Searcharxiv⌕ Search

arXiv subjects

Alexander Khodjamirian

Publications and source records attributed to Alexander Khodjamirian.

At least 37 records · Page 2Linked to original sources
arXiv2016

Form factors in $B\to ππ\ell\barν_\ell$ from QCD light-cone sum rules

The form factors of the semileptonic $B\to ππ\ell\barν$ decay are calculated from QCD light-cone sum rules with the distribution amplitudes of dipion states. This method is valid in the kinematical region, where the hadronic dipion state has a small invariant mass and simultaneously a large recoil. The derivation of the sum rules is complicated by the presence of an additional variable related to the angle between the two pions. In particular, we realize that not all invariant amplitudes in the underlying correlation function can be used, some of them generating kinematical singularities in the dispersion relation. The two sum rules that are free from these ambiguities are obtained in the leading twist-2 approximation, predicting the $\bar{B}^0\to π^+π^0$ form factors $F_{\perp}$ and $F_{\parallel}$ of the vector and axial $b\to u$ current, respectively. We calculate these form factors at the momentum transfers $0<q^2\lesssim 12 $ GeV$^2$ and at the dipion mass close to the threshold $4m_π^2$. The sum rule results indicate that the contributions of the higher partial waves to the form factors are suppressed with respect to the lowest $P$-wave contribution and that the latter is not completely saturated by the $ρ$-meson term.

hep-ph↗
arXiv2015

Hadronic effects and observables in $B\to π\ell^+\ell^-$ decay at large recoil

We calculate the amplitude of the rare flavour-changing neutral-current decay $B\to π\ell^+\ell^-$ at large recoil of the pion. The nonlocal contributions in which the weak effective operators are combined with the electromagnetic lepton-pair emission are systematically taken into account. These amplitudes are calculated at off-shell values of the lepton-pair mass squared, $q^2<0$, employing the operator-product expansion, QCD factorization and light-cone sum rules. The results are fitted to hadronic dispersion relations in $q^2$, including the intermediate vector meson contributions. The dispersion relations are then used in the physical region $q^2>0$. Our main result is the process-dependent addition $ΔC^{(Bπ)}_9(q^2)$ to the Wilson coefficient $C_9$ obtained at $4m_\ell^2<q^2\lesssim m_{J/ψ}^2$. Together with the $B\to π$ form factors from light-cone sum rules, this quantity is used to predict the differential rate, direct CP-asymmetry and isospin asymmetry in $B\to π\ell^+\ell^-$. We also estimate the total rate of the rare decay $B\to πν\barν$.

hep-ph↗
arXiv2015

Direct probes of flavor-changing neutral currents in $e^+e^-$ collisions

We propose a novel method to study flavor-changing neutral currents in the $e^+e^-\to D^{*0}$ and $e^+e^-\to B_{s}^* $ transitions, tuning the energy of $e^+e^-$- collisions to the mass of the narrow vector resonance $D^{*0}$ or $ B_{s}^*$. We present a thorough study of both short-distance and long-distance contributions to $e^+e^-\to D^{*0}$ in the Standard Model and investigate possible contributions of new physics in the charm sector. This process, albeit very rare, has clear advantages with respect to the $D^0 \to e^+e^-$ decay: the helicity suppression is absent, and a richer set of effective operators can be probed. Implications of the same proposal for $B_{s}^*$ are also discussed.

hep-ph↗
arXiv2015

Decay constants of heavy-light vector mesons from QCD sum rules

We revisit QCD sum rules for the decay constants of heavy-light mesons. In the sum rules for the vector mesons B^*_(s) and D^*_(s) we improve the accuracy of OPE, taking into account the O(alpha_s^2) terms in the perturbative part and calculating the O(alpha_s) corrections to the quark-condensate contribution. With this accuracy, we obtain the ratios of decay constants: f_B^*/f_B=1.02 +0.07 -0.03, f_D^*/f_D=1.20 +0.10 -0.07. The sum rule predictions for the decay constants of pseudoscalar mesons are updated with the results f_B=(207 +17 -9) MeV, f_B_s=(242 +17 -12) MeV, f_D=(201 +12 -13) MeV, f_D_s=(238 +13 -23) MeV. In order to assess the sensitivity of our calculation to the form of the sum rule, we consider alternative versions such as the power moments and Borel sum rules with different weights of the spectral density. We also investigated the heavy quark limit of the sum rules for vector and pseudoscalar mesons, estimating the violations of the heavy-quark spin and flavour symmetry.

hep-ph↗
arXiv2015

Extrapolation and unitarity bounds for the $B \to π$ form factor

We address the problem of extrapolating the vector form factor $f_{Bπ}^+$, which is relevant to $B\to π\ell ν_\ell$ decays, from the region of small to the region of large momentum transfer. As input, we use the QCD light-cone sum rule at small momentum transfer. We carry out a comprehensive Bayesian uncertainty analysis and obtain correlated uncertainties for the normalization and shape parameters of the form factor. The $z$-series parametrization for $f_{Bπ}^+$ is employed to extrapolate our results to large momentum transfer, and to compare with the lattice QCD results. To test the validity of our extrapolation we use the upper and lower bounds from the unitarity and positivity of the two-point correlator of heavy-light quark currents. This correlator is updated by including the NNLO perturbative term and the NLO correction to the quark condensate contribution. We demonstrate that an additional input including the form factor, its first and second derivative calculated at one value of momentum transfer from the light-cone sum rules, considerably improves the bounds. This only holds when the correlations between the form factor parameters are taken into account. We further combine our results with the latest experimental measurements of $B\to π\ell ν_\ell$ by the BaBar and Belle collaborations, and obtain $|V_{ub}|= (3.32^{+0.26}_{-0.22}) \cdot 10^{-3}$ from a Bayesian analysis.

hep-ph↗
arXiv2014

Disentangling the Decay Observables in $B^- \to π^+π^-\ell^-\barν_\ell$

We study the semileptonic $b\to u$ transition in the decay mode $B^-\to π^+π^-\ell^-\barν_\ell$. We define $B\to ππ$ form factors in the helicity basis, and study their properties in various kinematic limits, including form factor relations in the heavy-mass and large-energy limits, the decomposition into partial waves of the dipion system, and the resonant contribution of vector and scalar mesons. We show how angular observables in $B^-\to π^+π^-\ell^-\barν_\ell$ can be used to measure dipion form factors or to perform null tests of the Standard Model.

hep-ph↗
arXiv2013

Applications of QCD Sum Rules to Heavy Quark Physics

In these lectures, I present several important applications of QCD sum rules to the decay processes involving heavy-flavour hadrons. The first lecture is introductory. As a study case, the sum rules for decay constants of the heavy-light mesons are considered. They are relevant for the leptonic decays of $B$-mesons. In the second lecture I describe the method of QCD light-cone sum rules used to calculate the heavy-to-light form factors at large hadronic recoil, such as the $B\to π\ell ν_\ell$ form factors. In the third lecture, the nonlocal hadronic amplitudes in the flavour-changing neutral current decays $B\to K^{(*)}\ell\ell$ are discussed. Light-cone sum rules provide important nonfactorizable contributions to these amplitudes.

hep-ph↗
arXiv2011

Form Factors and Long-Distance Effects in $B\to V(P) \ell^+\ell^-$ and $B\to V γ$

I overview the hadronic input for the exclusive flavour-changing neutral-current $B$-decays with a vector ($V=K^*,ρ$) or pseudoscalar ($P=K,π$) meson in the final state. After presenting the current status of $B\to P,V$ form factors, I discuss the estimate of the charm-loop effect in $B\to K^{(*)} \ell^+\ell^-$ and $B\to K^* γ$.

hep-ph↗
arXiv2009

Upper bounds on $f_D$ and $f_{D_s}$ from two-point correlation function in QCD

The correlation function of two pseudoscalar charmed quark currents with a positive hadronic spectral density is employed to obtain upper bounds on the decay constants of $D$ and $D_s$ mesons. Including all known terms of the operator-product-expansion of this correlation function in QCD and taking into account the estimated uncertainties, we obtain $f_{D}<230 $ MeV and $f_{D_s}<270$ MeV. Comparison with the decay constants determined from $D\to l ν_l$ and $D_s\to l ν_l$ measurements, reveals a tension between the bound and current experimental value of $f_{D_s}$.

hep-ph↗
arXiv2007

Form Factors from Light-Cone Sum Rules with B-Meson Distribution Amplitudes

New sum rules for $B\to π,K $ and $B\to ρ,K^*$ form factors are derived from the correlation functions expanded near the light-cone in terms of B-meson distribution amplitudes. The contributions of quark-antiquark and quark-antiquark-gluon components in the B meson are taken into account. Models for the B-meson three-particle distribution amplitudes are suggested, based on QCD sum rules in HQET. Employing the new light-cone sum rules we calculate the form factors at small momentum transfers, including $SU(3)_{fl}$ violation effects. The results agree with the predictions of the conventional light-cone sum rules.

hep-ph↗
arXiv2006

Exclusive Charmless $B$ Decays in QCD

The problem of hadronic input in charmless nonleptonic B decays is discussed. QCD sum rules and their light-cone versions (LCSR) provide an important part of this input, such as the decay constant $f_B$ and $B\to π$ form factor. Employing the LCSR technique, the $B\to ππ$ hadronic matrix elements with emission, penguin and annihilation topologies are calculated, with no evidence for large nonfactorizable effects and/or strong phases.

hep-ph↗
arXiv2005

$B$-Meson Distribution Amplitude from the $B\to π$ Form Factor

Employing the light-cone sum rule approach in QCD, we relate the $B$-meson distribution amplitude to the $B\to π$ form factor at zero momentum transfer. In leading order, the sum rule is converted into a simple expression for the inverse moment $λ_B$ of the distribution amplitude $ϕ_{+}^B$. Using as an input the $B\toπ$ form factor calculated from the light-cone sum rule in terms of pion distribution amplitudes, we obtain an estimate: $λ_B =460 \pm 160 $ MeV. We investigate how this result is modified by the $B$-meson three-particle distribution amplitudes.

hep-ph↗
arXiv2004

Modeling the Pion and Kaon Form Factors in the Timelike Region

New, accurate measurements of the pion and kaon electromagnetic form factors are expected in the near future from experiments at electron-positron colliders,using the radiative return method. We construct a model for the timelike pion electromagnetic form factor, that is valid also at momentum transfers far above the $ρ$ resonance. The ansatz is based on vector dominance and includes a pattern of radial excitations expected from dual resonance models.The form factor is fitted to the existing data in the timelike region, continued to the spacelike region and compared with the measurements there and with the QCD predictions. Furthermore, the model is extended to the kaon electromagnetic form factor. Using isospin and SU(3)-flavour symmetry relations we extract the isospin-one contribution and predict the kaon weak form factor accessible in semileptonic $τ$ decays.

hep-ph↗
arXiv2003

Flavour SU(3) Symmetry in Charmless B Decays

QCD sum rules are used to estimate the flavour SU(3)-symmetry violation in two-body B decays to pions and kaons. In the factorizable amplitudes the SU(3)-violation manifests itself in the ratio of the decay constants f_K/f_pi and in the differences between the B->K, B_s->K and B->pi form factors. These effects are calculated from the QCD two-point and light-cone sum rules, respectively, in terms of the strange quark mass and the ratio of the strange and nonstrange quark-condensate densities. Importantly, QCD sum rules predict that SU(3) breaking in the heavy-to-light form factors can be substantial and does not vanish in the heavy-quark mass limit. Furthermore, we investigate the strange-quark mass dependence of nonfactorizable effects in the B->K pi decay amplitudes. Taking into account these effects we estimate the accuracy of several SU(3)-symmetry relations between charmless B-decay amplitudes.

hep-ph↗
arXiv2003

Gluonic Penguins in $B\toππ$ from QCD Light-Cone Sum Rules

The $B\to ππ$ hadronic matrix element of the chromomagnetic dipole operator $O_{8g}$ (gluonic penguin) is calculated using the QCD light-cone sum rule approach. The resulting sum rule for $<ππ|O_{8g}|B>$ contains, in addition to the $O(α_s)$ part induced by hard gluon exchanges, a contribution due to soft gluons. We find that in the limit $m_b\to \infty$ the soft-gluon contribution is suppressed as a second power of $1/m_b$ with respect to the leading-order factorizable $B\to ππ$ amplitude, whereas the hard-gluon contribution has only an $α_s$ suppression. Nevertheless, at finite $m_b$, soft and hard effects of the gluonic penguin in $B\to ππ$ are of the same order. Our result indicates that soft contributions are indispensable for an accurate counting of nonfactorizable effects in charmless B decays. On the phenomenological side we predict that the impact of gluonic penguins on $\bar{B}^0_d\to π^+π^-$ is very small, but is noticeable for $\bar{B}^0_d\to π^0π^0$.

hep-ph↗