Dissociation of virtual photons in events with a leading proton at HERA

The ZEUS detector has been used to study dissociation of virtual photons in events with a leading proton, , in e ⁺ p collisions at HERA. The data cover photon virtualities in two ranges, 0.03 < Q ² < 0.60 GeV² and 2 < Q ² < 100 GeV², with M _X > 1.5 GeV, where M _X is the mass of the hadronic final state, X. Events were required to have a leading proton, detected in the ZEUS leading proton spectrometer, carrying at least 90% of the incoming proton energy. The cross section is presented as a function of t, the squared four-momentum transfer at the proton vertex, , the azimuthal angle between the positron scattering plane and the proton scattering plane, and Q ². The data are presented in terms of the diffractive structure function, . A next-to-leading-order QCD fit to the higher-Q ² data set and to previously published diffractive charm production data is presented.Received: 10 August 2004, Revised: 4 October 2004, Published online: 9 November 2004     

On the determination of double diffraction dissociation cross section at HERA

The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processesep→e′p′ρ ⁰,e′p′J/ψ,e′p′X at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in theJ/ψ photoproduction, electroproduction of theρ ⁰ at largeQ ² and diffraction dissociation of real and virtual photons into high mass statesX the contamination of the double diffraction dissociation can be as large as ∼30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the protonproton collisions.     

Equation of state of hadronic matter and electromagnetic radiation from relativistic heavy ion collisions

We study the radiation of thermal photons and dileptons likely to be produced in relativistic heavy ion collisions. We find that the thermal photon multiplicity scales with the charged pion multiplicity (dN_ch/dy) as (dN_ch/dy)^α with α1.2 for a transversely expanding system, contrary to the general belief of a quadratic dependence. The scaling is shown to be valid, both for real and virtual photons. The coefficient of proportionality at a given energy shows, however, strong dependance on the equation of state of hot hadronic matter produced in such collisions.     

Energy Dependence of Mean Charged Multiplicity in Hadron-hadron Collision and Modified HEISENBERG'S Approach

A semi empirical formulation for the energy dependence of multiplicity in hadronic collisions as proposed in a recent paper by Ghosh et al. has been applied to account for the experimental multiplicity data of p?p, p?n and π^±n collisions. A remarkable agreement has been found over the entire region.     

Multiplicity structure of inclusive diffraction in À+ p andK + p interactions at 250 GeV/c

Results on the multiplicity structure of diffractively excited meson and proton systems in À⁺/K⁺ p interactions at 250 GeV/c are presented for diffractive masses up to about 9 GeV. The energy dependence of the average charge multiplicity and the shape of the multiplicity distribution in terms of KNO-scaling and negative binomial distribution are investigated. The diffractive systems are compared toe ⁺ e ^–,lh and non-diffractivehh final states as suggested by modern approaches of the Pomeron-hadron collision. Systematic differences are found between diffractive meson and proton systems but also between diffraction and the reactions compared to.     

Possible signatures of the hadronisation scale in parton jets

Models for hadron production in hard collisions differ widely in the energy scale characteristic of the transition from the primary partonic to the secondary hadronic phase of jet evolution. We investigate possible experimental signatures for the existence of both phases. In particular, we consider multiplicity and energy moments, long range charge correlations and angular correlations as a function of total energy or near the exclusive two body limit ine ⁺ e ^? annihilation and deep inelastic scattering processes. The possibility of a dual correspondence between hadronic and partonic states is discussed.     

Transverse momentum dependence of the e/π ratio and the decay of continuum vector states into di-leptons

The recently observed rise of the e/π ratio at small transverse momentum in pp collisions can be understood in a model with continuum hadronic vector states which decay into di-leptons. The mass and momentum distribution of the continuum hadronic states—which we derive from the distribution of produced quarks—is consistent with the distribution of hadronic clusters as determined from multi hadron correlation studies.     

A theoretical approach to low multiplicity diffractive dissociation

We investigate the dynamics of low mass inelastic diffractive production in the framework of the “1/N dual unitarization” scheme. The smallness of inelastic diffractive dissociation is explicitly demonstrated by incorporating a Deck type mechanism with the crucial planar bootstrap equation. Although both inelastic and elastic pomeron couplings are of the same order in 1/N, the origin for their smallness, however, is not identical. Our work further confirms the validity of the iterative procedure, where the elastic amplitude is first generated from only non-diffractive intermediate states (except possibly for central collisions). Using a previous study of the “Cylinder” strength, we present also a semi-quantitative results for the integrated cross-section for low multiplicity diffractive production and compare it with the elastic cross-section at very high energies.     

Inclusive production of charged hadrons and {\rm K}^0_{\rm S} mesons in photon–photon collisions

The production of charged hadrons and mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in interactions with -proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions. Received: 4 June 1998 / Published online: 6 November 1998     

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     

Measurement of the Q and energy dependence of diffractive interactions at HERA

Diffractive dissociation of virtual photons, , has been studied in ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 < Q ² < 0.70 GeV² and 3 < Q ² < 80 GeV² with 3 < M_X < 38 GeV, where M_X is the mass of the hadronic final state. Diffractive events were selected by two methods: the first required the detection of the scattered proton in the ZEUS leading proton spectrometer (LPS); the second was based on the distribution of M_X. The integrated luminosities of the low- and high-Q² samples used in the LPS-based analysis are 0.9 pb^-1 and 3.3 pb^-1, respectively. The sample used for the M_X-based analysis corresponds to an integrated luminosity of 6.2 pb^-1. The dependence of the diffractive cross section on W, the virtual photon-proton centre-of-mass energy, and on Q² is studied. In the low-Q² range, the energy dependence is compatible with Regge theory and is used to determine the intercept of the Pomeron trajectory. The W dependence of the diffractive cross section exhibits no significant change from the low-Q² to the high-Q² region. In the low-Q² range, little Q² dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q ² > 3 GeV². The ratio of the diffractive to the virtual photon-proton total cross section is studied as a function of W and Q². Comparisons are made with a model based on perturbative QCD. Received: 27 March 2002 / Published online: 9 August 2002     

Measurements of transverse energy flow in deep-inelastic scattering at HERA

Measurements of transverse energy flow are presented for neutral current deep-inelastic scattering events produced in positron-proton collisions at HERA. The kinematic range covers squared momentum transfers from 3.2 to 2 200 GeV, the Bjorken scaling variable x from to 0.11 and the hadronic mass W from 66 to 233 GeV. The transverse energy flow is measured in the hadronic centre of mass frame and is studied as a function of , x, W and pseudorapidity. A comparison is made with QCD-based models. The behaviour of the mean transverse energy in the central pseudorapidity region and an interval corresponding to the photon fragmentation region are analysed as a function of and W. Received: 15 July 1999 / Published online: 27 January 2000     

On the determination of double diffraction dissociation cross section at HERA

The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processes epMe'p'A⁰,r e'p'J/O,r e'p'X at HERA. We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in the J/O photoproduction, electroproduction of the A⁰ at large Q² and diffraction dissociation of real and virtual photons into high mass states X the contamination of the double diffraction dissociation can be as large as ~30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the proton-proton collisions.     

Soft QCD in ATLAS: Minimum bias and diffraction studies

We present measurements of charged particle production in proton-proton collisions at centre-of-mass energies of √s = 0.9, 2.36 and 7 TeV recorded with the ATLAS detector at the Large Hadron Collider. Events were collected using a single-arm minimum bias trigger, charged tracks are measured with high precision in the inner tracking system. Minimum bias analysis uses data samples at all three energies, while diffractive events are studied using a sample of events at √s = 7 TeV. To study diffractive interactions, the events that have hits on exactly one side of the ATLAS detector were selected. The charged particle multiplicity, pseudorapidity and transverse momentum spectra are analyzed and compared to the predictions by various Monte Carlo models.     

On the model-dependence of the relation between minimum-bias and inelastic proton-proton cross sections

The model-dependence of the relation between the inelastic and various minimum-bias proton-proton cross sections is analyzed, paying a special attention to the sensitivity of minimum-bias triggers to diffractive collisions. Concentrating on the trigger selections of the ATLAS experiment, the measured cross sections are compared to predictions of a number of hadronic Monte Carlo models used in the cosmic ray field. It is demonstrated that the ATLAS results are able to discriminate between different models and between certain theoretical approaches for soft multi-particle production. On the other hand, the strong model-dependence of the selection efficiency of the minimum-bias triggers prevents one from inferring high mass diffraction rate from the discussed data. Moreover, the measured cross sections prove to be insensitive to the production of low mass diffractive states in proton-proton collisions. Consequently, a reliable determination of the total inelastic cross section requires forward proton tracking by a dedicated experiment.     

One-prong \tau decays with kaons

Cross-sections for hadronic, and lepton pair final states in collisions at =183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at =130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation. Received: 3 July 1998 / Published online: 15 October 1998     

High-p jets in diffractive electroproduction

The diffractive production of high-p jets in deep-inelastic scattering is studied in the semiclassical approach. The p-spectra of and diffractive final states are found to be qualitatively different. For fina states, which are produced by ‘hard’ colour-singlet exchange, the p-spectrum is much softer than for final states, where the colour neutralization is ‘soft’. Furthermore, the two different final states can be clearly distinguished by their diffractive mass distributions.