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Absolute charge-transfer cross sections for collisions of protons with hydrogen and deuterium atoms have been measured within the energy range 0.1 to 150 eV using the merging-beams technique. The results are in excellent agreement with a fully quantum-mechanical treatment of this reaction. Earlier measurements which extended down to about 10 eV lie somewhat above the present values.
A measurement of the dijet production cross section is reported based on proton–proton collision data collected in 2016 at $\sqrt{s}$ = 13 TeV by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of up to 36.3 fb –1 . Jets are reconstructed with the anti-$k$ T algorithm for distance parameters of R = 0.4 and 0.8. Cross sections are measured double-differentially (2D) as a function of the largest absolute rapidity |y| max of the two jets with the highest transverse momenta $p$ T and their invariant mass $m$ 1,2 , and triple-differentially (3D) as a function of the rapidity separation $y$*, the total boost $y$ b , and either $m$ 1,2 or the average $p$ T of the two jets. The cross sections are unfolded to correct for detector effects and are compared with fixed-order calculations derived at next-to-next-to-leading order in perturbative quantum chromodynamics. The impact of the measurements on the parton distribution functions and the strong coupling constant at the mass of the Z boson is investigated, yielding a value of $α$ S ($m$ Z ) = 0.1179 ± 0.0019.
Recent discovery by STEREO A/B of energetic neutral hydrogen is spurring an interest and need for reliable estimates of electron capture cross sections at few MeVs per nucleon as well as for multi-electron ions. Required accuracy in such estimates necessitates detailed and involved quantum-mechanical calculations or expensive numerical simulations. For ENA modeling and similar purposes, a semi-classical approach offers a middle-ground approach. Kuang's semiclassical formalism to calculate electron-capture cross sections for single and multi-electron ions is an elegant and efficient method, but has so far been applied to limited and specific laboratory measurements and at somewhat lower energies. Our goals are to test and extend Kuang s method to all ion-atom and ion-ion collisions relevant to ENA modeling, including multi-electron ions and for K-shell to K-shell transitions.
Nuclear medium effects on 𝐵 + meson production are studied using the binary-collision scaled cross section ratio between events of different charged-particle multiplicities from proton-lead collisions. Data, collected by the CMS experiment in 2016 at a nucleon-nucleon center-of-mass energy of $\sqrt{𝑠_{NN}}$ = 8.16 TeV, corresponding to an integrated luminosity of 175 nb −1 , were used. The scaling factors in the ratio are determined using a novel approach based on the 𝑍 → 𝜇 − 𝜇 + cross sections measured in the same events. The scaled ratio for 𝐵 + is consistent with unity for all event multiplicities, putting stringent constraints on nuclear modification for heavy flavor.
The corrections to the transport cross-sections and collision integrals for Coulomb interactions arising from the application of realistic interaction energies of the ions of nitrogen and oxygen are investigated. Accurate potential-energy curves from an ab initio electronic-structure calculation and a semiclassical description of the scattering are used to determine the difference between the cross-sections for the real interaction forces and a Coulomb force for large values of the Debye shielding parameter. Graphs of the correction to the diffusion and viscosity-collision integrals are presented for temperatures from about 10,000 K to 150,000 K. This correction can be combined with tabulations of the collision integrals for shielded Coulomb potentials to determine the contribution of N(+)-N(+), N(+)-O(+), and O(+)-O(+) interactions to the transport properties of high-temperature air. Analytical forms are fitted to the calculated results to assist this application.
The cross section of top quark pair production is measured in the $\mathrm{t\bar{t}}\to (\ell\nu_{\ell})(\tau_\mathrm{h}\nu_{\tau})\mathrm{b\bar{b}}$ final state, where $\tau_\mathrm{h}$ refers to the hadronic decays of the $\tau$ lepton, and $\ell$ is either an electron or a muon. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected in proton-proton collisions at $\sqrt{s}=$ 13 TeV with the CMS detector. The measured cross section is $\sigma_{\mathrm{t\bar{t}}} =$ 781 $\pm$ 7 (stat) $\pm$ 62 (syst) $\pm$ 20 (lum) pb, and the ratio of the partial width $\Gamma($t$\to\tau\nu_{\tau}$b) to the total decay width of the top quark is measured to be 0.1050 $\pm$ 0.0009 (stat) $\pm$ 0.0071 (syst). This is the first measurement of the $\mathrm{t\bar{t}}$ production cross section in proton-proton collisions at $\sqrt{s}=$ 13 TeV that explicitly includes $\tau$ leptons. The ratio of the cross sections in the $\ell\tau_\mathrm{h}$ and $\ell\ell$ final states yields a value $R_{\ell\tau_\mathrm{h}/\ell\ell}=$ 0.973 $\pm$ 0.009 (stat) $\pm$ 0.066 (syst), consistent with lepton universality.
The ratios of the Bc(2S)+ to Bc+, Bc*(2S)+ to Bc+, and Bc*(2S)+ to Bc(2S)+ production cross sections are measured in proton-proton collisions at s=13 TeV, using a data sample collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 143 fb-1. The three measurements are made in the Bc+ meson phase space region defined by the transverse momentum pT>15 GeV and absolute rapidity |y|<2.4, with the excited Bc(*)(2S)+ states reconstructed through the Bc(*)+π+π−, followed by the Bc+→J/ψπ+ and J/ψ→μ+μ− decays. The Bc(2S)+ to Bc+, Bc*(2S)+ to Bc+, and Bc*(2S)+ to Bc(2S)+ cross section ratios, including the unknown Bc(*)(2S)+→Bc(*)+π+π− branching fractions, are (3.47±0.63(stat)±0.33(syst))%, (4.69±0.71(stat)±0.56(syst))%, and 1.35±0.32(stat)±0.09(syst), respectively. None of these ratios shows a significant dependence on the pT or |y| of the Bc+ meson. The normalized dipion invariant mass distributions from the decays Bc(*)(2S)+→Bc(*)+π+π− are also reported.
Three charged particles 1, 2, 3 collide according to the reaction 1+(2+3) yields (1+3)+2, where (2+3) and (1+3) are hydrogenlike bound states. It is shown when (1+3) is in a highly excited state n, due to the repulsive potential, the cross section in the first Born approximation behaves as 1/n which makes the total cross section to diverge like ln n. The total cross sections in the higher orders of the Born approximation are similarly divergent logarithmically.
The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum (p$^{γ}_{T}$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $\sqrt{s_{NN}}$ = 5.02 TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/c, depending on the collision system and on the Pb–Pb centrality class. The result extends to lower p$^{γ}_{T}$ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is |η γ | < 0.67. The isolation selection is based on a charged particle isolation momentum threshold p$^{iso, ch}_{T}$ GeV/c within a cone of radii R = 0.2 and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z 0 bosons from the CMS experiment, which are all found to be in agreement.
The inclusive \( b\overline{b} \) - and \( c\overline{c} \) -dijet production cross-sections in the forward region of pp collisions are measured using a data sample collected with the LHCb detector at a centre-of-mass energy of 13 TeV in 2016. The data sample corresponds to an integrated luminosity of 1.6 fb - 1 . Differential cross-sections are measured as a function of the transverse momentum and of the pseudorapidity of the leading jet, of the rapidity difference between the jets, and of the dijet invariant mass. A fiducial region for the measurement is defined by requiring that the two jets originating from the two b or c quarks are emitted with transverse momentum greater than 20 GeV/ c , pseudorapidity in the range 2 . 2 < η < 4 . 2, and with a difference in the azimuthal angle between the two jets greater than 1.5. The integrated \( b\overline{b} \) -dijet cross-section is measured to be 53 . 0 ± 9 . 7 nb, and the total \( c\overline{c} \) -dijet cross-section is measured to be 73 ± 16 nb. The ratio between \( c\overline{c} \) - and \( b\overline{b} \) -dijet cross-sections is also measured and found to be 1 . 37 ± 0 . 27. The results are in agreement with theoretical predictions at next-to-leading order.
The interpretation of cosmic antiproton flux measurements from space-borne experiments is currently limited by the knowledge of the antiproton production cross-section in collisions between primary cosmic rays and the interstellar medium. Using collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest in the proximity of the interaction region of the LHCb experiment, the ratio of antiprotons originating from antihyperon decays to prompt production is measured for antiproton momenta between 12 and 110 GeV/c. The dominant antihyperon contribution, namely ${\overline{\varLambda }} \rightarrow {\overline{{p}}} {{\pi } ^+}$ decays from promptly produced $\overline{\varLambda }$ particles, is also exclusively measured. The results complement the measurement of prompt antiproton production obtained from the same data sample. At the energy scale of this measurement, the antihyperon contributions to antiproton production are observed to be significantly larger than predictions of commonly used hadronic production models.
This paper describes a measurement of the jet radius dependence of the dijet momentum balance between leading back-to-back jets in 1.72 nb -1 of Pb + Pb collisions collected in 2018 and 255 pb -1 of pp collisions collected in 2017 by the ATLAS detector at the LHC. Both datasets were collected at $\sqrt{s_{NN}}$ = 5.02 TeV. Jets are reconstructed using the anti-k t algorithm with jet radius parameters R = 0.2, 0.3, 0.4, 0.5, and 0.6. The dijet momentum balance distributions are constructed for leading jets with transverse momentum p T from 100 to 562 GeV for R = 0.2, 0.3, and 0.4 jets, and from 158 to 562 GeV for R = 0.5 and 0.6 jets. The absolutely normalized dijet momentum balance distributions are constructed to compare measurements of the dijet yields in Pb + Pb collisions directly to the dijet cross sections in pp collisions. For all jet radii considered here, there is a suppression of more balanced dijets in Pb + Pb collisions compared with pp collisions, while for more imbalanced dijets there is an enhancement. There is a jet radius dependence to the dijet yields, being stronger for more imbalanced dijets than for more balanced dijets. Additionally, jet pair nuclear modification factors are measured. The subleading jet yields are found to be more suppressed than leading jet yields in dijets. A jet radius dependence of the pair nuclear modification factors is observed, with the suppression decreasing with increasing jet radius. These measurements provide new constraints on jet quenching scenarios in the quark-gluon plasma.
We present the first measurements of the forward and midrapidity 𝜂-meson cross sections from 𝑝 + 𝑝 collisions at $\sqrt{𝑠}$ = 500 and 510 GeV, respectively. We also report the midrapidity 𝜂/𝜋 0 ratio at 510 GeV. The forward cross section is measured differentially in 𝜂-meson transverse momentum (𝑝 𝑇 ) from 1.0 to 6.5 GeV/𝑐 for pseudorapidity 3.0 < |𝜂| < 3.8. The midrapidity cross section is measured from 3.5 to 44 GeV/𝑐 for pseudorapidity |𝜂| < 0.35. Both cross sections serve as critical inputs to an updated global analysis of the 𝜂-meson fragmentation functions.
Measurements are presented of the W and Z boson production cross sections in proton-proton collisions at a center-of-mass energy of 13.6 TeV. Data collected in 2022 and corresponding to an integrated luminosity of 5.01 fb −1 with one or two identified muons in the final state are analyzed. The results for the products of total inclusive cross sections and branching fractions for muonic decays of W and Z bosons are 11.93 ± 0.08 (syst) ± 0.17 (lumi) $^{+0.07}_{−0.07}$ (acceptance) nb for W + boson production, 8.86 ± 0.06 (syst) ± 0.12 (lumi) $^{+0.05}_{−0.06}$ (acceptance) nb for W − boson production, and 2.021 ± 0.009 (syst) ± 0.028 (lumi) $^{+0.011}_{−0.013}$ (acceptance) nb for the Z boson production in the dimuon mass range of 60–120 GeV, all with negligible statistical uncertainties. Furthermore, the corresponding fiducial cross sections, as well as cross section ratios for both fiducial and total phase space, are provided. The ratios include charge-separated results for W boson production (W + and W − ) and the sum of the two contributions (W ± ), each relative to the measured Z boson production cross section. Additionally, the ratio of the measured cross sections for W + and W − boson production is reported. All measurements are in agreement with theoretical predictions, calculated at next-to-next-to-leading order accuracy in quantum chromodynamics.
The study was undertaken to determine the effects of reduced mass and differences in asymmetry on the collision integrals and thermal diffusion factors of isotopic hydrogen systems. Each system selected for study consisted of two diatoms, one in the j = 0 rotation state and the other in the j = 1 state. The molecules interacted with a Lennard-Jones type potential modified to include angular terms. A set of cross sections and collision integrals were obtained for each system.
Using the Weizsacher-Williams method of virtual quanta, total cross section estimates for the photoproduction of neutral subthreshold pins in carbon-carbon collisions at incident energies below 300 MeV/nucleon are made. Comparisons with recent experimental data indicate that the photoproduction mechanism makes an insignificant contribution to these measured cross sections.
Hybrid simulation frameworks combining Computational Fluid Dynamics (CFD) and Direct Simulation Monte Carlo (DSMC) are frequently employed to efficiently perform high-fidelity solutions of environments containing combined continuum/rarified flow. The use of DSMC, a stochastic, particle-based method, is necessary for high-Knudsen flow where continuum-based assumptions governing CFD break down. However, the DSMC methodology is generally very computationally inefficient to model the continuum regime. In a CFD/DSMC hybrid approach, obtaining an accurate, high-fidelity solution hinges on the consistent treatment of transport properties and the used thermo-chemical models employed within the two solvers. In principle, in regions where CFD and DSMC are both employed, the same gas mixture under the same conditions should have the same properties, regardless of simulation type. Observed differences should be due to non-equilibrium processes, rather than differences in physical models. While the transport models governing CFD and DSMC simulations are starkly different, they can effectively be linked via their use of reduced Chapman-Enskog collision integrals. In CFD, these integrals are typically stored as fitted polynomial expressions and used to directly compute gas transport properties via mixing rules or the full Chapman-Enskog formulation. In DSMC, they can be used to derive the collision parameters needed for the phenomenological collision cross-section models that govern particle interactions, via a Nelder-Mead optimization scheme. The goal of this work is to provide a unified DSMC transport database encompassing the vast majority of known gas species encountered during atmospheric entry, on Earth or any other Solar body. This goal is largely possible due to recently performed ab-initio quantum chemistry calculations. Combined with other high-fidelity literature sources, the planned database will consist of collision integral data for over 200 neutral and ionized species and over 17000 binary collisions. From these collision integrals, Nelder-Mead optimization is used to compute Variable Soft Sphere (VSS) collision model parameters for DSMC, fitted from 300 K to 20000 K. Initial comparisons of transport properties of relevant equilibrium gas mixtures show great agreement between CFD and DSMC-derived results. The completed database will be able to be readily applied to model binary collisions of any gas mixture containing the included species over the specified temperature range, making it a valuable tool for future planetary probe modeling efforts. An example is shown below. Equilibrium mixture transport properties for a 19-species Titan atmospheric model [4] are computed using both fitted VSS parameters and the original CFD collision integral values. Deviations in computed properties between the two approaches is less than 5% for the entire temperature range.
A differential cross section for pi-meson production in peripheral heavy-ion collisions is formulated within the context of a particle-hole model in the Tamm-Dancoff approximation. This is the first attempt at a fully quantum-mechanical particle-hole calculation for pion production in relativistic heavy-ion collisions. The particular reaction studied is an O-16 projectile colliding with a C-12 target at rest. In the projectile a linear combination of isobar-hole states is formed, with the possibility of a coherent isobar giant resonance. The target can be excited to its giant M1 resonance (J-pi = 1(+), T = 1) at 15.11 MeV, or to its isobar analog neighbors, B-12 at 13.4 MeV and N-12 at 17.5 MeV. The theory is compared to recent experimental results.