g_1 at low x and low Q^2 with Polarized ep colliders

Measurements of at low x and low are expected to provide a sensitive probe of the transition from Regge to perturbative QCD dynamics, offering a new testing ground for models of small x physics. We discuss the potential of polarized ep colliders (Polarized HERA and eRHIC) to investigate this physics – varying between 0.01 and 1 GeV– and to constrain the high-energy part of the Drell-Hearn-Gerasimov sum-rule for polarized photoproduction. Received: 28 August 2000 / Revised version: 9 November 2000 / Published online: 21 December 2000     

Ranges of interaction inp\bar p scattering at low energies

Thep¯ p cross sections at low energies are analysed in the framework of the optical model. It is shown that the annihilation range is closely related to the size of the proton as measured in electromagnetic and hadronic interactions. At larger distances thep¯ p interaction has the similar character as the NN one.     

Forward-jet production in deep inelastic ep scattering at HERA

Forward jet cross sections have been measured in neutral current deep inelastic scattering at low Bjorken-x with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb^-1. Measurements are presented for inclusive forward jets as well as for forward jets accompanied by a dijet system. The explored phase space, with jet pseudorapidity up to 4.3 is expected to be particularly sensitive to the dynamics of QCD parton evolution at low x. The measurements are compared to fixed-order QCD calculations and to leading-order parton-shower Monte Carlo models.     

Deep-inelastic neutron scattering determination of the single-particle kinetic energy in solid and liquid 3He

An experimental determination of the single-particle mean kinetic energies for 3He along the T = 2.00 K isotherm in the dense liquid and in the solid hcp and bcc phases is reported. Deep-inelastic neutron scattering measurements at exchanged wave vectors ranging from 90.0 to 140.0 A(-1) have been performed in order to evaluate, within the framework of the impulse approximation, the molar volume dependence of the kinetic energy. The results are found in excellent agreement with recent diffusion Monte Carlo simulations.     

Charged hadron multiplicities and inclusive π− distributions in inelastic ep scattering

Electroproduction of hadrons is studied in the kinematic region W < 2.8 GeV and 0.3 < Q² < 1.4 GeV² using the DESY streamer chamber. Prong cross sections, charged-particle multiplicities and inclusive π^? distributions are presented. The average charged multiplicity is found to be independent of Q² in the Q² range studied here; however it is lower than in photoproduction. The fraction of forward π^? is found to be significantly less in electroproduction than in photoproduction. The 〈p_⊥²〉 for inclusive π^? is, for all x values, similar to that found in photoproduction.     

The inclusive proton spectrum for\bar pd \to 3\pi p at rest re-examined

We re-analyze the spectator proton distribution for ¯pd→π ⁺ π^? π^? p. For this channel previous calculations underestimated the cross section for proton momenta of 300 to 600 MeV/c by about a factor 7 while the channels with higher pion multiplicities and the ¯pd→K¯ Kmπp reactions were well explained. By using detailed experimental information on ¯pn and ¯pp annihilation, meson rescattering can in fact explain the cross section also for the 3π case. The¯ pp annihilation followed by charge exchange pion rescattering is much larger than in the 5π case. Vector meson exchange is small but not negligible at high spectator momenta.     

The generalised vector dominance/colour-dipole picture of deep-inelastic scattering at low x

We give a detailed account of the recently formulated generalised vector dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low , including photoproduction. The approach, based on transitions, propagation and diffractive scattering via the generic structure of the two-gluon exchange, provides a unique and quantitatively successful theory for the total cross section, , at low x. The GVD/CDP is shown to imply the empirical low-x scaling law, with , that was established by a model-independent analysis of the experimental data. Received: 16 February 2001 / Published online: 3 May 2001     

Search for a narrow charmed baryonic state decaying to $D^{\ast \pm} p^\mp$in ep collisions at HERA

A resonance search has been made in the invariant-mass spectrum with the ZEUS detector at HERA using an integrated luminosity of . The decay channels and (and the corresponding antiparticle decays) were used to identify mesons. No resonance structure was observed in the mass spectrum from more than 60 000 reconstructed mesons. The results are not compatible with a report of the H1 Collaboration of a charmed pentaquark, .Received: 14 September 2004, Revised: 29 September 2004, Published online: 9 November 2004     

Multijet production in neutral current deep inelastic scattering at HERA and determination of $\alpha_s$

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities . The data were taken at the ep collider HERA with centre-of-mass energy using the ZEUS detector and correspond to an integrated luminosity of . Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy ( ), pseudorapidity ( ) and Q² with and . Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant , determined from the ratio of the trijet to dijet cross sections, is . Received: 1 April 2005, Revised: 28 June 2005, Published online: 30 August 2005     

Search for QCD-instanton induced events in deep inelastic ep scattering at HERA

A search for QCD-instanton-induced events in deep inelastic ep scattering has been performed with the ZEUS detector at the HERA collider, using data corresponding to an integrated luminosity of 38 pb^-1. A kinematic range defined by cuts on the photon virtuality, Q² > 120 GeV², and on the Bjorken scaling variable, x > 10^-3, has been investigated. The QCD-instanton induced events were modelled by the Monte Carlo generator QCDINS. A background-independent, conservative 95% confidence level upper limit for the instanton cross section of 26 pb is obtained, to be compared with the theoretically expected value of 8.9 pb.Received: 17 December 2003, Published online: 2 April 2004     

Inclusive measurement of diffractive deep-inelastic ep scattering

A measurement is made of the cross section for the process ep → eXY in deep-inelastic scattering with the H1 detector at HERA. The cross section is presented in terms of a differential structure function F ₂ ^D(3) (x _P, β,Q ²) of the proton over the kinematic range 4.5 < Q ² < 75 GeV². The dependence of F ₂ ^D(3) on x _P is found to vary with β, demonstrating that a factorisation of F ₂ ^D(3) with a single diffractive flux independent of β and Q ² is not tenable. An interpretation in which a leading diffractive exchange and a subleading reggeon contribute to F ₂ ^D(3) reproduces well the x _P dependence of F ₂ ^D(3) with values for the pomeron and subleading reggeon intercepts of α_P (0) = 1.203±0.020(stat.)±0.013(sys.) _-0.035 ^+0.030 (model) and α _R (0) = 0.50 ± 0.11(stat.) ± 0.11(sys.) _-0.10 ^+0.09 (model), respectively. A fit is performed of the data using a QCD motivated model, in which parton distributions are assigned to the leading and subleading exchanges. In this model, the majority of the momentum of the pomeron must be carried by gluons in order for the data to be well described.     

A two-com ponent model for $\gamma^*$- p scattering at small Bjorken variable x

We extend the Golec-Biernat-Wüsthoff model for virtual photon-proton scattering to include the resolved photon component explicitly. The parameters of the resolved photon component are taken from the literature, while the parameters of the dipole-nucleon interaction are fitted to the HERA data in a selected limited range of x and Q ². A good agreement with the experimental data is obtained beyond the region of the fit.Received: 16 July 2003, Published online: 17 October 2003     

Off-diagonal generalized vector dominance and inelastic ep scattering

By also taking into account off-diagonal contributions to the virtual forward Compton amplitude, scaling in ep scattering is obtained from scaling of the total cross section for e⁺e^? annihilation into hadrons. The parameter which sets the scale for the q² dependence in ep scattering is calculated to be $\text{m}{}^2\text{? 0.61 m}{}_{\mathrm{?}}{}^2\text{? 0.36}\text{GeV}{}^2$.