Searcharxiv⌕ Search

arXiv subjects

D. Melikhov

Publications and source records attributed to D. Melikhov.

At least 19 recordsLinked to original sources
arXiv2014

QCD sum-rule results for heavy-light meson decay constants and comparison with lattice QCD

Updated predictions for the decay constants of the D, Ds, B and Bs mesons obtained from Borel QCD sum rules for heavy-light currents are presented and compared with the recent lattice averages performed by the Flavor Lattice Averaging Group. An excellent agreement is obtained in the charm sector, while some tension is observed in the bottom sector. Moreover, available lattice and QCD sum-rule calculations of the decay constants of the vector D*, Ds*, B* and Bs* mesons are compared. Again some tension in the bottom sector is observed.

hep-ph↗
arXiv2008

Extracting Hadron Parameters from Dispersive Sum Rules

Puzzled or surprised by the almost incredible accuracy occasionally claimed in the literature to be achievable for numerical outcomes of QCD sum-rule analyses, we scrutinized the usual procedure employed for the extraction of the parameters of individual bound states from dispersive sum rules by taking advantage of the exact solvability of a quantum-mechanical harmonic-oscillator model: It turns out that the determination of the ground-state parameters (that is, decay constant and form factor) by requiring independence from the Borel mass in its stability window does not necessarily yield their exact numerical values. For instance, the comparison of the sum-rule predictions for bound-state parameters with their numerical values known precisely in our harmonic-oscillator model reveals that standard sum-rule procedures underestimate the ground-state decay constant by some 4% and its form factor by almost 15%; such systematic uncertainties cannot be inferred from our correlators' accuracy better than 1% in the window of Borel stability: they are uncontrollable.

hep-ph↗
arXiv2008

B, D and K decays

With the advent of the LHC, we will be able to probe New Physics (NP) up to energy scales almost one order of magnitude larger than it has been possible with present accelerator facilities. While direct detection of new particles will be the main avenue to establish the presence of NP at the LHC, indirect searches will provide precious complementary information, since most probably it will not be possible to measure the full spectrum of new particles and their couplings through direct production. In particular, precision measurements and computations in the realm of flavour physics are expected to play a key role in constraining the unknown parameters of the Lagrangian of any NP model emerging from direct searches at the LHC. The aim of Working Group 2 was twofold: on one hand, to provide a coherent, up-to-date picture of the status of flavour physics before the start of the LHC; on the other hand, to initiate activities on the path towards integrating information on NP from high-pT and flavour data.

hep-ph↗
arXiv2006

Quark-hadron duality and hadron properties from correlators of pseudoscalar and axial currents

We study the operator product expansion (OPE) and quark-hadron duality for 2- and 3-point correlators of the axial (A) and pseudoscalar (P) currents of the light quarks. In the chiral limit these correlators are often dominated by nonperturbative power corrections leading to subtleties of quark-hadron duality relations and of the extraction of properties of light pseudoscalars. For the 2-point correlators, we show the sum rule for $ $ to be sensitive to the excited light pseudoscalar. For the 3-point correlators, we derive the Ward identites which provide the normalization of the pion electromagnetic form factor at zero momentum transfer. For large momentum transfer, we demonstrate the way the correct behaviour of the pion form factor in agreement with perturbative QCD emerges from condensate terms in the OPE for the $ $ and $ $ correlators. The local-duality sum rule for $ $ is shown to lead to the pion form factor with the required properties for all values of the momentum transfer.

hep-ph↗
arXiv2004

Width of the J^P=1/2^+ pentaquark in the quark-diquark model

We analyse the width of the $θ(\frac12^+)$ pentaquark assuming that it is a bound state of two extended spin-zero $ud$-diquarks and the $\bar s$ antiquark (the Jaffe-Wilczek scenario). The width obtained when the size parameters of the pentaquark wave function are taken to be close to the parameters of the nucleon is found to be $\simeq 150$ MeV, i.e. it has a normal value for a $P$-wave hadron decay with the corresponding energy release.However, we found a strong dynamical suppression of the decay width if the pentaquark has an asymmetric "peanut" structure with the strange antiquark in the center and the two diquarks rotating around. In this case a decay width of $\simeq$ 1 MeV is a natural possibility.

hep-ph↗
arXiv2003

Masses and couplings of vector mesons from the pion electromagnetic, weak, and πγtransition form factors

We analyse the pion electromagnetic, charged-current, and $πγ$ transition form factors at timelike momentum transfers $q$, $q^2=s\le 1.4$ GeV$^2$, using a dispersion approach. We discuss in detail the propagator matrix of the photon-vector-meson system and define certain reduced amplitudes, or vertex functions, describing the coupling of this system to final states. We then apply the derived analytic expressions to the analysis of the recent $e^+e^-\to π^+π^-$, $τ^-\to π^-π^0ν_τ$, and $e^+e^-\to π^0γ$ data. We find the reduced amplitudes for the coupling of the photon and vector mesons to two pseudoscalars to be constant, independent of $s$, in the range considered, indicating a "freezing" of the amplitudes for $s\le 1$ GeV. The fit to the form factor data leads to the following values of the Breit-Wigner resonance masses m_{ρ^-}=775.3\pm 0.8 MeV, m_{ρ^0}=773.7\pm 0.6 MeV and m_ω=782.43\pm 0.05 MeV, where the errors are only statistical.

hep-ph↗
arXiv2003

The pion form factor at timelike momentum transfers in a dispersion approach

We consider a model with $ρππ$, $ρKK$, and gauge-invariant $ργ$ couplings and obtain the pion form factor $F_π$ at timelike momentum transfers by resummation of pion and kaon loops. We use a dispersion representation for the loop diagrams and analyse ambiguities related to subtraction constants. The resulting representation for $F_π$ is shown to have the form of the conventional vector meson dominance formula with one important distinction - the effective $ρ$-meson decay constant $f^{\rm eff}_ρ$ turns out to depend on the momentum transfer. For the electromagnetic pion form factor we include in addition the $ρ-ω$ mixing effects. We apply the representations obtained to the analysis of the data on the pion form factors from $e^+e^-$ annihilation and $τ$ decay and extract the $ρ^-$, $ρ^0$ and $ω$ masses and coupling constants.

hep-ph↗
arXiv2003

End-point singularities of Feynman graphs on the light cone

We show that some Lorentz components of the Feynman integrals calculated in terms of the light-cone variables may contain end-point singularities which originate from the contribution of the big-circle integral in the complex k_ plane. These singularities appear in various types of diagrams (two-point functions, three-point functions, etc) and provide the covariance of the Feynman integrals on the light-cone. We propose a procedure for calculating Feynman integrals which guarantees that the end-point singularities do not appear in the light-cone representations of the invariant amplitudes.

hep-ph↗
arXiv2002

Proceedings to the Workshops ''What comes beyond the Standard model'' 2000,2001, VOLUME 1: Festschrift dedicated to the 60th birthday of Holger Bech Nielsen

The present volume is the collection of contributions by friends of Holger Bech Nielsen for his 60th birthday. Contents: 1.Unified internal space of spins and charges (N. Mankoc Borstnik) 2.Semitopological Q-Rings (M. Axenides) 3.Non-local axial anomalies in the Standard model (D. Melikhov, B. Stech) 4.Could there be a fourth generation? (C.D. Froggatt) 5.Another complex Bateman equation (D.B. Fairlie) 6.Non-associative loops for Holger Bech Nielsen (P.H. Frampton) 7.Particles, fluids and vortices (J.W. van Holten) 8.Results on 2D current algebras (J.L. Petersen) 9.Phase transition in gauge theories and the Planck scale physics (L.V. Laperashvili and D.A. Ryzhikh) 10.Some remarks on the 'classical' large N limit (P. Olesen) 11.String theory and the size of hadrons (L. Susskind) 12.Relativity and $c/sqrt3$ (S.I. Blinnikov, L.B. Okun and M.I. Vysotsky) 13.Vortices (Jeff Greensite)

hep-ph↗
arXiv2002

Electromagnetic form factors in the light-front formalism and the Feynman triangle diagram: spin-0 and spin-1 two-fermion systems

The connection between the Feynman triangle diagram and the light-front formalism for spin-0 and spin-1 two-fermion systems is analyzed. It is shown that in the limit q+ = 0 the form factors for both spin-0 and spin-1 systems can be uniquely determined using only the good amplitudes, which are not affected by spurious effects related to the loss of rotational covariance present in the light-front formalism. At the same time, the unique feature of the suppression of the pair creation process is maintained. Therefore, a physically meaningful one-body approximation, in which all the constituents are on their mass-shells, can be consistently formulated in the limit q+ = 0. Moreover, it is shown that the effects of the contact term arising from the instantaneous propagation of the active constituent can be canceled out from the triangle diagram by means of an appropriate choice of the off-shell behavior of the bound state vertexes; this implies that in case of good amplitudes the Feynman triangle diagram and the one-body light-front result match exactly. The application of our covariant light-front approach to the evaluation of the rho-meson elastic form factors is presented.

hep-ph↗
arXiv2001

Non-local anomaly of the axial-vector current for bound states

We demonstrate that the amplitude $<ργ|\partial_ν(\bar qγ_νγ_5 q)|0>$ does not vanish in the limit of zero quark masses. This represents a new kind of violation of the classical equation of motion for the axial current and should be interpreted as the axial anomaly for bound states. The anomaly emerges in spite of the fact that the one loop integrals are ultraviolet-finite as guaranteed by the presence of the bound-state wave function. As a result, the amplitude behaves like $\sim 1/p^2$ in the limit of a large momentum $p$ of the current. This is to be compared with the amplitude $<γγ|\partial_ν\bar qγ_νγ_5 q|0>$ which remains finite in the limit $p^2\to\infty$. The observed effect leads to the modification of the classical equation of motion of the axial-vector current in terms of the non-local operator and can be formulated as a non-local axial anomaly for bound states.

hep-ph↗
arXiv2001

Weak annihilation in the rare radiative $B\to ργ$ decay

The ampitude of the $B\toργ$ decay induced by the flavour-changing neutral currents contains the penguin contribution and the weak-annihilation contribution generated by the 4-quark operators in the effective Hamiltonian. The penguin contribution is known quite well. We analyse the weak-annihilation which is suppressed by the heavy-quark mass compared to the penguin contribution. In the factorization approximation, the weak annihilation amplitude is represented in terms of the leptonic decay constants and the meson-photon matrix elements of the weak currents. The latter contain the $Bγ$, $ργ$ transition form factors and contact terms determined by the equations of motion. We calculate the $Bγ$ and $ργ$ form factors within the relativistic dispersion approach and obtain numerical estimates for the weak annihilation amplitude.

hep-ph↗
arXiv2001

Duality in semileptonic inclusive B-decays in potential models: regular versus singular potentials

Making use of the nonrelativistic potential model for the description of mesons, and working in the Shifman-Voloshin limit, we compare the integrated rate $Γ(B\to X_clν)$ calculated as a sum of the individual decay rates to the quantum-mechanical analog of the OPE. In the case of a potential regular at the origin, we find a well-defined duality violation, which is however exponentially small. It corresponds to the charm resonances kinematically forbidden in the decay process, but apparently picked up by the OPE. For singular potentials, we do not obtain a full OPE series, but only a limited Taylor expansion, since the cofficients become infinite beyond some order. In this case, we do not find an indication of duality violation: the difference is smaller than the last term of the limited expansion. This emphasizes that the case of singular potentials, which may be relevant for QCD, deserves further study.

hep-ph↗
arXiv2000

Semileptonic inclusive heavy meson decay: duality in a nonrelativistic potential model in the Shifman-Voloshin limit

Quark-hadron duality in the inclusive semileptonic decay $B\to X_c lν$ in the Shifman-Voloshin limit $Λ\ll δm=m_b - m_c \ll m_b, m_c$ is studied within a nonrelativistic potential model. The integrated semileptonic decay rate is calculated in two ways: first, by constructing the Operator Product Expansion, and second by a direct summation of the exclusive channels. Sum rules (Bjorken, Voloshin, etc.) for the potential model are derived, providing a possibility to compare the two representations for $Γ(B\to X_c lν)$. An explicit difference between them referred to as duality-violation effect is found. The origin of this effect is related to higher charm resonances which are kinematically forbidden in the decay process but are nevertheless picked up by the OPE. Within the considered $1/m_c^2$ order the OPE and the sum over exclusive channels match each other, up to the contributions of higher resonances, by virtue of the sum rules. In particular this is true for the terms of order $δm^2/m_c^2$ and $Λδm/m_c^2$ which are present in each of the decay channels and cancel in the sum of these channels due to the Bjorken and Voloshin sum rules, respectively. The size of the duality violation effects is estimated to be of the order $O(Λ^{2+b}/m_c^2δm^b)$ with $b>0$ depending on the details of the potential. Constraints for a better accuracy are discussed.

hep-ph↗
arXiv2000

Weak form factors for heavy meson decays: an update

We calculate the form factors for weak decays of $B_{(s)}$ and $D_{(s)}$ mesons to light pseudoscalar and vector mesons. To reveal the intimate connection between different decay modes and to be able to perform the calculations in the full physical $q^2$-region we use a relativistic dispersion approach based on the constituent quark picture. This approach gives the form factors as relativistic double spectral representations in terms of the wave functions of the initial and final mesons. The form factors have the correct analytic properties and satisfy general requirements of nonperturbative QCD in the heavy quark limit. The disadvantages of quark models related to ill-defined effective quark masses and not precisely known meson wave functions are reduced by fitting the quark model parameters to lattice QCD results for the $B\to ρ$ transition form factors at large momentum transfers and to the measured total $D\to (K,K^*)lν$ decay rates. This allows us to predict numerous form factors for all kinematically accessible $q^2$ values.

hep-ph↗
arXiv2000

Duality in the non-relativistic harmonic oscillator quark model in the Shifman-Voloshin limit : a pedagogical example

The detailed way in which duality between sum of exclusive states and the free quark model description operates in semileptonic total decay widths, is analysed. It is made very explicit by the use of the non relativistic harmonic oscillator quark model in the SV limit, and a simple interaction current with the lepton pair. In particular, the Voloshin sum rule is found to eliminate the mismatches of order $δm/m_b^2$.

hep-ph↗