Soft Hadronic Collisions Research Articles

Modeling Photon Radiation in Soft Hadronic Collisions

Modeling Photon Radiation in Soft Hadronic Collisions

Increasing effective intensity of soft strong interactions

We suggest definition of effective interaction intensity for soft hadron collisions and discuss its energy dependence in the preasymptotic region. Practical importance of this quantity consists in separation of rising interaction radius from effective interaction intensity increase both contributing to the total cross-section growth. It would be helpful for understanding the origin of this growth at the accelerator energies. The essential feature is that the effective interaction intensity is an experimentally measurable quantity.

Diffractive scattering

We discuss basic concepts and properties of diffractive phenomena in soft hadron collisions and in deep-inelastic scattering at low Bjorken-x.

A geometrical estimation of saturation of partonic densities

We propose a new criterium for saturation of the density of partons both in nucleons and nuclei. It is applicable to any multiple scattering model which would be used to compute the number of strings exchanged in ep and eA collisions. The criterium is based on percolation of strings, and the onset of percolation is estimated from expectations coming from the study of heavy-ion collisions at high energies. We interpret this onset as an indication of saturation of the density of partons in the wave function of the hadron. In order to produce quantitative results, a particular model fitted to describe present HERA data and generalized to the nuclear case is used. Nevertheless, with the number of scatterings controlled by the relation between inclusive and diffractive processes, conclusions are weakly model-dependent as long as different models are tuned to describe the experimental data. This constitutes a new approach, based on the eikonal description of soft hadronic collisions, and different from others which employ either perturbative QCD ideas or semiclassical methods. It offers an alternative picture for saturation in the small Q 2 region.

Monte Carlo study of the dynamical-fluctuation property of the hadronic system inside jets

A study of the dynamical fluctuation property of jets is carried out using Monte Carlo method. The results suggest that, unlike the average properties of the hadronic system inside jets, the anisotropy of dynamical fluctuations in these systems changes abruptly with the variation of the cut parameter $\yct$. A transition point exists, where the dynamical fluctuations in the hadronic system inside jet behave like those in soft hadronic collisions, i.e. being circular in the transverse plan with repect to dynamical fluctuations. This finding obtained from Jetset and Herwig Monte Carlo is encouraged to be checked by experiments.

Non-perturbative scales in soft hadronic collisions at high energy

We investigate the role of non-perturbative quark-gluon dynamics in soft high energy processes. In order to reproduce differential and total cross sections for elastic proton-proton and proton-antiproton scattering at high energy and small momentum transfer it turns out that we need two scales, the gluonic correlation length and a confinement scale. We find a small gluonic correlation length, a ∼- 0.2 fm, in accordance with recent lattice QCD results.

A String Model for Hadron Interactions at High Energies

An iterative string cascade model is extended to the SPS energies, where high order terms in the fragmentation series are considered. The derived fragmentation series converges rapidly. Terms of second and third order have significant values only at SPS energies, while the zero and the first order terms are sufficient for the hadron interactions at a few GeV. The charged particle multiplicity and the relative cross section of the production of the resonance and the prompt hadrons are reproduced with fair agreement with experimental data. 2 > It was found that the perturbative series for the total cross section converges reasonably fast. On the other hand, the problem of the multi-particle production in pp collision is dealt by different models based on soft hadronic collisions, 3 > the 1/N expansion of the amplitude for processes in QCD and string type models, describing quark transi­ tions into hadrons. At high energies there exist diagrams of complicated topology which correspond to processes with the exchange of several Pomerons. 4 > The contri­ bution of such diagrams to the scattering amplitude increases rapidly with energy more than the one Pomeron exchange contribution. The properties of iterative cascade jets with emphasis on the longitudinal degrees of freedom are studied by Field and Feynman through a dynamical picture in space-time and energy-momentum space, based upon string dynamics. In this model a scheme is considered which is symmetric with respect to particle generation from the quark and antiquark ends in a two-jet system. The particle production is considered as a tunneling process in a constant force field. Because of the three-gluon coupling, the color flux lines will not spread out over all space, as the electromagnetic field lines do, but rather be con­ strained to a thin tube like region. Within this tube, new qij pairs can be created from the available field energy. Thus the original system breaks into smaller and smaller pieces, until only ordinary hadrons remain. In the field behind the original, outgoing quark Qo a new quark pair Q1 ii1 is produced, so that the Qo may join with ih to form a hadron Qo fh, leaving the Q1 unpaired. The production of another pair qd(2 will give a hadron Q1 fh, etc. To denote this flavor ordering of the hadrons, the concept of rank is introduced. 5 >' 6 > Q;-1 q; is the ith rank hadron. From these assump­ tions one may find the resulting particle spectra in a jet. A fragmentation function

Extrapolation of hadron production in nucleus-nucleus collisions to energies reached at the BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider with the two-component dual parton model.

We present a Monte Carlo version of the dual parton model for the description of particle production in hadron-nucleus and nucleus-nucleus collisions. Each nucleon-nucleon collision is described by the two-component dual parton model. Soft hadronic collisions and hard perturbative collisions (minijets) are the two components included in the unitarization scheme. Depending on the transverse momentum cutoff we find up to several thousand minijets in collisions of heavy ions at the TeV energies of the future CERN Large Hadron Collider (LHC). We get predictions up to energies reached at the LHC for the rapidity and transverse momentum distributions and the minijet component of the hadronic energy density.

A probabilistic model of interacting strings

Interactions among colour strings produced in high-energy soft hadronic collisions are introduced via a probabilistic picture, in which strings fuse as soon as they overlap in the transverse area available. The possibility of formation of new strings, which have more colour and fragment with higher average transverse momentum and heavy flavour content, is studied. The obtained hadronic amplitude satisfies the s-channel unitarity. Predictions of the model for the main observables are compared with the experimental data and the results obtained in the framework of independent string fragmentation.

Dual Parton Model with complete formation zone intranuclear cascade and a minijet component for the description of particle production in heavy ion collisions at collider energies

We present a Monte Carlo version of the Dual Parton Model for the description of particle production in hadron-nucleus and nucleus-nucleus collisions with a complete formation zone intranuclear cascade. Each nucleon nucleon collision is described by the two component Dual Parton Model. Soft hadronic collisions and hard perturbative collisions (minijets) are the two components included in the unitarization scheme. Depending on the transverse momentum cut off we find up to several thousand minijets in collisions of heavy ions at the TeV energies of the future LHC collider. The model is compared with rapidity distributions and grey-particle production in hadron-nucleus interactions as well as with results from nucleus-nucleus collisions. We present predictions for the rapidity and transverse momentum distributions as well as for the hadronic energy density at LHC energies. We also study the cascade of the partons from the perturbative component.

Heavy quark production in soft hadron collisions at superhigh energies

Production cross sections of heavy quarks are calculated in the framework of QCD motivated phenomenology. Obtained values of σ( c /c) and σ( b /b) are in a good agreement with data. Predictions for σ( t /t) are made.

Heavy quark production in soft hadron collisions at superhigh energies

Production cross sections of heavy quarks are calculated in the framework of QCD motivated phenomenology. Obtained values of σ(cc) and σ(bb) are in a good agreement with data. Predictions for σ(tt) are made.

Exploring the Lipatov equation

Properties of the Lipatov equation for the small x behaviour in deep inelastic scattering are explored. In particular, the effect of imposing upper and lower cutoffs on the transverse momentum integral in the equation is analysed. The lower cutoff, needed to avoid integration over momenta where the gluon propagators are nonperturbative, does not change the output power. Imposing both a lower and an upper cutoff, so as to take account of kinematic limitations at subasymptotic values of Q 2, has an important effect even at HERA energies. This will make it difficult to separate the perturbative pomeron from the nonperturbative one seen already in soft hadronic collisions.

Multijet events in soft and semihard hadronic collisions

Various phenomena suggest the importance of a hard component in soft hadronic collisions with large cross sections already at moderate energies below\(\sqrt s \sim 30 GeV\) which involve the valence quarks. We consider Born term models based on one gluon exchange and hadron quark couplings either pointlike or with additional largepT damping. A resolution dependent multijet analysis-performed in a way similar toe+e−-annihilation-can yield important information about the elements of a perturbative approach. Predictions are presented for multijet rates and properties of 2 and 3 jet events in the low resolution regime. Some results for 2 jet events do also apply for higher energies and high resolution.

String dynamics in hadronic matter

Hadron production in soft hadronic collisions is successfully described by a longitudinal excitation and subsequent decay of color flux tubes. We consider the dynamics of interacting unstable strings as a generalization designed forhA andAA interactions at ultrarelativistic energies. The constituent quarks at the ends of the decaying strings and the produced hadrons can interact with the surrounding matter. The effect of secondary interactions in molecular dynamics calculations forAA collisions at CERN energies (200A GeV) can be seen in an enhancement of transverse energy, particle production and the mean transverse momenta. The results agree very well with the experimental measurements at ultrarelativistic beam energies inpp, hA and the recentAA collisions.

Analytic model for soft and hard hadronic collisions

Starting from a dual analytic model that interpolates soft and hard collisions, we give an explicit expression for its impact parameter amplitude at high energy. The mean effective interaction radius is evaluated and compared with experiment. Constraints on the parameters of the model are discussed.

Soft quark annihilation contribution to the low mass dimuon production in high-p T jet events

The soft quark annihilation contribution to the low-mass, unlike-sign dimuon production in high-p T jet events, which has recently been observed by the UA1 collaboration at the CERN protonantiproton collider, is estimated within a model that successfully describes the anomalous dilepton production in soft hadronic collisions. A method is proposed for disentangling the various production mechanisms, based on comparing the mean multiplicity of hadrons accompanying a dimuon with a quantity calculated from the lowest multiplicity moments in events without dimuons.

La physique hadronique des collisions molles

We discuss some recent experimental results concerning the physics of soft hadronic collisions. We compare them with earlier results and with existing models.[Journal translation]

Monte Carlo simulation of general hard processes with coherent QCD radiation

In this paper we extend our previous work on the simulation of coherent soft-gluon radiation to hard collisions that involve incoming as well as outgoing coloured partons. Existing simulations correctly sum the leading collinear singularities for initial- and final-state radiation, and in some cases the leading infrared contributions from outgoing partons, but not those for incoming (or the interference between incoming and outgoing). Asymptotically, however, the leading infrared and collinear contributions are comparable, the bulk of gluon emission occuring in the soft region. Furthermore, a correct treatment of leading infrared terms is necessary for the inclusive cancellation of singularities in the Sudakov form factor. We show how such a treatment ,ay be formulated in terms of an angular ordering procedure applicable to all hard processes. We then describe a new Monte Carlo program which incorporates this procedure, together with other new features such as azimuthal correlations due to gluon polarization and interference. The program is designed as a general-purpose event generator, simulating hard lepton-lepton, lepton-hadron and hadron-hadron scattering in a single package. Simulation of soft hadronic collisions and underlying events is also included. We present the predictions of the program for a wide variety of processes, and compare them with analytical results and experimental data.

Hadronic Interactions in the Energy Region of Future Hadron Colliders, Dual Topological Unitarization of Hard and Soft Hadronic Cross Sections

AbstractThe dual topological unitarization of hard and soft hadronic collisions is formulated as a Monte Carlo event generator for events containing both the soft (low p ∂) and hard (jets, minijets) component of hadron production. The parameters of the model are determined from fits to the energy dependence of the total and inelastic hadron cross‐sections and from the predictions of the QCD‐parton model for the perturbative hard constituent scattering cross sections. The predictions of the model for TeV colliders are presented. Interesting changes of the produced multiparticle system are found when selecting classes of events with and without hard jets or minijets.