arXiv subjects

B. Z. Kopeliovich

Publications and source records attributed to B. Z. Kopeliovich.

At least 19 recordsLinked to original sources

arXiv2025

Dipole-pion cross section in the saturation regime

The scale-dependent dipole-pion cross section is analyzed as a function of the dipole size $r$ and the impact parameter $b$. This analysis relies on the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equation in $μ{\sim} 1/r +μ_{0}$ at the next-to-leading order (NLO) approximation, with a specific initial condition at $μ_{0}$. The dipole-pion cross section at small Bjorken variable $β$ is being considered over a wide range of transverse separations $r$. Using the Laplace transformation technique, we describe the determination of the dipole-pion cross section based on the gluon distribution at the initial scale $μ_{0}$ within a kinematic region characterized by low values of the Bjorken variable $β$. We found that geometric scaling for the dipole-pion cross section holds approximately within a wide kinematic region of $rQ_{s}$. The cross section saturates at large dipole sizes.

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arXiv2024

Flavor-independent yield of high-$p_T$ hadrons from nuclear collisions

Data on high-$p_T$ hadron production in heavy ion collisions at Feynman $x_F=0$ indicate at universality of the observed nuclear suppression. Our analysis of the production mechanisms demonstrates important role of the color transparency effects which make the survival probability of a quark-antiquark dipole independent of the quark flavor, provided that the hadron wave function is formed outside the medium. The latter condition imposes restrictions on the range of $p_T$, which should be sufficiently high to make the nuclear suppression universal. We also found that the in-medium broadening rate $\hat q$ (frequently called transport coefficient) significantly depends on the quark flavor, diminishing for heavy quarks.

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arXiv2024

Coulomb-Nuclear Interference in polarized pA scattering

We made the first attempt to understand the observed unusual t dependence of single-spin asymmetry observed in the HJET experiment at RHIC. Usually, the interaction of hadrons is presented as a long-range Coulomb interaction and a short-range strong interaction with Coulomb corrections. Such a division gives rise to a Coulomb phase of the hadronic term. Conversely, here we consider short-range hadronic interaction as a correction to the long-range electromagnetic term, i.e., we treat it as an absorptive correction. This significantly affects the Coulomb-nuclear interference, which is a source of single-spin azimuthal asymmetry at small angles.

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arXiv2023

Single-Spin Asymmetry of Neutrons in Polarized pA Collisions

Absorptive corrections, which are known to suppress proton-neutron transitions with a large fractional momentum z -> 1 in pp collisions, become dramatically strong on a nuclear target, and they push the partial cross sections of leading neutron production to the very periphery of the nucleus. The mechanism of the pion and axial vector a1-meson interference, which successfully explains the observed single-spin asymmetry in a polarized pp -> nX, is extended to the collisions of polarized protons with nuclei. When corrected for nuclear effects, it explains the observed single-spin azimuthal asymmetry of neutrons that is produced in inelastic events, which is where the nucleus violently breaks up. This single-spin asymmetry is found to be negative and nearly A-independent.

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arXiv2023

Unconventional mechanisms of heavy quark fragmentation

Heavy and light quarks produced in high-$p_T$ partonic collisions radiate differently. Heavy quarks regenerate their color field, stripped-off in the hard reaction, much faster than the light ones and radiate a significantly smaller fraction of the initial quark energy. This peculiar feature of heavy-quark jets leads to a specific shape of the fragmentation functions observed in $e^+e^-$ annihilation. Differently from light flavors, the heavy quark fragmentation function strongly peaks at large fractional momentum $z$, i.e. the produced heavy-light mesons, $B$ or $D$, carry the main fraction of the jet momentum. This is a clear evidence of the dead-cone effect, and of a short production time of a heavy-light mesons. Contrary to propagation of a small $q\bar q$ dipole, which survives in the medium due to color transparency, a heavy-light $Q\bar q$ dipole promptly expands to a large size. Such a big dipole has no chance to remain intact in a dense medium produced in relativistic heavy ion collisions. On the other hand, a breakup of such a dipole does not affect much the production rate of $Q\bar q$ mesons, differently from the case of light $q\bar q$ meson production.

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arXiv2023

Ultra-peripheral nuclear collisions as a source of heavy quarkonia

Heavy quarkonium production in ultra-peripheral nuclear collisions (UPC) is described within the QCD dipole formalism. Realistic quarkonium wave functions in the $Q\bar Q$ rest frame are calculated by solving the Schrödinger equation with a subsequent Lorentz boost to high energy. We rely on several realistic $Q\bar Q$ potentials, which allow to describe well the quarkonium masses and decay widths, as well as data on diffractive electroproduction of quarkonia on protons. Nuclear effects are calculated with the phenomenological dipole cross sections fitted to DIS data. The higher twist quark shadowing related to the lowest $Q\bar Q$ Fock component of the photon, as well as the leading twist gluon shadowing, related to higher components containing gluons, are included. The results for coherent and incoherent photoproduction of charmonia and bottomonia in UPC of heavy nuclei are in good accord with available data from the LHC. They can also be verified in future experiments at electron-ion colliders.

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arXiv2022

Modeling photon radiation in soft hadronic collisions

Soft hadronic collisions with multiple production of (anti)quarks accompanied with soft photon radiation are described in terms of higher Fock states of the colliding hadrons, which contain a photon component as well. The Fock state distribution functions are shaped with the Quark-Gluon String Model. Photon radiation by quarks is described within the color-dipole phenomenology. The results of calculations are in a good accord with available data in a wide range of transverse momenta of the photons.

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arXiv2022

The Low theorem for diffractive bremsstrahlung and the soft photon puzzle

The anomalous excess of small-kT photons radiated along with multi-hadron production, is challenging the physics community over four decades, but no solution has been proposed so far. We argue that the problem is rooted in the comparison with an incorrect model, usually called bremsstrahlung model. It is believed to be an extension of the Low theorem from the 2 -> 2+gamma process to radiative multi-particle production 2 -> n+gamma, where either initial, or final charged hadrons participate in radiation. We demonstrate that this breaks down unitarity of the S-matrix, so contradicts the optical theorem.

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arXiv2022

Nuclear effects in coherent photoproduction of heavy quarkonia

Coherent photoproduction of heavy quarkonia on nuclear targets is studied within the QCD color dipole formalism including several main phenomena: i) The correlation between impact parameter of a collision $\vec b$ and dipole orientation $\vec r$; ii) The higher-twist nuclear shadowing related to the $\bar QQ$ Fock state of the photon; iii) The leading-twist gluon shadowing corresponding to higher Fock components of the photon containing gluons; iv) Reduced effects of quantum coherence in a popular Balitsky-Kovchegov equation compared to calculations, which are frequently presented in the literature. Our calculations of differential cross sections are in good agreement with recent ALICE data on charmonium production in ultra-peripheral nuclear collisions. We present also predictions for coherent photoproduction of other quarkonium states ($ψ^{\,\prime}$(2S), $Υ$(1S) and $Υ^{\,\prime}$(2S)) that can be verified by future measurements at the LHC.

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arXiv2022

Coherent photoproduction of heavy quarkonia on nuclei

The differential cross section of coherent photo-production of heavy quarkonia on nuclear targets is calculated within the QCD color dipole formalism. The higher-twist nuclear shadowing corresponding to the $\bar QQ$ Fock component of the photon, is calculated including the correlation between dipole orientation $\vec r$ and impact parameter of a collision $\vec b$, which is related to the transverse momentum transfer via Fourier transform. We also included the leading twist gluon shadowing corresponding to higher Fock components of the photon containing gluons, which have specifically short coherence time, especially for multi-gluon components, even at very high energies. The contribution of such fluctuating gluonic dipole is calculated employing the path-integral technique. Our results are in good agreement with recent ALICE data on charmonium production in ultra-peripheral nuclear collisions.

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arXiv2021

Probing the Pomeron spin structure with Coulomb-nuclear interference

Polarized pp elastic scattering at small angles in the Coulomb-nuclear interference (CNI) region offers a unique opportunity to study the spin structure of the Pomeron. Electromagnetic effects in elastic amplitude can be equivalently treated either as Coulomb corrections to the hadronic amplitude (Coulomb phase), or as absorption corrections to the Coulomb scattering amplitude. We perform the first calculation of the Coulomb phase for the spin-flip amplitude and found it significantly exceeding the widely used non-flip Coulomb phase. The alternative description in terms of absorption corrections, though equivalent, turned out to be a more adequate approach for the Coulomb corrected spin-flip amplitude. Inspired by the recent high statistics measurements of single-spin asymmetry in the fixed-target HJET experiment at the BNL, we also performed a Regge analysis of data, aiming at disentangling the Pomeron contribution. However, in spite of an exceptional accuracy of the data, they do not allow to single out the Pomeron term, which strongly correlates with the major sub-leading Reggeons. A stable solution can be accessed only by making additional ad hoc assumptions, e.g. assuming the Pomeron to be a simple Regge pole, or fixing some unknown parameters. Otherwise, in addition to the STAR data at $\sqrt{s}=$200 GeV new measurements, say at 100 GeV or 500 GeV, could become decisive.

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arXiv2021

Electroproduction of heavy quarkonia: significance of dipole orientation

The differential cross section $dσ/dq^2$ of diffractive electroproduction of heavy quarkonia on protons is a sensitive study tool for the interaction dynamics within the dipole representation. Knowledge of the transverse momentum transfer $\vec q$ provides a unique opportunity to identify the reaction plane, due to a strong correlation between the directions of $\vec q$ and impact parameter $\vec b$. On top of that, the elastic dipole-proton amplitude is subject to a strong correlation between $\vec b$ and dipole orientation $\vec r$. Most of models for $b$-dependent dipole cross section either completely miss this information, or make unjustified assumptions. We perform calculations basing on a realistic model for $\vec r$-$\vec b$ correlation, which significantly affect the $q$-dependence of the cross section, in particular the ratio of $ψ^{\,\prime}(2S)$ to $J/ψ$ yields. We rely on realistic potential models for the heavy quarkonium wave function, and the Lorentz-boosted Schrödinger equation. Good agreement with data on $q$-dependent diffractive electroproduction of heavy quarkonia is achieved.

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arXiv2021

Probing the Pomeron spin-flip with Coulomb-nuclear interference

Brand-new high-precision data for single-spin asymmetry $A_N(t)$ in small angle elastic $pp$ scattering from the fixed target experiment HJET at BNL at $E_{lab}=100$ and $255 \mbox{ GeV}$, as well as high energy STAR measurements at $\sqrt{s}=200 \mbox{ GeV}$, for the first time allowed to determine the spin-flip to non-flip ratio $r_5(t)$ in a wide energy range. We introduced an essential modification in the Coulomb-nuclear interference (CNI) mechanism, missed in previous analyses. It can be formulated either as a modification of the Coulomb phase, which is much larger for the spin-flip compared with non-flip amplitudes, or as absorptive corrections to the electromagnetic interaction of hadrons. The Regge analysis singles out the Pomeron contribution to the spin-flip amplitude, which steeply rises with energy. We found the spin-flip to non-flip ratio of the Pomeron amplitudes to be nearly $-10\%$, steeply rising with energy in accordance with theoretical expectations.

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arXiv2020

Spacetime development of in-medium hadronization: Scenario for leading hadrons

We present a perturbative QCD based model for vacuum and in-medium hadronization. The effects of induced energy loss and nuclear absorption have been included. The main objective is the determination of the relative contribution of these mechanisms to the multiplicity ratio observable, measured in semi-inclusive deep-inelastic scattering off deuterium and nuclear targets. This is directly related to the determination of the production length, $Lp$, necessary for a quark to turn into a prehadron. We compare our results with HERMES data for multiplicity ratio and $p_t$-broadening, and show that the description of the whole data set, keeping the model parameters fixed, puts strong constrains on $Lp$. Contrary to induced-energy-loss based models, we find an important contribution from nuclear absorption at HERMES energies. Finally, we discuss some consequences of our study for the LHC physics, and we present the model predictions for the future EIC experiment.

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arXiv2020

Strangeness Enhancement due to String Fluctuations

We study string fragmentation in high multiplicity proton-proton collisions in a model where the string tension fluctuates. These fluctuations produce exponential pion spectra which are fitted to the transverse momentum distributions of charged particles for different multiplicities. For each multiplicity the so obtained hadronic slope parameter defines the magnitude of the string fluctuations which in turn determines the produced ratio of strange to light quarks. PYTHIA string decay simulations are used to convert each ratio of strange to light quarks to the appropriate ratio of strange hadrons to pions.

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arXiv2019

Heavy quarkonium in saturated environment of high-multiplicity pp collisions

High-multiplicity pp collisions exhibit features, traditionally associated with nuclear effects. Coherence motivates to treat high-multiplicity pp, pA and AA collisions on an equal footing. We rely on the phenomenological parametrization for mean multiplicities of light hadrons and J/psi, assuming their linear dependence on N_{coll} in pA collisions. The results of this approach underestimate the recently measured production rate of J/psi at very high hadronic multiplicities. The linear dependence of J/psi multiplicity on N_{coll} is subject to predicted nonlinear corrections, related to mutual boosting of the saturation scales in colliding dense parton clouds. A parameter-free calculation of the non-linear corrections allows to explain data for pT-integrated yield of J/psi at high hadronic multiplicities. Calculations are in a good accord with data binned in several pT-intervals as well. As was predicted, Upsilon and J/psi are equally suppressed at forward rapidities in pA collisions. Consequently, their fractional multiplicities at forward rapidities in pp collisions are equal as well, and their magnitude agrees with data.

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arXiv2019

Survival of heavy flavored mesons in a hot medium

Hadronization of heavy quarks reveals various unusual features. Gluon radiation by a heavy quark originated from a hard process, ceases shortly on a distance of the order of few fm. Due to the dead-cone effect a heavy quark radiates only a small fraction of its energy. This is why the measured fragmentation function D(z) peaks at large z. Hadronization finishes at very short distances, well shorter than 1 fm, by production of a colorless small-size Qq-bar dipole. This ensures dominance of a perturbative mechanism and makes possible factorization of short and long distances. The latter corresponds to final state interactions of the produced dipole propagating through a dense medium. The results provide good description of data on beauty and charm suppression in heavy ion collisions, fixing the transport coefficient for b-quarks about twice smaller than for charm, and both significantly lower that the values determined from data on suppression of high-pT light hadrons. We relate this to reduction of the QCD coupling at higher scales, and suppression of radiation by the dead-cone effect.

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arXiv2019

Distinctive features of hadronizing heavy quarks

The color field of a quark, stripped off in a hard reaction, is regenerated via gluon radiation. The space-time development of a jet is controlled by the coherence time of gluon radiation, which for heavy quarks is subject to the dead-cone effect, suppressing gluons with small transverse momenta. As a result, heavy quarks can radiate only a small fraction of the initial energy. This explains the peculiar shape of the measured heavy quark fragmentation function, which strongly peaks at large fractional momenta z. The fragmentation length distribution, related to the fragmentation function in a model independent way, turns out to be concentrated at distances much shorter than the confinement radius. This implies that the mechanisms of heavy quark fragmentation is pure perturbative.

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