Diffractive Structure Function Research Articles

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (ϰγOBS) and pomeron (βOBS) momentum participating in the production of the dijet system. The observed ϰγOBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section da/dβOBS increases as βOBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states.

Towards a unified description of total and diffractive structure functions at DESY HERA in the QCD dipole picture

It is argued that the QCD dipole picture allows us to build a unified theoretical description, based on Balitskii-Fadin-Kuraev-Lipatov dynamics, of the total and diffractive nucleon structure functions. This description is in qualitative agreement with the present collection of data obtained by the H1 Collaboration. More precise theoretical estimates, in particular the determination of the normalizations and proton transverse momentum behavior of the diffractive components, are shown to be required in order to reach definite conclusions.

Multiplicity structure of the hadronic final state in diffractive deep-inelastic scattering at HERA

The multiplicity structure of the hadronic system X produced in deep-inelastic processes at HERA of the type ep -> eXY, where Y is a hadronic system with mass M_Y< 1.6 GeV and where the squared momentum transfer at the pY vertex, t, is limited to |t|<1 GeV^2, is studied as a function of the invariant mass M_X of the system X. Results are presented on multiplicity distributions and multiplicity moments, rapidity spectra and forward-backward correlations in the centre-of-mass system of X. The data are compared to results in e+e- annihilation, fixed-target lepton-nucleon collisions, hadro-produced diffractive final states and to non-diffractive hadron-hadron collisions. The comparison suggests a production mechanism of virtual photon dissociation which involves a mixture of partonic states and a significant gluon content. The data are well described by a model, based on a QCD-Regge analysis of the diffractive structure function, which assumes a large hard gluonic component of the colourless exchange at low Q^2. A model with soft colour interactions is also successful.

Diffractive color-dipole nucleon scattering

We determine the diffractive scattering amplitude of a color dipole on a nucleon using a nonperturbative model of QCD which contains only parameters taken from low-energy physics. This allows us to relate specific features of the confinement mechanisms with diffractive electroproduction processes and structure functions. The agreement with phenomenological data is satisfactory.

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

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with χ L > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section ont is measured in the interval 0.073 < |t| < 0.4 GeV2 and is found to be described by an exponential shape with the slope parameterb = 7.2 ± 1.1(stat.) +0.7−0.9 (syst.) GeV−2. The diffractive structure function FD (4) is presented as 0.9 a function of χ H ≃ 1 − χ L and β, the momentum fraction of the struck quark with respect to χ H , and averaged over thet interval 0.073 < |t′ < 0.4 GeV2 and the photon virtuality range 5 <Q 2 < 20 GeV2. In the kinematic range 4 × 104 < χ p < 0.03 and 0.015 < β < 0.5, the χ p dependence ofF D(4) is fitted with a form (1/χ p )α , yieldinga − 1.00 ± 0.09 (stat.) +0.11−0.05 (syst.). Upon integration overL, the structure functionF 2 D(3) is determined in a kinematic range extending to higher χ p and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

Open charm production in diffractive ep collisions

The cross section for the diffractive reactionγ + p → cc + p with a real or virtual photon is calculated in the nonperturbative two-gluon exchange model of Landshoff and Nachtmann. Numerical predictions are given for cross sections and spectra at typical HERA values of c.m. energy and photon virtuality. The contribution of charm to the diffractive structure function is evaluated and found to be rather small in the model, and the ratio between the production rates for bb and cc is tiny.

Twist-4 effects and Q2 dependence of diffractive DIS

In this letter we report the direct perturbative QCD evaluation of twist-4 effects in diffractive DIS. They are large and have a strong impact on the Q 2 dependence of diffractive structure functions at large β. Based on the AGK rules, we comment on the possible contribution from diffractive higher twists to ∝ 1 Q 2 corrections to proton structure function at small x. These corrections to the longitudinal structure function F L may be particularly large.

Study on the Pomeron structure function

A study about the Pomeron structure function based on diffractive deep inelastic scattering data measured at the DESY HERA experiments is performed. In this study it is assumed that the diffractive structure function is given by the product of the Pomeron flux factor and the Pomeron structure function. In order to obtain the Pomeron structure function, two different forms of the Pomeron flux factor are employed, one that is usually obtained within the Regge pole phenomenology and another which comes from the renormalization of the standard Regge flux factor. It is shown that these two schemes of analysis lead to very different conclusions about the Pomeron constitution in terms of quarks and gluons.

Measurement of the diffractive structure function

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with χ L > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section ont is measured in the interval 0.073 < |t| < 0.4 GeV2 and is found to be described by an exponential shape with the slope parameterb = 7.2 ± 1.1(stat.) +0.7−0.9 (syst.) GeV−2. The diffractive structure function FD (4) is presented as 0.9 a function of χ H ≃ 1 − χ L and β, the momentum fraction of the struck quark with respect to χ H , and averaged over thet interval 0.073 < |t′ < 0.4 GeV2 and the photon virtuality range 5 <Q 2 < 20 GeV2. In the kinematic range 4 × 104 < χ p < 0.03 and 0.015 < β < 0.5, the χ p dependence ofF D(4) is fitted with a form (1/χ p )α , yieldinga − 1.00 ± 0.09 (stat.) +0.11−0.05 (syst.). Upon integration overL, the structure functionF 2 D(3) is determined in a kinematic range extending to higher χ p and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_gamma^OBS) and pomeron (beta^OBS) momentum participating in the production of the dijet system. The observed x_gamma^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d(sigma)/d(beta^OBS) increases as beta^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

Reggeon and pion contributions in semiexclusive diffractive processes at DESY HERA

A detailed analysis of semi-exclusive diffractive processes in ep DIS at HERA, with the diffractive final states in the forward direction is presented. The contributions of the subleading f2, !, a2, � reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the (a2,�) reggeons are entirely responsible for the forward neutron production

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states.

Diffraction in deep inelastic scattering: the hadronic nature of quarks

In the framework of Anisotropic Chromo Dynamics, a non-perturbative model of QCD based on magnetic condensation in the vacuum, we develope a picture of the Pomeron. Within this model we are able to calculate the diffractive contribution to deep inelastic scattering in the small Bjorken x region, actually probed at Hera. We calculate the diffractive structure function of the proton and make the comparison with the experimental data of Zeus and H1 collaborations. Good agreement with the experimental data, both for the diffractive structure function $F_2^D$ and for the ''structure function'' of the Pomeron, is achieved.

Diffractive Higgs production at the LHC

We use diffractive parton distributions obtained from fits to the diffractive structure function measured at HERA to predict cross sections for single diffractive Higgs production at the LHC. The dominant background processes are also considered. Although some 5% - 15% of Higgs events are predicted to be diffractive in this model, the ratio of signal to background is not significantly improved.

Production of charm quark jets in DIS diffractive dissociation

We present a calculation of open charm quark production in diffractive deep inelastic electron-proton scattering in a perturbative QCD framework. The cross section is proportional to the square of the gluon density and explicitly displays breaking of Regge factorization. Jet cross sections as well as the charm contribution to the diffractive structure function are calculated. As a consequence of the steep rise of the gluon density at small x the charm contribution to $F_2^{D}$ rises with decreasing x_P.

Unified description of rapidity gaps and energy flows in DIS final states

We show that the ‘orthogonal’ characteristics of the observed rapidity gaps and large forward energy flows in deep inelastic scattering at HERA, can be described within a single framework. Our Monte Carlo model is based on perturbative QCD matrix elements and parton showers together with Lund string model hadronization, but has in addition a new mechanism for soft colour interactions which modifies the perturbative colour structure and thereby the hadronization. Effects of perturbative multiparton emission are investigated and the non-perturbative treatment of the proton remnant is discussed and comparison to the observed transverse energy flow is made. We investigate the resulting diffractive-like properties of the model; such as rapidity gap events, t- and MX-distributions and the diffractive structure function in comparison to H1 data.

Diffractive leptoproduction of small masses in QCD

In this paper we consider the process of diffraction dissociation in deep inelastic scattering in which a small mass is produced. This process is analyzed by calculating qq and qqG production. We show that the small distance contributions ( r⊥ ∝ 1/ Q) to the longitudinal polarized virtual photon dominate the diffractive channels. Formulae for the cross section using the gluon density are written within the framework of perturbative QCD. We show that the production of small masses by a transverse polarized photon is concentrated at moderate values of r⊥ ≈ 1 GeV −1, where the pQCD approach is valid. It is shown that only qq pair production contributes to diffraction dissociation with β > 0.4, and the possibility to extract the values of the gluon density from the measurements in this kinematic region is discussed. Shadowing corrections are assessed for both longitudinal and transverse polarized photons, and estimates of the different damping factors are given. The relation between diffractive production and the corrections to F 2 is alluded to. The evolution of the diffractive structure function is studied and a solution to the diffractive evolution equations is proposed.

Gluon radiation in diffractive electroproduction

Order α s corrections to the diffractive structure functions F L D and F 2 D at large Q 2 and small x are evaluated in the semiclassical approach, where the initial proton is treated as a classical color field. The diffractive final state contains a fast gluon in addition to a quark-antiquark pair. Two of these partons may have large transverse momentum. Our calculations lead to an intuitive picture of deep inelastic diffractive processes which is very similar to Bjorken's aligned-jet model. Both diffractive structure functions contain leading twist contributions from high- p⊥ jets.

Subleading Reggeons in deep inelastic diffractive scattering at DESY HERA

The contribution of subleading Reggeons to the diffractive structure function dF{sub 2}{sup D}/dx{sub P}dt is estimated from soft physics data. This contribution leads in a natural way to the violation of the factorization property of the diffractive structure function. {copyright} {ital 1997} {ital The American Physical Society}