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K. Keri Vos

Publications and source records attributed to K. Keri Vos.

At least 37 records · Page 2Linked to original sources
arXiv2024

Inclusive semileptonic $B_{s}^{0}$ meson decays at the LHC via a sum-of-exclusive modes technique: possibilities and prospects

We propose an approach for measuring the moments of the hadronic invariant mass distribution in semileptonic $B_{s}^{0}$ meson decays using a sum-of-exclusive technique. Using the present and foreseen knowledge about exclusive semileptonic $B_{s}^{0}$ decays, we estimate the uncertainties on moments of the kinematic distribution. Semileptonic $B_{s}^{0}$ decays can be described, as their $B^{0}$ counterpart, using the Heavy Quark Expansion (HQE), with the only difference between the $B_{s}^{0}$ and $B^{0}$ mesons being the $SU(3)_F$ breaking effects that change the numerical values of the non-perturbative HQE parameters. We extract the HQE parameters for the $B_{s}^{0}$ decays from our estimates of the moments, showing the potential of the proposed method. We identify a set of required measurements for a future precision measurement.

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arXiv2024

Quark-Hadron Duality Violations and Higher-Order $1/m_b$ Corrections in Inclusive Semileptonic $B$ Decays

The theoretical description and data for inclusive semileptonic $B$ decays have reached incredible precision. This motivated us to re-animate the discussion of possible Quark-Hadron Duality violations. There seems that there is currently no evidence of a failure of the Heavy Quark Expansion (HQE) used to compute observables for these decays. However, we might arrive at a point where an asymptotic behaviour of the HQE would limit a further increase of precision. We discuss this possibility and suggest a simple model, which can be used to study the effects of higher orders in the $1/m_b$ expansion and possible quark-hadron duality violations. We devise observables sensitive only to such higher-order effects to test the behaviour of the HQE. Using these observables we obtain a first estimate of possible quark-hadron duality violations using the measured $q^2$ moments.

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arXiv2024

CP Violation in $B \to K\ell^+\ell^-$ Decays: New Opportunities in the High-Precision Era

Experimental data on rare $B$-meson decays indicate deviations from Standard Model predictions. In studies of these decays, possible new sources of CP violation are often neglected. We discuss CP violation in the rare $B$-meson decays $B\to K\ell^+\ell^- (\ell = μ,e)$ and point to two phenomena that arise when new sources of CP violation are included. First, the Wilson coefficients $C_{9\ell}$ and $C_{10\ell}$ become complex, and we show how we can extract their values from measurements of direct and mixing-induced CP asymmetries. Second, new sources of CP violation can generate nontrivial lepton flavour universality violation. Such a violation is usually measured through ratios like $R_K$ and $R_{K^*}$, but we show that measuring only these ratios leaves a large parameter space unexplored. These results bring exciting opportunities to reveal New Physics effects in the high-precision era.

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arXiv2023

New determination of $|V_{ub}/V_{cb}|$ from $B_s^0\to \lbrace K^-, D_s^- \rbrace μ^+ν$

We update the full set of $\bar{B}_s\rightarrow K$ form factors using light-cone sum rules with an on-shell kaon. Our approach determines the relevant sum rule parameters -- the duality thresholds -- from a Bayesian fit for the first time. Using a modified version of the Boyd-Grinstein-Lebed parametrisation, we combine our sum rule results at low momentum transfer $q^2$ with more precise lattice QCD results at large $q^2$. We obtain a consistent description of the form factors in the full $q^2$ range. Applying these results to a recent LHCb measurement of branching ratios for the decays $B_s^0 \to \lbrace K^-, D_s^-\rbrace μ^+ν_μ$, we determine the ratio of Cabibbo-Kobayashi-Maskawa elements $$ \notag \left|\frac{V_{ub}}{V_{cb}}\right|_{q^2<7\; \textrm{GeV}^2} = 0.0681\pm 0.0040 \quad \text{and} \quad \left|\frac{V_{ub}}{V_{cb}}\right|_{q^2>7 \;\textrm{GeV}^2} = 0.0801\pm 0.0047 \ , $$ which are mutually compatible at the $1.9σ$ level. We further comment on the sensitivity to Beyond the Standard Model effects through measurements of the shape of $B_s^0 \to K^- μ^+ν_μ$ decays, in light of recent limits on such effects from other exclusive $b\to u\ellν$ processes.

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arXiv2023

New Probes of Electron--Muon Universality in $B \to K\ell^+\ell^-$ Decays

In the pursuit of physics beyond the Standard Model, a promising path is the study of B-meson decays caused by the transition $b \to s\ell^+\ell^-$. A key observable in such decays is the ratio $R_K$, which measures electron--muon universality in $B \to K μ^+μ^-/e^+e^-$. At first sight, the recent LHCb measurement of $R_K \sim 1$ may seem to largely constrain deviations from universality in these decays. However, we show that this is actually not the case: new sources of CP violation allow for significant universality violation consistent with $R_K \sim 1$. This provides an exciting new opportunity to search for New Physics by measuring differences between CP asymmetries in $B \to Kμ^+μ^-$ and $B \to K e^+e^-$.

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arXiv2023

QCD Light-Cone Distribution Amplitudes of Heavy Mesons from boosted HQET

Light-cone distribution amplitudes (LCDAs) frequently arise in factorization theorems involving light and heavy mesons. The QCD LCDA for heavy mesons includes short-distance physics at energy scales of the heavy-quark mass. In this paper we achieve the separation of this perturbative scale from the purely hadronic effects by matching the QCD LCDA to the convolution of a perturbative function with the universal, quark-mass independent LCDA defined in heavy-quark effective theory. This factorization allows to resum potentially large logarithms between $Λ_{\rm QCD}$ and $m_Q$ as well as between $m_Q$ and the scale $Q$ of the hard process in the production of boosted heavy mesons at colliders. As an application we derive new theoretical predictions for the branching ratio of the decay $W^\pm \to B^\pm γ$. Furthermore, we provide phenomenological models for the QCD LCDAs of both the $\bar{B}$ and $D$ mesons expressed as expansions in Gegenbauer polynomials.

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arXiv2023

New physics contributions to moments of inclusive $b \to c$ semileptonic decays

Inclusive semileptonic $B\to X_c \ell \barν_\ell$ decays, where $\ell = μ,e$, are by now standard candles in the determination of the CKM element $|V_{cb}|$. These determinations rely on the heavy-quark expansion and use moments of decay spectra to extract the non-perturbative parameters directly from data under the standard model assumption. At the same time, new physics could influence the moments of the inclusive decay. In this paper, we compute power-corrections and next-to-leading order corrections in the strong coupling constant using the full basis of dimension-six new physics operators for the inclusive $B \to X_c \ell \barν$ decay. We provide predictions for lepton energy, hadronic and leptonic invariant mass moments, and perform a phenomenological study to show the possible impact of new physics. Our results could be used to perform a global fit including new physics contributions.

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arXiv2023

Light-Cone Sum Rules for $S$-wave $B\to Kπ$ Form Factors

We derive a set of light-cone sum rules relating the $S$-wave $B\to K π$ hadronic form factors to the $B$-meson light-cone distribution amplitudes (LCDAs), taking into account the complete set of LCDAs up to and including twist four. These results complement the sum rules for the $P$-wave $B\to K π$ form factors obtained earlier. We then use the new sum rules to estimate the $S$-wave contributions to $B\to K π\ell\ell$ decays as a function of the $Kπ$ invariant mass. We pay particular attention to the fact that the $S$-wave $Kπ$ spectrum cannot be modelled by a sum of Breit-Wigner resonances, and employ a more consistent dispersive coupled-channel approach. We compare our predictions for branching ratios and angular observables with LHCb measurements in two different kinematic regions, around $K^*(892)$ and $K^*_0(1430)$. We observe an overall compatibility and discuss possible improvements of our model to obtain a better description of the $B\to Kπ$ form factors over a large kinematic range.

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arXiv2023

New Perspectives for Testing Electron-Muon Universality

Intriguing results for tests of the universality of electrons and muons through measurements of rates of $B\to K \ell^+ \ell^-$ and similar decays have been in the spotlight for years. The LHCb collaboration has recently reported new results which are in agreement with Lepton Flavour Universality, while the individual decay rates are found below their Standard Model predictions. In view of this new situation, we explore how much space is left for a violation of electron-muon universality. Considering new sources of CP violation and taking the new LHCb measurements into account, we show that significant differences between the short-distance coefficients for electronic and muonic final states are actually allowed by the current data. These patterns can be revealed through CP asymmetries in neutral and charged $B\to K \ell^+ \ell^-$ decays. We obtain correlations between these observables and map them to the short-distance coefficients. This results in regions in New Physics parameter space with large differences between CP asymmetries of the decays with final-state electrons and muons, thereby leaving a lot of room for possible surprises in the future high-precision era.

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arXiv2023

Alternative Treatment of the Quark Mass in the Heavy Quark Expansion

The treatment of the quark mass plays an important role when it comes to increasing the precision of the predictions of the heavy quark expansion for inclusive heavy hadron decays. Various short-distance mass schemes have been invented to minimize the uncertainties induced by the quark mass, which needs to be extracted from other, independent observables. We suggest to replace the quark mass directly by an observable such as e.g. the inverse moments of the cross section for $e^+ e^- \to $ hadrons. We investigate this alternative strategy and study its impact on the perturbative series.

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arXiv2022

Fingerprinting CP-violating New Physics with $B \to Kμ^+μ^-$

The $B \to K \ell^+ \ell^-$ system is particularly interesting to explore physics beyond the Standard Model. Such effects could provide new sources of CP violation, thereby giving rise to Wilson coefficients which could not only be real, as assumed in most analyses, but also be complex. We propose a new strategy to extract the complex $C_{9 μ}$, $C_{10 μ}$ from the data, utilising the complementarity of direct CP violation in $B^+\to K^+ μ^+μ^-$ and mixing-induced CP violation arising - in addition to direct CP violation - in $B^0_d \to K_{S} μ^+μ^-$, as well as the differential rate in appropriate $q^2$ bins. The long-distance effects, which play a key role to generate the direct CP asymmetries, cannot be reliably calculated due to their non-perturbative nature. In order to describe them, we apply a model by the LHCb Collaboration employing experimental data. Assuming specific scenarios, we demonstrate the fingerprinting of CP-violating New Physics and the distinction between ambiguities in the model of the long-distance contributions. Finally, New Physics could couple differently to muons and electrons, as probed through the lepton flavour universality ratio $R_K$. We discuss these effects in the presence of CP violation and present a new way to measure the direct CP asymmetries of the $B \to K e^+ e^-$ channels.

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arXiv2022

Zooming into CP violation in $B_{(s)}\to hh$ Decays

The LHCb collaboration has recently reported the first observation of CP violation in the penguin-dominated $B^0_s\to K^-K^+$ decay and further new measurements, indicating differences between the direct CP asymmetries of both the $B^0_s\to K^-K^+$, $B^0_d\toπ^-K^+$ and the $B^0_d\to π^-π^+$, $B^0_s\to K^-π^+$ modes. We show that these puzzling differences can be accommodated through sizeable penguin annihilation and exchange topologies in the Standard Model, and constrain them. Utilising the $U$-spin symmetry, we extract the angle $γ$ of the unitarity triangle from the CP asymmetries in the $B^0_s\to K^-K^+$, $B^0_d\to π^-π^+$ system alone, finding $γ=(65^{+11}_{-7})^\circ$, in perfect agreement with the determination from tree-level $B\to DK$ decays. The $B^0_s$--$\bar B^0_s$ mixing phase $ϕ_s$ can be extracted from CP violation measurements in $B^0_s\to K^-K^+$ in a clean way. We present a new strategy and extract $ϕ_s=-(3.6\pm 5.4)^\circ$. This result is in agreement with the determination from $B^0_s\to J/ψϕ$ decays. New CP-violating contributions would influence these determinations differently. Hence it is interesting to keep monitoring both as the experimental picture sharpens.

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arXiv2022

Standard Model predictions for Lepton Flavour Universality ratios of inclusive semileptonic $B$ decays

We present Standard Model predictions for lepton flavour universality ratios of inclusive $B\to X_{(c)} \ell \barν_\ell$. For the $\ell=μ,e$, these ratios are very close to unity as expected. For the $τ$ mode, we update the SM prediction for the branching ratio including power-corrections in the heavy-quark expansion up to $1/m_b^3$. These inclusive ratios serve as an important cross-check of the exclusive $B\to D^{(*)}\ell\barν_\ell$ modes, in which tensions exists between the predictions and measurements in those modes.

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arXiv2022

First extraction of inclusive $V_{cb}$ from $q^2$ moments

We present the first determination of $V_{cb}$ from inclusive $B\to X_c \ell \barν_\ell$ using moments of the dilepton invariant mass, $q^2$. These moments are reparametrization invariant quantities and depend on a reduced set of non-perturbative parameters. This reduced opens a new path to extract these parameters up to $1/m_b^4$ purely from data and thereby reducing the uncertainty on $V_{cb}$. In this paper, we present our first determination of $V_{cb}$ using this method. Combining the recent measurements of $q^2$ moments by Belle and Belle II, our default fit gives $|V_{cb}| = (41.69\pm 0.63)\cdot 10^{-3}$. This results presents an important independent cross check of, and is consistent with, the previous state-of-the-art inclusive determinations using lepton energy and hadronic invariant mass moments.

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arXiv2021

EOS -- A Software for Flavor Physics Phenomenology

EOS is an open-source software for a variety of computational tasks in flavor physics. Its use cases include theory predictions within and beyond the Standard Model of particle physics, Bayesian inference of theory parameters from experimental and theoretical likelihoods, and simulation of pseudo events for a number of signal processes. EOS ensures high-performance computations through a C++ back-end and ease of usability through a Python front-end. To achieve this flexibility, EOS enables the user to select from a variety of implementations of the relevant decay processes and hadronic matrix elements at run time. In this article, we describe the general structure of the software framework and provide basic examples. Further details and in-depth interactive examples are provided as part of the EOS online documentation.

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arXiv2021

Lepton flavour violation in rare $Λ_b$ decays

Lepton flavour violation (LFV) naturally occurs in many new physics models, specifically in those explaining the $B$ anomalies. While LFV has already been studied for mesonic decays, it is important to consider also baryonic decays mediated by the same quark transition. In this paper, we study LFV in the baryonic $Λ_b \to Λ\ell_1 \ell_2$ using for the first time a full basis of New Physics operators. We present expected bounds on the branching ratio in a model-independent framework and using two specific new physics models. Finally, we point out the interplay and orthogonality between the baryonic and mesonic LFV searches.

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arXiv2021

Light-cone distribution amplitudes of light mesons with QED effects

We discuss the generalization of the leading-twist light-cone distribution amplitude for light mesons including QED effects. This generalization was introduced to describe virtual collinear photon exchanges above the strong-interaction scale $Λ_{\rm QCD}$ in the factorization of QED effects in non-leptonic $B$-meson decays. In this paper we study the renormalization group evolution of this non-perturbative function. For charged mesons, in particular, this exhibits qualitative differences with respect to the well-known scale evolution in QCD only, especially regarding the endpoint-behaviour. We analytically solve the evolution equation to first order in the electromagnetic coupling $α_{\rm em}$, which resums large logarithms in QCD on top of a fixed-order expansion in $α_{\rm em}$. We further provide numerical estimates for QED corrections to Gegenbauer coefficients as well as inverse moments relevant to (QED-generalized) factorization theorems for hard exclusive processes.

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