arXiv subjects

J. Nemchik

Publications and source records attributed to J. Nemchik.

At least 19 recordsLinked to original sources

arXiv2025

Searching for gluon saturation effects in the momentum transfer dependence of coherent charmonium electroproduction off nuclei

We study for the first time the transverse momentum transfer distributions $d\sigma/dt$ in coherent production of charmonia in nuclear ultra-peripheral and electron-ion collisions within the QCD color dipole approach based on a rigorous Green function formalism. This allows us to treat properly the color transparency effects, as well as the higher and leading-twist shadowing corrections associated with the $|Q\bar Q\rangle$ and $|Q\bar QnG\rangle$ Fock components of the photon. While the multi-gluon photon fluctuations represent the dominant source of nuclear shadowing at kinematic regions related to the recent LHC and its future upgrade to LHeC, the upcoming electron-ion collider at RHIC will additionally require the proper incorporation of reduced quark shadowing. The latter effect leads to a significant decrease in the differential cross sections $d\sigma/dt$ compared to standard calculations based on the eikonal form for the dipole-nucleus amplitude. The leading-twist shadowing corrections, corresponding to a non-linear QCD evolution of a partial dipole-nucleus amplitude, reduce substantially charmonium $t$-distributions in the LHeC energy range. We predict a non-monotonic energy dependence of $d\sigma/dt$ suggesting so possible gluon saturation effects with increased onset at larger $t$-values. In addition to shadowing corrections, we study how the color transparency effects affect the shape of $t$-dependent nuclear modification factor. We also briefly discuss several aspects that can modify the charmonium production rate and thus may have a large impact on the search for gluon saturation effects.

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arXiv2024

Path integral treatment of coherence effects in charmonium production in nuclear ultra-peripheral collisions

We present for the first time a revised study of charmonium production in nuclear ultra-peripheral collisions (UPC) based on a rigorous Green function formalism. This formalism allows for the proper incorporation of the effects of color transparency, as well as the quantum coherence inherent in the higher twist quark shadowing related to the $Q\bar Q$ Fock component of the photon. The significance of this effect gradually decreases towards forward and/or backward rapidities. In the LHC kinematic region we additionally incorporate within the same formalism the leading twist gluon shadowing corrections related to higher multi-gluon photon fluctuations. They represent a dominant source of nuclear phenomena in the mid-rapidity region. Model predictions for the rapidity distributions $d\sigma/dy$ are in good agreement with available UPC data on coherent charmonium production at RHIC and the LHC. They can also be verified by future measurements at the LHC, as well as at EIC.

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arXiv2024

Coherent photo- and electroproduction of charmonia on nuclear targets revisited: Green function formalism

We study for the first time the production of charmonia in nuclear ultra-peripheral and electron-ion collisions based on a rigorous Green function formalism. Such formalism allows to incorporate properly formation effects (color transparency), as well as the quantum coherence inherent in higher twist shadowing corrections related to the $|Q \bar{Q}\rangle$ Fock component of the photon. The leading twist gluon shadowing associated with multi-gluon photon fluctuations is also included within the same formalism. The later effect represents the dominant source of shadowing at mid rapidities in the LHC kinematic region, while the reduced effect of quark shadowing leads to a significant modification of differential cross sections $d\sigma/dy$ at forward and/or backwards rapidities. Model calculations for $d\sigma/dy$ are in a good agreement with available UPC data on coherent charmonium production at RHIC and the LHC. In addition, we also perform predictions for nuclear effects in the electroproduction of charmonia, which can be verified by new data from electron-ion colliders.

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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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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

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

Momentum transfer dependence of heavy quarkonium electroproduction

We investigate the momentum transfer dependence of differential cross sections $dσ/dt$ in diffractive electroproduction of heavy quarkonia on proton targets. Model predictions for $dσ/dt$ within the light-front QCD dipole formalism are based on a realistic model for a proper correlation between the impact parameter $\vec b$ of a collision and color dipole orientation $\vec r$. We demonstrate a significance of $\vec b-\vec r$ correlation by comparing with a standard simplification $\vec{b}\parallel\vec{r}$, frequently used in the literature.

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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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arXiv2017

Novel scenario for production of heavy flavored mesons in heavy ion collisions

The observed strong suppression of heavy flavored hadrons produced with high $p_T$, is caused by final state interactions with the created dense medium. Vacuum radiation of high-pT heavy quarks ceases at a short time scale, as is confirmed by pQCD calculations and by LEP measurements of the fragmentation functions of heavy quarks. Production of a heavy flavored hadrons in a dense medium is considerably delayed due to prompt breakup of the hadrons by the medium. This causes a strong suppression of the heavy quark yield because of the specific shape of the fragmentation function. The parameter-free description is in a good accord with available data.

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arXiv2014

Quenching of high-pT hadrons: a non-energy-loss scenario

A parton produced with a high transverse momentum in a hard collision is regenerating its color field, intensively radiating gluons and losing energy. This process cannot last long, if it ends up with production of a leading hadron carrying the main fraction z_h of the initial parton momentum. So energy conservation imposes severe constraints on the length scale of production of a single hadron with high pT. As a result, the main reason for hadron quenching observed in heavy ion collisions, is not energy loss, but attenuation of the produced colorless dipole in the created dense medium. The latter mechanism, calculated with the path-integral method, explains well the observed suppression of light hadrons and the elliptic flow in a wide range of energies, from the lowest energy of RHIC up to LHC, and in a wide range of transverse momenta. The values of the transport coefficient extracted from data range within 1-2 GeV^2/fm, dependent on energy, and agree well with the theoretical expectations.

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arXiv2013

High-pT hadrons from nuclear collisions: Unifying pQCD with hydrodynamics

Hadrons inclusively produced with large pT in high-energy collisions originate from the jets, whose initial virtuality and energy are of the same order, what leads to an extremely intensive gluon radiation and dissipation of energy at the early stage of hadronization. Besides, these jets have a peculiar structure: the main fraction of the jet energy is carried by a single leading hadron, so such jets are very rare. The constraints imposed by energy conservation enforce an early color neutralization and a cease of gluon radiation. The produced colorless dipole does not dissipate energy anymore and is evolving to form the hadron wave function. The small and medium pT region is dominated by the hydrodynamic mechanisms of hadron production from the created hot medium. The abrupt transition between the hydrodynamic and perturbative QCD mechanisms causes distinct minima in the pT dependence of the suppression factor R_{AA} and of the azimuthal asymmetry v2. Combination of these mechanisms allows to describe the data through the full range of pT at different collision energies and centralities.

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arXiv2013

Predictions for $p+$Pb Collisions at sqrt s_NN = 5 TeV

Predictions for charged hadron, identified light hadron, quarkonium, photon, jet and gauge bosons in p+Pb collisions at sqrt s_NN = 5 TeV are compiled and compared. When test run data are available, they are compared to the model predictions.

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arXiv2012

Quenching of high-pT hadrons: Energy Loss vs Color Transparency

High-pT hadrons produced in hard collisions and detected inclusively bear peculiar features: (i) they originate from jets whose initial virtuality and energy are of the same order; (ii) such jets are rare and have a very biased energy sharing among the particles, namely, the detected hadron carries the main fraction of the jet energy. The former feature leads to an extremely intensive gluon radiation and energy dissipation at the early stage of hadronization, either in vacuum or in a medium. As a result, a leading hadron must be produced on a short length scale. Evaluation within a model of perturbative fragmentation confirms the shortness of the production length. This result is at variance with the unjustified assumption of long production length, made within the popular energy loss scenario. Thus we conclude that the main reason of suppression of high-pT hadrons in heavy ion collisions is the controlled by color transparency attenuation of a high-pT dipole propagating through the hot medium. Adjusting a single parameter, the transport coefficient, we describe quite well the data from LHC and RHIC for the suppression factor R_{AA} as function of pT, collision energy and centrality. We observe that the complementary effect of initial state interaction causes a flattening and even fall of R_{AA} at large pT. The azimuthal anisotropy of hadron production, calculated with no further adjustment, also agrees well with data at different energies and centralities.

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arXiv2011

Direct Photon Production in Proton-Nucleus and Nucleus-Nucleus Collisions

Prompt photons produced in a hard reaction are not accompanied with any final state interaction, either energy loss or absorption. Therefore, besides the Cronin enhancement at medium transverse momenta pT and small isotopic corrections at larger pT, one should not expect any nuclear effects. However, data from PHENIX experiment exhibit a significant large-pT suppression in central d+Au and Au+Au collisions that cannot be accompanied by coherent phenomena. We demonstrate that such an unexpected result is subject to the energy sharing problem near the kinematic limit and is universally induced by multiple initial state interactions. We describe production of photons in the color dipole approach and find a good agreement with available data in p+p collisions. Besides explanation of large-pT nuclear suppression at RHIC we present for the first time predictions for expected nuclear effects also in the LHC energy range at different rapidities. We include and analyze also a contribution of gluon shadowing as a leading twist shadowing correction modifying nuclear effects at small and medium pT.

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arXiv2010

Nuclear suppression of dileptons at forward rapidities

Data from E772 and E866 experiments on the Drell-Yan process exhibit a significant nuclear suppression at large Feynman xF. We show that a corresponding kinematic region does not allow to interpret this as a manifestation of coherence or a Color Glass Condensate. We demonstrate, however, that this suppression can be treated alternatively as an effective energy loss proportional to initial energy. To eliminate suppression coming from the coherence, we perform predictions for nuclear effects also at large dilepton masses. Our calculations are in a good agreement with available data. Since the kinematic limit can be also approached in transverse momenta pT, we present in the RHIC energy range corresponding predictions for expected large-pT suppression as well. Since a new experiment E906 planned at FNAL will provide us with more precise data soon, we present also predictions for expected large-xF nuclear suppression in this kinematic region.

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arXiv2010

High-pT paradigms revisited

We present an attempt at a critical overview of the current status of modeling for high-pT processes in nuclei. The paper covers several topics including coherence phenomena, in particular gluon shadowing and CGC; nuclear effects related to the restrictions imposed by energy conservation at large xL and xT ; space-time development of hadronization of highly virtual light and heavy partons and the related time scales; and the role of early production and subsequent attenuation of pre-hadrons in a dense medium. We identify several intriguing problems in the current paradigms for high-pT processes and propose solutions for some of them.

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