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

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 ${\cal C}_{9'μ}$ and ${\cal C}_{10'μ}$. It also strengthens our earlier observation that a lepton flavour universality violating (LFUV) left-handed lepton coupling (${\cal C}_{9μ}^{\rm V}=-{\cal C}_{10μ}^{\rm V}$), 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 $b\to cτν$ anomalies, showing a preference of $\approx 7\,σ$ 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 $B\to K^*μμ$ angular distribution released in 2020 by the LHCb collaboration.

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arXiv2020

Light-Cone Sum Rules for $B\to Kπ$ Form Factors and Applications to Rare Decays

We derive a set of light-cone sum rules relating the hadronic form factors relevant for $B\to Kπ\ell^+\ell^-$ decays to the $B$-meson light-cone distribution amplitudes (LCDAs). We obtain the sum rule relations for all $B\to Kπ$ form factors of (axial)vector and (pseudo)tensor $b\to s$ currents with a $P$-wave $Kπ$ system. Our results reduce to the known light-cone sum rules for $B\to K^*$ form factors in the limit of a single narrow-width resonance. We update the operator-product expansion for the underlying correlation function by including a more complete set of $B$-meson LCDAs with higher twists, and produce numerical results for all $B\to K^*$ form factors in the narrow-width limit. We then use the new sum rules to estimate the effect of a non-vanishing $K^*$ width in $B\to K^*$ transitions, and find that this effect is universal and increases the factorizable part of the rate of $B\to K^*X$ decays by a factor of $20\%$. This effect, by itself, goes in the direction of increasing the current tension in the differential $B\to K^*μμ$ branching fractions. We also discuss $B\to Kπ$ transitions outside the $K^*$ window, and explain how measurements of $B\to Kπ\ell\ell$ observables above the $K^*$ region can be used to further constrain the $B\to K^*$ form factors.

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arXiv2019

Exact NLO Matching and Analyticity in $b\to s\ell\ell$

Exclusive rare decays mediated by $b\to s\ell\ell$ transitions receive contributions from four-quark operators that cannot be naively expressed in terms of local form factors. Instead, one needs to calculate a matrix element of a bilocal operator. In certain kinematic regions, this bilocal operator obeys some type of Operator Product Expansion, with coefficients that can be calculated in perturbation theory. We review the formalism and, focusing on the dominant SM operators ${\cal O}_{1,2}$, we perform an improved calculation of the NLO matching for the leading dimension-three operators. This calculation is performed completely analytically in the two relevant mass scales (charm-quark mass $m_c$ and dilepton squared mass $q^2$), and we pay particular attention to the analytic continuation in the complex $q^2$ plane. This allows for the first time to study the analytic structure of the non-local form factors at NLO, and to calculate the OPE coefficients far below $q^2=0$, say $q^2 \lesssim -10\,{\rm GeV}^2$. We also provide explicitly the contributions proportional to different charge factors, which obey separate dispersion relations.

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arXiv2019

Computing Tools for the SMEFT

The increasing interest in the phenomenology of the Standard Model Effective Field Theory (SMEFT), has led to the development of a wide spectrum of public codes which implement automatically different aspects of the SMEFT for phenomenological applications. In order to discuss the present and future of such efforts, the "SMEFT-Tools 2019" Workshop was held at the IPPP Durham on the 12th-14th June 2019. Here we collect and summarize the contents of this workshop.

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arXiv2019

The CKM parameters in the SMEFT

The extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix from flavour observables can be affected by physics beyond the Standard Model (SM). We provide a general roadmap to take this into account, which we apply to the case of the Standard Model Effective Field Theory (SMEFT). We choose a set of four input observables that determine the four Wolfenstein parameters, and discuss how the effects of dimension-six operators can be included in their definition. We provide numerical values and confidence intervals for the CKM parameters, and compare them with the results of CKM fits obtained in the SM context. Our approach allows one to perform general SMEFT analyses in a consistent fashion, independently of any assumptions about the way new physics affects flavour observables. We discuss a few examples illustrating how our approach can be implemented in practice.

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arXiv2018

B physics Beyond the Standard Model at One Loop: Complete Renormalization Group Evolution below the Electroweak Scale

General analyses of $B$-physics processes beyond the Standard Model require accounting for operator mixing in the renormalization-group evolution from the matching scale down to the typical scale of $B$ physics. For this purpose the anomalous dimensions of the full set of local dimension-six operators beyond the Standard Model are needed. We present here for the first time a complete and non-redundant set of dimension-six operators relevant for $B$-meson mixing and decay, together with the complete one-loop anomalous dimensions in QCD and QED. These results are an important step towards the automation of general New Physics analyses.

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arXiv2018

Patterns of New Physics in $b\to s\ell^+\ell^-$ transitions in the light of recent data

In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available $b\to s\ell^+\ell^-$ data ($\ell=e,μ$) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by $5\,σ$ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4$\,σ$ level. We discuss the impact of LFU-violating New Physics on the observable $P_5^\prime$ from $B \to K^*μ^+μ^-$ and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.

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arXiv2017

CP Violation in Multibody $B$ Decays from QCD Factorization

We test a data-driven approach based on QCD factorization for charmless three-body $B$-decays by confronting it to measurements of CP violation in $B^- \to π^- π^+ π^-$. While some of the needed non-perturbative objects can be directly extracted from data, some others can, so far, only be modelled. Although this approach is currently model dependent, we comment on the perspectives to reduce this model dependence. While our model naturally accommodates the gross features of the Dalitz distribution, it cannot quantitatively explain the details seen in the current experimental data on local CP asymmetries. We comment on possible refinements of our simple model and conclude by briefly discussing a possible extension of the model to large invariant masses, where large local CP asymmetries have been measured.

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arXiv2017

Timelike-helicity $B\to ππ$ form factor from light-cone sum rules with dipion distribution amplitudes

We complete the set of QCD light-cone sum rules for $B\to ππ$ transition form factors, deriving a new sum rule for the timelike-helicity form factor $F_t$ in terms of dipion distribution amplitudes. This sum rule, in the leading twist-2 approximation, is directly related to the pion vector form factor. Employing a relation between $F_t$ and other $B\to ππ$ form factors we obtain also the longitudinal-helicity form factor $F_0$. In this way, all four (axial-)vector $B\to ππ$ form factors are predicted from light-cone sum rules with dipion distribution amplitudes. These results are valid for small dipion masses with large momentum.

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arXiv2017

Gauge-invariant implications of the LHCb measurements on Lepton-Flavour Non-Universality

We study the implications of the recent measurements of $R_K$ and $R_{K^*}$ by the LHCb collaboration. We do that by adopting a model-independent approach based on the Standard Model Effective Field Theory (SMEFT), in which the dominant new physics effects are encoded in the coefficients of dimension-six operators respecting the full Standard Model (SM) gauge symmetry. After providing simplified expressions for $R_K$ and $R_{K^\ast}$, we determine the implications of the recent LHCb results for these observables on the coefficients of the SMEFT operators at low and high energies. We also take into account all $b\to s \ell\ell$ data, which combined lead to effective New Physics (NP) scenarios with SM pulls in excess of 5~$σ$. Thus the operators discussed in this paper would be the first dimension-six terms in the SM Lagrangian to be detected experimentally. Indirect constraints on these operators are also discussed. The results of this paper transcend the singularity of the present situation, and set a standard for future analyses in $b\to s$ transitions when the NP is assumed to lie above the electroweak scale.

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arXiv2017

Long-distance effects in $B\to K^*\ell\ell$ from Analyticity

We discuss a novel approach to systematically determine the long-distance contribution to $B\to K^*\ell\ell$ decays in the kinematic region where the dilepton invariant mass is below the open charm threshold. This approach provides the most consistent and reliable determination to date and can be used to compute Standard Model predictions for all observables of interest, including the kinematic region where the dilepton invariant mass lies between the $J/ψ$ and the $ψ(2S)$ resonances. We illustrate the power of our results by performing a New Physics fit to the Wilson coefficient $C_9$. This approach is systematically improvable from theoretical and experimental sides, and applies to other decay modes of the type $B\to V\ell\ell$, $B\to P\ell\ell$ and $B\to Vγ$.

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arXiv2017

DsixTools: The Standard Model Effective Field Theory Toolkit

We present DsixTools, a Mathematica package for the handling of the dimension-six Standard Model Effective Field Theory. Among other features, DsixTools allows the user to perform the full one-loop Renormalization Group Evolution of the Wilson coefficients in the Warsaw basis. This is achieved thanks to the SMEFTrunner module, which implements the full one-loop anomalous dimension matrix previously derived in the literature. In addition, DsixTools also contains modules devoted to the matching to the $ΔB = ΔS = 1,2$ and $ΔB = ΔC = 1$ operators of the Weak Effective Theory at the electroweak scale and their QCD and QED Renormalization Group Evolution below the electroweak scale.

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arXiv2017

$B\toππ$ Form Factors from Light-Cone Sum Rules with $B$-meson Distribution Amplitudes

We study $B\toππ$ form factors using QCD light-cone sum rules with $B$-meson distribution amplitudes. These form factors describe the semileptonic decay $B\to ππ\ell\barν_{\ell}$, and constitute an essential input in $B\to ππ\ell^+\ell^-$ and $B\to πππ$ decays. We employ the correlation functions where a dipion isospin-one state is interpolated by the vector light-quark current. We obtain sum rules where convolutions of the $P$-wave $\bar{B}^0\to π^+π^0$ form factors with the time-like pion vector form factor are related to universal $B$-meson distribution amplitudes. These sum rules are valid in the kinematic regime where the dipion state has a large energy and a low invariant mass, and reproduce analytically the known light-cone sum rules for $B\to ρ$ form factors in the limit of $ρ$-dominance with zero width, thus providing a systematics for so-far-unaccounted corrections to $B\toρ$ transitions. Using data for the pion vector form factor, we estimate finite width-effects and the contribution of excited $ρ$-resonances to the $B\toππ$ form factors. We find that these contributions amount up to $\sim 20\%$ in the small dipion mass region where they can be effectively regarded as a nonresonant ($P$-wave) background to the $B\toρ$ transition.

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arXiv2016

Phenomenology of an $SU(2) \times SU(2) \times U(1)$ model with lepton-flavour non-universality

We investigate a gauge extension of the Standard Model in light of the observed hints of lepton universality violation in $b \to c \ell ν$ and $b \to s \ell^+ \ell^-$ decays at BaBar, Belle and LHCb. The model consists of an extended gauge group $\mathrm{SU(2)}_{1} \times \mathrm{SU(2)}_{2} \times \mathrm{U(1)}_Y$ which breaks spontaneously around the TeV scale to the electroweak gauge group. Fermion mixing effects with vector-like fermions give rise to potentially large new physics contributions in flavour transitions mediated by $W^{\prime}$ and $Z^{\prime}$ bosons. This model can ease tensions in $B$-physics data while satisfying stringent bounds from flavour physics, tau decays, and electroweak precision data. Possible ways to test the proposed new physics scenario with upcoming experimental measurements are discussed. Among other predictions, the lepton flavour violating ratios $R_M$, with $M = K^*, ϕ$, are found to be reduced with respect to the Standard Model expectation $R_M \simeq 1$.

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arXiv2016

Charmless Non-Leptonic Multi-Body B decays

I review the theoretical and phenomenological status of two- and three-body charmless non-leptonic B decays, with an emphasis on factorization approaches. Most of the material presented here is based on talks given by the participants of the workshop "Future Challenges in Non-Leptonic B Decays: Theory and Experiment", held at Bad Honnef in February 2016.

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arXiv2016

$B \to K^*(\to Kπ) \ell^+\ell^-$ theory and the global picture: What's next?

The present status of the LHC anomalies found in exclusive semileptonic $b\to s\ell^+\ell^-$ decays is discussed with special emphasis on the exclusive 4-body angular distribution $B \to K^*(\to Kπ)\ell^+\ell^-$. The treatment of hadronic uncertainties in this mode is briefly reviewed, and some of the analyses in the literature are critically reassessed. The global picture provided by the global fit points to a coherent pattern of deviations with a significance substantially above 4$σ$ for different New Physics scenarios. Finally, we propose as the next step in the field to focus on the study of optimized observables that compare electron and muon modes, sensitive to lepton-flavour universality violations and free from hadronic uncertainties (including charm) in the SM, the so called $Q_i$ observables.

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arXiv2016

Global analysis of $b\to s\ell\ell$ anomalies

We present a detailed discussion of the current theoretical and experimental situation of the anomaly in the angular distribution of $B \to K^*(\to Kπ)μ^+μ^-$, observed at LHCb in the $1~\text{fb}^{-1}$ dataset and recently confirmed by the $3~\text{fb}^{-1}$ dataset. The impact of this data and other recent measurements on $b\to s\ell^+\ell^-$ transitions ($\ell=e,μ$) is considered. We review the observables of interest, focusing on their theoretical uncertainties and their sensitivity to New Physics, based on an analysis employing the QCD factorisation approach including several sources of hadronic uncertainties (form factors, power corrections, charm-loop effects). We perform fits to New Physics contributions including experimental and theoretical correlations. The solution that we proposed in 2013 to solve the $B\to K^*μ^+μ^-$ anomaly, with a contribution ${\mathcal C}_9^{\rm NP}\simeq -1$, is confirmed and reinforced. A wider range of New-Physics scenarios with high significances (between 4 and 5 $σ$) emerges from the fit, some of them being particularly relevant for model building. More data is needed to discriminate among them conclusively. The inclusion of $b\to s e^+ e^-$ observables increases the significance of the favoured scenarios under the hypothesis of New Physics breaking lepton flavour universality. Several tests illustrate the robustness of our conclusions.

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arXiv2016

The b --> s l^+ l^- anomalies and their implications for new physics

Recently, the LHC has found several anomalies in exclusive semileptonic b --> s l^+ l^- decays. In this proceeding, we summarize the most important results of our global analysis of the relevant decay modes. After a discussion of the hadronic uncertainties entering the theoretical predictions, we present an interpretation of the data in terms of generic new physics scenarios. To this end, we have performed model-independent fits of the corresponding Wilson coefficients to the data and have found that in certain scenarios the best fit point is prefererred over the Standard Model by a global significance of more than 4σ. Based on the results, the discrimination between high-scale new physics and low-energy QCD effects as well as the possibility of lepton-flavour universality violation are discussed.

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