Explore the vast range of titles available.

A bstract Two-particle azimuthal correlations have been measured in neutral current deep inelastic ep scattering with virtuality Q 2 > 5 GeV 2 at a centre-of-mass energy s = 318 GeV recorded with the ZEUS detector at HERA. The correlations of charged particles have been measured in the range of laboratory pseudorapidity − 1 . 5 < η < 2 . 0 and transverse momentum 0 . 1 < p T < 5 . 0 GeV and event multiplicities N ch up to six times larger than the average 〈 N ch 〉 ≈ 5. The two-particle correlations have been measured in terms of the angular observables c n {2}  = 〈〈 cosn Δ φ 〉〉, where n is between 1 and 4 and ∆ φ is the relative azimuthal angle between the two particles. Comparisons with available models of deep inelastic scattering, which are tuned to reproduce inclusive particle production, suggest that the measured two-particle correlations are dominated by contributions from multijet production. The correlations observed here do not indicate the kind of collective behaviour recently observed at the highest RHIC and LHC energies in high-multiplicity hadronic collisions.

A new measurement of inclusive-jet cross sections in the Breit frame in neutral current deep inelastic scattering using the ZEUS detector at the HERA collider is presented. The data were taken in the years 2004–2007 at a centre-of-mass energy of 318 GeV and correspond to an integrated luminosity of 347 pb - 1 . The jets were reconstructed using the k t -algorithm in the Breit reference frame. They have been measured as a function of the squared momentum transfer, Q 2 , and the transverse momentum of the jets in the Breit frame, p ⊥ , Breit . The measured jet cross sections are compared to previous measurements and to perturbative QCD predictions. The measurement has been used in a next-to-next-to-leading-order QCD analysis to perform a simultaneous determination of parton distribution functions of the proton and the strong coupling, resulting in a value of α s ( M Z 2 ) = 0.1142 ± 0.0017 (experimental/fit) - 0.0007 + 0.0006 (model/parameterisation) - 0.0004 + 0.0006 (scale) , whose accuracy is improved compared to similar measurements. In addition, the running of the strong coupling is demonstrated using data obtained at different scales.

A bstract Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity ep scattering at a centre-of-mass energy s = 318 GeV with the ZEUS detector at HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged-particle multiplicities N ch ≥ 20 are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.

A bstract The exclusive photoproduction reactions γp → J/ψ (1 S ) p and γp → ψ (2 S ) p have been measured at an ep centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 373 pb − 1 . The measurement was made in the kinematic range 30 < W < 180 GeV, Q 2 < 1 GeV 2 and | t | < 1 GeV 2 , where W is the photon-proton centre-of-mass energy, Q 2 is the photon virtuality and t is the squared four-momentum transfer at the proton vertex. The decay channels used were J/ψ (1 S ) → μ + μ − , ψ (2 S ) → μ + μ − and ψ (2 S ) → J/ψ (1 S ) π + π − with subsequent decay J/ψ (1 S ) → μ + μ − . The ratio of the production cross sections, R = σ ψ (2 S ) /σ J/ψ (1 S ) , has been measured as a function of W and | t | and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.

The azimuthal correlation angle,$$\\Delta \\phi $$Δ ϕ , between the scattered lepton and the leading jet in deep inelastic$$e^{\\pm }p$$e ± p scattering at HERA has been studied using data collected with the ZEUS detector at a centre-of-mass energy of$$\\sqrt{s} = 318 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}$$s = 318 Ge V , corresponding to an integrated luminosity of$$326 \\,\\text {pb}^{-1}$$326 pb - 1 . A measurement of jet cross sections in the laboratory frame was made in a fiducial region corresponding to photon virtuality$$10 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}^2< Q^2 < 350 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}^2$$10 Ge V 2 < Q 2 < 350 Ge V 2 , inelasticity$$0.04< y < 0.7$$0.04 < y < 0.7 , outgoing lepton energy$$E_e > 10 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}$$E e > 10 Ge V , lepton polar angle$$140^\\circ< \\theta _e < 180^\\circ $$140 ∘ < θ e < 180 ∘ , jet transverse momentum$$2.5 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}< p_\\textrm{T,jet} < 30 {\\,\\text {Ge}\\hspace{-0.66666pt}\\text {V}}$$2.5 Ge V < p T,jet < 30 Ge V , and jet pseudorapidity$$-1.5< \\eta _\\textrm{jet} < 1.8$$- 1.5 < η jet < 1.8 . Jets were reconstructed using the$$k_\\textrm{T}$$k T algorithm with the radius parameter$$R = 1$$R = 1 . The leading jet in an event is defined as the jet that carries the highest$$p_\\textrm{T,jet}$$p T,jet . Differential cross sections,$$d\\sigma /d\\Delta \\phi $$d σ / d Δ ϕ , were measured as a function of the azimuthal correlation angle in various ranges of leading-jet transverse momentum, photon virtuality and jet multiplicity. Perturbative calculations at$$\\mathcal {O}(\\alpha _{s}^2)$$O ( α s 2 ) accuracy successfully describe the data within the fiducial region, although a lower level of agreement is observed near$$\\Delta \\phi \\rightarrow \\pi $$Δ ϕ → π for events with high jet multiplicity, due to limitations of the perturbative approach in describing soft phenomena in QCD. The data are equally well described by Monte Carlo predictions that supplement leading-order matrix elements with parton showering.

A bstract Charm production in charged current deep inelastic scattering has been measured for the first time in e ± p collisions, using data collected with the ZEUS detector at HERA, corresponding to an integrated luminosity of 358 pb −1 . Results are presented separately for e + p and e − p scattering at a centre-of-mass energy of s = 318 GeV within a kinematic phase-space region of 200 GeV 2 < Q 2 < 60000 GeV 2 and y < 0.9, where Q 2 is the squared four-momentum transfer and y is the inelasticity. The measured cross sections of electroweak charm production are consistent with expectations from the Standard Model within the large statistical uncertainties.

A bstract Isolated photons with high transverse energy have been studied in deep inelastic ep scattering with the ZEUS detector at HERA, using an integrated luminosity of 326 pb −1 in the range of exchanged-photon virtuality 10-350 GeV 2 . Outgoing isolated photons with transverse energy 4 <  E T γ  < 15 GeV and pseudorapidity − 0.7 <  η γ  < 0.9 were measured with accompanying jets having transverse energy and pseudorapidity 2.5 <  E T jet  < 35 GeV and −1 . 5 < η jet < 1 . 8, respectively. Differential cross sections are presented for the following variables: the fraction of the incoming photon energy and momentum that is transferred to the outgoing photon and the leading jet; the fraction of the incoming proton energy transferred to the photon and leading jet; the differences in azimuthal angle and pseudorapidity between the outgoing photon and the leading jet and between the outgoing photon and the scattered electron. Comparisons are made with theoretical predictions: a leading-logarithm Monte Carlo simulation, a next-to-leading-order QCD prediction, and a prediction using the k T -factorisation approach.

Measurements of open charm and beauty production cross sections in deep inelastic ep scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections are obtained in the kinematic range of negative four-momentum transfer squared of the photon 2.5GeV2≤Q2≤2000GeV2 and Bjorken scaling variable 3·10-5≤xBj≤5·10-2. The combination method accounts for the correlations of the statistical and systematic uncertainties among the different datasets. Perturbative QCD calculations are compared to the combined data. A next-to-leading order QCD analysis is performed using these data together with the combined inclusive deep inelastic scattering cross sections from HERA. The running charm- and beauty-quark masses are determined as mc(mc)=1.290-0.041+0.046(exp/fit)-0.014+0.062(model)-0.031+0.003(parameterisation) GeV and mb(mb)=4.049-0.109+0.104(exp/fit)-0.032+0.090(model)-0.031+0.001(parameterisation)GeV.

The HERAPDF2.0 ensemble of parton distribution functions (PDFs) was introduced in 2015. The final stage is presented, a next-to-next-to-leading-order (NNLO) analysis of the HERA data on inclusive deep inelastic ep scattering together with jet data as published by the H1 and ZEUS collaborations. A perturbative QCD fit, simultaneously of αs(MZ2) and the PDFs, was performed with the result αs(MZ2)=0.1156±0.0011(exp)-0.0002+0.0001(model+parameterisation)±0.0029(scale). The PDF sets of HERAPDF2.0Jets NNLO were determined with separate fits using two fixed values of αs(MZ2), αs(MZ2)=0.1155 and 0.118, since the latter value was already chosen for the published HERAPDF2.0 NNLO analysis based on HERA inclusive DIS data only. The different sets of PDFs are presented, evaluated and compared. The consistency of the PDFs determined with and without the jet data demonstrates the consistency of HERA inclusive and jet-production cross-section data. The inclusion of the jet data reduced the uncertainty on the gluon PDF. Predictions based on the PDFs of HERAPDF2.0Jets NNLO give an excellent description of the jet-production data used as input.