Collinear Parton Model Research Articles

PRODUCTION OF ηc WITH TWO-PHOTON DECAY IN THE GPM AT THE ENERGIES OF NICA COLLIDER

The article discuss the production of c-mesons with decays into the photon pairs at the energy of the NICA collider, s =27 GeV, in the generalized parton model and the leading order approximation of the perturbation theory of quantum chrodynamics. The hadronization of a cc-pair into c-meson is described in the color singlet model and in the color evaporation model. In the calculation of the background process of two photon production with invariant mass near the mass of c-meson, we take into account direct as well as fragmentation mechanisms of the prompt photon production. The results of calculations are compared with predictions of the collinear parton model. We study the signal/background ratio as a function of thedifferent kinematical variables.

Production of three isolated photons in the parton Reggeization approach at high energies

We study a large-$p_T$ three-photon production in proton-proton collisions at the LHC. We use the leading order (LO) approximation of the parton Reggeization approach consistently merged with the next-to-leading order corrections originated from the emission of additional jet. For numerical calculations we use the parton-level generator KaTie and modified KMR-type unintegrated parton distribution functions which satisfy exact normalization conditions for arbitrary $x$. We compare our prediction with data from ATLAS collaboration at the center-of-mass energy $\sqrt{s}=8$ TeV. We find that the inclusion of the real next-to-leading-order corrections leads to a good agreement between our predictions and data with the same accuracy as for the next-to-next-to-leading calculations based on the collinear parton model of QCD. At higher energies ($\sqrt{s}=13$ and 27 TeV) parton Reggeization approach predicts larger cross sections, up to $\sim 15$ \% and $\sim 30$ \%, respectively.

Production of three isolated photons in the high-energy factorization approach

We study large-p_TpT three-photon production at the LHC at the center-of-mass energy \sqrt{s}=8s=8 TeV. We use the LO approximation of the parton Reggeization approach consistently merged with the real NLO corrections. For numerical calculations use the parton-level generator KaTie and modified KMR-type unintegrated parton distribution functions. We find good agreement between our predictions and data with the same accuracy as in the NNLO calculations based on the collinear parton model of QCD. At higher energies (\sqrt{s}=13s=13 and 27 TeV) parton Reggeization approach predicts larger cross sections, up to \sim 10∼10 % and \sim 20∼20 %, respectively.

PRODUCTION OF ISOLATED PHOTONS WITH LARGE TRANSVERSE MOMENTA AT LHC IN THE REGGE LIMIT OF QCD

The article discusses the production of prompt isolated photons with large transverse momenta at the LHC at energies s = 8 and 13 TeV in the parton Reggeization approach, which is based on the factorizationtheorem for hard processes at high energies and the effective theory of Reggeized gluons and quarks by L.N. Lipatov. Unintegrated parton distributions in the parton Reggeization approach were obtained in the modified KimberMartinRyskin model proposed earlier by the authors of the article. In numerical calculations, only the contribution of the main parton process, R +Q( Q) + q(q), is taken into account,since the contribution of other processes does not exceed 510 %. The calculation results are compared with the predictions obtained in the collinear parton model. Good agreement of calculations in the partonReggeization approach with experimental data obtained by the ATLAS collaboration is shown.

Double prompt J/ψ production at hadron colliders

In this paper, we give a brief review of recent theoretical studies of double prompt [Formula: see text] production at the FNAL Tevatron and the CERN LHC, to which both single- and double-parton-scattering processes contribute. In single parton scattering, we mainly present the results computed within the framework of non-relativistic quantum chromodynamics factorization combined with one of the two approaches to describe the parton inside the (anti)proton, the collinear parton model and the parton Reggeization approach. The contributions from double parton scattering calculated with different values of [Formula: see text] are also discussed. We conclude that, in double prompt [Formula: see text] hadroproduction, there remain some differences between theoretical predictions and experimental measurements.

High-energy factorization for the Drell-Yan process in pp and pp¯ collisions with new unintegrated PDFs

The formalism for uniform description of Drell-Yan transverse-momentum spectrum is presented in a framework of High-Energy Factorization, which smoothly interpolates between Collins-Soper-Sterman formalism at $|{\bf q}_T|\ll Q$ and usual Collinear Parton Model at $|{\bf q}_T|\sim Q\ll \sqrt{S}$. The new formula for deriving Unintegrated Parton Distribution Functions(UPDFs) from collinear ones is introduced, which leads to excellent description of the shape of $Z$-boson $|{\bf q}_T|$-spectrum at high energies up to $|{\bf q}_T|/\sqrt{S}\simeq 0.02$. Description of normalized $|{\bf q}_T|$-distributions at low energies is achieved via the fit of non-perturbative parameters of quark UPDFs. Reasonable description of angular distributions of leptons in the dilepton center-of-mass frame is also obtained with new UPDFs.

Spectra and polarizations of prompt J/ψ at the NICA within collinear parton model and parton Reggeization approach

We study prompt production of J/ψ at the energy range of the NICA collider within the parton Reggeization approach (PRA) applying the non-perturbative QCD (NRQCD) factorization model for transition. We calculate transverse momentum distribution for direct J/ψ as well as for J/ψ from decays of higher-lying charmonium states. Production of polarized J/ψ is studied and parameter λθ as a function of transverse momentum is calculated. The comparison with predictions obtained in calculations based on the next-to-leading approximation of the collinear parton model is done.

Evidence in favor of single parton scattering mechanism in ϒ and D associated production at the LHC

Associated production of prompt $\Upsilon(1S)$ and $D^{+,0}$ mesons has been proposed as a golden channel for the search of Double Parton Scattering, because Single Parton Scattering contribution to the cross-section is believed to be negligible on a basis of leading order calculations in the Collinear Parton Model. We study this process in the leading order approximation of the Parton Reggeization Approach. Hadronization of $b\bar{b}$-pair into bottomonium states is described within framework of the NRQCD-factorization approach while production of $D$ mesons is described in the fragmentation model with scale-dependent fragmentation functions. We have found good agreement with LHCb data for various normalized differential distributions, except for the case of spectra on azimuthal angle differences at the small $\Delta\varphi$ values. Crucially, the total cross-section in our Single Parton Scattering model accounts for more than one half of observed cross-section, thus dramatically shrinking the room for Double Parton Scattering mechanism.

From LO to NLO in the parton Reggeization approach

We present recent developments of the parton Reggeization approach (PRA), which is based on high-energy factorization of hard processes in the multi-Regge kinematics and Lipatov’s effective theory of Reggeized gluons and Reggeized quarks. The scheme of calculations in the leading order (LO) of the PRA is discussed. We present important examples of LO PRA applications for cross section calculations of multi-scale hard processes, such as pair production ofBB¯-mesons, pair production of photons, and pair production of jets. Also the problem of matching of NLO calculations in PRA with NLO results in the Collinear Parton Model is discussed.

DIS structure functions in the NLO approximation of the Parton Reggeization Approach

The main ideas of the NLO calculations in Parton Reggeization Approach are illustrated on the example of the simplest NLO subprocess, which contributes to DIS: $\gamma^\star+R\to q + \bar{q}$. The double counting with the LO contribution $\gamma^\star + Q \to q$ is resolved. The problem of matching of the NLO results for single-scale observables in the Parton Reggeization Approach on the corresponding NLO results in the Collinear Parton Model is considered. In the developed framework, the usual NLO PDFs in the $\overline{MS}$-scheme can be consistently used as the collinear input for the NLO calculation in the Parton Reggeization Approach.

Two-photon dilepton production in proton-proton collisions: Two alternative approaches

We investigate different methods to incorporate the effect of photons in hard processes. We compare two different approaches used for calculating cross sections for two-photon $p p \to l^+ l^- X$ process. In one of the approaches photon is treated as a collinear parton in the proton. In the second approach recently proposed a $k_T$-factorization method is used. We discuss how results of the collinear parton model depend on the initial condition for the QCD evolution and discuss an approximate treatment where photon is excluded from the combined QCD-QED evolution. We demonstrate that it is not necessary to put photon into the evolution equation as often done recently but it is sufficient to use a simplified approach in which photon couples to quarks and antiquarks which by themselves undergo DGLAP evolution equations. We discuss sensitivity of the results to the choice of structure function parametrization and experimental cuts in the $k_T$-factorization approach. A new optimal structure function parametrization is proposed. We compare results of our calculations with recent experimental data for dilepton production and find that in most cases the contribution of the photon-photon mechanism is rather small. We discuss how to enhance the photon-photon contribution. We also compare our results to those of recent measurements of exclusive and semi-exclusive $e^+ e^-$ pair production with certain experimental data by the CMS collaboration.

Рождение $D$-мезонов на Большом адронном коллайдере в реджевском пределе КХД

В работе рассматривается инклюзивное рождение очарованных $D^{0}$, $D^{+}$, $D^{\star+}$ и $D_{s}^{+}$ мезонов в протон-антипротонных соударениях на коллайдере Тэватрон и протон-протонных соударениях на Большом адронном коллайдере (БАК). Теоретические расчeты проведены в лидирующем порядке теории реджезованных партонов (ТРП), основанной на эффективной квантовой теории поля Л. Н. Липатова и подходе $k_{T}$-факторизации при высоких энергиях. В работе были использованы универсальные функции фрагментации (ФФ), полученные фитированием данных рождения $D$-мезонов на линейном электрон-позитронном коллайдере LEP1. Нами описаны распределения $D$-мезонов по поперечному импульсу, измеренные в центральной области быстрот коллаборациями CDF на Тэватроне ($|y|<1$) и ALICE на БАК ($|y|<0.5$) в пределах ошибок измерений и без привлечения свободных параметров. На стадии численных расчeтов нами использовались неинтегрированные глюонные функции распределения (ФР) Кимбера, Мартина и Рыскина (КМР).

On the role of quark orbital angular momentum in the proton spin

We study the covariant version of the quark-parton model, in which the general rules of the angular momentum composition are accurately taken into account. We demonstrate how these rules affect the relativistic interplay between the quark spins and orbital angular momenta, which collectively contribute to the proton spin. The spin structure functions $g_{1}$ and $g_{2}$ corresponding to the many-quark state $J=1/2$ are studied and it is shown they satisfy constraints and relations well compatible with the available experimental data including proton spin content $\Delta\Sigma\lesssim1/3$. The suggested Lorentz invariant 3D approach for calculation of the structure functions is compared with the approach based on the conventional collinear parton model.

Proton spin in leading order of the covariant approach

We study the covariant version of the quark-parton model, in which the general rules of the angular momentum composition are accurately taken into account. We demonstrate how these rules affect the relativistic interplay between the quark spins and orbital angular momenta, which collectively contribute to the proton spin. The spin structure functions $g_{1}$ and $g_{2}$ corresponding to the many-quark state $J=1/2$ are studied and it is shown they satisfy constraints and relations well compatible with the available experimental data including proton spin content $\Delta\Sigma\lesssim1/3$. The suggested Lorentz invariant 3D approach for calculation of the structure functions is compared with the approach based on the conventional collinear parton model.

Heavy quarkonium production in the regge limit of quantum chromodynamics

Heavy quarkonium \((c\bar c,b\bar b)\) hadronic production at Tevatron (run I and run II) and LHC energies is considered in the framework of nonrelativistic quantum chromodynamics in the leading order with respect to α s and v using the quasi-multi-Regge kinematics approach. Fitting of p T spectra of different S and P wave heavy quarkonium states at Tevatron (run I and run II) energies is carried out. The obtained set of octet nonperturbative matrix elements is used for prediction of heavy quarkonium production at LHC energies. The results obtained in the framework of quasi-multi-Regge kinematics are compared with predictions of the collinear parton model.

Relative contributions of fusion and fragmentation mechanisms inJ/ψphotoproduction at high energy

We study J/psi photoproduction via the fusion and fragmentation mechanisms at the HERA Collider within the frameworks of the collinear parton model and the quasi-multi-Regge kinematics approach using the factorization formalism of non-relativistic QCD at leading order in the strong-coupling constant alpha_s and the relative velocity v of the bound quarks. It is shown that the fusion production mechanism dominates over the fragmentation production mechanism at the all relevant J/psi transverse momenta. The J/psi meson p_T-spectra in the fragmentation and fusion production at the asymptotically large p_T have equal slopes in the quasi-multi-Regge kinematics approach, oppocite the collinear parton model.

AssociatedΥ+b+b¯production at the Fermilab Tevatron and CERN LHC

In the framework of perturbative QCD and nonrelativistic bound state formalism, we calculate the production of {upsilon} mesons associated with open-beauty particles at the Tevatron and LHC conditions. We show that the contribution from the {upsilon}+b+b channel becomes comparable with that of {upsilon} sole production at the LHC at high enough transverse momenta, but is small at the Tevatron. We investigate the {upsilon} polarization effects and kinematic correlations between {upsilon} mesons and the accompanying open-beauty mesons. We show that the correlations are sensitive to the initial k{sub t} of the colliding partons, and so, can be considered as an additional test discriminating the collinear parton model and the k{sub t}-factorization approach.

Topics in associatedJ/ψ+c+c¯production at modern colliders

In the framework of perturbative QCD and nonrelativistic bound state formalism, we calculate the production of $J/\ensuremath{\psi}$ mesons associated with open-charm particles at HERA and Tevatron conditions. We show that the contribution from $J/\ensuremath{\psi}+c+\overline{c}$ channel becomes comparable with that of $J/\ensuremath{\psi}$ sole production at high $J/\ensuremath{\psi}$ transverse momenta (approximately, at ${p}_{\ensuremath{\psi},T}>10\text{ }\text{ }\mathrm{GeV}$). We investigate the $J/\ensuremath{\psi}$ polarization effects and kinematic correlations between $J/\ensuremath{\psi}$ mesons and the accompanying open-charmed mesons. We show that the correlations are sensitive to the initial ${k}_{t}$ of the colliding partons, and so, can be considered as an additional test discriminating the collinear parton model and the ${k}_{t}$-factorization approach.

Electroproduction of D*± mesons at high energies

A comparative analysis of the predictions of the collinear parton model and the kT-factorization approach is performed for the case of the D*-meson electroproduction at the HERA ep collider. It is shown that, owing to effectively taking into account, in noncollinear distributions, next-order corrections in the strong coupling constant αs, the kT-factorization approach increases, in contrast to the predictions of the collinear parton model, the absolute value of the cross sections for charmed-meson electroproduction by approximately a factor of 1.5 to 2. As a result, the agreement with experimental data is improved. This is not so only for the pseudorapidity spectrum, whose shape differs considerably from the experimental one and depends greatly on the choice of parametrization of the noncollinear gluon distribution within the proton.

Production of [formula omitted] mesons via fragmentation in the [formula omitted]-factorization approach

In the framework of the kT-factorization approach we have calculated in the fragmentation model the pT-spectra of Bc mesons at the energies of the Tevatron and the LHC Colliders and at the large pT domain. We compare the obtained results with the existing experimental data and with the predictions obtained in the collinear parton model.