arXiv subjects

Alexander Khodjamirian

Publications and source records attributed to Alexander Khodjamirian.

At least 19 recordsLinked to original sources

arXiv2026

Light-cone sum rules with $B$-meson distribution amplitudes for the $B\to p$ form factors in $B$-mesogenesis models

New decay modes of $B$-meson into a baryon and invisible dark antibaryon $Ψ$ are among the most distinctive signatures of the $B$-mesogenesis scenario. We concentrate on the proton mode and consider two versions of the underlying interaction of $Ψ$ with quarks, the so-called models $(d)$ and $(b)$. To estimate the width of the $B^+\to p Ψ$ decay, we obtain the $B^+\to p$ transition form factors, applying QCD light-cone sum rules (LCSRs) with $B$-meson distribution amplitudes with an accuracy up to twist-5, while interpolating the proton with a current. This method is independent of the previously applied one, which was based on the nucleon distribution amplitudes. We estimate the partial width of the $B^+\to p Ψ$ decay as a function of the dark antibaryon mass. Furthermore, we use the ratio of this width to the inclusive $B\to X_N Ψ$ width, the latter predicted earlier using the heavy quark expansion method. This ratio, which is independent of the effective coupling, when combined with the minimal inclusive branching fraction of $O(10^{-4})$, necessary for the feasibility of $B$-mesogenesis, yields lower limits on the $B^+\to p Ψ$ branching fraction. We confront these limits with the most recent upper bounds obtained from BaBar and Belle/Belle II searches for the decays of $B^+$-meson into a proton and missing energy. The comparison indicates that experimental upper bounds on the branching fraction of $B\to p Ψ$ at the level of $10^{-8}-10^{-7}$ are needed for a decisive probe of this invisible mode of $B$ decays.

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arXiv2025

Non-perturbaitve effects in Higgs boson decays to electroweak vector bosons and photons

We estimate the magnitude of the leading non-perturbative QCD corrections to the decays of the Higgs boson to the $γZ$ and $γγ$ final states. These corrections originate from the light-quark contributions to such decays. We show that the non-perturbative effects are suppressed by the small Yukawa couplings of light quarks, but that there is no further quark-mass suppression. This is at variance with what is found in the standard perturbative calculations of the light-quark contributions. We demonstrate that the non-perturbative corrections modify the $H \to γZ$ and $H \to γγ$ decay rates by $O(10^{-5})$, well below the expected precision with which such decays can be studied both at the high-luminosity LHC and at future colliders.

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arXiv2025

$D\to P \ell^+\ell^-$ decays assisted by QCD light-cone sum rules

We suggest a new method to analyse the rare $D_{(s)}\to P \ell^+\ell^-$ decays ($P=π,K$), combining QCD light-cone sum rules (LCSR) with hadronic dispersion relations. As our main study case, we consider the $D^+\toπ^+\ell^+\ell^-$ mode which attracts much of interest from the point of view of GIM cancellation and potential new sources of the FCNC $c\to u$ transitions. The hadronic amplitude of this decay is dominated by the combination of weak annihilation with the emission of a virtual photon. This $D^+\to π^+γ^*$ amplitude is calculated at spacelike photon virtualities, $-q^2 \gg Λ_{QCD}^2$, using LCSR with pion distribution amplitudes. The LCSR results are then fitted to the hadronic dispersion relation in the variable $q^2$. The fit allows us to determine relative phases of the $ρ$-,$ω$- and $ϕ$-meson terms and to estimate the contributions of heavier hadronic states. The latter are parameterized in two different ways: first, with a $z$-expansion and second, using a model with excited vector-meson resonances. The resulting $D^+\toπ^+\ell^+\ell^-$ width obtained from the LCSR-assisted dispersion relation is not much smaller than the current upper bound measured by LHCb collaboration. In comparison with our result for the dominant $D^+\to π^+γ^*$ transition, the contribution induced by the short distance $c\to u \ell^+\ell^-$ quark transition generated by the effective $O_9$ operator in Standard Model turns out to be practically invisible. To detect its effect, one readily needs a enhancement of the coefficient $C_9$ from non-standard effects by many orders of magnitude. We also establish new $U$-spin symmetry relations between the amplitudes of $D\to P \ell^+\ell^-$ decays and apply our method to the Cabibbo-favoured modes $D^+_s\toπ^+\ell^+\ell^-$ and $D^0\to\bar{K}^0\ell^+\ell^-$ which proceed via the same weak annihilation mechanism.

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arXiv2023

$B\to D_0^$ and $B_s\to D_{s0}^$ form factors from QCD light-cone sum rules

We present the first application of QCD light-cone sum rules (LCSRs) with $B_{(s)}$-meson distribution amplitudes to the $B_{(s)}\!\to\! D_{(s)0}^*$ form factors, where $D_{(s)0}^*$ is a charmed scalar meson. We consider two scenarios for the $D_0^*$ spectrum. In the first one, we follow the Particle Data Group and consider a single broad resonance $D_0^*(2300)$. In the second one, we assume the existence of two scalar resonances, $D_0^*(2105)$ and $D_0^*(2451)$, as follows from a recent theoretically motivated analysis of $B\to Dππ$ decays. The $B\!\to\! D_0^*$ form factors are calculated in both scenarios, also taking into account the large total width of $D_0^*(2300)$. Furthermore, we calculate the $B_s\!\to\! D_{s0}^*$ form factors, considering in this case only the one-resonance scenario with $D_{s0}(2317)$. In this LCSRs calculation, the $c$-quark mass is kept finite and the $s$-quark mass is taken into account. We also include contributions of the two- and three-particle distribution amplitudes up to twist-four. Our predictions for semileptonic $B\!\to\! D_0^*\ellν_\ell$ and $B_s\!\to\! D_{s0}^*\ellν_\ell$ branching ratios are compared with the available data and HQET-based predictions. As a byproduct, we also obtain the $D_0^*$- and $D_{s0}^*$-meson decay constants and predict the lepton flavour universality ratios $R(D_0^*)$ and $R(D_{s0}^*)$.

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arXiv2023

A guide to the QCD light-cone sum rule for $b$-quark decays

We overview the current status and future perspectives of the QCD-based method of light-cone sum rules. The two main versions of these sum rules, using light-meson and $B$-meson distribution amplitudes are introduced and the most important applications of the method are discussed. We also outline open problems and future perspectives of this method.

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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 Sum Rules for the $B_c \to J/ψ$ Form Factors

We derive new sum rules for the form factors of the $B_c\to J/ψ\, \ell \barν_\ell$ semileptonic transitions, employing the vacuum-to-$B_c$ correlation function of the $J/ψ$-interpolating and $b\to c$ weak currents. In the heavy quark limit and at a space-like momentum transfer to the weak current, a local operator-product expansion is valid for this correlation function. As a result, in the leading power, the non-perturbative input is reduced to the decay constant of $B_c$ meson. Furthermore, applying hadronic dispersion relation in the $J/ψ$ channel, we find that a non-vanishing OPE spectral density in the duality interval of $J/ψ$ emerges only at $\mathcal{O}(α_s)$. We calculate this density in the relevant kinematic regime. The $B_c\to J/ψ$ form factors at space-like momentum transfer are then calculated from the new sum rules.

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arXiv2022

$B$-meson decay into a proton and dark antibaryon from QCD light-cone sum rules

The recently developed $B$-Mesogenesis scenario predicts decays of $B$ mesons into a baryon and hypothetical dark antibaryon $Ψ$. We suggest a method to calculate the amplitude of the simplest exclusive decay mode $B^+\to p Ψ$. Considering two models of $B$-Mesogenesis, we obtain the $B\to p$ hadronic matrix elements by applying QCD light-cone sum rules with the proton light-cone distribution amplitudes. We estimate the $B^+\to p Ψ$ decay width as a function of the mass and effective coupling of the dark antibaryon.

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arXiv2022

$B\to D_1(2420)$ and $B\to D_1'(2430)$ form factors from QCD light-cone sum rules

We perform the first calculation of form factors in the semileptonic decays $B\!\to\! D_1(2420)\ellν_\ell$ and $B \to D_1^\prime (2430)\ell ν_\ell$ using QCD light-cone sum rules (LCSRs) with $B$-meson distribution amplitudes. In this calculation the $c$-quark mass is finite. Analytical expressions for two-particle contributions up to twist four are obtained. To disentangle the $D_1$ and $D_1^\prime$ contributions in the LCSRs, we suggest a novel approach that introduces a combination of two interpolating currents for these charmed mesons. To fix all the parameters in the LCSRs, we use the two-point QCD sum rules for the decay constants of $D_1$ and $D_1^\prime$ mesons augmented by a single experimental input, that is the $B \to D_1(2420)\ellν_\ell$ decay width. We provide numerical results for all $B\to D_1$ and $B\to D_1^\prime$ form factors. As a byproduct, we also obtain the $D_1$- and $D_1'$-meson decay constants and predict the lepton-flavour universality ratios $R(D_1)$ and $R(D_1')$.

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arXiv2021

QCD-based estimate of direct $CP$ asymmetry in charm decays

I discuss the calculation of the direct CP-asymmetry in $D\to π^+π^-$ and $D\to K^+K^- $ decays with the method of QCD light-cone sum rules. The main result is the upper limit for the difference of the two asymmetries $\left|Δa_{CP}^{dir}\right| < 0.020\pm 0.003\%$ which is significantly smaller than the recent measurement of this quantity by the LHCb collaboration

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arXiv2021

The $D^D π$ and $B^Bπ$ couplings from light-cone sum rules

We revisit the calculation of the strong couplings $D^*Dπ$ and $B^*Bπ$ from the QCD light-cone sum rules using the pion light-cone distribution amplitudes. The accuracy of the correlation function, calculated from the operator product expansion near the light-cone, is upgraded by taking into account the gluon radiative corrections to the twist-3 terms. The double spectral density of the correlation function, including the twist-2, 3 terms at ${\cal O} (α_s)$ and the twist-4 LO terms, is presented in an analytical form for the first time. This form allows us to use various versions of the quark-hadron duality regions in the double dispersion relation underlying the sum rules. We predict $g_{D^*Dπ}=14.1^{+1.3}_{-1.2}$ and $g_{B^*Bπ}=30.0^{+2.6}_{-2.4}$ when the decay constants of heavy mesons entering the light-cone sum rule are taken from lattice QCD results. We compare our results with the experimental value for the charmed meson coupling and with the lattice QCD calculations.

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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

Inverse moment of the $B_s$-meson distribution amplitude from QCD sum rule

We derive a QCD sum rule for the inverse moment of the $B_s$-meson light-cone distribution amplitude in HQET. Within this method, the $SU(3)_{fl}$ symmetry violation is traced to the strange quark mass and to the difference between strange and nonstrange quark condensate densities. We predict the ratio of inverse moments $λ_{B_s}/λ_B= 1.19 \pm 0.14$ which can be used in various applications of these distribution amplitudes to the analyses of $B_{s}$-meson decays, provided an accurate value of $λ_B$ is available from other sources, such as the $B\to \ell ν_\ell γ$ decay.

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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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arXiv2017

Direct CP asymmetry in $D\to π^-π^+$ and $D\to K^-K^+$ in QCD-based approach

We present the first QCD-based calculation of hadronic matrix elements with penguin topology determining direct CP-violating asymmetries in $D^0\to π^-π^+$ and $D^0\to K^- K^+$ nonleptonic decays. The method is based on the QCD light-cone sum rules and does not rely on any model-inspired amplitude decomposition, instead leaning heavily on quark-hadron duality. We provide a Standard Model estimate of the direct CP-violating asymmetries in both pion and kaon modes and their difference and comment on further improvements of the presented computation.

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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

$B_{s}\to K \ell ν_\ell$ 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

$B\toππ$ 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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