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B. L. Ioffe

Publications and source records attributed to B. L. Ioffe.

At least 55 records · Page 3Linked to original sources
arXiv1999

Quark distributions in extended QCD sum rule approach

The improved calculation method of quark distributions in hadrons in the framework of QCD sum rules is presented. The imaginary part of the virtual photon 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 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 the previous calculations. Valence quark distributions in the pion were found in a good agreement with the data, determined from the Drell-Yan process.

hep-ph↗
arXiv1999

Correlator of Topological Charge Densities in Instanton Model in QCD

The QCD sum rule for the correlator of topological charge densities and related to it longitudinal part of the correlator of singlet axial currents is considered in the framework of instanton model. The coupling constant of eta'-meson with the singlet axial current is determined. Its value appears to be in a good coincidence with the value determined recently from the connection of the part of proton spin, carried by u,d,s quarks, with the derivative of QCD topological susceptibility. From the same sum rule eta-eta' mixing angle is found in the framework of two mixing angles model. Its value is close to that found in the chiral effective theory. The correlator of topological charge densities at large momenta is calculated.

hep-ph↗
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.

hep-ph↗
arXiv1999

Improvement of perturbation theory in QCD for e^+e^- -> hadrons and the problem of α_s freezing

We develope the method of improvement of perturbative theory in QCD, applied to any polarization operator. The case of polarization operator Π(q^2), corresponding to the process e^+e^- -> hadrons is considered in details. Using the analytical properties of Π(q^2) and perturbative expansion of Π(q^2) at q^2<0, ImΠ(q^2) at q^2>0 is determined in such a way, that the infared pole is eliminated. The convergence of perturbative series for R(q^2)=σ(e^+e^- -> hadrons)/(e^+e^- -> μ^+μ^-) is improved. After substitution of R(q^2) into dispersion relation the improved Adler function D(q^2) with no infrared pole and frozen α_s(q^2) has been obtained. A good agreement with experiment has been achieved.

hep-ph↗
arXiv1999

Correlator of topological charge densities at low $Q^2$ in QCD: connection with proton spin problem

Vacuum correlator of topological charge densities $χ(q^2)$ at low $q^2$ in QCD is discussed: its value $χ(0)$ and first derivative $χ'(0)$ at $q^2=0$ and the contributions of pseudoscalar quasi-Goldstone bosons to $χ(q^2)$ at low $q^2$. The QCD sum rule \cite{1}, giving the connection of $χ'(0)$ (for massless quarks) with the part of the proton spin $Σ$, carried by $u,d,s$ quarks, is presented. From the requirement of selfconsistancy of the sum rule the values of $χ'(0)$ and $Σ$ were found. The same value of $χ'(0)$ follows also from the experimental data on $Σ$. The contributions of $π$ and $η$ to $χ(q^2)$ are calculated basing on low energy theorems. In such calculation the $π-η$ mixing, expressed in terms of quark mass ratios is of importance.

hep-ph↗
arXiv1998

The Correlator of Topological Charge Densities at low $Q^2$ in QCD

The correlator of topological charge densities $χ(Q^2)$ in QCD is calculated in the domain of low $Q^2$. Basing on the Ward identities the low energy theorems are proved. The value of the first derivative $χ^{\prime}(0)$ found recently by QCD sum rules \cite{1} is used. The contributions of pseudoscalar quasi-Goldstone bosons as intermediate states in the correlator are calculated with the account of $η-π$ mixing.

hep-ph↗
arXiv1998

Proton spin content, axial coupling constant 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, $Σ= 0.33\pm 0.20$, $χ^{\prime}(0) = (2.4\pm 0.5)\cdot 10^{-3} GeV.$ The values of $g_A = 1.37 \pm 0.10$ and $g^8_A = 0.65 \pm 0.15$ are also determined.

hep-ph↗
arXiv1998

Properties of $ρ$-mesons produced in heavy ion collisions

The mass shift and width broadening of $ρ$-mesons produced in heavy ion collisions is estimated. It is found that the mass increases by some tens of MeV but the width becomes large, increasing by 200 MeV at beam energies of a few GeV$\cdot$A and by twice that amount at beam energies of about a hundred GeV$\cdot$A.

hep-ph↗
arXiv1998

Mass Shift and Width Broadening of Rho-Mesons Produced in Heavy Ion Collisions

The mass shift and width broadening of rho-mesons produced in heavy ion collisions is estimated using general formulae which relate the in-medium mass shift of a particle to the real part of the forward scattering amplitude of this particle on constituents of the medium and the width broadening to the corresponding cross section. It is found that the mass increases by some tens of MeV but, more importantly, the width becomes large, increasing by several hundred MeV at beam energies of a few GeV per nucleon and by twice that amount at beam energies of about a hundred GeV per nucleon.

nucl-th↗
arXiv1998

In-medium modification of $ρ$-mesons produced in heavy ion collisions

The mass shift $Δm_ρ$ and width broadening $ΔΓ_ρ$ of $ρ$ mesons produced in heavy ion collisions is estimated using a general formula which relates the in-medium mass shift of a particle to the real part of the forward scattering amplitude ${\rm Re} f(E)$ of this particle on constituents of the medium and $ΔΓ$ to the corresponding cross section. It is found that in high energy ($E/A \ga 100~$GeV) heavy ion collisions the $ρ$ width broadening is large and $ρ$ (or $ω$) peak could hardly be observed in $e^+e^-(μ^+μ^-)$ effective mass distributions. In low energy collisions ($E/A \sim$ a few GeV) a broad (a few hundred MeV) enhancement is expected at the position of the $ρ$ peak.

hep-ph↗
arXiv1998

Nucleon spin content and axial coupling constants in QCD sum rules approach

The review of current experimental situation in the measurements of the first moment $Γ_{p,n}$ of spin dependent nucleon structure functions $g_{1;p,n}(x,Q^2)$ is presented. The results of the calculations of twist-4 corrections to $Γ_{p,n}$ are discussed and their accuracy is estimated. 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 $χ' (0)$. The comparison with the experimental data on $Σ$ gives $χ'(0)= (2.3 \pm 0.6) \times 10^{-3} GeV^2$. The limits on $Σ$ and $χ'(0)$ are found from selfconsistency of the sum rule, $Σ\ga 0.05, χ' (0) \ga 1.6 \times 10^{-3} GeV^2$. The values of $g_A = 1.37 \pm 0.10$ and $g^8_A = 0.65 \pm 0.15$ are also determined from the corresponding sum rules.

hep-ph↗
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.

hep-ph↗
arXiv1997

On the thermal mass shift of nucleons

The mass shift $Δm_N$ of a nucleon in the finite temperature pion gas is calculated in the case of an arbitrary nucleon momentum. A general formula is used which relates the in-medium mass shift $Δm(E)$ of a particle to the real part of the forward scattering amplitude $Re f(E)$ of this particle on constituents of the medium and its applicability domain is formulated. The mass shift of the nucleon in thermal equilibrium with pion gas is also calculated.

hep-ph↗
arXiv1997

The Sum Rules for Structure Functions of Polarized $e(μ)N$ Scattering: Theory vs. Experiment

The determination of twist-4 corrections to the structure functions of polarized $e(μ)N$ scattering by QCD sum rules is reviewed and critically analyzed. It is found that in the case of the Bjorken sum rule the twist-4 correction is small at $Q^2 > 5 GeV^2$. However, the accuracy of the today experimental data is insufficient to reliably determine $α_s$ from the Bjorken sum rule. For the singlet sum rule -- $p+n$ -- the QCD sum rule gives only the order of magnitude of twist-4 correction. At low and intermediate $Q^2$ the model is presented which realizes a smooth connection of the Gerasimov-Drell-Hearn sum rules at $Q^2 = 0$ with the sum rules for $Γ_{p,n}(Q^2)$ at high $Q^2$. The model is in a good agreement with the experiment.

hep-ph↗
arXiv1997

The $Q^2$ Dependence of the Sum Rules for Structure Functions of Polarized $e(μ)N$ Scattering

The nonperturbative $Q^2$- dependence of the sum rules for the structure functions of polarized $e(μ)N$ scattering is discussed. The determination of twist-4 corrections to the structure functions at high $Q^2$ by QCD sum rules is reviewed and critically analyzed. It is found that in the case of the Bjorken sum rule the twist-4 correction is small at $Q^2 > 5 GeV^2$ and does not influence the value of $α_s$ determined from this sum rule. However, the accuracy of the today experimental data is insufficient to reliably determine $α_s$ from the Bjorken sum rule. For the singlet sum rule -- $p+n$ -- the QCD sum rule gives only the order of magnitude of twist-4 correction. At low and intermediate $Q^2$ the model is presented which realizes a smooth connection of the Gerasimov-Drell-Hearn sum rules at $Q^2 = 0$ with the sum rules for $Γ_{p,n}(Q^2)$ at high $Q^2$. The model is in a good agreement with the experiment.

hep-ph↗
arXiv1997

Reply Comment on "Meson Masses in Nuclear Matter"

We reply to the Comment by Hatsuda and Lee on our recent paper "Meson Masses in Nuclear Matter" (Phys.Rev.Lett. 78, 1010, (1997)). We argue that, contrary to the claim in the Comment, the short distance operator product expansion is not applicable for calculation of meson mass shifts in nuclear matter, since they are determined by long distance physics. We also remark that the suggestion made in the Comment that we overlooked in our paper the mean field description of the mass shifts of low-q mesons is perhaps due to a misunderstanding.

hep-ph↗
arXiv1997

Comment on "Remark on the external-field method in QCD sum rules"

It is proved, that suggested by Jin modified formalism in the external-field method in QCD sum rules exactly coincides with the formalism used before. Therefore, unlike the claims of ref.1, this formalism cannot improve the predictability and reliability of external-field sum rule calculations in comparison with those, done by the standard approach. PACS number(s): 12.38.Lg, 11.55.Hx

hep-ph↗