arXiv subjects

Blaženka Melić

Publications and source records attributed to Blaženka Melić.

17 recordsLinked to original sources

arXiv2026

New Predictions for the Lifetimes of Doubly Heavy Baryons and the $B_c$ Meson

We present updated predictions for the lifetimes of all weakly decaying doubly heavy baryons, including $bb$, $cc$, and $bc$ baryons, as well as for the $B_c$ meson. The analysis includes NNLO corrections to the leading dimension-three contribution, NLO corrections to the chromomagnetic term, and the complete set of currently known NLO corrections to the dimension-six heavy-light quark spectator contributions, including penguin terms. We also compare the results in the $\overline{\rm MS}$, kinetic, and, where applicable, $Υ$ mass schemes. For the $bc$ baryons, we present predictions for both possible ground-state diquark-spin assignments, $S_{bc}=0$ and $S_{bc}=1$. In the kinetic scheme we obtain the lifetime hierarchies $τ(Ξ^0_{bb})<τ(Ξ^-_{bb})\simeqτ(Ω^-_{bb})$, $τ(Ξ^+_{cc})<τ(Ω^+_{cc})<τ(Ξ^{++}_{cc})$, $τ(Ξ^0_{bc})\lesssimτ(Ω^0_{bc})<τ(Ξ^+_{bc})$ for $S_{bc}=0$, and $τ(Ξ^{\prime\, 0}_{bc})<τ(Ω^{\prime\, 0}_{bc})<τ(Ξ^{\prime\, +}_{bc})$ for $S_{bc}=1$. We also revisit the $B_c$ lifetime and discuss the impact of the newly included Darwin term.

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arXiv2026

Revisiting lifetimes of doubly charmed baryons

We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO $α_s$ contributions and newly-computed terms in the $1/m_c$ series. Our improved results confirm the expected hierarchy $$τ(Ξ_{cc}^{+}) < τ(Ω_{cc}^{+}) < τ(Ξ_{cc}^{++}) \,, $$ while the predicted lifetime $τ(Ξ_{cc}^{++}) = 0.32 \pm 0.05 ^{+0.08}_{-0.07} \,\textrm{ps} $ is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the $Ξ_{cc}^{+}$ and $Ω_{cc}^+$ baryons relative to the $Ξ_{cc}^{++}$ baryon, namely $τ(Ξ_{cc}^{+})/τ(Ξ_{cc}^{++})=0.22\pm 0.05\pm 0.04$ and $τ(Ω_{cc}^{+})/τ(Ξ_{cc}^{++})=0.52\pm 0.13^{+0.03}_{-0.02}$.

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arXiv2025

Nonperturbative Parameters of Inclusive $Λ_b$ Decays from Small Velocity Sum Rules

We investigate the nonperturbative parameters entering the heavy quark expansion for the inclusive decay rates of the $Λ_b$ baryon, focusing on the kinetic term $\hatμ_π^2$ and the Darwin term $\hatρ_D^3$. Recent evaluations have yielded an unexpectedly large Wilson coefficient associated with the dimension-six Darwin operator, which motivates a more detailed examination of the associated hadronic matrix element. We constrain $\hatμ_π^2$ and $\hatρ_D^3$ using Small Velocity Sum Rules for the inclusive semileptonic decay $Λ_b \to X_c e^- \barν_e$. These sum rules relate moments of hadronic structure functions, computed via the Operator Product Expansion, to hadronic matrix elements of relevant exclusive transitions. Truncating the hadronic side to include the lowest-lying charmed baryon states with spin-parity $ J^P = 1/2^+, 1/2^-, 3/2^- $, we employ the corresponding lattice QCD form factors near zero recoil as inputs. By analysing zeroth and higher moments, we derive inequalities that define an allowed region in the $(\hatμ_π^2, \hatρ_D^3)$ plane. The derived constraints are consistent, within uncertainties, with estimates from spectroscopic relations and the nonrelativistic constituent quark model. This work refines the determination of key HQE parameters for $Λ_b$, with implications for precision predictions of heavy baryon lifetimes and inclusive decays of heavy baryons in general.

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arXiv2025

Form factors and phenomenology of $\boldsymbol{B_{(s)}}$ and $\boldsymbol{D_{(s)}}$ semileptonic decays to $\boldsymbolη$ and $\boldsymbol{η^\prime}$

Motivated by more precise recent measurements of the $B \to η\ell ν$ and $D_{(s)} \to η^{(\prime)}\ell ν$ decays, we employ state-of-the-art parametrizations to describe the $B_{(s)}, D_{(s)} \to η^{(\prime)}$ form factors across the full $q^2$ range, fitting them to the latest light-cone sum rule results. Using these results, we compute the branching ratios for all relevant decays and compare them with experimental data, finding good agreement. Additionally, we examine the validity and precision of the extracted $η$-$η'$ mixing angle in the context of heavy meson decays. By combining our predictions for the $B, D, D_s \to η^{(\prime)}$ form factors with recent semileptonic decay measurements, we extract the CKM elements $|V_{ub}|$, $V_{cs}$, and $V_{cd}$. We also provide tests of the lepton flavour universality in the $B, D, D_s \to η^{(\prime)} \ell ν$ decays and present results for the forward-backwards asymmetries in these decays. Our findings indicate that the extracted values are becoming comparable in precision to those obtained from more conventional semileptonic decay analyses.

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arXiv2025

Nonperturbative Dynamics in D-meson Mixing

Theoretical predictions for the $D^0\overline{D^0}$ mixing parameters fall significantly short of experimental measurements, with discrepancies spanning several orders of magnitude. This gap is mainly due to the Glashow-Iliopoulos-Maiani (GIM) mechanism, which suppresses leading-order contributions by high powers of $m_s/m_c$. However, higher-order corrections and nonperturbative effects could reduce this suppression, especially through flavor $SU(3)_F$ symmetry breaking. In this work, we investigate the long-distance contributions from QCD condensates, including, for the first time, the effects of mixed quark-gluon and four-quark condensates. Our results show an increase in the predicted values of $D^0\overline{D^0}$ mixing parameters by two orders of magnitude compared to perturbative NLO result, providing valuable insights into nonperturbative QCD dynamics. Although the theoretical estimates still fall below experimental values, this study represents an important step toward narrowing the gap between theory and observation, highlighting the significance of higher-order $1/m_c$ QCD effects in understanding $D^0\overline{D^0}$ mixing.

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arXiv2024

$D^0\overline{D^0}$ mixing from nonlocal condensate contributions

A significant discrepancy, spanning multiple orders of magnitude, exists between the leading order contribution to the $D^0\overline{D^0}$ mixing parameters and experimental values. This is largely due to the Glashow-Iliopoulos-Maiani (GIM) mechanism, which results in substantial suppression of the theoretical predictions. To bridge this gap, various efforts have been made to account for higher-order terms and nonperturbative effects, which, although suppressed in the operator product expansion (OPE), could potentially lead to a larger contribution by weakening the GIM cancellation through flavour SU(3) symmetry breaking. In this work, we compute the long-distance contributions of nonlocal QCD condensates within different models and, for the first time, determine the impact of the mixed condensate. Although our results still fall short of the experimental value, they represent an improvement over current theoretical estimates by an order of magnitude.

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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

Toward a complete description of $b \to u \ell^- \barν$ decays within the Weak Effective Theory

We fit the available data on exclusive semileptonic $b\to u\ell^-\barν$ decays within the Standard Model and in the Weak Effective Theory. Assuming Standard Model dynamics, we find $|V_{ub}| = 3.59^{+0.13}_{-0.12} \times 10^{-3}$. Lifting this assumption, we obtain stringent constraints on the coefficients of the $ub\ellν$ sector of the Weak Effective Theory. Performing a Bayesian model comparison, we find that a beyond the Standard Model interpretation is favoured over a Standard Model interpretation of the available data. We provide a Gaussian mixture model that enables the efficient use of our fit results in subsequent analyses beyond the Standard Model, within and beyond the framework of the Standard Model Effective Field Theory.

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arXiv2023

Charmed hadron lifetimes

We provide updated predictions of the lifetimes of singly charmed baryons and mesons within the heavy quark expansion, with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} τ(Ξ_c^0) < τ(Λ_c^+)< τ(Ω_c^0) < τ(Ξ_c^+)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic branching ratios are also in agreement, within uncertainties, with a recent comprehensive study and with experimental results.

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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

Lifetimes of singly charmed hadrons

We provide an extensive study of the lifetimes of singly charmed baryons and mesons, within the heavy quark expansion with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. We give our predictions for lifetimes, lifetime ratios, and semileptonic branching ratios of singly charmed baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} τ\left(Ξ_c^{0}\right) < τ\left(Λ_c^{+}\right)< τ\left(Ω_c^{0}\right) < τ\left(Ξ_c^{+}\right)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic decay rates are in agreement with a recent comprehensive study and experimental results within uncertainties.

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arXiv2021

The $\bar{B}\to π$ form factors from QCD and their impact on $|V_{ub}|$

We revisit light-cone sum rules with pion distribution amplitudes to determine the full set of local $B \to π$ form factors. To this end, we determine all duality threshold parameters from a Bayesian fit for the first time. Our results, obtained at small momentum transfer $q^2$, are extrapolated to large $q^2$ where they agree with precise lattice QCD results. We find that a modification to the commonly used BCL parametrization is crucial to interpolate the scalar form factor between the two $q^2$ regions. We provide numerical results for the form factor parameters -- including their covariance -- based on simultaneous fit of all three form factors to both the sum rule and lattice QCD results. Our predictions for the form factors agree well with measurements of the $q^2$ spectrum of the semileptonic decay $\bar{B}^0\to π^+\ell^-\barν$. From the world average of the latter we obtain $|V_{ub}| = (3.77 \pm 0.15)\cdot 10^{-3}$, which is in agreement with the most recent inclusive determination at the $1\,σ$ level.

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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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arXiv2019

A new angle on an old problem: Helicity approach to neutron beta decay in the Standard Model

We elaborate on the dichotomy between the description of the semileptonic decays of heavy hadrons on the one hand and the semileptonic decays of light hadrons such as neutron $β$ decays on the other hand. For example, almost without exception the semileptonic decays of heavy baryons are described in cascade fashion as a sequence of two two-body decays $B_1 \to B_2 + W_{\rm off-shell}$ and $W_{\rm off-shell} \to \ell + ν_\ell$ whereas neutron $β$ decays are analyzed as true three-body decays $n \to p + e^- +\bar ν_e$. Within the cascade approach it is possible to define a set of seven angular observables for polarized neutron $β$ decays as well as the longitudinal, transverse and normal polarization of the decay electron. We determine the dependence of the observables on the usual vector and axial vector form factors. In order to be able to assess the importance of recoil corrections we expand the rate and the $q^2$ averages of the observables up to NLO and NNLO in the recoil parameter $δ=(M_n-M_p)/(M_n+M_p)= 0.689\cdot 10^{-3}$. Remarkably, we find that the rate and three of the four parity conserving polarization observables that we analyze are protected from NLO recoil corrections when the second class current contributions are set to zero.

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arXiv2017

Seeking the CP-odd Higgs via $h\rightarrow η_{c,b}\ell^+\ell^-$

We show that the decay rates of the Higgs boson to a pseudoscalar quarkonium and a pair of leptons, $h\to P\ell^+\ell^-$ ($P\in\{η_c,η_b\}$), can be substantially enhanced in a scenario with two Higgs doublets with a softly broken $\mathbb{Z}_2$ symmetry (2HDM) when the CP-odd Higgs A is light, i.e. $m_A\lesssim m_h$. Depending on the type of 2HDM the enhancement of $\mathcal{B}(h\to η_{c,b}τ^+τ^-)$ with respect to its Standard Model value can be an order of magnitude larger, i.e. $\mathcal{O}(10^{-6}\div10^{-5})$. The decays $h\to P\ell^+\ell^-$ could therefore provide an efficient channel to investigate the presence of a light CP-odd Higgs $A$ and help to disentangle among various 2HDM scenarios.

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arXiv2017

Exploring the Top-Higgs FCNC Couplings at Polarized Linear Colliders with Top Spin Observables

We study the nature of the flavor changing neutral couplings of the top quark with the Higgs boson and the up/charm quark in the $t\bar{t}$ production at linear colliders. There are previous bounds on such tqH couplings at both, linear and hadronic colliders, with the assumption that it couples equally to the left and the right handed fermions. In this paper we examine the chirality of the tqH coupling and construct different observables which will be sensitive to it. The kinematics of the emitted q from t $\rightarrow$ qH in $t\bar{t}$ production is discussed and it was found that the polar angle distribution of q is sensitive to the chiral nature of tqH couplings. The observables in the context of top-antitop spin correlations, which are sensitive to new physics in the top decay are considered using different spin-quantization bases. It was found that in particular the off-diagonal basis can be useful to distinguish among the chiral tqH couplings. The sensitivity of the unpolarized ILC in probing the couplings at the 3$σ$ level at $\sqrt{s}$ = 500 GeV and L = 500 fb$^{-1}$ is also studied, resulting in predicted BR(t $\rightarrow$ qH) < 1.19 $\times 10^{-3}$. This limit is further improved to BR(t $\rightarrow$ qH) < 8.84 $\times 10^{-4}$ with the inclusion of initial beam polarization of left handed electrons and right handed positrons.

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arXiv2013

Pionic Dark Matter

We study a phenomenological model where the lightest dark matter (DM) particles are the pseudo-Goldstone excitations associated with a spontaneously broken symmetry, and transforming linearly with respect to an unbroken group H. For definiteness we take H = SU(N) and assume the Goldstone particles are bosons; in parallel with QCD, we refer to these particles as dark-matter pions. This scenario is in contrast to the common assumption that DM fields transform linearly under the full symmetry of the model. We illustrate the formalism by treating in detail the case of H = SU(2), in particular we calculate all the interactions relevant for the Boltzmann equations, which we solve numerically; we also derive approximate analytic solutions and show their consistency with the numerical results. We then compare the results with the constraints derived from the cold DM and direct detection experiments and derive the corresponding restrictions on the model parameters.

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