arXiv subjects

Blazenka Melic

Publications and source records attributed to Blazenka Melic.

18 recordsLinked to original sources

arXiv2021

Form factors of B, Bs -> eta, eta' and D, Ds -> eta, eta' transitions from QCD light-cone sum rules

In the framework of the QCD light-cone sum rules (LCSRs) we present the analysis of all $B, B_{s}\to η^{(\prime)}$ and $D, D_{s}\to η^{(\prime)}$ form factors ($f^+, f^0$ and $f^T$) by including $m_{η^{(\prime)}}^2$ corrections in the leading (up to the twist-four) and next-to-leading order (up to the twist-three) in QCD, and two-gluon contributions to the form factors at the leading twist. The SU(3)-flavour breaking corrections and the axial anomaly contributions to the distribution amplitudes are also consistently taken into account. The complete results for the $f^0$ and $f^T$ form factors of $B,B_s \to η^{(\prime)}$ and $D, D_{s} \to η^{(\prime)}$ relevant for processes like $B \to η^{(\prime)} τν_τ$ or $B_{s} \to η^{(\prime)} l^+ l^-$ are given for the first time, as well as the two-gluon contribution to the tensor form factors. The values obtained for the $f^+$ form factors are as follows: $f^+_{Bη}(0)= 0.168^{+0.042}_{-0.047}$, $|f^+_{B_sη}(0)|= 0.212^{+0.015}_{-0.013}$, $f^+_{Bη^\prime}(0)= 0.130^{+0.036}_{-0.032}$, $f^+_{B_sη^\prime}(0)= 0.252^{+0.023}_{-0.020}$ and $f^+_{Dη}(0)= 0.429^{+0.165}_{-0.141}$, $|f^+_{D_sη}(0)|= 0.495^{+0.030}_{-0.029}$, $f^+_{Dη^\prime}(0)= 0.292^{+0.113}_{-0.104}$, $f^+_{D_sη^\prime}(0)= 0.558^{+0.047}_{-0.045}$. Also phenomenological predictions for semileptonic $B, B_{s}\to η^{(\prime)}$ and $D, D_{s}\to η^{(\prime)}$ decay modes are given.

hep-ph↗

arXiv2020

$|V_{ub}|$ determination and testing of lepton flavour universality in semileptonic $B_c \rightarrow D^{(\ast)}$ decays

In light of prospects for measurements of $B_c \rightarrow D^{(\ast)} l ν$ decays in the upcoming Upgrade II of the LHC, we show that by using calculated $B_c \rightarrow D^{(\ast)}$ form factors competitive extraction of the $|V_{ub}|$ CKM matrix element from the $B_c \to D μ\barν_μ$ decay might be possible. To minimize experimental and theoretical uncertainties we provide the ratio $|V_{ub}|/|V_{cb}|$ by normalizing the $B_c \rightarrow D^{(\ast)} μ\barν_μ$ to $B_c \to J/ψμ\barν_μ$ decay. We also briefly examine the suggestion to extract $|V_{ub}|/|V_{cs}|$ from the theoretically interesting ratio of $B_c \rightarrow D^0 e \barν_{e}$ and $B_c \rightarrow B_s e \barν_{e}$ decay rates in the zero-recoil limit. With the present average value of $|V_{ub}|$, the predicted branching ratios are estimated to be $BR(B_c \to D^0 μ\barν_μ) = (2.4\pm 0.4)\cdot 10^{-5} $ and $BR(B_c \to D^{\ast} μ\barν_μ) = (7\pm3)\cdot 10^{-5} $, and the semileptonic ratios for testing the lepton flavour universality in these $B_c$ decays are $R_c(D^0) =0.64 \pm 0.05$ and $R_c(D^{\ast}) = 0.55 \pm 0.05$. We also provide $q^2$ distributions and various angular observables of $B_c \rightarrow D^{(\ast)} l ν$ decays.

hep-ph↗

arXiv2019

On lepton flavour universality in semileptonic $B_c \to η_c, J/ψ$ decays

We discuss $B_c \to η_c$ and $B_c \to J/ψ$ semileptonic decays within the Standard Model (SM) and beyond. The relevant transition form factors, being the main source of theoretical uncertainties, are calculated in the sum rule approach and are provided in a full $q^2$ range. We calculate the semileptonic branching fractions and find for the ratios, $R_{η_c}|_{\rm SM}= 0.32 \pm 0.02$, $R_{J/ψ}|_{\rm SM} = 0.23 \pm 0.01$. Both predictions are in agreement with other existing calculations and support the current tension in $R_{J/ψ}$ at 2$σ$ level with the experiment. To extend the potential of testing the SM in the semileptonic $B_c$ decays, we consider the forward-backward asymmetry and polarization observables. We also study the 4-fold differential distributions of $B_c \rightarrow J/ψ(J/ψ\to \tilde{\ell}^-\tilde{\ell}^+ ) \ell^- \barν_\ell$, where $\tilde{\ell} = e, μ$, in the presence of different new physics scenarios and find that the new physics effects can significantly modify the angular observables and can also produce effects which do not exist in the SM. Using the constraints on the new physics couplings from the recent combined analysis of BaBar, Belle and LHCb data on semileptonic $B \to D^{(\ast)}$ decays, where the effects of new physics could be visible, we find that these different new physics scenarios are not able to simultaneously explain the current experimental value of $R_{J/ψ}$.

hep-ph↗

arXiv2018

Testing spin-2 mediator by angular observables in $b\rightarrow s μ^+ μ^-$

We consider effects of spin-2 particle in the $b\rightarrow s μ^+ μ^-$ transition assuming that the spin-2 particle couples in a flavour non-universal way to $b$ and $s$ quarks and in the leptonic sector couples only to the muons, thereby only contributing to the process $b\rightarrow s μ^+ μ^-$. The $B_s - \bar B_s$ transition gives the strong constraint on the coupling of the spin-2 mediator and $b$ and $s$ quarks while the observed discrepancy from the SM prediction for the muon anomalous magnetic moment $ (g-2)_μ$ serves us to constrain the $μ$-coupling to spin-2 particle. We find that the spin-2 particle can modify the angular observables in the $B \to K μ^+ μ^-$ and $B \to K^* μ^+ μ^-$ decays and produce effects which do not exist in the SM. The generated forward-backward asymmetries in these processes can reach $15\%$, while other observables for these decays receive tiny effects.

hep-ph↗

arXiv2017

Revisiting the decoupling effects in the running of the Cosmological Constant

We revisit the decoupling effects associated with heavy particles in the renormalization group running of the vacuum energy in a mass-dependent renormalization scheme. We find the running of the vacuum energy stemming from the Higgs condensate in the entire energy range and show that it behaves as expected from the simple dimensional arguments meaning that it exhibits the quadratic sensitivity to the mass of the heavy particles in the infrared regime. The consequence of such a running to the fine-tuning problem with the measured value of the Cosmological Constant is analyzed and the constraint on the mass spectrum of a given model is derived. We show that in the Standard Model (SM) this fine-tuning constraint is not satisfied while in the massless theories this constraint formally coincides with the well known Veltman condition. We also provide a remarkably simple extension of the SM where saturation of this constraint enables us to predict the radiative Higgs mass correctly. Generalization to constant curvature spaces is also given.

hep-th↗

arXiv2014

Lattice QCD and QCD Sum Rule determination of the decay constants of eta_c, J/psi and hc states

We compute the decay constants of the lowest ccbar-states with quantum numbers J(PC)=0(-+) [eta_c], 1(--) [J/psi], and 1(+-) [hc] by using lattice QCD and QCD sum rules. We consider the coupling of J/psi to both the vector and tensor currents. Lattice QCD results are obtained from the unquenched (Nf=2) simulations using twisted mass QCD at four lattice spacings, allowing us to take the continuum limit. On the QCD sum rule side we use the moment sum rules. The results are then used to discuss the rate of eta_c --> gamma gamma decay, and to comment on the factorization in B --> X K decays, with X being either eta_c or J/psi.

hep-ph↗

arXiv2013

Updated S3 Model of Quarks

A model proposed in 2004 using the non-Abelian discrete symmetry S3 for understanding the flavor structure of quarks and leptons is updated, with special focus on the quark and scalar sectors. We show how the approximate residual symmetries of this model explain both the pattern of the quark mixing matrix and why the recently observed particle of 126 GeV at the Large Hadron Collider is so much like the one Higgs boson of the Standard Model. We identify the strongest phenomenological bounds on the scalar masses of this model, and predict a possibly observable decay b -> s tau- mu+, but not b -> s tau+ mu-.

hep-ph↗

arXiv2012

Discerning New Physics in Top-Antitop Production using Top Spin Observables at Hadron Colliders

Copious production of top-antitop quark pairs at hadron colliders has enabled various probes into the properties and interactions of top quarks. Among the various presently measured observables, the forward-backward asymmetry (FBA) in t tbar production measured at the Tevatron significantly deviates from the standard model predictions, and many models of new physics have been invented to explain the puzzle. We consider the consistency of the simplified single-resonance models containing a color octet axial-vector ("axigluon"), color triplet or sextet weak singlet scalars, weak isodoublet scalar, flavor-changing neutral Z', or charged W' vector boson with existing t tbar production measurements. Among the considered models only an axigluon can reproduce all Tevatron observables, without being in severe tension with the recent LHC results on t tbar production cross section, charge asymmetry and top-spin correlations. The LHC charge asymmetry measurements exclude the W' and Z' explanations of the Tevatron FBA anomaly. On the other hand, all scalar models predict notable deviations in several top spin observables, and the recent top spin correlation measurement using the "helicity" spin quantization axis by ATLAS already provides a significant constraint on possible explanations of the Tevatron FBA anomaly. Future precise measurements of top spin correlations and especially top polarization could differentiate between scalar t(u)-channel models, while they are less sensitive to pure axigluon contributions.

hep-ph↗

arXiv2008

B -> pi, B(s) -> K form factors and Vub determination

We present a QCD light-cone sum rule (LCSR) estimation of the B -> pi, B -> K, and Bs -> K form factors calculating gluon radiative corrections at next-to-leading order. The MS-bar b-quark mass is used, instead of the one-loop pole mass employed in the previous analyses. For B -> K and Bs -> K form factors the SU(3)-symmetry breaking corrections are included, both in the hard-scattering kernels and in the distribution amplitudes (DAs). By combining the predicted value for f^+_{Bpi}(0) with the product |Vub f^+_{Bpi}| extracted from the B -> pi l nu measurement, we obtain the LCSR prediction for Vub CKM matrix element.

hep-ph↗

arXiv2004

LCSR analysis of exclusive two-body B decay into charmonium

We analyze B -> K H_c (where H_c = eta_c, J/psi, chi_{cJ(J=0,1)}) decays estimating nonfactorizable contributions from the light-cone sum rules (LCSR). Nonfactorizable contributions are sizable for B -> K J/psi and particularly for B -> K chi_{c1} decay. For the B decays into a (pseudo)scalar charmonia we find small nonfactorizable contributions which cannot accommodate relatively large branching ratios obtained by measurements. This specially concerns the puzzling B -> K chi_{c0} decay.

hep-ph↗

arXiv2003

Estimate of the charming penguin contributions to B -> pi pi

We consider the problem of factorization in B decays and illustrate the calculation of nonfactorizable contributions employing the QCD light-cone sum rule method. We present a more detailed calculation of the ``charming penguin'' contributions as a potential source of the substantial nonfactorizable O(1/m_b) effects in the B -> pi pi decay. Although the predicted corrections are not sizable, by calculating the CP asymmetry we illustrate how such corrections can accumulate to a visible effect. In conclusion, nonfactorizable contributions in nonleptonic B decays into charmonium are briefly discussed.

hep-ph↗

arXiv2003

Nonfactorizable corrections to B -> J/psi K

We apply the method of QCD light-cone sum rules to calculate nonfactorizable contributions to the B -> J/psi K decay and estimate soft nonfactorizable corrections to the a_2 parameter. The corrections appear to be positive, favoring the positive sign of a_2, in agreement with recent theoretical considerations and experimental data. Our result also confirms expectations that in the color-suppressed decay nonfactorizable corrections are sizable.

hep-ph↗

arXiv2002

Heavy Quarks on the Lattice

I review the basic ideas behind lattice QCD calculations that involve charm and bottom quarks. I report on the progress in getting the correct hyperfine splitting in charmonium from lattice QCD. Some of the basic technology behind numerical lattice QCD calculations is explained by studying some specific examples: computation of the charm quark mass, and the calculation of fB.

hep-lat↗

arXiv2002

BLM scale for the pion transition form factor

We review the determination of the NLO Brodsky-Lepage-Mackenzie (BLM) renormalization scale for the pion transition form factor. We argue that the prediction for the pion transition form factor is independent of the factorization scale at every order in the strong coupling constant.

hep-ph↗

arXiv2002

BLM scale for the pion transition form factor

The NLO Brodsky-Lepage-Mackenzie (BLM) scale for the pion transition form factor has been determined. To achieve that, a consistent calculation up to nf-proportional NNLO contributions to both the hard-scattering amplitude and the perturbatively calculable part of the pion distribution amplitude has been performed. By combining and matching the results obtained for these two amplitudes, a proper cancellation of collinear singularities has been established and the gamma5 ambiguity problem (related to the use of the dimensional regularization method) has been resolved by using the naive-gamma5 as well as the 't Hooft-Veltman (HV) schemes. It has been demonstrated that the prediction for the pion transition form factor is independent of the factorization scale muF at every order in the strong coupling constant, making it possible to use the simplest choice muF^2=Q^2 at the intermediate steps of the calculation. Assuming the pion asymptotic distribution amplitude and working in the \bar{MS} scheme, we have found the BLM scale to be muR^2=muBLM^2 ~ Q^2/9. Based on the same distribution, the complete NLO prediction for the pion transition form factor has been calculated in the alphaV definition of the QCD coupling renormalized at muR^2=muV^2 = e^{5/3} muBLM^2 ~ Q^2/2. It is in good agreement with the presently available experimental data.

hep-ph↗

arXiv1999

On the perturbative approach to the penguin-induced B -> pi phi decay

Using a modified perturbative approach that includes the Sudakov resummation and transverse degrees of freedom we analyze the penguin-induced B -> pi phi decay. The perturbative method enables us to include nonfactorizable contributions and to control virtual momenta appearing in the process. The calculation supports the results obtained in the standard BSW factorization approach, illustrating the electroweak penguin dominance and the branching ratio of order O(10^{-8}). However, the estimated prediction of 16% for CP asymmetry is much larger than that obtained in the factorization approach.

hep-ph↗

arXiv1999

Determination of heavy-meson wave functions from B decays

We extract the B, D, and D* meson wave functions from the CLEO data of the decays B -> K*γand B -> D^(*)πin the perturbative QCD framework. In this formalism, various logarithmic corrections are organized to give the Wilson evolution from the W boson mass down to the characteristic scale of a decay process, which is of order of the b quark mass, and the Sudakov evolution from the characteristic scale to a lower factorization scale of order Lambda_{QCD}. With large logarithms organized, the b quark decay amplitudes are evaluated reliably in perturbation theory. Below the factorization scale, QCD dynamics is regarded as being nonperturbative, and absorbed into meson wave functions. Because of their universality, the heavy-meson wave functions determined in this work, can be employed to make predictions of other decay modes. We also observe that the dependence of heavy meson wave functions on intrinsic parton transverse momenta plays an important role in the explanation of data.

hep-ph↗

arXiv1998

Perturbative approach to the penguin-induced $B \to πϕ$

Using a modified perturbative approach that includes the Sudakov resummation and transverse degrees of freedom we analyze the penguin-induced $B^{-} \to π^{-}ϕ$ decay by applying the next-to-leading order effective weak Hamiltonian. The modified perturbative method enables us to include nonfactorizable contributions and to control virtual momenta appearing in the process. Besides, we apply the three-scale factorization theorem for nonleptonic processes that offers the possibility of having the scale-independent product of short- and long-distance parts in the amplitude of the weak Hamiltonian. The calculation supports the results obtained in the BSW factorization approach, illustrating the electroweak penguin dominance and the branching ratio of order ${\cal O}(10^{-8})$. However, the estimated prediction of 16% for the CP asymmetry is much larger than that obtained in the factorization approach.

hep-ph↗