Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity neutral meson production in virtual photon proton collision at high energies. Due to the opposite -parities of photon and meson () this process probes the -channel odderon exchange which is described here as noninteracting three–gluon exchange. Estimates for the cross section of inelastic diffractive process are presented. The total cross section of diffractive meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons () in collisions are of the same order if the photon virtuality is . Received: 30 July 1997     

Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ^LT for the diffractive production of open charm (cc) from longitudinally and transversely polarised photons (of virtuality Q2) incident at high energy (√S) on a proton target. We study both the Q ² and M ² dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ^T, as well as for σ^L, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the square of the gluon density g(x, K²), and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for x = (Q ² + M ²)/s and scale K2 = (M ²c + )(1 + Q ²/M ²) where the average quark transverse momentum squared 〈k ₂ ^t ~ (M ²c . As compared to diffractive J/#x03C8; production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/#x03C8; wave function. We estimate the fraction of diffractive events that arise from cc¯ production.     

Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q²<0.01 GeV²) and deep-inelastic scattering processes (DIS, 4<Q²<80 GeV²). The event topology is given by ep→eXY, in which the system X, containing at least two jets, is separated from a leading low-mass baryonic system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5±0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.     

Small-angle photon scattering on complex nuclei

Photons of 3 GeV and 5 GeV were scattered on 7 different elements, ranging from Be to Au, and detected with a pair spectrometer. The angular distributions show diffractive patterns consistent with known nuclear sizes. Forward cross sections are 20–30% lower than expected from an A² dependence. This shadowing effect is qualitatively explained by photon interactions via intermediate hadronic states.     

Diffractive Scattering Factorization from J/ψ and γ Associated Production in HERA ep Collisions

Using three sets of Pomeron structure functions, the cross sections of J/ψ and γ associated productionvia resolved photon and proton diffractive scatting in ep collision are investigated. It is found that the cross sectionscalculated with various gluon distribution functions of Pomeron are different. The discrepancy is about 1.8 times fordifferential cross sections and 1.7 times for total cross sections. The experimental studies of the process could give valuableinsight in the diffractive production mechanism and test the color-octet mechanism for heavy quarkonium production ina new environment.     

Diffractive jet production in deep-inelastic e^+p collisions at HERA

A measurement is presented of dijet and 3-jet cross sections in low- diffractive deep-inelastic scattering interactions of the type , where the system X is separated by a large rapidity gap from a low-mass baryonic system Y. Data taken with the H1 detector at HERA, corresponding to an integrated luminosity of 18.0 pb, are used to measure hadron level single and double differential cross sections for and . The energy flow not attributed to jets is also investigated. The measurements are consistent with a factorising diffractive exchange with trajectory intercept close to 1.2 and tightly constrain the dominating diffractive gluon distribution. Viewed in terms of the diffractive scattering of partonic fluctuations of the photon, the data require the dominance of over states. Soft colour neutralisation models in their present form cannot simultaneously reproduce the shapes and the normalisations of the differential cross sections. Models based on 2-gluon exchange are able to reproduce the shapes of the cross sections at low values. Received: 21 December 2000 / Published online: 6 April 2001     

A study of elastic photoproduction of low massK + K − pairs from hydrogen in the energy range 2.8–4.8 GeV

The results of an experiment to study elasticK ⁺ K ^? photoproduction are presented. Differential cross sections and spin density matrix elements for ?(1.019) production are stddied as a function of incident photon energy and over a wide range of momentum transfer,t (t _min >t>?1.5(GeV/c)²). Helicity conserving amplitudes are observed to dominate ? production throughout this range and the differential cross sections exhibit a forward diffractive peak which cannot be understood in terms of a simple exponential dependence. A new value of the photon ? coupling constant is determined and shown to be consistent withe ⁺ e ^? annihilation measurements. A detailed study of the energy dependence of the differential cross sections is made, including other experimental data, and the extracted effective Regge trajectory compared with other diffractive processes. A study of the dependence of theK ⁺ K ^? decay angular distribution on invariant mass reveals evidence for ans wave contribution interfering with thep wave ? which may be attributable to theS ^* meson.     

Correlation with large transverse momentum photons and the gluon structure function

The inclusive two-particle cross section for the production of largep _T photons and opposite side charged hadrons in proton-proton collisions is examined in detail in the framework of QCD. The model, with the parametrization as in our previous work, agrees with the measured single photon spectra. Quantitative predictions are made for two-particle distributions to be measured at the ISR. The contributions due to the photon bremsstrahlung and the effects coming from the intrinsic constituent motion are estimated. It is shown that indeed the direct subprocess gluon+quark→photon+quark gives the dominant contribution in the experimentally relevant region of phase space, and that direct information about the gluon structure function can be obtained from such measurements.     

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction where PS denotes a pseudoscalar meson , , , or and X either a proton or resonance or continuum state into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in pp and forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering. Received: 2 February 1999 / Revised version: 22 March 1999 / Published online: 28 May 1999     

High Energy P+P(P)→P+P(P)+Xj(cc) Process and Two Methods for Calculating Double-Pomeron Exchange Mechanism

Using the X_j(cc) double diffractive production processes in high energy P + P(P) collisions, we have comparatively discussed the D-L phenomenological method of calculating the double-pomeron exchange (DPE) and the method based on the exchange of a pair of nonperturbative color singlet gluons. From the energy dependence of the total cross sections calculated with the two methods we find the two methods fitting well. We conclude that the two methods are consistent.     

Electroproduction of ω-mesons as a test of the ϱωπ-vertex

A simple model for electroproduction of ω-mesons is presented. The non-diffractive part is described by elementary OPE without corrections, combined with vector-meson dominance. For the diffractive production amplitudes we assume helicity conservation in the direct channel. We discuss the differential cross section and the spin-density matrix of the ω-mesons, particularly emphasizing the dependence on the mass of the virtual photon and the possibility to test the mass-dependence of the ?ωπ coupling constant.     

A comparison between pp and pp diffractive peaks at and beyond ISR energies

Although a substantial difference between pp differential cross section at ISR energies and $\text{p}\text{p}$ differential cross section at SPS energy has been reported, we demonstrate that both differential cross sections, at least up to |t| ～ 1.2 GeV², can be satisfactory accounted for within the usual eikonal model in which the diffractive component is crossing even. Thus at a given ultra high energy, there is no suggestive evidence for a difference between pp and $\text{p}\text{p}$ diffractive peaks.     

Diffractive photoproduction of charm at the HERA ep collider

For various kinematical domains, the cross section for the diffractive photoproduction of D* mesons at the HERA ep collider is calculated within a model based on perturbation theory. The results obtained by using different models of the Pomeron are compared.     

The diffractive component of multihadron production processes at high energies

Multihadron production at high energies proceeds through two mechanisms: A nondiffractive mechanism of a general multiperipheral nature and a diffractive mechanism. Assuming that the diffractive mechanism is dominated at present energies (s < 3000 BeV²) by single diffraction excitation we explore its theoretical and phenomenological implications. In particular we study the question of the rising total cross section. We show that “high mass diffraction excitation” leads to the formation of a central plateau in the inclusive single pion distribution and that the height of this plateau rises with energy and obeys scaling when normalized by the total cross section. We compute the multiplicity distribution for the diffractive component, and deduce the properties of the average multiplicity, its moments, and the statistical mechanics analogue of the “diffractive” Feynman gas. Two-particle correlations are computed and shown to be of a long range nature. Finally we outline an “improved” two component model into which some of the detailed features of the diffractive part are incorporated.     

Inclusive single and double diffractive dissociations inK + p interactions at 32 GeV/c

We study single and double inclusive diffractive production in a 32 GeV/c K ⁺ p experiment in MIRABELLE at the Serpukhov accelerator. From reactionsK ⁺ p→K ⁺+X ⁺ andK ⁺ p→X ⁺+p we determine the total proton and kaon single diffractive dissociation cross sections \(\sigma (p\xrightarrow{{K^ + }}p_D ) = 0.90 \pm 0.12 mb\) and \(\sigma (K^ + \xrightarrow{p}K_D^ + ) = 0\) . 90±0.17 mb, respectively. In either case the only notable contributions come from dissociations into 1 and 3 charged particles. Kaon dissociation exhibits a pronounced slope-mass correlation. The search for double diffractive production in reactionsK ⁺ p→(K ⁺π^?π⁺)+X ⁺ andK ⁺ p→(pπ ⁺ π ^?)+X ⁺ leads in either case to an estimated total double diffractive cross section σ(K ⁺p→K _D ⁺ p_D) of ?220 μb. The double dissociation differential cross section exhibits a large slope of ?10GeV^?2 in the nearthreshold mass region, rapidly decreasing to ?4 GeV^?2 with increasing excitation mass. At our c.m. energy \((\sqrt s \simeq 8 GeV)\) the ratio σ_{inel dif}/σ_el is 0.85±0.10, the total diffractive cross section σ_dif≡σ_el+σ_{inel dif}=4.41±0.24 mb and the fraction σ_dif(K ⁺ p)/σ_tot(K ⁺ p) is 25±2%. TheK ⁺ andK ^? diffractive excitation mass spectra, differential cross sections and total diffractive cross sections are very similar for both single and double dissociations.     

Measurement and QCD analysis of the diffractive deep-inelastic scattering cross section at HERA

A detailed analysis is presented of the diffractive deep-inelastic scattering process ep→eXY, where Y is a proton or a low mass proton excitation carrying a fraction 1-x_IP>0.95 of the incident proton longitudinal momentum and the squared four-momentum transfer at the proton vertex satisfies |t|<1 GeV². Using data taken by the H1 experiment, the cross section is measured for photon virtualities in the range 3.5≤Q²≤1600 GeV², triple differentially in x_IP, Q² and β=x/x_IP, where x is the Bjorken scaling variable. At low x_IP, the data are consistent with a factorisable x_IP dependence, which can be described by the exchange of an effective pomeron trajectory with intercept α_IP(0)=1.118±0.008(exp.)^+0.029 _-0.010(model). Diffractive parton distribution functions and their uncertainties are determined from a next-to-leading order DGLAP QCD analysis of the Q² and β dependences of the cross section. The resulting gluon distribution carries an integrated fraction of around 70% of the exchanged momentum in the Q² range studied. Total and differential cross sections are also measured for the diffractive charged current process e⁺p→ν̄_eXY and are found to be well described by predictions based on the diffractive parton distributions. The ratio of the diffractive to the inclusive neutral current ep cross sections is studied. Over most of the kinematic range, this ratio shows no significant dependence on Q² at fixed x_IP and x or on x at fixed Q² and β.     

The pomeron-particle total cross section and diffractive production of showers at very high energies

The experimental data of ISR and NAL for the process pp → pX are used for the determination of the pomeron-particle total cross section and the triple-pomeron vertex. Estimates of the cross sections of the diffractive production of one and several showers of particles are given. It is shown that at s ～ 10³ GeV² the cross section of all inelastic diffractive processes is ～ 6 mb and grows with energy. A theoretical estimate of the amplitude of pomeron-pomeron scattering is given.     

Photoproduction of large transverse momentum mesons and production of large-pT photons and leptons in proton-proton collisions

The photoproduction of large transverse momentum mesons is examined in a leading particle model using the techniques of the covariant parton model. In the leading particle model the large-p_T meson is produced in a high-energy, wide-angle scattering: γ + q → M + q. Numerical results such as inclusive cross sectioons, angular distributions and particle ratios are presented for the pseudoscalar mesons. The Landshoff-Polking-horne quark-fusion model is then applied to the production of large transverse momentum photons and leptons in proton-proton collisions. In the lepton case this model differs from the usual Drell-Yan mechanism in that the lepton pair has a small invariant mass but comes from a large-p_T virtual photon.     

Study of Strangeness Photoproduction on the Neutron in the Threshold Region

Photoproduction of the neutral kaon on the deuteron has been investigated at the Research Center for Electron Photon Science, Tohoku University. We constructed the Neutral Kaon Spectrometer-2 for the detection of charged particles from the decay of the neutral kaon and the hyperon. We obtained a momentum distribution of K ⁰ with the inclusive measurement. It was consistent with the previous measurement. The total cross section of γ + d → K ⁰ + Λ + p was estimated from the measured integral cross section of γ + d → Λ + X. The total cross section with respect to the photon energy was compared with the theoretical calculations. It favored the Saclay-Lyon A model calculation with the ratio of the neutral to charged coupling constants of the axial-vector meson, K ₁, as ~ ?1.5. The energy dependence and the magnitude of the total cross section were similar to the total cross section of γ + p → K ⁺ Λ.