arXiv subjects

Joaquim Matias

Publications and source records attributed to Joaquim Matias.

At least 37 records · Page 2Linked to original sources

arXiv2021

A new $B$-flavour anomaly in $B_{d,s}\to K^{0}\bar{K}^{0}$: anatomy and interpretation

In the context of the recently measured non-leptonic decays $B_{d}\to K^{*0}\bar{K}^{*0}$ and $B_{s}\to K^{*0}\bar{K}^{*0}$ we analyse the anatomy of the $L_{VV}$ observable that compares the longitudinal components of $B_s \to VV$ and $B_d \to VV$ decays. This observable is cleaner than the longitudinal polarisation fraction as it is afflicted only at subleading order in a $1/m_b$ expansion by the theoretical uncertainties arising in the transverse components entering the polarisation fraction. Focusing on the particular case of $B_{d}\to K^{*0}\bar{K}^{*0}$ and $B_{s}\to K^{*0}\bar{K}^{*0}$, we discuss the main sources of hadronic uncertainty in the SM. We find for the SM prediction $L_{K^*\bar{K}^*}=19.5^{+9.3}_{-6.8}$, which implies a $2.6σ$ tension with respect to the most recent data, pointing to a deficit in the $b \to s$ transition of the non-leptonic decay versus the corresponding $b \to d$ transition. We discuss possible New Physics explanations for this deviation, first at the level of the Weak Effective Theory and we identify that the two Wilson coefficients ${\cal C}_{4}$ and ${\cal C}_{8g}$ can play a central role in explaining this anomaly. Finally, we briefly explore two different simplified New Physics models which can explain the anomaly through a contribution either in ${\cal C}_4$ (Kaluza-Klein gluon) or in ${\cal C}_{8g}$, with a significant amount of fine tuning, but possible connections to the $b \to s \ell \ell$ anomalies.

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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

Symmetries in $B \to D^* \ell ν$ angular observables

We apply the formalism of amplitude symmetries to the angular distribution of the decays $B \to D^* \ell ν$ for $\ell=e,μ,τ$. We show that the angular observables used to describe the distribution of this class of decays are not independent in absence of New Physics contributing to tensor operators. We derive sets of relations among the angular coefficients of the decay distribution for the massless and massive lepton cases which can be used to probe in a very general way the consistency among the angular observables and the underlying New Physics at work. We use these relations to access the longitudinal polarisation fraction of the $D^*$ using different angular coefficients from the ones used by Belle experiment. This in the near future can provide an alternative strategy to measure $F_L^{D^*}$ in $B \to D^* τν$ and to understand the relatively high value measured by the Belle experiment. Using the same symmetries, we identify three observables which may exhibit a tension if the experimental value of $F_L^{D^*}$ remains high. We discuss how these relations can be exploited for binned measurements. We also propose a new observable that could test for specific scenarios of New Physics generated by light right-handed neutrinos. Finally we study the prospects of testing these relations based on the projected experimental sensitivity of new experiments.

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arXiv2019

What $R_K$ and $Q_5$ can tell us about New Physics in $b\to s\ell\ell$ transitions?

The deviations with respect to the Standard Model that are currently observed in $b \to s \ell\ell$ transitions, or $B$ anomalies, can be interpreted in terms of different New Physics (NP) scenarios within a model-independent effective approach. We identify a set of internal tensions of the fit that require further attention and whose theoretical or experimental nature could be determined with more data. In this landscape of NP, we discuss possible ways to discriminate among favoured NP hypotheses in the short term thanks to current and forthcoming observables. While an update of $R_K$ should help to disentangle the type of NP we may be observing (Lepton-Flavour Universality Violating and/or Lepton Flavour Universal), additional observables, in particular $Q_5$, turn out to be central to determine which NP hypothesis should be preferred. We also analyse the preferences shown by the current global fit concerning various NP hypotheses, using two different tools: the behaviour of the pulls of individual observables under NP scenarios and the directions favoured by approximate quadratic parametrisations of the observables in terms of Wilson coefficients.

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arXiv2019

Are we overlooking Lepton Flavour Universal New Physics in $b\to s\ell\ell$ ?

The deviations with respect to the Standard Model (SM) that are currently observed in $b \to s \ell\ell$ transitions (the so-called flavour anomalies) can be interpreted in terms of different New Physics (NP) scenarios within a model-independent effective approach. We reconsider the determination of NP in global fits from a different perspective by removing one implicit hypothesis of current analyses, namely that NP is only Lepton-Flavour Universality Violating (LFUV). We examine the roles played by LFUV NP and Lepton-Flavour Universal (LFU) NP altogether, providing new directions to identify the possible theory beyond the SM responsible for the anomalies observed. New patterns of NP emerge due to the possibility of allowing at the same time large LFUV and LFU NP contributions to $C_{10μ}$, which provides a different mechanism to obey the constraint from the $B_s \toμ^+μ^-$ branching ratio. In this landscape of NP, we discuss how to discriminate among these scenarios in the short term thanks to current and forthcoming observables. While the update of $R_K$ will be a major milestone to confirm the NP origin of the flavour anomalies, additional observables, in particular the LFUV angular observable $Q_5$, turn out to be central to assess the precise NP scenario responsible for the observed anomalies.

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arXiv2018

Hadronic uncertainties in $B\to K^*μ^+μ^-$: a state-of-the-art analysis

In the absence of direct evidence for New Physics at present LHC energies, the focus is set on the anomalies and discrepancies recently observed in rare $b \to s\ell\ell$ transitions which can be interpreted as indirect hints. Global fits have shown that an economical New Physics solution can simultaneously alleviate the tensions in the various channels and can lead to a significant improvement in the description of the data. Alternative explanations within the Standard Model for part of the observed anomalies have been proposed in terms of (unexpectedly large) hadronic effects at low dilepton invariant mass and attributing tensions in protected observables to statistical fluctuations or experimental errors. We review the treatment of hadronic uncertainties in this kinematic regime for one of the most important channels, $B \to K^*μ^+μ^-$, in a pedagogical way. We provide detailed arguments showing that factorisable power corrections cannot account for the observed anomalies and that an explanation through long-distance charm contributions is disfavoured. Some optimized observables at very low dilepton invariant mass are shown to be protected against contributions from the semileptonic coefficient $C_9$ (including any associated long-distance charm effects), enhancing their sensitivity to New Physics contributions to other Wilson coefficients. Finally, we discuss how the recent measurement of $Q_5$ by Belle (and in the future by LHCb and Belle-II) may provide a robust cross-check of our arguments.

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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

Searching for New Physics with $b\to sτ^+τ^-$ processes

In recent years, intriguing hints for the violation of Lepton Flavour Universality (LFU) have been accumulated in semileptonic $B$ decays, both in the neutral-current transitions $b\to s\ell^+\ell^-$ (i.e., $R_K$ and $R_{K^*}$) and the charged-current transitions $b\to c\ell^-\barν_\ell$ (i.e., $R_D$, $R_{D^*}$ and $R_{J/ψ}$). LHCb has reported deviations from the Standard Model (SM) expectations in $b\to sμ^+μ^-$ processes as well as in the ratios $R_K$ and $R_{K^*}$, which together point at New Physics (NP) affecting muons with a high significance. Furthermore, hints for LFU violation in $R_{D^{(*)}}$ and $R_{J/ψ}$ point at large deviations from the SM in processes involving tau leptons. Together, these hints for NP motivate the possibility of huge LFU-violating effects in $b\to sτ^+τ^-$ transitions. In this article we predict the branching ratios of $B\to Kτ^+τ^-$, $B\to K^{*}τ^+τ^-$ and $B_s\to ϕτ^+τ^-$ taking into account NP effects in the Wilson coefficients $C_{9(')}^{ττ}$ and $C_{10(')}^{ττ}$. Assuming a common NP explanation of $R_{D^{}}$ , $R_{D^{(*)}}$ and $R_{J/ψ}$, we show that a very large enhancement of $b\to sτ^+τ^-$ processes, of around three orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of $B_s\to τ^+τ^-$, $B_s\to ϕτ^+τ^-$ and $B\to K^{(*)}τ^+τ^-$ under these assumptions are in the observable range for LHCb and Belle II.

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arXiv2017

Status of new physics in CP violation and rare B decays

We briefly report on some particular aspects of new physics searches concerning, on the one side, the CKM paradigm and, on the other, the global fit to rare $b\to s \ell\ell$ observables including lepton flavour universality ones. We put special emphasis on the state-of-the-art of hadronic uncertainties of $b\to sμμ$ observables and LFUV in SM and in presence of New Physics. Finally, we discuss the latest experimental and theoretical developments concerning long-distance charm contributions.

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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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arXiv2016

Assessing lepton-flavour non-universality from $B\to K^*\ell\ell$ angular analyses

The $B\to K^*μμ$ decay exhibits deviations with respect to Standard Model expectations and the measurement of the ratio $R_K$ hints at a violation of lepton-flavour universality in $B\to K\ell\ell$ transitions. Both effects can be understood in model-independent fits as a short-distance contribution to the Wilson coefficient $C_{9μ}$, with some room for similar contributions in other Wilson coefficients for $b\to sμμ$ transitions. We discuss how a full angular analysis of $B\to K^*ee$ and its comparison with $B\to K^*μμ$ could improve our understanding of these anomalies and help confirming their interpretation in terms of short-distance New Physics. We discuss several observables of interest in this context and provide predictions for them within the Standard Model as well as within several New Physics benchmark scenarios. We pay special attention to the sensitivity of these observables to hadronic uncertainties from SM contributions with charm loops.

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arXiv2015

Lepton-flavour violating $B$ decays in generic $Z^\prime$ models

LHCb has reported deviations from the Standard Model in $b\to sμ^+μ^-$ transitions for which a new neutral gauge boson is a prime candidate for an explanation. As this gauge boson has to couple in a flavour non-universal way to muons and electrons in order to explain $R_K$, it is interesting to examine the possibility that also lepton flavour is violated, especially in the light of the CMS excess in $h\toτ^\pmμ^\mp$. In this article, we investigate the perspectives to discover the lepton-flavour violating modes $B\to K^{(*)}τ^\pmμ^\mp$, $B_s\to τ^\pmμ^\mp$ and $B\to K^{(*)} μ^\pm e^\mp$, $B_s\to μ^\pm e^\mp$. For this purpose we consider a simplified model in which new-physics effects originate from an additional neutral gauge boson ($Z^\prime$) with generic couplings to quarks and leptons. The constraints from $τ\to3μ$, $τ\toμν\barν$, $μ\to eγ$, $g_μ-2$, semi-leptonic $b\to sμ^+μ^-$ decays, $B\to K^{(*)}νν$ and $B_s$--$\overline{B}_s$ mixing are examined. From these decays, we determine upper bounds on the decay rates of lepton flavour violating $B$ decays. $Br(B\to Kνν)$ limits the branching ratios of LFV $B$ decays to be smaller than $8\times 10^{-5} (2\times 10^{-5})$ for vectorial (left-handed) lepton couplings. However, much stronger bounds can be obtained by a combined analysis of $B_s$--$\overline{B}_s$, $τ\to3μ$, $τ\toμν\barν$ and other rare decays. The bounds depend on the amount of fine-tuning among the contributions to $B_s$--$\overline{B}_s$ mixing. Allowing for a fine-tuning at the percent level we find upper bounds of the order of $10^{-6}$ for branching ratios into $τμ$ final states, while $B_s\to μ^\pm e^\mp$ is strongly suppressed and only $B\to K^{(*)} μ^\pm e^\mp$ can be experimentally accessible (with a branching ratio of order $10^{-7}$).

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arXiv2015

Exploiting the Symmetries of P and S wave for B --> K^* mu^+ mu^-

After summarizing the current theoretical status of the four-body decay B --> K^*(--> K pi) mu^+ mu^-, we apply the formalism of spin-symmetries to the full angular distribution, including the S-wave part involving a broad scalar resonance K0^*. While we recover in the P-wave sector the known relation between the angular observables Pi('), we find in the S-wave sector two new relations connecting the coefficients of the S-wave angular distribution and reducing the number of independent S-wave observables from six to four. Included in the experimental data analysis, these relations can help to reduce the background from S-wave pollution. We further point out the discriminative power of the maximum of the angular observable P2 as a charm-loop insensitive probe of right-handed currents. Moreover, we show that in absence of right-handed currents the angular observables P4' and P5' fulfill the relation P4' = beta P5' at the position where P2 reaches its maximum.

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arXiv2015

Theoretical status of $B \to K^* μ^+μ^-$: The path towards New Physics

We summarize the status of the analysis of the 4-body angular distribution $B \to K^*(\to K π) μ^+μ^-$ using a basis of optimized observables. We provide a New Physics interpretation of the pattern of deviations observed in the 1fb$^{-1}$ dataset with the coefficient of the semileptonic operator $O_9$ being the main responsible for this pattern. Also other scenarios involving more Wilson coefficients are briefly discussed. Further, we present a detailed description of each of the hadronic uncertainties entering our predictions and suggest possible tests for alternative hadronic explanations. Our most accurate SM predictions for the optimized observables in various bins including all uncertainties are also provided.

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arXiv2014

QCD uncertainties in the prediction of B --> K^* mu^+ mu^- observables

The recent LHCb angular analysis of the exclusive decay B --> K^* mu^+ mu^- has indicated significant deviations from the Standard Model expectations. In order to give precise theory predictions, it is crucial that uncertainties from non-perturbative QCD are under control and properly included. The dominant QCD uncertainties originate from the hadronic B --> K^* form factors and from charm loops. We present a systematic method to include factorisable power corrections to the form factors in the framework of QCD factorisation and study the impact of the scheme chosen to define the soft form factors. We also discuss charm-loop effects.

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arXiv2014

On the impact of power corrections in the prediction of B->K*mu+mu- observables

The recent LHCb angular analysis of the exclusive decay B->K^*mu+mu- has indicated significant deviations from the Standard Model expectations. Accurate predictions can be achieved at large K*-meson recoil for an optimised set of observables designed to have no sensitivity to hadronic input in the heavy-quark limit at leading order in alpha_s. However, hadronic uncertainties reappear through non-perturbative Lambda_QCD/m_b power corrections, which must be assessed precisely. In the framework of QCD factorisation we present a systematic method to include factorisable power corrections and point out that their impact on angular observables depends on the scheme chosen to define the soft form factors. Associated uncertainties are found to be under control, contrary to earlier claims in the literature. We also discuss the impact of possible non-factorisable power corrections, including an estimate of charm-loop effects. We provide results for angular observables at large recoil for two different sets of inputs for the form factors, spelling out the different sources of theoretical uncertainties. Finally, we comment on a recent proposal to explain the anomaly in B->K*mu+mu- observables through charm-resonance effects, and we propose strategies to test this proposal identifying observables and kinematic regions where either the charm-loop model can be disentangled from New Physics effects or the two options leave different imprints.

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