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S. P. Baranov

Publications and source records attributed to S. P. Baranov.

At least 19 recordsLinked to original sources
arXiv2025

Tree-level NLO corrections to inclusive $ψ'$ production in High Energy Factorization

We consider inclusive $ψ(2S)$ production in proton-proton collisions at collider energies in the framework of non-relativistic QCD and high-energy factorization beyond the low-order approximation. We utilise a matching scheme proposed earlier to merge the leading order and tree-level next-to-leading order production amplitudes and now extend it to low transverse momenta and/or forward rapidities. With the improved scheme, we examine the possibility to simultaneously describe all unpolarized LHC data in the full kinematic range accessible to experimental measurements and try different Transverse Momentum Dependent (TMD) gluon distributions in the proton. A global fit to the data is carried out to extract the color octet long-distance matrix elements for $ψ(2S)$ mesons. We show that taking the NLO corrections into account leads to better description of the data.

hep-ph↗
arXiv2024

Forward $J/ψ+ J/ψ$ and $J/ψ+ ψ^\prime$ production with High Energy Factorization

We calculate the cross sections of associated $J/ψ+ ψ^\prime$ and $J/ψ+ J/ψ$ production in $pp$ collisions at $\sqrt s = 13$ TeV in the forward kinematic region. The High Energy Factorization ($k_T$-factorization) framework supplemented with the Catani-Ciafaloni-Fiorani-Marchesini evolution of gluon densities in a proton is applied. We demonstrate that latest data on $J/ψ+ J/ψ$ production and first experimental data on $J/ψ+ ψ^\prime$ events taken very recently by the LHCb Collaboration can be described well by the color singlet terms and contributions from the double parton scattering (DPS) with the standard choice for $σ_{\rm eff}$ parameter. The relative production rate $σ(J/ψ+ ψ^\prime)/σ(J/ψ+ J/ψ)$ is found to be sensitive to the DPS terms as well as to feeddown contributions.

hep-ph↗
arXiv2024

Double parton scattering versus jet quenching

A novel observable, the double nuclear modification factor, is proposed to probe simultaneously the initial and final state effects in nucleus-nucleus collisions. An interesting competition between the combinatorial enhancement in the double parton scattering and the suppression due to parton energy loss can be observed in the production rate of two hard particles. In particular, the production of $J/ψ$ mesons in association with a $W$ boson is not suppressed but is enhanced in the region of moderate transverse momenta, unlike the case of unassociated (inclusive) $J/ψ$ production. At the same time, in the region of high enough transverse momenta the nuclear modification factor for associated $J/ψ+W$ production converges to that of unassociated $J/ψ$.

nucl-th↗
arXiv2023

Towards higher-order calculations of quarkonia production with $k_T$-factorization: $P$-wave charmonia

Inclusive $P$-wave charmonia production in hadronic collisions at high energies is discussed in the framework of non-relativistic QCD and $k_T$-factorization formalism. We present two consistent approches to merge the usual leading order $k_T$-factorization calculations with tree-level next-to-leading order off-shell amplitudes. Using these prescriptions, we extracted long-distance matrix elements for $χ_c$ mesons from a combined fit to available Tevatron and LHC data. In contrast to previous (leading order) calculations, our fits do not contradict equal color singlet wave functions of $χ_{c1}$ and $χ_{c2}$ states. The extracted values of long-distance matrix elements are employed to analyse the $χ_c$ polarization data reported recently by the CMS Collaboration. Our predictions are in a reasonably good agreement with the Tevatron and LHC measurements within the theoretical and experimental uncertainties.

hep-ph↗
arXiv2022

The role of initial gluon emission in double $J/ψ$ production at central rapidities

We consider the process of double $J/ψ$ production in $pp$ collisions at the LHC in the framework of $k_T$-factorization approach. We focus on the gluon fragmentation mechanism which is related to multiple gluon emission in the initial state. The initial state emission is treated according to Catani-Ciafaloni-Fiorani-Marchesini evolution equation, and applies to both single and double parton scattering cases. We show the importance of fragmentation contributions to $J/ψ$ pair production and perform a comparison between theoretical predictions and the latest ATLAS data collected at $\sqrt{s} = 8$ TeV. We find that the effects of multiple gluon emission are essential for both single- and multiparton interaction processes. Finally, we highlight the problem of correct choice of the factorization scale with respect to numerical stability of the calculations and the consistency with non-collinear evolution equations.

hep-ph↗
arXiv2021

Discriminating the heavy jet production mechanisms in associated $Z$ + heavy flavor events at the LHC

We reconsider the associated $Z$ boson and charm or beauty jet production at the LHC with paying special attention to the formation dynamics of heavy jets. Two different approaches are studied: first one, where heavy quarks are produced in the hard scattering subprocesses, implemented in the Monte-Carlo generator PEGASUS, and another scenario, where the hard scattering is calculated at NLO with MadGraph5_aMC@NLO and TMD parton shower is included (implemented in the Monte-Carlo generator CASCADE3). We compare the predictions obtained in both schemes with latest experimental data for associated $Z + b$ production cross sections and the relative production rate $σ(Z + c)/σ(Z + b)$ collected by the ATLAS and CMS Collaborations at $\sqrt s = 13$~TeV. We introduce two kinematic observables (denoted as $z_b$ and $p_T^{\rm rel}$) which can be used to discriminate the heavy jet production mechanisms. Using these variables we trace the shape of the simulated $b$-jet events and recommend that these observables be taken into consideration in the forthcoming experimental analyses.

hep-ph↗
arXiv2021

Charm fragmentation and associated $J/ψ+ Z/W^\pm$ production at the LHC

We consider the production of electroweak $Z$ or $W^\pm$ bosons associated with $J/ψ$ mesons at the LHC conditions. Our attention is focused on new partonic subprocesses which yet have never been considered in the literature, namely, the charmed or strange quark excitation subprocesses followed by the charmed quark fragmentation $c\to J/ψ+c$. Additionally we take into account the effects of multiple quark and gluon radiation in the initial and final states. We find that the contributions from the new mechanisms are important and significantly reduce the gap between the theoretical and experimental results on the $J/ψ+ Z$ and $J/ψ+ W^\pm$ production cross sections.

hep-ph↗
arXiv2020

Revisiting the production of $J/ψ$ pairs at the LHC

We consider the prompt double $J/ψ$ production in $pp$ collisions at the LHC in the framework of $k_T$-factorization QCD approach. Using the fragmentation mechanism, we evaluate the color octet contributions to the production cross sections taking into account the combinatorial effects of multiple gluon radiation in the initial state driven by the Ciafaloni-Catani-Fiorani-Marchesini evolution equation. We demonstrate the importance of these contributions in a certain kinematical region covered by the CMS and ATLAS measurements. On the other hand, the experimental data taken by the LHCb Collaboration at forward rapidities and moderate transverse momenta can be described well by ${\cal O}(α_s^4)$ color singlet terms and contributions from the double parton scattering mechanism. The extracted value of the effective cross section $σ_{\rm eff} = 17.5 \pm 4.1$~mb is compatible with many other estimations based on different final states.

hep-ph↗
arXiv2020

$χ_{c1}$ and $χ_{c2}$ polarization as a probe of color octet channel

We analyze the first LHC data on $χ_{c1}$ and $χ_{c2}$ polarization obtained very recently by the CMS Collaboration at $\sqrt s = 8$~TeV. We describe the perturbative production of $c\bar c$ pair with $k_T$-factorization approach and use nonrelativistic QCD formalism for the formation of bound states. We demonstrate that the polar anisotropy of $χ_{c1}$ and $χ_{c2}$ mesons is strongly sensitive to the color octet contributions. We extract the long-distance matrix elements for $χ_{c1}$ and $χ_{c2}$ mesons from the first CMS polarization measurement %on the polarization parameters $λ_θ^{χ_{c1}}$ and $λ_θ^{χ_{c2}}$ obtained at $\sqrt s = 8$~TeV together with available LHC data on the $χ_{c1}$ and $χ_{c2}$ transverse momentum distributions (and their ratios) collected at $\sqrt s = 7$~TeV. Our fit points to unequal color singlet wave functions of $χ_{c1}$ and $χ_{c2}$ states.

hep-ph↗
arXiv2019

Particle Event Generator: A Simple-in-Use System PEGASUS version 1.0

PEGASUS is a parton-level Monte-Carlo event generator designed to calculate cross sections for a wide range of hard QCD processes at high energy $pp$ and $p\bar p$ collisions, which incorporates the dynamics of transverse momentum dependent (TMD) parton distributions in a proton. Being supplemented with off-shell production amplitudes for a number of partonic subprocesses and provided with necessary TMD gluon density functions, it produces weighted or unweighted event records which can be saved as a plain data file or a file in a commonly used Les Houches Event format. A distinctive feature of PEGASUS is an intuitive and extremely user friendly interface, allowing one to easily implement various kinematical cuts into the calculations. Results can be also presented "on the fly" with built-in tool \textsc{pegasus plotter}. A short theoretical basis is presented and detailed program description is given.

hep-ph↗
arXiv2019

Are there any challenges in the charmonia production and polarization at the LHC?

We analyze the inclusive prompt production of $ψ^\prime$, $χ_c$, $J/ψ$ and $η_c$ mesons at the LHC using the $k_T$-factorization approach. Our consideration is based on the off-shell production amplitudes for hard partonic subprocesses, nonrelativistic QCD formalism for the formation of bound states and transverse momentum dependent (or unintegrated) gluon densities in a proton derived from Catani-Ciafaloni-Fiorani-Marchesini equation. The nonperturbative color octet transitions are treated in terms of the multipole radiation theory. We extract the corresponding long-distance matrix elements from a combined fit to transverse momentum distributions measured at various LHC experiments. We make predictions for the polarization parameters $λ_θ$, $λ_ϕ$, $λ_{θϕ}$ and the frame-independent parameter $λ^*$ and compare them to the available $ψ^\prime$ and $J/ψ$ data. Finally, we present a universal set of parameters that provides a reasonable simultaneous description for the whole body of the LHC data (on the $p_T$ distributions, relative production rates and polarization observables) for the whole charmonium family.

hep-ph↗
arXiv2019

Prompt $η_c$ meson production at the LHC in the NRQCD with $k_T$-factorization

In the framework of the $k_T$-factorization, the prompt production of $η_c$ mesons at the LHC conditions is studied. Our consideration is based on the off-shell amplitudes for hard partonic subprocesses and on the nonrelativistic QCD (NRQCD) formalism for the formation of bound states. We try two latest parametrizations for noncollinear, or transverse momentum dependent (TMD) gluon densities derived from the Catani-Ciafaloni-Fiorani-Marchesini (CCFM) equation. We use the values of the nonperturbative matrix elements obtained from a combined fit of the $η_c$ and $J/ψ$ differential cross sections. Finally, we show an universal set of parameters that provides a reasonable simultaneous description for all of the available data on the prompt $J/ψ$ and $η_c$ production at the LHC.

hep-ph↗
arXiv2019

Fragmentation of charm to charmonium in $e^+e^-$ and $pp$ collisions

We perform numerical comparison of the fragmentation mechanism of charmonium production ($g\,g\to c\,\bar{c}$ followed by $c\toψ\,c$) with the full leading order calculation ($g\,g\toψ\,c\,\bar{c}$ at $Ø (α_s^4)$). We conclude that the non-fragmentation contributions remain important up to $J/ψ$ transverse momenta about as large as 50 GeV, thus making questionable the applicability of the fragmentation approximation at smaller transverse momenta.

hep-ph↗
arXiv2018

Associated non-prompt $J/ψ+ μ$ and $J/ψ+ J/ψ$ production at LHC as a test for TMD gluon density

We consider the associated production of $J/ψ$ mesons and muons originating from the $b$-flavored hadron decays and non-prompt double $J/ψ$ production at the LHC using the $k_T$-factorization approach. To describe the inclusive $b$-hadron decays into the different charmonium states we apply fragmentation approach and adopt fragmentation functions based on the non-relativistic QCD factorization. The transverse momentum dependent (TMD) gluon densities in a proton are determined using the Catani-Ciafaloni-Fiorani-Marchesini equation and Kimber-Martin-Ryskin prescription. We investigate the effects coming from parton showers, estimate the double parton scattering contribution and compare our predictions with the first experimental data taken by the ATLAS and LHCb Collaborations at $\sqrt s = 8$~TeV. These data can serve as an additional test for TMD gluon density function in a proton.

hep-ph↗
arXiv2017

Associated production of $Z$ bosons and $b$-jets at the LHC in the combined $k_T$ + collinear QCD factorization approach

We consider the production of $Z$ bosons associated with beauty quarks at the LHC using a combined $k_T$ + collinear QCD factorization approach, that interpolates between small $x$ and large $x$ physics. Our consideration is based on the off-shell gluon-gluon fusion subprocess $g^* g^* \to Z Q\bar Q$ at the leading order ${\cal O}(αα_s^2)$ (where the $Z$ boson further decays into a lepton pair), calculated in the $k_T$-factorization approach, and several subleading ${\cal O}(αα_s^2 )$ and ${\cal O}(αα_s^3 )$ subprocesses involving quark-antiquark and quark-gluon interactions, taken into account in conventional (collinear) QCD factorization. The contributions from double parton scattering are discussed as well. The transverse momentum dependent (or unintegrated) gluon densities in a proton are derived from Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation. We achieve reasonably good agreement with the latest data taken by CMS and ATLAS Collaborations. The comparison of our results with next-to-leading-order pQCD predictions, obtained in the collinear QCD factorization, is presented. We discuss the uncertainties of our calculations and demonstrate the importance of subleading quark involving contributions in describing the LHC data in the whole kinematic region.

hep-ph↗
arXiv2017

Charmonia production from $b$-hadron decays at LHC with $k_T$-factorization: $J/ψ$, $ψ(2S)$ and $J/ψ+ Z$

We consider the production of $J/ψ$ and $ψ(2S)$ mesons originating from the decays of $b$-flavored hadrons at the LHC using the $k_T$-factorization approach. Our analysis covers both inclusive charmonia production and production of $J/ψ$ mesons in association with $Z$ bosons. We apply the transverse momentum dependent (or unintegrated) gluon density in a proton derived from Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation and adopt fragmentation functions based on the non-relativistic QCD factorization to describe the inclusive $b$-hadron decays into the different charmonium states. Our predictions agree well with latest experimental data taken by the CMS, ATLAS and LHCb Collaborations at $\sqrt s = 7$, $8$ and $13$~TeV. The contributions from double parton scattering to the associated non-prompt $J/ψ+ Z$ production are estimated and found to be small.

hep-ph↗
arXiv2016

Prompt charmonia production and polarization at LHC in the NRQCD with $k_T$-factorization. Part III: $J/ψ$ meson

In the framework of $k_T$-factorization approach, the production and polarization of prompt $J/ψ$ mesons at the LHC energies is studied. Our consideration is based on the non-relativistic QCD formalism for bound states and off-shell amplitudes for hard partonic subprocesses. Both the direct production mechanism and feed-down contributions from $χ_c$ and $ψ(2S)$ decays are taken into account. The transverse momentum dependent (or unintegrated) gluon densities in a proton were derived from Ciafaloni-Catani-Fiorani-Marchesini evolution equation or, alternatively, were chosen in accordance with Kimber-Martin-Ryskin prescription. The non-perturbative color-octet matrix elements were first deduced from the fits to the latest CMS data on $J/ψ$ transverse momentum distributions and then applied to describe the ATLAS and LHCb data on $J/ψ$ production and polarization at $\sqrt s = 7$, $8$ and $13$ TeV. We perform an estimation of polarization parameters $λ_θ$, $λ_ϕ$ and $λ_{θϕ}$ which determine $J/ψ$ spin density matrix and demonstrate that treating the soft gluon emission as a series of explicit color-electric dipole transitions within NRQCD leads to unpolarized $J/ψ$ production at high transverse momenta, that is in qualitative agreement with the LHC data.

hep-ph↗