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A. G. Oganesian

Publications and source records attributed to A. G. Oganesian.

At least 19 recordsLinked to original sources
arXiv2015

Matching lightcone- and anomaly-sum-rule predictions for the pion-photon transition form factor

The pion-photon transition form factor is studied by employing two types of Sum Rules: Light Cone Sum Rules (LCSR) and Anomaly Sum Rules (ASR). By comparing the predictions for the pion-photon transition form factor, obtained from these two approaches, the applicability limit of the LCSRs at low momenta is determined. Reciprocally, the ASR threshold dependence on the momentum was extracted using our LCSR-based method in combination with two different types of pion distribution amplitudes and found that at higher Q2 it approaches a constant.

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arXiv2009

Dispersive Approach to Abelian Axial Anomaly, Mixing of Pseudoscalar Mesons and Symmetries

We suggest a rigorous generalization of the pseudoscalar mesons mixing description in SU(3) basis. It is shown that the appearance of extra massive state nicknamed in the paper as glueball is unavoidable in any scheme with more than one angle. In this framework we develop the dispersive approach to Abelian axial anomaly of isoscalar non-singlet current. Combining it with the analysis of experimental data of charmonium radiative decays ratio we get the number of quite strict constraints for mixing parameters. Our analysis favors the equal values of axial currents coupling constants which may be considered as a manifestation of SU(3) symmetry and possible violation of chiral symmetry based predictions.

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arXiv2008

Dispersive Approach to Abelian Axial Anomaly and $η-η'$ Mixing

We investigate what can be learnt about the $η$ -$η'$ mixing by means of dispersive representation of axial anomaly. We show that our method leads to the strong bounds for the $η-η'$ mixing angle: $θ= -15.3^o \pm 1^o$. Moreover, our result manifests also a dramatic dependence of the width $Γ_{η\to 2γ}$ on the mixing angle $θ$. This property explains how the relatively small mixing strongly effects the decay width.

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arXiv2008

Heavy quark distribution function in hadrons

The moments of the heavy quark-parton distribution functions in a heavy pseudoscalar meson, obtained in QCD sum rules, are expanded in the inverse heavy quark. Comparison with the finite mass results reveals that while the heavy mass expansion works reasonably well for the $b$ quark, one has to take into account terms of higher than $(1/m_c)^2$ order for the $c$ quark.

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arXiv2007

Moments of the heavy-quark parton distribution function from QCD sum rules

The moments of the heavy quark-parton distribution functions in a heavy pseudoscalar meson are calculated from QCD sum rules. Expanding these sum rules in the inverse heavy quark mass we obtain the heavy-mass limits of the moments. Comparison with the finite mass results reveals that while the heavy mass expansion works reasonably well for the $b$ quark, one has to take into account terms of higher than $(1/m_c)^2$ order for the $c$ quark. This result can provide a quantitative assessment of $c$ and $b$ quark fragmentation models based on the heavy-quark mass limit.

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arXiv2007

Axial anomaly and the precise value of the $π^0 \to 2 γ$ decay width

The anomaly in the vacuum expectation value of the product of axial and two vector currents (AVV) in QCD is investigated. The goal is to determine from its value the $π^0 \to 2 γ$ decay width with high precision. The sum rule for AVV formfactor is studied. The difference $f_{π^0} - f_{π^+}$ caused by strong interaction is calculated and appears to be small. The $π^0 - η$ mixing is accounted. The $π^0 \to 2 γ$ decay width determined theoretically from the axial anomaly is found to be $Γ(π^0 \to 2 γ) = 7.93 eV$ with an error $\sim 1.5%$. The measurement of the $π^0 \to 2 γ$ decay width at the same level of accuracy would allow one to achieve a high precision test of QCD.

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arXiv2006

Quark distributions in QCD sum rules: unexpected features and paradoxes

Some very unusual features of the hadron structure functions, obtained in the generalized QCD sum rules, like the surprisingly strong difference between longitudinally and transversally polarized $ρ$ mesons structure functions and the strong suppression of the gluon sea in longitudinally polarized $ρ$ mesons are discussed. Also the problem of exact zero contribution of gluon condensates to pion and longitudinally polarized $ρ$ meson quark distributions is discussed.

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arXiv2006

Calculation of the pentaquark width by QCD sum rule

The pentaquark width is calculated in QCD sum rules. Result for $Γ_Θ$ show, that $Γ_Θ$ can vary in the region less than 1$MeV$. The main conclusion is, that if pentaquark is genuine states then sum rules really predict the narrow width of pentaquark $θ^+$, and the suppression of the width is both parametrical and numerical.

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arXiv2005

Quark distributions in polarized and nonpolarized hadrons

Valence quark distributions in pion, polarizied $ρ$ mesons and nucleon in the region of intermediate $x$ are obtained by generalizied QCD sum rules. It is shown, that polarization effects are very significant. The strong suppression of quark and gluon sea distributions in longitudinally polarizied $ρ$ mesons is found.

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arXiv2005

Small size pentaquark in QCD sum rule approach

The analysis of the sum rules (for two differentchoices of 5-quark currents) indicate that to explain experimental results it is necessary to have some one-particle lower state (and pentaquark is a reliable candidate to this state) while the two-particle coupled lower state ($NK$ system) should be excluded. Obtained sum rules have good stability and correspond to pentaquark mass $m_θ$=1.54 $GeV$ at the appropriate choice of continuum threshold. Also the features of the sum rules for 2-point correlator with 5-quark currents are studied. This features should be taken into account to obtain sum rules correctly.

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arXiv2005

Pentaquark decay is suppressed by chirality conservation

It is shown, that if the pentaquark $Θ^+ = uudd\bar{s}$ baryon can be represented by the local quark current $η_Θ$, its decay $Θ^+ \to n K^+ (p K^0)$ is forbidden in the limit of chirality conservation. The $Θ^+$decay width $Γ$ is proportional to $α^2_s < 0 | \bar{q} q | 0 >^2$, where $<0 | \bar{q} q | 0 >$, $q = u,d,s$ is quark condensate, and, therefore, is strongly suppressed. Also the polarization operator of the pentaquark current with isospin 1 is calculated using the operator product expansion and estimation for it mass is obtained .

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arXiv2004

Exotic baryon states in QCD sum rule

It is shown, that the small decay width of $Θ^+ = uudd\bar{s}$ baryon is suppressed by chirality violation. It is shown that $Θ^+$ decay width $Γ$ is proportional to $α^2_s < 0 | \bar{q} q | 0 >^2$, for any pentaquark current without derivatives.

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arXiv2002

Valence quark distributions in nucleon at low $Q^2$ in QCD

Valence u- and d-quarks distributions in proton are calculated in QCD at low $Q^2$ and intermediate x, basing on the operator product expansion (OPE). The imaginary part of the virtual photon scattering amplitude on quark current with proton quantum numbers is considered. The initial and final virtualities $p^2_1$ and $p^2_2$ of the currents are assumed to be large, negative and different, $p^2_1 \not= p^2_2$. The OPE in $p^2_1$, $p^2_2$ up to dimension 6 operators was performed. Double dispersion representations in $p^2_1, p^2_2$ of the amplitudes in terms of physical states contributions are used. Putting them to be equal to those calculated in QCD, the sum rules for quark distributions are found. The double Borel transformations are applied to the sum rules. Leading order perturbative corrections are accounted. Valence quark distributions are found: $u(x)_v$ at $0.15<x<65$,~ $d(x)_v$ at $0.25<x<0.55$ with an accuracy $\sim 30%$ in the middles and $\sim 50%$ at the ends of these intervals. The quark distributions obtained are in agreement with those found from the analysis of hard processes data.

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arXiv2002

Quark distributions in polarized rho meson and its comparison with those in pion

Valence quark distributions in transversally and longitudinally polarizied rho mesons in the region of intermediate x are obtained by generalizied QCD sum rules. Power corrections up to d=6 are taken into account. Comparison of the results for pi and rho mesons shows, that polarization effects are very significant and SU(6) symmetry of distribution functions is absent. The strong suppression of quark and gluon sea distributions in longitudinally polarizied rho mesons is found.

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arXiv2000

Valence quark distributions in mesons in generalized QCD sum rules

The method for calculation of valence quark distributions at intermediate $x$ is presented. The imaginary part of the virtual photon forward scattering amplitude on the quark current with meson quantum number is considered. Initial and final virtualities $p^2_1$ and $p^2_2$ of the currents are assumed to be large, negative and different, $p^2_1 \not= p^2_2$. The operator product expansion (OPE) in $p^2_1, p^2_2$ up to dimension 6 operators is performed. Double dispersion representations in $p^2_1, p^2_2$ of the amplitude in terms of physical states contributions are used. Equalling them to those calculated in QCD by OPE the desired sum rules for quark distributions in mesons are found. The double Borel transformations are applied to the sum rules, killing non-diagonal transition terms, which deteriorated the accuracy in the previous calculations of quark distributions in nucleon. Leading order perturbative corrections are accounted. Valence quark distributions in pion, longitudinally and transversally polarized $ρ$-mesons are calculated at intermediate $x$, $0.2 \la x \la 0.7$ and normalization points $Q^2 = 2-4 GeV^2$ with no fitting parameters. The use of the Regge behaviour at small $x$ and quark counting rules at large $x$ allows one to find the first and the second moments of valence quark distributions. The obtained quark distributions may be used as an input for evolution equations. In the case of pion the quark distribution is in agreement with those found from the data on the Drell-Yan process.

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arXiv1999

Improved Calculations of Quark Distributions in Hadrons: the case of pion

The earlier introduced method of calculation of quark distributions in hadrons, based on QCD sum rules, is improved. The imaginary part of the virtual photon forward scattering amplitude on some hadronic current is considered in the case, when initial and final virtualities of the current $p^2_1$, and $p^2_2$ are different, $p^2_1\not= p^2_2$. The operator product expansion (OPE) in $p^2_1$, $p^2_2$ is performed. The sum rule for quark distribution is obtained using double dispersion representation of the amplitude on one side in terms of calculated in QCD OPE and on the other side in terms of physical states contributions. Double Borel transformation in $p^2_1$, $p^2_2$ is applied to the sum rule, killing background non-diagonal transition terms, which deteriorated the accuracy in previous calculations. The case of valence quark distribution in pion is considered, which was impossible to treat by the previous method. OPE up to dimension 6 operators is performed and leading order perturbative corrections are accounted. Valence $u$-quark distribution in $π^+$ was found at intermediate $x$, $0.15 < x < 0.7$ and normalization point $Q^2=2 GeV^2$. These results may be used as input for evolution equations.

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arXiv1998

Proton spin content and QCD topological susceptibility

The part of the proton spin $Σ$ carried by $u, d, s$ quarks is calculated in the framework of the QCD sum rules in the external fields. The operators up to dimension 9 are accounted. An important contribution comes from the operator of dimension 3, which in the limit of massless $u, d, s$ quarks is equal to the derivative of QCD topological susceptibility $χ^{\prime} (0)$. The comparison with the experimental data on $Σ$ gives $χ^{\prime}(0)= (2.3 \pm 0.6) \times 10^{-3} ~ GeV^2$. The limits on $Σ$ and $χ^{\prime}(0)$ are found from selfconsistency of the sum rule, $Σ\ga 0.05,~~ χ^{\prime} (0) \ga 1.6 \times 10^{-3} ~ GeV^2$. The values of $g_A = 1.37 \pm 0.10$ and $g^8_A = 0.65 \pm 0.15$ are also determined.

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