arXiv subjects

K. Keri Vos

Publications and source records attributed to K. Keri Vos.

At least 19 recordsLinked to original sources

arXiv2026

Extending the Kinetic Mass to Higher Orders in $1/m_Q$

Currently, the kinetic mass is defined in terms of the pole mass and operators at order $1/m_Q^2$, which are known to N$^3$LO accuracy in $α_s$. At the same time, the Heavy Quark Expansion (HQE) for inclusive semileptonic decays is known up to and including terms of order $1/m_Q^5$. Therefore, it is desirable to extend the definition of the kinetic mass to higher orders in $1/m_Q$. The original kinetic mass is based on the hadron-mass formula in Heavy Quark Effective Theory (HQET). However, the HQE is formulated in terms of matrix elements defined in full QCD to avoid the appearance of non-local matrix elements. To avoid this, we develop a definition of the kinetic mass rooted in full QCD. Starting from the hadron-mass formula derived from the energy-momentum tensor of full QCD, we define a relation between a general mass and the pole mass. Using a simple cut-off scheme, we compute a generalized kinetic mass at one loop to all powers of $1/m_Q$, which reproduces the well-known results for the kinetic mass up to $1/m_Q^2$. Our approach opens the road to a consistent use of the kinetic mass at higher-orders in the heavy quark expansion.

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arXiv2026

CP Violation in Charmed Meson Decays into Final States with $η'$

We derive Standard Model predictions for the CP asymmetries of singly-Cabibbo suppressed $D\rightarrow Pη'$ decays, where $P=K,π,η$. Our predictions are based on the approximate SU(3)$_F$ symmetry of QCD and include first-order symmetry-breaking effects in a systematic way. The underlying symmetry leads to correlations between different decay modes. To this end we predict the correlations between $ a_{\rm CP}^{\rm dir}(D^+\to π^+ η')$ and $a_{\rm CP}^{\rm dir}(D_s^+ \to K^+ η')$ as well as $a_{\rm CP}^{\rm dir}(D^0\to π^0 η')$ and $ a_{\rm CP}^{\rm dir}(D^0\to ηη')$. Our results can be used to probe the Standard Model with future measurements by LHCb, Belle II and BESIII. At the same time, such future measurements permit the extraction of the key theory parameter related to the ratio of color-suppressed to color-favoured contributions to the decay amplitudes. Future data on both branching ratios and CP asymmetries will automatically improve our predictions and thereby also their sensitivity to physics beyond the Standard Model.

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arXiv2026

A Standard Model analysis of $D^0 \to π^- π^+, K^- K^+, K_{\rm S}^0 K_{\rm S}^0$

We investigate the Standard Model (SM) description of the singly Cabibbo-suppressed charm decays $D^0\to π^-π^+$, $D^0\to K^-K^+$ and $D^0\to K_{\rm S}^0K_{\rm S}^0$. Using factorisation together with isospin symmetry, we constrain non-factorisable effects and the associated $U$-spin breaking required by the measured branching fractions. We find that corrections of order $50\%$ are sufficient to describe all modes, which although sizeable, are not unexpected for hadronic charm decays. Using the constraints from the branching fractions, we derive SM predictions for the direct CP asymmetry in $D^0\to K_{\rm S}^0K_{\rm S}^0$, finding it to be at most at the per-mille level in our benchmark scenario, providing motivation for future precision measurements.

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arXiv2026

Extracting production fractions of $b$ hadrons from exclusive semi-leptonic decays

Ratios of production fractions of $b$ hadrons are a dominant source of uncertainty in many LHC analyses, in particular in measurements with $B_s$ mesons. The currently used value for $f_s/f_d$ is based on a combination of hadronic and semi-inclusive semi-leptonic decays, and relies in part on assumptions about the underlying decays that are hard to quantify. We propose an independent alternative method to obtain this quantity by measuring ratios of the exclusive semi-leptonic decays $\bar B_{(s)} \to D_{(s)}^{(*)} \ell \barν$. This method benefits from significant cancellations of both experimental and theoretical uncertainties, as well as robustness against potential contamination from heavy physics beyond the Standard Model. As a proof of principle, we show that current measurements constrain $f_s/f_d$ with an uncertainty of $7\%$, dominated by present experimental uncertainties. This method can also be applied to other ratios of production fractions involving heavier $b$ hadrons, such as $B_c$ or $Λ_b$.

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arXiv2026

CP Violation in $B_{(s)}\toϕK$ Decays: Standard Model Benchmarks and Isospin-Breaking New Physics

The penguin loop-suppressed $B\toϕK$ decays are highly sensitive to contributions of hypothetical heavy new particles. Particularly interesting probes for testing the Standard Model and revealing such phenomena are provided by CP violation in the $B^0_d\toϕK_{\rm S}$ decay. Standard-Model estimates for the corresponding CP-violating observables are theoretically limited by doubly Cabibbo-suppressed penguin contributions. We study these effects using a factorization approach, and provide predictions for CP asymmetries, to be contrasted with future measurements. To gain additional insight into these hadronic effects, we propose the $B_s^0\toϕK_{\rm S}$ decay as a new channel. We predict the observables for this decay for which currently no measurements exist. By comparing $B^0_d\toϕK_{\rm S}$ and $B^+ \to ϕK^+$, we further derive state-of-the-art constraints on isospin observables within the Standard Model. The same framework enables probing of possible New-Physics contributions, including general effects and those with non-trivial isospin structure. Interesting prospects arise for the high-precision era of flavour physics ahead.

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arXiv2025

Standard Model Benchmarks for $D^0\to K^- K^+, π^-π^+, K^0_{\rm S} K^0_{\rm S}$ Decays

The non-leptonic $D^0\to K^- K^+$ and $D^0\to π^-π^+$ decays are powerful probes of the Standard Model and are related to each other through the $U$-spin symmetry of the strong interaction. Using lattice QCD inputs we calculate the corresponding colour-allowed tree amplitudes in factorisation and demonstrate that non-factorisable contributions and $U$-spin-breaking effects at the level of 50% allow us to accommodate the measured branching ratios in the Standard Model. An exciting direct probe of such non-factorisable and $U$-spin breaking effects is provided by the $D^0\to K^0_{\rm S} K^0_{\rm S}$ channel. This decay is governed by non-factorisable exchange topologies and essentially vanishes in the $U$-spin limit, although it is experimentally well established with a prominent branching ratio. Extrapolating our $D^0\to K^- K^+$ results using the isospin symmetry, we find a consistent benchmark picture. Specifically, we can accommodate the measured $D^0\to K^0_{\rm S} K^0_{\rm S}$ branching ratio with $U$-spin-breaking effects at the 50% level and exchange amplitudes at the level of 50% of the colour-allowed $D^0\to K^- K^+$, $D^0\to π^-π^+$ tree contributions. Finally, we explore the resulting range for direct CP violation in $D^0\to K^0_{\rm S} K^0_{\rm S}$, obtaining upper bounds in our benchmark scenarios of a few per mille, offering an exciting target for future measurements.

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arXiv2025

Inclusive $\bar{B}\to X_s \ell^+\ell^-$ at the LHC: theory predictions and new-physics reach

We present theoretical predictions for observables in inclusive $\bar{B}\to X_s \ell^+\ell^-$ suitable for measurements at hadron colliders through a sum-over-exclusive approach. At low $q^2$ we calculate the branching ratio and three angular observables. At high $q^2$ we provide the branching ratio and the ratio of the $\bar B \to X_s \ell^+\ell^-$ rate with respect to the inclusive $\bar B \to X_u \ell \barν$ rate with the same phase-space cut. We compare our predictions to the $B$ factory measurements and also to an extraction of the experimental rate through a sum-over-exclusive method using branching ratios of the exclusive $\bar B \to K^{(*)}μ^+μ^-$ modes measured at LHCb. We find a consistent picture comparing Standard Model theory and experiment. As such, our analysis does not support a recent claim about a deficit in the inclusive branching ratio in the high-$q^2$ region. Finally, we present current model-independent bounds on new physics and emphasize the potential of complementary analyses of $\bar{B}\to X_s \ell^+\ell^-$ at Belle II and the LHC.

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arXiv2025

Anatomy of Non-Leptonic Two-Body Decays of Charmed Mesons into Final States with $η'$

We show that $D\rightarrow Pη^\prime$ decay amplitudes, where $P=K,π,η$, cannot be simply related to their $D\rightarrow Pη$ counterparts with a single $η_0$--$η_8$ mixing angle. We propose a novel, consistent treatment of $η_0$--$η_8$ mixing for application in $D\to Pη$ and $D\to Pη^\prime$ decays. Using this framework, we perform a global analysis of $D\rightarrow Pη^\prime$ decays employing SU(3)$_F$ symmetry including linear SU(3)$_F$ breaking. We find that the assumption of 30\% SU(3)$_F$ breaking is in slight tension ($2.5σ$) with the data when compared to a fit that allows for 50\% SU(3)$_F$ breaking, the latter giving a perfect description of the data. In order to allow for further scrutinization of SU(3)$_F$-breaking effects in the future, we give branching ratio predictions for all $D\rightarrow Pη'$ modes. Our predictions deviate from the current data in case of the branching ratios $\mathcal{B}(D_s^+\rightarrow K^+η^\prime)$ and $\mathcal{B}(D^+\rightarrow K^+η')$. Future more precise measurements of these channels are therefore highly important in order to clarify the quality of the SU(3)$_F$ expansion in nonleptonic $D\rightarrow Pη^\prime$ decays.

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arXiv2025

Charming Darwin: the evolution of QCD parameters across different species

We explore the prospects of measurements of spectral moments of inclusive charm decays with BESIII. The rich and uniquely clean data set of charm mesons and baryons at BESIII offers a unique laboratory to study the evolution of Heavy Quark Expansion (HQE) parameters across different charm hadron species and to shed light on the interplay between heavy quark dynamics and light quark effects. The HQE in terms of inverse powers of the heavy meson mass is well-established in beauty decays, however due to the lighter charm mass its applicability to charm remains an open question. To date no determination of the HQE parameters, the kinetic energy, chromomagnetic moment and Darwin terms, has been attempted. A particularly important role here is given to the Darwin and weak annihilation operators, whose values are important to predict lifetimes of heavy hadrons. Using a fast simulation for the BESIII detector response and predictions for spectral moments, we investigate the sensitivity to HQE parameters with today's and possible future data sets. In addition, we discuss the theory challenges for the HQE in charm and the experimental limitations. We further investigate the sensitivity of determining the CKM matrix element $|V_{cs}|$ with inclusive semileptonic charm decays.

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arXiv2025

Probing New Physics Through CP Violation in $B_{(s)}\to Vμ^+μ^-$ Decays

Rare decays of the kind $B\to K^*μ^+μ^-$ and $B_s\to ϕμ^+μ^-$ are key players for testing the Standard Model. The current experimental data for their decay rates and angular observables show tensions with the theoretical predictions that may be indications of New Physics. We present a strategy to extract the relevant short-distance coefficients in the presence of new sources of CP violation, utilizing a synergy with $B\to Kμ^+μ^-$ decays. Using the current data as a guideline, we illustrate the new method to determine the complex coefficients $C_9^{(\prime)}$ and $C_{10}^{(\prime)}$ using only four angular observables. Interestingly, the current experimental picture leaves significant room for CP-violating New Physics. We discuss also the link to leptonic $B^0_s\toμ^+μ^-$ decays. We are looking forward to the implementation of these strategies at the future high-precision frontier of flavour physics.

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arXiv2025

Inclusive Semileptonic $b \to c \ell \barν$ Decays to Order $1/m_b^5$

Inclusive semileptonic $B\to X_c \ell\barν$ decays can be described in the Heavy Quark Expansion (HQE) and allow for a precision determination of the CKM element $|V_{cb}|$. We calculate the terms of $1/m_b^5$ and derive a ``trace formula'' which allows for the computation of the decay rate and kinematic moments of the spectrum up to this order in the HQE. We focus specifically on the reparametrization invariant (RPI) dilepton invariant mass $q^2$ moments of the spectrum, which depend on a reduced set of HQE parameters. At this order, ``intrinsic charm'' (IC) contributions proportional to $1/(m_b^3m_c^2)$ enter, which are numerically expected to be sizeable. Using the ``lowest-lying state saturation ansatz'' (LLSA), we estimate the size of these contributions. Within this approximation, we observe a partial cancellation between the IC and the ``genuine'' $1/m_b^5$ contributions, resulting in a small overall contribution.

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arXiv2024

Constraining $|V_{cs}|$ and physics beyond the Standard Model from exclusive (semi)leptonic charm decays

We study the available data on exclusive leptonic and semileptonic $c\to s\ell^+ν$ decays within the Standard Model and beyond. Our analysis accounts for theory correlations between the relevant hadronic matrix elements through application of dispersive bounds. We find that, within a global analysis, the dispersive bounds are generally well respected and only mildly affect the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $|V_{cs}|$. Assuming Standard Model dynamics, we obtain $$ |V_{cs}| = 0.957 \pm 0.003\,, $$ which is compatible with the HFLAV/PDG reference value $|V_{cs}| = 0.975 \pm 0.006$ at the $2.7\,σ$ level. Our findings lead to significant deficits in the second-row and second-column unitarity relations of the CKM matrix. Allowing for beyond the SM contributions in the Weak Effective Theory, we find very strong constraints on potential (pseudo)scalar and tensor effects. However, the data still permits sizeable CP-violating right-handed currents.

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arXiv2024

Pushing the Heavy Quark Expansion for $b\to cl\barν$ to Higher Order in $1/m_b$

The Heavy Quark Expansion (HQE) is the major tool to perform calculations for inclusive semileptonic $B\to X_cl\barν$ and, consequently, for precision determinations of the CKM matrix element $V_{cb}$. To further improve precision, we pushed the expansion to $1/m_b^5$ to include even higher order terms in the HQE. Notably, at $1/m_b^5$, ``intrinsic charm'' (IC) contributions proportional to $1/(m_b^3m_c^2)$ occur, which are numerically expected to be sizeable. We will therefore firstly discuss the determination of a reduced set of HQE parameters at $1/m_b^5$, employing Reparametrisation Invariance (RPI) to this end. Consequently, we will show how the IC and ``genuine" $1/m_b^5$ contribute to the different kinematical moments of $B\to X_cl\barν$, focusing in this work on the lepton energy $E_l$ moments. Using the ``lowest-lying state saturation ansatz'' (LLSA), we estimate the size of these contributions and observe a partial cancellation between the IC and ``genuine'' $1/m_b^5$ contributions, leading to an overall small contribution.

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arXiv2024

Targeting (Pseudo)-Scalar CP Violation with $B_s \to μ^+μ^-$

The leptonic decay $B_s \to μ^+μ^-$ is both rare and theoretically clean, making it an excellent probe for New Physics searches. Due to its helicity suppression in the Standard Model, this decay is particularly sensitive to new (pseudo)-scalar contributions. We present a new strategy for detecting CP-violating New Physics contributions of this kind, exploiting two observables: $\mathcal{A}_{ΔΓ_s}^{μμ}$, which is accessible due to the sizeable decay width difference of the $B_s$ system, and the mixing-induced CP asymmetry $\mathcal{S}_{μμ}$. The strategy also uses information from $B\to K^{(*)} μ^+μ^-$ and $B_s\to ϕμ^+μ^-$ decays. We find remarkably constrained regions in the $\mathcal{A}_{ΔΓ_s}^{μμ}$-$\mathcal{S}_{μμ}$ plane that serve as promising targets for future measurements.

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arXiv2024

Kolya: an open-source package for inclusive semileptonic $B$ decays

We introduce the code kolya, an open-source tool for phenomenological analyses of inclusive semileptonic $B$ meson decays. It contains a library to compute predictions for the total rate and various kinematic moments within the framework of the heavy quark expansion, utilizing the so-called kinetic scheme. The library currently includes power corrections up to $1/m_b^5$. All available QCD perturbative corrections are implemented via interpolation grids for fast numerical evaluation. We also include effects from new physics parameterised as Wilson coefficients of dimension-six operators in the weak effective theory below the electroweak scale. The library is interfaced to CRunDec for easy evaluation of the quark masses and strong coupling constant at different renormalization scales. The library is developed in Python and does not require compilation. It can be used in an interactive Jupyter notebook session.

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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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