arXiv subjects

Aleksey V. Rusov

Publications and source records attributed to Aleksey V. Rusov.

At least 19 recordsLinked to original sources

arXiv2026

Predictions for $b$-baryon lifetimes at NNLO-QCD

Motivated by recent and forthcoming experimental progress, we provide updated predictions for the total decay rates of $b$-baryons, their lifetime ratios and their lifetimes normalised to that of the $B^0_d$ meson within the framework of the heavy quark expansion (HQE). We include, for the first time, next-to-next-to-leading-order QCD corrections to the free $b$-quark decay, which significantly reduce the theoretical uncertainties in the total decay rates. In addition, we also include, for the first time, the complete next-to-leading-order QCD corrections to the dimension-five contributions. While these corrections have only a minor effect on the total decay rates, they induce a sizeable shift in the lifetime ratios, improving the agreement between HQE predictions and experimental data. Overall, we find excellent agreement between the HQE predictions and current experimental measurements for both total decay rates and lifetime ratios within the quoted uncertainties.

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arXiv2026

One-loop matching of QCD currents to power-suppressed two-jet operators

We compute the matching of QCD quark-antiquark currents onto the set of the two-particle and three-particle two-jet operators in soft-collinear effective theory (SCET) at next-to-leading order (NLO) in the perturbative QCD series, including for the first time operators up to second order in the power expansion in the transverse momentum over energy. These results contribute to the ongoing programme of computing power corrections and summing power-suppressed logarithmically enhanced terms for event shapes in the two-jet region and deep-inelastic scattering in the Bjorken-$x\to 1$ limit. The three-particle operators depend on the partonic momentum fractions of two partons moving into the same direction. When one of the momentum fractions approaches zero, the coefficient functions are shown to satisfy endpoint factorization relations, which allow for a consistent cancellation of endpoint singularities among various terms in the complete factorization formula for power corrections.

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arXiv2026

$B$-meson decay width up to $1/m_b^3$ corrections within and beyond the Standard Model

Starting from the most general effective $|ΔB| = 1$ Hamiltonian describing non-leptonic $b$-quark decays $b\to q_1 \bar q_2 q_3$, we compute analytic expressions for all matching coefficients of the two-quark operator contributions in the heavy quark expansion~(HQE) of a $B$ meson, up to mass-dimension-six. In addition, we calculate the weak-annihilation contributions, which enter the matching of four-quark operators in the HQE at dimension-six and were previously missing. Our results complete the calculation of beyond Standard Model (BSM) effects in non-leptonic, tree-level, $b$-quark decays relevant for $B$ meson lifetimes and lifetime ratios such as $τ(B^0_s)/τ(B^0_d)$. Such BSM contributions naturally arise in generic extensions of the Standard Model (SM) that aim to address the observed tensions between experimental measurements and theoretical predictions based on QCD factorisation in several colour-allowed non-leptonic $B$-meson decays. As a by-product of our calculation, we also determine the matching coefficients in the HQE induced by the QCD-penguin operators within the SM, including both the interference between current-current and penguin operators and the contributions quadratic in the penguin operators. Owing to the suppression of the QCD-penguin Wilson coefficients within the SM, these effects are typically regarded as corrections of order $α_s$ and $α_s^2$ in the strong coupling, respectively. Our results reproduce the known expressions at dimension-three and provide new results for the coefficients of the chromomagnetic operator at dimension-five and of the Darwin operator at dimension-six.

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arXiv2024

Total decay rates of $B$ mesons at NNLO-QCD

We update the Standard Model (SM) predictions for the lifetimes of the $B^+$, $B_d$ and $B_s$ mesons within the heavy quark expansion (HQE), including the recently determined NNLO-QCD corrections to non-leptonic decays of the free $b$-quark. In addition, we update the HQE predictions for the lifetime ratios $τ(B^+)/τ(B_d)$ and $τ(B_s)/τ(B_d)$, and provide new results for the semileptonic branching fractions of the three mesons entirely within the HQE. We obtain a considerable improvement of the theoretical uncertainties, mostly due to the reduction of the renormalisation scale dependence when going from LO to NNLO, and for all the observables considered, we find good agreement, within uncertainties, between the HQE predictions and the corresponding experimental data. Our results read, respectively, $Γ(B^+) = 0.587^{+0.025}_{-0.035}~{\rm ps}^{-1}$, $Γ(B_d) = 0.636^{+0.028}_{-0.037}~{\rm ps}^{-1}$, $Γ(B_s) = 0.628^{+0.027}_{-0.035}~{\rm ps}^{-1}$, for the total decay widths, $τ(B^+)/τ(B_d) = 1.081^{+0.014}_{-0.016}$, $τ(B_s)/τ(B_d) = 1.013^{+0.007}_{-0.007}$, for the lifetime ratios, and ${\cal B}_{\rm sl} (B^+) = (11.46^{+0.47}_{-0.32}) \%$, ${\cal B}_{\rm sl} (B_d) = (10.57^{+0.47}_{-0.27}) \%$, ${\cal B}_{\rm sl} (B_s) = (10.52^{+0.50}_{-0.29}) \%$, for the semileptonic branching ratios. Finally, we also provide an outlook for further improvements of the HQE determinations of the $B$-meson decay widths and of their ratios.

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arXiv2024

Towards a SM prediction for CP violation in charm

We provide an overview of the current experimental and theoretical status of charm CP violation and discuss recent progress in obtaining a Standard Model prediction for $Δa_{\rm CP}^{\rm dir}$ using the framework of light-cone sum rules. Furthermore, we present new results for the ratios of the direct CP asymmetries and of the branching fractions for the modes $D^0 \to π^+ π^-$ and $D^0 \to K^+ K^-$.

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arXiv2023

Two body non-leptonic $D^0$ decays from LCSR and implications for $Δa^{\rm dir}_{\rm CP}$

Motivated by the recent measurements of CP violating effects in singly Cabibbo suppressed $D^0$ decays, we revisit the theoretical predictions of these channels. Using up-to-date values for the decay constants and form factors, we find already within naive QCD factorisation surprisingly good agreement between the central values of the branching ratios and the corresponding experimental data. We further extend the study of these modes by employing the method of light-cone sum rules (LCSR) with light-meson light-cone distribution amplitudes. Using for the first time this framework to compute the leading contribution to the decay amplitude, we can again describe well the experimental branching ratios for the modes $D^0 \to π^+ K^-$, $D^0 \to K^+ K^-$, $D^0 \to π^+ π^- $ and $D^0 \to K^+ π^-$. The combination of our results with known predictions for the penguin contributions, obtained with LCSR, leads to an upper bound for the value of direct CP violation expected in the Standard Model of $|Δa_{\rm CP}^{\rm dir}| \leq {2.4} \times 10^{-4} \,,$ which is approximately a factor six smaller than the current measurement.

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arXiv2023

Quark-hadron duality at work: lifetimes of bottom baryons

In the 1990s, very low experimental values for the lifetime ratio $τ(Λ_b)/τ(B_d)$ triggered a considerable amount of doubt in the applicability of the heavy quark expansion (HQE), which is based on the assumption of quark-hadron duality (QHD) for inclusive total decay rates. However, these low values turned out to be the result of purely experimental problems, and the current HFLAV average reads $τ(Λ_b)/τ(B_d) = 0.969(6)$. In this work, we present the Standard Model predictions for the $b$-baryon lifetimes within the framework of the HQE. In particular, we include for the first time the contribution of the Darwin term and we update the estimates for the matrix elements of the dimension-six four-quark operators. Within experimental and theoretical uncertainties, we find excellent agreement between the data and the HQE predictions, and thus no indication for any visible violation of QHD. Our numerical results can be summarised by the ratios $τ(Λ_b)/τ(B_d) = 0.955(14)$, $τ(Ω_b^-)/τ(B_d) = 1.081(42)$, and $τ(Ξ_b^0)/τ(Ξ_b^-) = 0.929(28)$.

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arXiv2022

Taming New Physics in $b \to c \bar u d (s) $ with $τ(B^+)/τ(B_d)$ and $a_{sl}^d$

Inspired by the recently observed tensions between the experimental data and the theoretical predictions, based on QCD factorisation, for several colour-allowed non-leptonic $B$-meson decays, we study the potential size of new physics (NP) effects in the decay channels $b \to c \bar{u} d(s)$. Starting from the most general effective Hamiltonian describing the $b \to c \bar{u} d(s)$ transitions, we compute NP contributions to the theoretical predictions of $B$-meson lifetime and of $B$-mixing observables. The well-known lifetime ratio $τ(B^+)/τ(B_d)$ and the experimental bound on the semi-leptonic CP asymmetry $a_{sl}^d$, provide strong, complementary constraints on some of the NP Wilson coefficients.

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arXiv2022

Disintegration of beauty: a precision study

We update the Standard Model (SM) predictions for $B$-meson lifetimes within the heavy quark expansion (HQE). Including for the first time the contribution of the Darwin operator, SU(3)$_F$ breaking corrections to the matrix element of dimension-six four-quark operators and the so-called eye-contractions, we obtain for the total widths $Γ(B^+) = (0.58^{+0.11}_{-0.07}) \, \mbox{ps}^{-1}$, $Γ(B_d) = (0.63^{+0.11}_{-0.07}) \, \mbox{ps}^{-1}$, $Γ(B_s) = (0.63^{+0.11}_{-0.07}) \, \mbox{ps}^{-1}$, and for the lifetime ratios $τ(B^+) / τ(B_d) = 1.086 \pm 0.022$, $τ(B_s) / τ(B_d) = 1.003 \pm 0.006 \, (1.028 \pm 0.011)$. The two values for the last observable arise from using two different sets of input for the non-perturbative parameters $μ_π^2(B_d)$, $μ_G^2(B_d)$, and $ρ_D^3(B_d)$ as well as from different estimates of the SU(3)$_F$ breaking in these parameters. Our results are overall in very good agreement with the corresponding experimental data, however, there seems to emerge a tension in $τ(B_s)/τ(B_d)$ when considering the second set of input parameters. Specifically, this observable is extremely sensitive to the size of the parameter $ρ_D^3 (B_d)$ and of the SU(3)$_F$ breaking effects in $μ_π^2$, $μ_G^2$ and $ρ_D^3$; hence, it is of utmost importance to be able to better constrain all these parameters. In this respect, an extraction of $μ_π^2 (B_s), μ_G^2 (B_s), ρ_D^3 (B_s)$ from future experimental data on inclusive semileptonic $B_s$-meson decays or from direct non-perturbative calculations, as well as more insights about the value of $ρ_D^3 (B)$ extracted from fit, would be very helpful in reducing the corresponding theory uncertainties.

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arXiv2021

Testing the Standard Model with CP-asymmetries in flavour-specific non-leptonic decays

Motivated by recent indications that the rates of colour-allowed non-leptonic channels are not in agreement with their Standard Model expectations based on QCD factorisation, we investigate the potential to study CP asymmetries with these decays. In the Standard Model, these flavour-specific decays are sensitive to CP violation in $B^0_{(s)}$--$\bar{B}^0_{(s)}$ mixing, which is predicted with low uncertainties and can be measured precisely with semileptonic decays. If there are beyond Standard Model contributions to the non-leptonic decay amplitudes, there could be significant enhancements to the CP asymmetries. Measurements of these quantities therefore have potential to identify BSM effects without relying on Standard Model predictions that might be affected by hadronic effects. We discuss the experimental prospects, and note the excellent potential for a precise determination of the CP asymmetry in $\bar{B}_s \to D_s^+ π^-$ decays by the LHCb experiment.

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arXiv2021

Revisiting Inclusive Decay Widths of Charmed Mesons

Determining for the first time the Darwin operator contribution for the non-leptonic charm-quark decays and using new non-perturbative results for the matrix elements of $ΔC=0$ four-quark operators, including eye-contractions, we present a comprehensive study of the lifetimes of charmed mesons and inclusive semileptonic decay rates as well as the ratios, within the framework of the Heavy Quark Expansion (HQE). We find good agreement with experiment for the ratio $τ(D^+)/τ(D^0)$, for the total $D_s^+$-meson decay rate, for the semileptonic rates of all three mesons $D^0$, $D^+$ and $D_s^+$, and for the semileptonic ratio $Γ_{sl}^{D^+}/Γ_{sl}^{D^0}$. The total decay rates of the $D^0$ and $D^+$ mesons are underestimated in our HQE approach and we suspect that this is due to missing higher-order QCD corrections to the free charm quark decay and the Pauli interference contribution. For the $SU(3)_F$ breaking ratios $τ(D_s^+) / τ(D^0) $ and $Γ_{sl}^{D_s^+}/Γ_{sl}^{D^0} $ our predictions lie closer to one than experiment. This might originate from the poor knowledge of the non-perturbative parameters $μ_G^2$, $μ_π^2$ and $ρ_D^3$ in the $D^0$ and $D_s^+$ systems. These parameters could be determined by experimental studies of the moments of inclusive semileptonic $D$ meson decays.

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arXiv2021

Contribution of the Darwin operator to non-leptonic decays of heavy quarks

We compute the Darwin operator contribution ($1/m_b^3$ correction) to the width of the inclusive non-leptonic decay of a $B$ meson ($B^+$, $B_d$ or $B_s$), stemming from the quark flavour-changing transition $b \to q_1 \bar q_2 q_3$, where $q_1,q_2 = u, c$ and $q_3 = d, s$. The key ideas of the computation are the local expansion of the quark propagator in the external gluon field including terms with covariant derivative of the gluon field strength tensor and the standard technique of the Heavy Quark Expansion (HQE). We confirm the previously known expressions of the $1/m_b^3$ contributions to the semi-leptonic decay $b \to q_1 \ell \bar ν_\ell$, with $\ell = e, μ, τ$ and of the $1/m_b^2$ contributions to the non-leptonic modes. We find that this new term can give a sizeable correction of about $- 4 \, \%$ to the non-leptonic decay width of a $B$ meson. For $B_d$ and $B_s$ mesons this turns out to be the dominant correction to the free b-quark decay, while for the $B^+$ meson the Darwin term gives the second most important correction - roughly 1/2 to 1/3 of the phase space enhanced Pauli interference contribution. Due to the tiny experimental uncertainties in lifetime measurements the incorporation of the Darwin term contribution is crucial for precision tests of the Standard Model.

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arXiv2020

The pion light-cone distribution amplitude from the pion electromagnetic form factor

We suggest to probe the pion light-cone distribution amplitude, applying a dispersion relation for the pion electromagnetic form factor. Instead of the standard dispersion relation, we use the equation between the spacelike form factor $F_π(Q^2)$ and the integrated modulus of the timelike form factor. For $F_π(Q^2)$, the QCD light-cone sum rule with a dominant twist-2 term is used. Adopting for the pion twist-2 distribution amplitude a certain combination of the first few Gegenbauer polynomials, it is possible to fit their coefficients $a_{2,4,6,...}$ (Gegenbauer moments) from this equation, employing the measured pion timelike form factor. For the exploratory fit we use the data of the BaBar collaboration. The results definitely exclude the asymptotic twist-2 distribution amplitude. Also the model with a single $a_2\neq 0$ is disfavoured by the fit. Considering the models with $a_{n>2}\neq 0$, we find that the fitted values of the second and fourth Gegenbauer moments cover the intervals $a_2 (1 \mbox{GeV}) = (0.22 - 0.33) $, $a_4 (1 \mbox{GeV}) = (0.12 - 0.25) $. The higher moments starting from $a_{8}$ are consistent with zero, albeit with large uncertainties. The spacelike pion form factor obtained in two different ways, from the dispersion relation and from the light-cone sum rule, agrees, within uncertainties, with the measurement by the Jefferson Lab $F_π$ collaboration.

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arXiv2020

Probing New Physics in $b \to d$ Transitions

Recent experimental data on several observables in semileptonic $B$-meson decays are found to be in tension with the corresponding Standard Model predictions. Most of these deviations are related to $b \to c$ and $b \to s$ flavour changing transitions. In this work, we estimate possible New Physics effects in $b \to d μ^+ μ^-$ flavour changing neutral currents. We parametrize NP contributions in a model-independent way and determine the allowed ranges of corresponding Wilson coefficients from the data on the exclusive $B^\pm \to π^\pm μ^+ μ^-$ decays measured recently by the LHCb collaboration. Afterwards, we investigate the impact of these results on other $b \to d$ processes such as the leptonic $B^0 \to μ^+ μ^-$ decays and $B^0 - \bar B^0$ mixing. As an example, we consider a simplified $Z^\prime$ model that is found to be consistent with current $b \to d$ data in the certain regions of the NP parameter space. In addition, we estimate the correlations between the partial decay widths of $B \to πμ^+ μ^-$ and $B \to K μ^+ μ^-$ processes to be used for an independent determination of CKM matrix elements as well as for a combined New Physics analysis of both $b \to d$ and $b \to s$ transitions.

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arXiv2019

$ΔA_{CP}$ within the Standard Model and beyond

In light of the recent LHCb observation of CP violation in the charm sector, we review standard model (SM) predictions in the charm sector and in particular for $ΔA_{CP}$. We get as an upper bound in the SM $| ΔA_{CP} ^{\rm SM}| \leq 3.6 \times 10^{-4}$, which can be compared to the measurement of $ΔA_{CP} ^{\rm LHCb2019} = (-15.4 \pm 2.9) \times 10^{-4}$. We discuss resolving this tension within an extension of the SM that includes a flavour violating $Z'$ that couples only to $\bar{s}s$ and $\bar{c}u$. We show that for masses below 80 GeV and flavour violating coupling of the order of $10^{-4}$, this model can successfully resolve the tension and avoid constraints from dijet searches, $D^0-\overline{D}^0$ mixing and measurements of the $Z$ width.

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arXiv2017

$B_{s}\to K \ell ν_\ell$ and $B_{(s)} \to π(K) \ell^+\ell^-$ decays at large recoil and CKM matrix elements

We provide hadronic input for the $B$-meson semileptonic transitions to a light pseudoscalar meson at large recoil. The $B_s\to K$ form factor calculated from QCD light-cone sum rule is updated, to be used for a $|V_{ub}|$ determination from the $B_s\to K \ell ν$ width. Furthermore, we calculate the hadronic input for the binned observables of $B \to π\ell^+ \ell^-$ and $B \to K \ell^+ \ell^-$. In addition to the form factors, the nonlocal hadronic matrix elements are obtained, combining QCD factorization and light-cone sum rules with hadronic dispersion relations. We emphasize that, due to nonlocal effects, the ratio of branching fractions of these decays is not sufficient for an accurate extraction of the $|V_{td}/V_{ts}|$ ratio. Instead, we suggest to determine the Wolfenstein parameters $A,ρ,η$ of the CKM matrix, combining the branching fractions of $B \to K \ell^+ \ell^-$ and $B \to π\ell^+ \ell^-$ with the direct $CP$-asymmetry in the latter decay. We also obtain the hadronic matrix elements for a yet unexplored channel $B_s \to K \ell^+ \ell^-$.

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arXiv2017

Higher-Twist Effects in Light-Cone Sum Rule for the $B\toπ$ Form Factor

We calculate the higher-twist corrections to the QCD light-cone sum rule for the $B \to π$ transition form factor. The light-cone expansion of the massive quark propagator in the external gluonic field is extended to include new terms containing the derivatives of gluon-field strength. The resulting analytical expressions for the twist-5 and twist-6 contributions to the correlation function are obtained in a factorized approximation, expressed via the product of the lower-twist pion distribution amplitudes and the quark-condensate density. The numerical analysis reveals that new higher-twist effects for the $B \to π$ form factor are strongly suppressed. This result justifies the conventional truncation of the operator product expansion in the light-cone sum rules up to twist-4 terms.

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arXiv2017

Inclusive Semitauonic $B$ Decays to Order ${\cal O} (Λ_{QCD}^3/m_b^3)$

We calculate the decay width and the $τ$-lepton energy distribution as well as relevant moments for inclusive $\bar B \to X_c τ\bar ν_τ$ process including power corrections up to order $Λ_{QCD}^3/m_b^3$ and QCD corrections to the partonic level. We compare the result with the sum of the standard-model predictions of the branching fractions of the exclusive semileptonic $\bar B \to (D, D^*, D^{**}) τ\bar ν_τ$ decays as well as with the relevant experimental data. Our prediction is in agreement with the LEP measurement and is consistent with the standard-model calculation of the exclusive modes. We discuss the impact from physics beyond the Standard Model.

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