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A. V. Tarasov

Publications and source records attributed to A. V. Tarasov.

At least 37 records · Page 2Linked to original sources
arXiv2005

Challenge of lepton pair production in peripheral collisions of relativistic ions

The new approach to the lepton pair production in the Coulomb field of two highly relativistic nuclei was developed.Solving the operator equation for lepton scattering in arbitrary Coulomb field, we obtain the amplitudes for lepton scattering in the Coulomb potential in terms of light cone variables. Using the Watson expansion for the amplitude of lepton scattering on two centers we propose prescription which allows one to construct the amplitude for lepton pair production in the Coulomb field of two highly relativistic ions. We show that for the certain sums of finite terms of the Watson series numerous cancellations lead to infrared stability of amplitude.

hep-ph↗
arXiv2004

$e^+e^-$ pair production in relativistic ions collision and its correspondence to electron-ion scattering

It is shown that the amplitudes of electron-ion scattering and $e^+e^-$ pair production in the Coulomb field of two colliding ions are sxpressed in the terms of electron scattering amplitudes in the fields of the individual ions via the Watson expansion. We have obtained the compact expressions for these amplitudes valid in the high energy limit and discuss the crossing symmetry relations among the considered processes.

hep-ph↗
arXiv2004

The $e^+e^-$ pair production in relativistic ions collision and its correspondence to electron scattering

It is shown that the processes of electron(positron) scattering and $e^+e^-$ pair production in the Coulomb fields of two relativistic ions, can be expressed through the amplitudes of lepton scattering off the ion Coulomb field via Watson expansion. We obtained the compact expressions for these amplitudes valid in the high energy limit and discuss the crossing symmetry relations among the considered processes.

nucl-th↗
arXiv2002

Electroproduction of Charmonia off Protons and Nuclei

Elastic virtual photoproduction of charmonia on nucleons is calculated in a parameter free way with the light-cone dipole formalism and the same input: factorization in impact parameters, light-cone wave functions for the photons and the charmonia, and the universal phenomenological dipole cross section which is fitted to other data. The charmonium wave functions are calculated with four known realistic potentials, and two models for the dipole cross section are tested. Very good agreement with data for the cross section of charmonium electroproduction is found in a wide range of $s$ and $Q^2$. Using the ingredients from those calculations we calculate also exclusive electroproduction of charmonia off nuclei. Here new effects become important, (i) color filtering of the $c\bar c$ pair on its trajectory through nuclear matter, (ii) dependence on the finite lifetime of the $c\bar c$ fluctuation (coherence length) and (iii) gluon shadowing in a nucleus compared to the one in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as functions of $s$ are presented. The results can be tested with future electron-nucleus colliders or in the peripheral collisions of relativistic heavy ions.

hep-ph↗
arXiv2002

Nuclear effects in the Drell-Yan process at very high energies

We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and in nucleus-nucleus collisions within the light-cone color dipole formalism. This approach is especially suitable for predicting nuclear effects in the DY cross section for heavy ion collisions, as it provides the impact parameter dependence of nuclear shadowing and transverse momentum broadening, quantities that are not available from the standard parton model. For p(D)+A collisions we calculate nuclear shadowing and investigate nuclear modification of the DY transverse momentum distribution at RHIC and LHC for kinematics corresponding to coherence length much longer than the nuclear size. Calculations are performed separately for transversely and longitudinally polarized DY photons, and predictions are presented for the dilepton angular distribution. Furthermore, we calculate nuclear broadening of the mean transverse momentum squared of DY dileptons as function of the nuclear mass number and energy. We also predict nuclear effects for the cross section of the DY process in heavy ion collisions. We found a substantial nuclear shadowing for valence quarks, stronger than for the sea.

hep-ph↗
arXiv2002

Gluon Shadowing and Heavy Flavor Production off Nuclei

Gluon shadowing which is the main source of nuclear effects for production of heavy flavored hadrons, remains unknown. We develop a light-cone dipole approach aiming at simplifying the calculations of nuclear shadowing for heavy flavor production, as well as the cross section which does not need next-to-leading and higher order corrections. A substantial process dependence of gluon shadowing is found at the scale of charm mass manifesting a deviation from QCD factorization. The magnitude of the shadowing effect correlates with the symmetry properties and color state of the produced c-cbar pair. It is about twice as large as in DIS, but smaller than for charmonium production. The higher twist shadowing correction related to a nonzero size of the c-cbar pair is not negligible and steeply rises with energy. We predict an appreciable suppression by shadowing for charm production in heavy ion collisions at RHIC and a stronger effect at LHC. At the same time, we expect no visible difference between nuclear effects for minimal bias and central collisions, as is suggested by recent data from the PHENIX experiment at RHIC. We also demonstrate that at medium high energies when no shadowing is possible, final state interaction may cause a rather strong absorption of heavy flavored hadrons produced at large x_F.

hep-ph↗
arXiv2002

Cronin Effect in Hadron Production off Nuclei

Recent data from RHIC for high-$p_T$ hadrons in gold-gold collisions raised again the long standing problem of quantitatively understanding the Cronin effect, i.e. nuclear enhancement of high-$p_T$ hadrons due to multiple interactions in nuclear matter. In nucleus-nucleus collisions this effect has to be reliably calculated as baseline for a signal of new physics in high-$p_T$ hadron production. The only possibility to test models is to compare with available data for $pA$ collisions, however, all existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows to explain available data without fitting to them and to provide predictions for $pA$ collisions at RHIC and LHC. We point out that the mechanism causing Cronin effect drastically changes between the energies of fixed target experiments and RHIC-LHC. High-$p_T$ hadrons are produced incoherently on different nucleons at low energies, whereas the production amplitudes interfere if the energy is sufficiently high.

hep-ph↗
arXiv2002

Electroproduction of Charmonia off Nuclei

In a recent publication we have calculated elastic charmonium production in ep collisions employing realistic charmonia wave functions and dipole cross sections and have found good agreement with the data in a wide range of s and Q^2. Using the ingredients from those calculations we calculate exclusive electroproduction of charmonia off nuclei. Here new effects become important, (i) color filtering of the c-cbar pair on its trajectory through nuclear matter, (ii) dependence on the finite lifetime of the c-cbar fluctuation (coherence length) and (iii) gluon shadowing in a nucleus compared to the one in a nucleon. Total coherent and incoherent cross sections for C, Cu and Pb as functions of s and Q^2 are presented together with some differential cross sections. The results can be tested with future electron-nucleus colliders or in peripheral collisions of ultrarelativistic heavy ions.

hep-ph↗
arXiv2001

Color Transparency versus Quantum Coherence in Electroproduction of Vector Mesons off Nuclei

So far no theoretical tool for the comprehensive description of exclusive electroproduction of vector mesons off nuclei at medium energies has been developed. We suggest a light-cone QCD formalism which is valid at any energy and incorporates formation effects (color transparency), the coherence length and the gluon shadowing. At medium energies color transparency (CT) and the onset of coherence length (CL) effects are not easily separated. Indeed, although nuclear transparency measured by the HERMES experiment rises with Q^2, it agrees with predictions of the vector dominance model (VDM) without any CT effects. Our new results and observations are: (i) the good agreement with the VDM found earlier is accidental and related to the specific correlation between Q^2 and CL for HERMES kinematics; (ii) CT effects are much larger than have been estimated earlier within the two channel approximation. They are even stronger at low than at high energies and can be easily identified by HERMES or at JLab; (iii) gluon shadowing which is important at high energies is calculated and included; (iv) our parameter-free calculations explain well available data for variation of nuclear transparency with virtuality and energy of the photon; (v) predictions for electroproduction of ρand ϕare provided for future measurements at HERMES and JLab.

hep-ph↗
arXiv2001

Long-Range Coulomb Forces in DIS: Missed Radiative Corrections?

The Born approximation, one photon exchange, used for DIS is subject to virtual radiative corrections which are related to the long-range Coulomb forces. They may be sizeable for heavy nuclei since Zαis not a small parameter. So far these corrections are known only for two processes, elastic scattering and bremsstrahlung on the Coulomb field of a point-like target. While the former amplitude acquires only a phase, in the latter case the cross section is modified also. Although the problem of Coulomb corrections for DIS on nuclei is extremely difficult, it should be challenged rather than 'swept under the carpet'. The importance of these radiative corrections is questioned in present paper. We show that in the simplest case of a constant hadronic current the Coulomb corrections provide a phase to the Born amplitude, therefore the cross section remains the same. Inclusion of more realistic hadronic dynamics changes this conclusion. The example of coherent production of vector mesons off nuclei reveals large effects. So far a little progress has been made deriving exact lepton wave functions in the Coulomb field of an extended target. Employing available results based on the first-order approximation in Zα, we conclude that the Coulomb corrections are still important for heavy nuclei. We also consider an alternative approach for extended nuclear targets, the eikonal approximation, which we demonstrate to reproduce the known exact results for Coulomb corrections. Calculating electroproduction of vector mesons we again arrive at a large deviation from the Born approximation. We conclude that one should accept with caution the experimental results for nuclear effects in DIS based on analyses done in the Born approximation.

hep-ph↗
arXiv2001

Coherence Phenomena in Charmonium Production off Nuclei at the Energies of RHIC and LHC

In the energy range of RHIC and LHC the mechanisms of nuclear suppression of charmonia are expected to be strikingly different from what is known for the energy of the SPS. One cannot think any more of charmonium produced on a bound nucleon which then attenuates as it passes through the rest of the nucleus. The coherence length of charmonium production substantially exceeds the nuclear radius in the new energy range. Therefore the production amplitudes on different nucleons, rather than the cross sections, add up and interfere, i.e. shadowing is at work. So far no theoretical tool has been available to calculate nuclear effects for charmonium production in this energy regime. We develop a light-cone Green function formalism which incorporates the effects of the coherence of the production amplitudes and of charmonium wave function formation, and is the central result of this paper. We found a substantial deviation from QCD factorization, namely, gluon shadowing is much stronger for charmonium production than it is in DIS. We predict for nuclear effects $x_2$ scaling which is violated at lower energies by initial state energy loss which must be also included in order to compare with available data. In this paper only the indirect J/Psi originating from decay of P-wave charmonia are considered. The calculated x_F-dependence of J/Psi nuclear suppression is in a good accord with data. We predict a dramatic variation of nuclear suppression with x_F in pA and a peculiar peak at x_F=0 in AA collisions at RHIC.

hep-ph↗
arXiv2001

The color dipole approach to the Drell-Yan process in pA collisions

In the target rest frame and at high energies, Drell-Yan (DY) dilepton production looks like bremsstrahlung of massive photons, rather than parton annihilation. The projectile quark is decomposed into a series of Fock states. Configurations with fixed transverse separations are interaction eigenstates for pp scattering. The DY cross section can then be expressed in terms of the same color dipole cross section as DIS. This approach is especially suitable to describe nuclear effects, since it allows to apply Glauber multiple scattering theory. We go beyond the Glauber eikonal approximation by taking into account transitions between interaction eigenstates. We calculate nuclear shadowing at large Feynman-x_F for DY in proton-nucleus collisions, compare to existing data from E772 and make predictions for RHIC. Nuclear effects on the transverse momentum distribution are also investigated.

hep-ph↗
arXiv2001

The color dipole picture of the Drell-Yan process

At high energies, Drell-Yan (DY) dilepton production viewed in the target rest frame should be interpreted as bremsstrahlung and can be expressed in terms of the same color dipole cross section as DIS. We compute DY cross sections on a nucleon target with the realistic parameterization for the dipole cross section saturated at large separations. The results are compared to experimental data and predictions for RHIC are presented. The transverse momentum distribution of the DY process is calculated and energy growth is expected to be steeper at large than at small transverse momenta. We also calculate the DY angular distribution and investigate deviations from the 1+cos^2(θ) shape.

hep-ph↗
arXiv2000

The Coulomb Phase Revisited

Motivated by the forthcoming data from the E950 experiment at BNL for small angle polarized proton-carbon scattering we revisit the problem of Coulomb phase. The approximation usually used with the momentum transfer squared (q^2) small compared to the inverse elastic slope is not justified within the kinematics of the E950 experiment. We go beyond this approximation and derive a new rather simple expression which recovers the formula of Bethe at small q^2, but is valid at any momentum transfer.

hep-ph↗
arXiv2000

Broadening of Transverse Momentum of Partons Propagating through a Medium

Broadening of the transverse momentum of a parton propagating through a medium is treated using the color dipole formalism, which has the advantage of being a well developed phenomenology in deep-inelastic scattering and soft processes. Within this approach, nuclear broadening should be treated as color filtering, i.e. absorption of large-size dipoles leading to diminishing (enlarged) transverse separation (momentum). We also present a more intuitive derivation based on the classic scattering theory of Molière. This derivation helps to understand the origin of the dipole cross section, part of which comes from attenuation of the quark, while another part is due to multiple interactions of the quark. It also demonstrates that the lowest-order rescattering term provides an A-dependence very different from the generally accepted A^{1/3} behavior. The effect of broadening increases with energy, and we evaluate it using different phenomenological models for the unintegrated gluon density. Although the process is dominated by soft interactions, the phenomenology we use is tested using hadronic cross section data.

hep-ph↗
arXiv2000

Photoproduction of Charmonia and Total Charmonium-Proton Cross Sections

Elastic virtual photoproduction cross sections gamma^*p -> J/psi(psi')p and total charmonium-nucleon cross sections for J/psi, psi' and chi states are calculated in a parameter free way with the light-cone dipole formalism and the same input: factorization in impact parameters, light-cone wave functions for the gamma^* and the charmonia, and the universal phenomenological dipole cross section which is fitted to other data. The charmonium wave functions are calculated with four known realistic potentials, and two models for the dipole cross section are tested. Very good agreement with data for the cross section of charmonium photoproduction is found in a wide range of s and Q^2. The inclusion of the Melosh spin rotation increases the psi' photoproduction rate by a factor 2-3 and removes previously observed discrepancies in the psi' to J/psi ratio in photoproduction. We also calculate the charmonium-proton cross sections whose absolute values and energy dependences are found to correlate strongly with the sizes of the states.

hep-ph↗
arXiv2000

Nuclear Shadowing and Coherence Length for Longitudinal and Transverse Photons

Motivated by the recent results for DIS off nuclei from the HERMES experiment we have performed a systematic study of shadowing for transverse and longitudinal photons. We found that the coherence length which controls the onset of nuclear shadowing at small x_{Bj} is much longer for longitudinal than transverse photons, and is much shorter for shadowing of gluons. The light-cone Green function approach we apply properly treats shadowing in the transition region x_{Bj}>0.01. It also incorporates the nonperturbative effects and is legitimate at small Q^2. We calculate nuclear shadowing and compare with data from the HERMES and NMC experiments. Although we expect different nuclear shadowing for longitudinal and transverse photons, numerically it cannot explain the strong effect observed by the HERMES collaboration.

hep-ph↗