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HERAFitter is an open-source package that provides a framework for the determination of the parton distribution functions (PDFs) of the proton and for many different kinds of analyses in Quantum Chromodynamics (QCD). It encodes results from a wide range of experimental measurements in lepton–proton deep inelastic scattering and proton–proton (proton–antiproton) collisions at hadron colliders. These are complemented with a variety of theoretical options for calculating PDF-dependent cross section predictions corresponding to the measurements. The framework covers a large number of the existing methods and schemes used for PDF determination. The data and theoretical predictions are brought together through numerous methodological options for carrying out PDF fits and plotting tools to help to visualise the results. While primarily based on the approach of collinear factorisation, HERAFitter also provides facilities for fits of dipole models and transverse-momentum dependent PDFs. The package can be used to study the impact of new precise measurements from hadron colliders. This paper describes the general structure of HERAFitter and its wide choice of options.

Recent measurements of the W -boson charge asymmetry and of the Z -boson production cross sections, performed at the Tevatron collider in Run II by the D0 and CDF collaborations, are studied using the HERAFitter framework to assess their impact on the proton parton distribution functions (PDFs). The Tevatron measurements, together with deep-inelastic scattering data from HERA, are included in a QCD analysis performed at next-to-leading order, and compared to the predictions obtained using other PDF sets from different groups. Good agreement between measurements and theoretical predictions is observed. The Tevatron data provide significant constraints on the d -valence quark distribution.

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 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 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.

Sets of parton distribution functions (PDFs) of the proton are reported for the leading (LO), next-to-leading (NLO) and next-to-next-to-leading-order (NNLO) QCD calculations. The parton distribution functions are determined with the HERAFitter program using the data from the HERA experiments and preserving correlations between uncertainties for the LO, NLO and NNLO PDF sets. The sets are used to study cross-section ratios and their uncertainties when calculated at different orders in QCD. A reduction of the overall theoretical uncertainty is observed if correlations between the PDF sets are taken into account for the ratio of W W di-boson to Z boson production cross sections at the LHC.

An NLO photon parton parametrization is presented based on the existing measurements from e + e- data and the low-x proton structure function from ep interactions. Also included in the extraction of the NLO parton distribution functions are the dijets data coming from γp → j1 + j2 + X. The new parametrization is compared to other NLO parametrizations.

HERAFitter is an open-source package that provides a framework for the determination of the parton distribution functions (PDFs) of the proton and for many different kinds of analyses in Quantum Chromodynamics (QCD). It encodes results from a wide range of experimental measurements in lepton-proton deep inelastic scattering and proton-proton (proton-antiproton) collisions at hadron colliders. These are complemented with a variety of theoretical options for calculating PDF-dependent cross section predictions corresponding to the measurements. The framework covers a large number of the existing methods and schemes used for PDF determination. The data and theoretical predictions are brought together through numerous methodological options for carrying out PDF fits and plotting tools to help to visualise the results. While primarily based on the approach of collinear factorisation, HERAFitter also provides facilities for fits of dipole models and transverse-momentum dependent PDFs. The package can be used to study the impact of new precise measurements from hadron colliders. This paper describes the general structure of HERAFitter and its wide choice of options.

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.