Collinear Approach Research Articles

NLO corrections to the deeply virtual meson production revisited: impact on the extraction of generalized parton distributions

We revisit the next-to-leading order (NLO) perturbative QCD corrections for the deeply virtual meson production (DVMP) process, exploring its phenomenology both in isolation and in a multichannel fit combined with deeply virtual Compton scattering (DVCS). Our approach involves the conformal partial wave (CPaW) formalism, which allows for the straightforward inclusion of higher-order contributions and evolutionary effects. Our findings indicate that a description of the longitudinal component of the vector meson DVMP cross-section at high energies is achievable only at NLO within the standard collinear approach. Furthermore, we demonstrate a simultaneous description of DIS, DVCS, and DVMP processes, providing insights into the proton structure described at NLO by unique universal generalized parton distribution (GPD) functions.

Collinear approach for top quark production at ep colliders

An approximately analysis study of the collinear approach for $t\overline{t}$ production at the Future Circular Collider hadron-electron (FCC-he) and the Large Hadron electron Collider (LHeC) is performed. To study heavy quark production processes, the collinear generalized double asymptotic scaling (DAS) approach in a wide rang of $Q^{2}$ is used. The reduced cross sections for top quark pair production at the FCC-he (for $Q^{2}{\geq}m^{2}_{t}$) and LHeC (for $Q^{2}{\leq}m^{2}_{t}$) center-of-mass energies at the high inelasticity are derived. These results may be considering for both the future collider experiments at center-of-mass energy frontier and the improvement of the phenomenological models on kinematics of low values of the Bjorken variable $x$. The evolution of the gluon density for heavy-quark production in DIS is obtained at low $x$ to include effects of heavy-quark masses. A nonlinear modification of these results shown that the nonlinear corrections, with respect to the top quark mass, are very small for $Q^{2}{\propto}m^{2}_{t}$.

Far-forward production of charm mesons and neutrinos at forward physics facilities at the LHC and the intrinsic charm in the proton

We discuss production of far-forward $D$ mesons/antimesons and neutrinos/antineutrinos from their semileptonic decays in pp-collisions at the LHC. We include the gluon-gluon fusion $gg \to c\bar{c}$, the intrinsic charm (IC) $gc \to gc$ as well as the recombination $gq \to Dc$ partonic mechanisms. The calculations are performed within the $k_T$-factorization approach and the hybrid model using different unintegrated parton distribution functions (uPDFs) for gluons from the literature, as well as within the collinear approach. We compare our results to the LHCb data for forward $D^{0}$-meson production at $\sqrt{s} = 13$ TeV for different rapidity bins in the interval $2 < y < 4.5$. The IC and recombination models are negligible at the LHCb kinematics. Both the mechanisms start to be crucial at larger rapidities and dominate over the standard charm production mechanisms. At high energies there are so far no experiments probing this region. We present also energy distributions for forward electron, muon and tau neutrinos to be measured at the LHC by the currently operating FASER$\nu$ experiment, as well as by future experiments like FASER$\nu2$ or FLArE, proposed very recently by the Forward Physics Facility project. Again components of different mechanisms are shown separately. For all kinds of neutrinos (electron, muon, tau) the subleading contributions, i.e. the IC and/or the recombination, dominate over light meson (pion, kaon) and the standard charm production contribution driven by fusion of gluons for neutrino energies $E_{\nu} \gtrsim 300$ GeV. For electron and muon neutrinos both the mechanisms lead to a similar production rates and their separation seems rather impossible. On the other hand, for $\nu_{\tau} + {\bar \nu}_{\tau}$ neutrino flux the recombination is further reduced making the measurement of the IC contribution very attractive.

Diffractive semi-hard production of a J/psi or a Upsilon from single-parton fragmentation plus a jet in hybrid factorization

We investigate the inclusive hadroproduction of a heavy quarkonium (J/psi or Upsilon ), in association with a light-flavored jet, as a testing ground for the semi-hard regime of QCD. Our theoretical setup is the hybrid high-energy and collinear factorization, where the standard collinear approach is supplemented by the t-channel resummation of leading and next-to-leading energy logarithms à la BFKL. We present predictions for rapidity and azimuthal-angle differential distributions, hunting for stabilizing effects of the high-energy series under higher-order corrections. Our reaction represents an additional channel to test the production mechanisms of quarkonia at high energies and large transverse momenta and to possibly shed light on the transition region from heavy-quark pair production to single-parton fragmentation of J/psi and Upsilon states.

SoftMatch: Comparing Scanpaths Using Combinatorial Spatio-Temporal Sequences with Fractal Curves.

Recent studies matching eye gaze patterns with those of others contain research that is heavily reliant on string editing methods borrowed from early work in bioinformatics. Previous studies have shown string editing methods to be susceptible to false negative results when matching mutated genes or unordered regions of interest in scanpaths. Even as new methods have emerged for matching amino acids using novel combinatorial techniques, scanpath matching is still limited by a traditional collinear approach. This approach reduces the ability to discriminate between free viewing scanpaths of two people looking at the same stimulus due to the heavy weight placed on linearity. To overcome this limitation, we here introduce a new method called SoftMatch to compare pairs of scanpaths. SoftMatch diverges from traditional scanpath matching in two different ways: firstly, by preserving locality using fractal curves to reduce dimensionality from 2D Cartesian (x,y) coordinates into 1D (h) Hilbert distances, and secondly by taking a combinatorial approach to fixation matching using discrete Fréchet distance measurements between segments of scanpath fixation sequences. These matching “sequences of fixations over time” are a loose acronym for SoftMatch. Results indicate high degrees of statistical and substantive significance when scoring matches between scanpaths made during free-form viewing of unfamiliar stimuli. Applications of this method can be used to better understand bottom up perceptual processes extending to scanpath outlier detection, expertise analysis, pathological screening, and salience prediction.

Collisional excitation of C+(2P) spin-orbit levels by molecular hydrogen revisited

ABSTRACT Relaxation of the spin-orbit excited C+(2P3/2) ion by collisions with H2 is an important process in the interstellar medium. Previous calculations of rate coefficients for this process employed potential energies computed for only collinear and perpendicular approach of H2 to the ion. To capture the full angular dependence of the C+–H2 interaction, the angular variation of the potential has been obtained by quantum chemical calculations in this work. These data were used to compute rate coefficients for the de-excitation of the C+(2P3/2) level in collisions with H2 in its j = 0, 1, and 2 rotational levels. With the assumption that the para-H2 rotational levels are in Local Thermodynamic Equilibrium (LTE), rate coefficients were then calculated for de-excitation by para- and ortho-H2 for temperature ranging from 5 to 500 K. The rate coefficient for de-excitation by para-H2 is ca. 10 per cent higher at temperatures near 100 K but 10 per cent lower at temperatures greater than 300 K than the previous best calculation. By contrast, the de-excitation rate coefficient for ortho-H2 is 15 per cent higher at low temperatures but approximately equal as compared with the previous best calculation. The impact of these new rate coefficients is briefly tested in radiative transfer calculations.

Intrinsic charm in the nucleon and charm production at large rapidities in collinear, hybrid and kT-factorization approaches

We discuss the role of intrinsic charm (IC) in the nucleon for forward production of c-quark (or overline{c} -antiquark) in proton-proton collisions for low and high energies. The calculations are performed in collinear-factorization approach with on-shell partons, kT-factorization approach with off-shell partons as well as in a hybrid approach using collinear charm distributions and unintegrated (transverse momentum dependent) gluon distributions. For the collinear-factorization approach we use matrix elements for both massless and massive charm quarks/antiquarks. The distributions in rapidity and transverse momentum of charm quark/antiquark are shown for a few different models of IC. Forward charm production is dominated by gc-fusion processes. The IC contribution dominates over the standard pQCD (extrinsic) gg-fusion mechanism of coverline{c} -pair production at large rapidities or Feynman-xF. We perform similar calculations within leading-order and next-to-leading order kT-factorization approach. The kT-factorization approach leads to much larger cross sections than the LO collinear approach. At high energies and large rapidities of c-quark or overline{c} -antiquark one tests gluon distributions at extremely small x. The IC contribution has important consequences for high-energy neutrino production in the Ice-Cube experiment and can be, to some extent, tested at the LHC by the SHIP and FASER experiments by studies of the ντ neutrino production.

QCD predictions for open charm meson production at the LHCb in a fixed-target experiment

We investigate open charm meson production in fixed-target LHCb experiment at $\sqrt{s}=86.6$ GeV in $p+^4\!\mathrm{He}$ collisions. Theoretical calculations of charm cross section are done in the framework of the $k_{T}$-factorization approach. Its application in the kinematical range never examined before is carefully discussed. We consider different schemes for the calculations relevant for different unintegrated (transverse momentum dependent) parton densities in a proton. We include in the analysis both CCFM- and DGLAP-based models of unintegrated parton distributions appropriate for the considered kinematics. Integrated as well as differential cross sections as a function of $D^{0}$ meson rapidity and transverse momentum are shown and compared with the experimental data. As a reference point, predictions of next-to-leading order collinear approach are also presented and discussed. A very good agreement between the experimental data and the $k_{T}$-factorization predictions was obtained. Both the CCFM and the DGLAP-based frameworks for parton distributions in a proton are successfully used to explain the LHCb fixed-target open charm cross section.

Production of f0(980) meson at the LHC: Color evaporation versus color-singlet gluon-gluon fusion

The production of the f0(980) meson at high energies is not well understood. We investigate two different potential mechanisms for inclusive scalar meson production in the kt-factorization approach: color-singlet gluon-gluon fusion and color evaporation model. The γ⁎γ⁎→f0(980) form factor(s) can be constraint from the f0(980) radiative decay width. The g⁎g⁎→f0(980) form factors are obtained by a replacement of αem electromagnetic coupling constant by αs strong coupling constant and appropriate color factors. The form factors for the two couplings are parametrized with a function motivated by recent results for scalar quarkonia. The differential cross sections are calculated in the kt-factorization approach with modern unintegrated gluon distributions. Unlike for quarkonia it seems rather difficult to describe a preliminary ALICE data for inclusive production of f0(980) exclusively by the color singlet gluon-gluon fusion mechanism. Two different scenarios for flavor structure of f0(980) are considered in this context. We consider also mechanism of fusion of quark-antiquark associated with soft gluon emission in a phenomenological color evaporation model (CEM) used sometimes for quarkonium production. Here we use kt-factorization version of CEM to include higher-order contributions. In addition, for comparison we consider also NLO collinear approach with qq¯q and qq¯g color octet partonic final states. Both approaches lead to a similar result. However, very large probabilities are required to describe the preliminary ALICE data. The pomeron-pomeron fusion mechanism is also discussed and results are quantified.

Formation of H2 on graphene using Eley-Rideal and Langmuir-Hinshelwood processes.

A hydrogen atom can either physisorb or chemisorb onto a graphene surface. To describe the interaction of H with graphene, we trained the C-C, H-H, and C-H interactions of the ReaxFF CHO bond order potential to reproduce Density Functional Theory (DFT) generated values of graphene cohesive energy and lattice constant, H2 dissociation energy, H on graphene adsorption potentials, and H2 formation on graphene using the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) processes. The results, generated from the trained H-graphene potentials, are in close agreement with the corresponding results from DFT. The advantage of using optimized CH potentials is, for example, the inclusion of physisorption interactions and quantum mechanical features of chemical bonding in the functional forms of the potentials. The trained CH potentials are utilized to study the energetics of formation of an H2 molecule on graphene using the Eley-Rideal and Langmuir-Hinshelwood processes. Potential energy surfaces for the formation of H2 through ER are generated for the collinear and oblique approach of the second hydrogen atom. Energetics of the formation of H2 through LH is studied for a variety of cases such as when hydrogen atoms are chemisorbed or physisorbed and when hydrogen occupies ortho, meta, or para chemisorption sites. The likelihood of H2 formation through LH for various configurations is discussed. Furthermore, the tunneling probability of an atom through a continuous symmetric/asymmetric barrier is calculated and applied to an adsorbed hydrogen atom on graphene.

Charm quark and meson production in association with single-jet at the LHC

We discuss charm quark/antiquark and charmed meson production in association with one extra jet (gluon, quark, antiquark) at the LHC. The calculations are performed both in collinear and $k_T$-factorization approaches. Different unintegrated gluon distribution functions are used in the $k_T$-factorization approach. Several predictions for the LHC are presented. We show distributions in rapidity and transverse momenta of $c$/$\bar c$ (or charmed mesons) and the associated jet as well as some two-dimensional observables. Interesting correlation effects are predicted, \textit{e.g.} in azimuthal angles $\varphi_{c\bar{c}}$ and $\varphi_{c\mathrm{\textit{-jet}}}$. We have also discussed a relation of the $2 \to 2$ and $2 \to 3$ partonic calculations in the region of large transverse momenta of charm quarks/antiquarks as well as the similarity of the next-to-leading order collinear approach and the $k_T$-factorization approach with the KMR unintegrated parton distribution functions. Integrated cross sections for $D^{0}$ + jet production for ATLAS detector acceptance and for different cuts on jet transverse momenta are also presented.

An Experimental and Theoretical Investigation of the C((1)D) + N2 → C((3)P) + N2 Quenching Reaction at Low Temperature.

The gas-phase quenching reaction C((1)D) + N2 → C((3)P) + N2 has been investigated experimentally over the temperature range 50-296 K using a supersonic flow reactor. C((1)D) atoms were produced in situ by the pulsed multiphoton dissociation of CBr4 precursor molecules. Rate constants for this process were measured using a chemical tracer method whereby the C((1)D) + H2 → CH + H reaction was employed to follow C((1)D) decays by monitoring vacuum ultraviolet laser-induced fluorescence of the atomic hydrogen product at 121.567 nm. The deactivation rates are seen to increase at lower temperature, indicating the likely influence of the CNN intermediate complex lifetime on intersystem crossing for this system. We also performed electronic structure calculations of relevant C((3)P)-N2 and C((1)D)-N2 potential energy curves as well as triplet-singlet spin-orbit coupling terms using the explicitly correlated internally contracted multireference configuration interaction method (ic-MRCI-F12). The calculations were performed for the collinear and perpendicular approach of the C atom toward the N2 molecule, which allowed us to construct the approximate spherical (isotropic) potential model of C-N2(j = 0). The computed reduced dimensional potentials were used in quantum close coupling scattering calculations of the electronic quenching cross sections and rate constants. While the calculated potential energy curves form the basis for a good qualitative description of the reaction, the calculated rate constants are significantly smaller than the measured ones, and fail to reproduce the temperature dependence of the experimental results. Several possible reasons are provided to explain the origin of these differences.

Four-jet production in single- and double-parton scattering within high-energy factorization

We perform a first study of 4-jet production in a complete high-energy factorization (HEF) framework. We include and discuss contributions from both single-parton scattering (SPS) and double-parton scattering (DPS). The calculations are performed for kinematical situations relevant for two experimental measurements (ATLAS and CMS) at the LHC. We compare our results to those reported by the ATLAS and CMS collaborations for different sets of kinematical cuts. The results of the HEF approach are compared with their counterparts for collinear factorization. For symmetric cuts the DPS HEF result is considerably smaller than the one obtained with collinear factorization. The mechanism leading to this difference is of kinematical nature. We conclude that an analysis of inclusive 4-jet production with asymmetric $p_T$-cuts below 50 GeV would be useful to enhance the DPS contribution relative to the SPS contribution. In contrast to the collinear approach, the HEF approach nicely describes the distribution of the $\Delta S$ variable, which involves all four jets and their angular correlations.

Imaging Proton Transfer and Dihalide Formation Pathways in Reactions of F(-) + CH3I.

Ion–molecule reactions of the type X– + CH3Y are commonly assumed to produce Y– through bimolecular nucleophilic substitution (SN2). Beyond this reaction, additional reaction products have been observed throughout the last decades and have been ascribed to different entrance channel geometries differing from the commonly assumed collinear approach. We have performed a crossed beam velocity map imaging experiment on the F– + CH3I reaction at different relative collision energies between 0.4 and 2.9 eV. We find three additional channels competing with nucleophilic substitution at high energies. Experimental branching ratios and angle- and energy differential cross sections are presented for each product channel. The proton transfer product CH2I– is the main reaction channel, which competes with nucleophilic substitution up to 2.9 eV relative collision energy. At this level, the second additional channel, the formation of IF– via halogen abstraction, becomes more efficient. In addition, we present the first evidence for an [FHI]− product ion. This [FHI]− product ion is present only for a narrow range of collision energies, indicating possible dissociation at high energies. All three products show a similar trend with respect to their velocity- and scattering angle distributions, with isotropic scattering and forward scattering of the product ions occurring at low and high energies, respectively. Reactions leading to all three reaction channels present a considerable amount of energy partitioning in product internal excitation. The internally excited fraction shows a collision energy dependence only for CH2I–. A similar trend is observed for the isoelectronic OH– + CH3I system. The comparison of our experimental data at 1.55 eV collision energy with a recent theoretical calculation for the same system shows a slightly higher fraction of internal excitation than predicted, which is, however, compatible within the experimental accuracy.

Diffractive production of heavy mesons at the LHC

We discuss diffractive production of heavy mesons at the LHC. In addition to standard collinear approach, for a first time we propose a k t -factorization approach to the diffractive processes. The unintegrated (transverse momentum dependent) diffractive parton distributions in proton are calculated with the help of the Kimber-Martin-Ryskin prescription where collinear diffractive PDFs are used as input. Some correlation observables, like azimuthal angle correlation between c and c , and cc pair transverse momentum distribution were obtained for the first time. The results of the new approach are compared with those of the standard collinear one. Significantly larger cross sections are obtained in the k t -factorization approach where some part of higher-order effects is effectively included.

Nucleophilic substitution with two reactive centers: The CN(-) + CH3I case.

The nucleophilic substitution reaction CN(-) + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle- and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.

Enhanced light emission from carbon nanotubes integrated in silicon micro-resonator

Single-walled carbon nanotubes are considered a fascinating nanomaterial for photonic applications and are especially promising for efficient light emitters in the telecommunication wavelength range. Furthermore, their hybrid integration with silicon photonic structures makes them an ideal platform to explore their intrinsic properties. Here we report on the strong photoluminescence enhancement from carbon nanotubes integrated in silicon ring resonator circuits under two pumping configurations: surface-illuminated pumping at 735 nm and collinear pumping at 1.26 . Extremely efficient rejection of the non-resonant photoluminescence was obtained. In the collinear approach, an emission efficiency enhancement by a factor of 26 has been demonstrated in comparison with the classical pumping scheme. This demonstration paves the way for the development of integrated light sources in silicon based on carbon nanotubes.

Searching for and exploring double-parton scattering effects in four-jet production at the LHC

We discuss four-jet production at the LHC. We calculate cross section for both single-parton scattering (SPS) using the ALPGEN code and for double-parton scattering (DPS) in leading-order collinear approach. Our results are compared with experimental data obtained recently by the CMS Collaboration. We show that the ALPGEN code relatively well describes distributions in transverse momenta and rapidity of each of the four jets ordered by their transverse momenta (leading, subleading, etc.). The SPS mechanism does not explain the distributions at large rapidity for the leading jet. The DPS mechanism improves the agreement with the experimental data in this corner of the phase space. In order to enhance the relative DPS contribution we propose to impose different cuts. The relative DPS contribution increases when decreasing the lower cut on the jet transverse momenta as well as when a low lower cut on the rapidity distance between the most remote jets is imposed. We predict very flat distribution in azimuthal angle between the most remote jets with low lower cuts on jets transverse momentum. We find phase-space corners where the DPS content is enhanced relatively to the SPS one.

Production of two charm quark–antiquark pairs in single-parton scattering within the [formula omitted]-factorization approach

We present first results for the 2→4 single-parton scattering gg→cc¯cc¯ subprocess for the first time fully within the kt-factorization approach. In this calculation we have used the Kimber–Martin–Ryskin unintegrated gluon distribution which effectively includes some class of higher-order gluon emissions, and an off-shell matrix element squared calculated using recently developed techniques. The results are compared with our earlier result obtained within the collinear-factorization approach. Only slightly larger cross sections are obtained than in the case of the collinear approach. Inclusion of transverse momenta of gluons entering the hard process leads to a much stronger azimuthal decorrelation between cc and c¯c¯ than in the collinear-factorization approach. A comparison to predictions of double parton scattering (DPS) results and the LHCb data strongly suggests that the assumption of two fully independent DPS (gg→cc¯⊗gg→cc¯) may be too approximate.

Exploring Double-parton Scattering Effects for Jets with Large Rapidity Separation and in Four-jet Production at the LHC

We present an estimation of the contribution of double parton scattering (DPS) for jets widely separated in rapidity and for four-jet sample. In the case of four-jet production we calculate cross section for both single-parton scattering (SPS) using the code ALPGEN as well as for DPS in LO collinear approach. The DPS contribution is calculated within the so-called factorized Ansatz and each step of DPS is calculated in the LO collinear approximation. We show that the relative (with respect to SPS dijets and to the BFKL Mueller-Navelet (MN) jets) contribution of DPS is growing at large rapidity distance between jets. The calculated differential cross sections as a function of rapidity distance between the most remote in rapidity jets are compared with recent results of LL and NLL BFKL calculations for the Mueller-Navelet jet production at $\sqrt{s} = 7$ TeV. Our results for four-jet sample are compared with experimental data obtained recently by the CMS collaboration and a rather good agreement is achieved. We propose to impose different cuts in order to enhance the relative DPS contribution. The relative DPS contribution increases when decreasing the lower cut on the jet transverse momenta as well as when a low lower cut on the rapidity distance between the most remote jets is imposed.