Measurement of the diffractive structure functionF 2 D(4) at HERA

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Abstract:The cross sections for inelastic photoproduction of J/ψ and ψ ′ mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb −1 collected in the period 1996-2007. The ψ ′ to J/ψ cross section ratio was measured in the range 0.55 < z < 0.9 and 60 < W < 190 GeV as a function of W , z and p T . Here W denotes the photon-proton centre-of-mass energy, z is the fraction of the incident photon energy carried by the meson and p T is the transverse momentum of the meson with respect to the beam axis. The J/ψ cross sections were measured for 0.1 < z < 0.9, 60 < W < 240 GeV and p T > 1 GeV. Theoretical predictions within the nonrelativistic QCD framework including NLO colour-singlet and colour-octet contributions were compared to the data, as were predictions based on the k T -factorisation approach. 6 Systematic uncertainties 77 Results 8 7.1 ψ ′ to J/ψ cross section ratio 8 7.2 J/ψ differential cross sections 9 7.2.1 Comparison of NRQCD calculation 9 7.2.2 Comparison of k T -factorisation approach 10 7.3 Momentum flow along and against the J/ψ direction 10 Conclusions 11The ZEUS collaboration 27 IntroductionThe inelastic production of J/ψ and of ψ ′ has been studied for several years in hadron and electron-proton colliders and in fixed target experiments [1]. At HERA, the reactions ep → eJ/ψX, (1.1) and ep → eψ ′ X, (1.2) have been studied [2,3] for low virtuality of the exchanged photon (photoproduction) in the range z < 0.9, where z denotes the fraction of the incident photon energy carried by the meson in the proton rest frame, thus excluding the diffractive process for which z ∼ 1. In the HERA photoproduction regime, the production of inelastic J/ψ or ψ ′ mesons arises mostly from direct and resolved photon interactions. In leading-order (LO) Quantum Chromodynamics (QCD), the two processes can be distinguished; in directphoton processes the photon enters directly into the hard interaction; in resolved-photon processes the photon acts as a source of partons, one of which participates in the hard -1 - JHEP02(2013)071interaction. The inelastic process in the photoproduction region is dominated by photongluon fusion. In this direct-photon process the photon emitted from the incoming electron interacts with a gluon from the proton to produce a pair of charm-anticharm quarks, cc, which then turn into the J/ψ or the ψ ′ mesons. When the cc pair emerges from the hard process with the quantum numbers of the mesons, the reaction is described in the framework of perturbative Quantum Chromodynamics (pQCD) by models such as the Colour Singlet (CS) model. In the Colour Octet (CO) model, the cc pair emerges from the hard process with quantum numbers different from those of the mesons and emits one or more soft gluons before turning into the physical meson state. Examples of direct-photon LO diagrams with a CS and a CO hard subprocess are shown in figure 1.Full next-to-leading order (NLO) J/ψ cross section predictions using only the directphoton CS contributions have al...

Section: Jhep02(2013)071 3 Event Selection and Kinematic Variablesmentioning

confidence: 99%

Measurement of inelastic J/ψ and ψ′ photoproduction at HERA

Abramowicz

Koffeman

Kooijman

2013

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“…The hadronic system was reconstructed from energy-flow objects (EFOs) [70,71] which combine the information from calorimetry and tracking, corrected for energy loss in the detector material. The kinematic variables used in the cross-section measurements, Q 2 and x, were reconstructed using the double-angle method [72].…”

Section: Data Selectionmentioning

confidence: 99%

Measurement of beauty and charm production in deep inelastic scattering at HERA and measurement of the beauty-quark mass

Abramowicz¹,

Abt²,

Adamczyk³

et al. 2014

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The production of beauty and charm quarks in ep interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared 5 < Q 2 < 1000 GeV 2 using an integrated luminosity of 354 pb −1 . The beauty and charm content in events with at least one jet have been extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of Q 2 , Bjorken x, jet transverse energy and pseudorapidity were measured and compared with next-to-leading-order QCD calculations. The beauty and charm contributions to the proton structure functions were extracted from the double-differential cross section as a function of x and Q 2 . The running beauty-quark mass, m b at the scale m b , was determined from a QCD fit at next-to-leading order to HERA data for the first time and found to be m b (m b ) = 4.07 ± 0.14 (fit) +0.01 −0.07 (mod.) +0.05 −0.00 (param.) +0.08 −0.05 (theo.) GeV.Keywords: Lepton-Nucleon Scattering, QCD, Jets, B physics The ZEUS collaboration 53 IntroductionThe measurement of beauty and charm production in ep collisions at HERA is an important testing ground for perturbative Quantum Chromodynamics (pQCD), since the heavy-quark masses provide a hard scale that allows perturbative calculations to be made. At leading order, the dominant process for heavy-quark production at HERA is boson-gluon fusion (BGF). In this process, a virtual photon emitted by the incoming electron interacts with a gluon from the proton forming a heavy quark-antiquark pair. When the negative squared four-momentum of the virtual photon, Q 2 , is large compared to the proton mass, the interaction is referred to as deep inelastic scattering (DIS). For heavy-quark transverse momenta comparable to the quark mass, next-to-leading-order (NLO) QCD calculations based on the dynamical generation of the massive quarks [1][2][3][4] are expected to provide reliable predictions. Beauty and charm production in DIS has been measured using several methods by the H1 [5][6][7][8][9][10][11][12][13][14][15][16] and ZEUS [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Inclusive jet cross sections in beauty and charm events are used in the analysis presented here to extract the heavy-quark contribution to the proton structure function F 2 with high precision, and to measure the b-quark mass. For this purpose, the long lifetimes of the weakly decaying b and c hadrons, which make the reconstruction of their decay vertices possible, as well as their large masses were exploited. Two discriminating variables, the significance of the reconstructed decay length and the invariant mass of the charged tracks associated with the decay vertex (secondary vertex), were used. This inclusive tagging method leads to a substantial increase in statistics with respect to previous ZEUS measurements.Differential cross sections as a function of Q 2 , the Bjorken scaling variable, x, jet transverse energy, E jet T , and pseudorapidit...

“…3 Energy-flow objects [41][42][43] were constructed from calorimeter-cell clusters and tracks, associated when possible. 4 The upper limit was selected to retain distinguishable shower shapes between the hadronic background and the photon signal.…”

Section: Jhep01(2018)032mentioning

confidence: 99%

Further studies of isolated photon production with a jet in deep inelastic scattering at HERA

Abramowicz

Abt

Adamczyk

et al. 2018

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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 γ T < 15 GeV and pseudorapidity −0.7 < η γ < 0.9 were measured with accompanying jets having transverse energy and pseudorapidity 2.5 < E jet T < 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.