QCD corrections to inclusive jet photoproduction via direct photons

The direct chiral perturbation theory (ChPT) one-loop prediction for the production of two pions from two photons is compared with the recent Crystal Ball experimental data. The one-loop computation does not fit data even close to threshold because unitarity effects are important even at very low energies. However, when the constraints coming from unitarity and analyticity are included in a suitable way, the range of application of one-loop ChPT can be largely extended in agreement with the experiment.     

Resonance decays,correlations and intermittency in hadronic collisions

We study a very simple model of correlations and intermittency of identical final state pions in hadronic collisions. Final state pions are either products of resonance decays or they are “directly” produced. The “direct” production is simulated by an immediate decay of a resonance. For “direct pions” forming about a half of final state pions and for formation times of resonances less than 0.5fin/c we get density of sources which via Hanbury-Brown and Twiss interference leads to correlations of two identical pions consistent with recent data and shows intermittency patterns for the second factorial moment. The essential ingredient of the scheme is the combination of pions from resonance decays and direct pions. Due to life-times of resonances taken from experiment, pions from resonance decays are responsible for short-range correlations in the longitudinal momentum, whereas directly produced pions, due to their fast production, dominate in the region of longitudinal momentum difference of the order of 100 MeV/c. The combination of both can give an approximate scaling leading to intermittency.     

The three-dimensional (2 + 1)-fluid model for relativistic nuclear collisions

A new multi-fluid model is constructed for describing high-energy heavy-ion collisions. It is assumed that two baryonic fluids formed by the projectile and target nucleons produce a third hadronic fluid via inelastic nucleon-nucleon collisions. The production and expansion dynamics of the hadronic fluid are investigated in detail. Two equations of state for this fluid are considered: one corresponding to an ideal gas of pions and resonances and another one corresponding to an interacting hadron gas described by the relativistic mean-field model. The effects of freeze-out and non-zero pion chemical potential are investigated. The rapidity and transverse momentum spectra of secondary pions are compared with the experimental data forS + S collisions at 200 GeV/nucleon.The authors thank J. Schaffner for his most valuable assistance in the application of the mean-field model. The authors are also grateful to H. Sorge and A. Jahns for fruitful discussions. This work was supported by the Gesellschaft für Schwerionenforschung (GSI) and the Bundesministerium für Forschung und Technologie (BMFT).     

Partonic effects on pion interferometry at the relativistic heavy-ion collider

Using a multiphase transport model that includes both initial partonic and final hadronic interactions, we study the pion interferometry at the Relativistic Heavy-Ion Collider. We find that the two-pion correlation function is sensitive to the magnitude of the parton-scattering cross section, which controls the parton density at which the transition from the partonic to hadronic matter occurs. Also, the emission source of pions is non-Gaussian, leading to source radii that can be more than twice larger than the radius parameters extracted from a Gaussian fit to the correlation function.     

Simulation of Deuteron Fragmentation into Pions Produced in the Twice-Cumulative Region

Special features of the behavior of the pion-production cross section as a function of the mass number of the target nucleus are discussed for incident-deuteron fragmentation to pions produced in the twice-cumulative kinematic region. The fact that, for such pions to be produced in deuteron-nucleus collisions, the target nucleus should be heavier than the proton is the most peculiar to them. They are produced owing to the high-momentum component in both colliding nuclei. The dependence of the production cross section for twice-cumulative pions on the target mass number is simulated, and the effect of various models used to describe the structure of the target nucleus at small nucleon-nucleon distances is examined.     

Space-time noncommutativity, discreteness of time and unitarity

Violation of unitarity for noncommutative field theory on compact space-times is considered. Although such theories are free of ultraviolet divergences they still violate unitarity, while in a usual field theory such a violation occurs when the theory is nonrenormalizable. The compactness of space-like coordinates implies discreteness of the time variable which leads to the appearance of unphysical modes and violation of unitarity even in the absence of a star-product in the interaction terms. Thus, this conclusion holds also for other quantum field theories with discrete time. Violation of causality, among others, occurs also in the case of the nonvanishing of the commutation relations between observables at space-like distances with a typical scale of noncommutativity. While this feature allows for a possible violation of the spin-statistics theorem, such a violation does not rescue the situation but makes causality violation scale as the inverse of the mass appearing in the considered model, i.e., it becomes even more severe. We also stress the role of smearing over the noncommutative coordinates entering the field operator symbols. Received: 19 March 2001 / Published online: 29 June 2001     

Pion and nucleon thermal widths in the linear sigma model

We calculate the hadronic width (damping rate) of pions and nucleons at finite temperature in the framework of the linear sigma model, and to lowest order in the virial expansion. Results indicate a substantial growth of both widths even at moderately low temperatures.     

Momentum dependence of the pion cloud for ϱ-mesons in nuclear matter

We extend hadronic models for ϱ-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.     

How to see independent emission in hadronic production

Long-range correlations, as observed at the ISR, can easily be described by a picture of overlapping independent emission (OIE). In this picture, hadronic production is viewed as an incoherent superposition of elementary bremsstrahlung components. By selecting events with fixed values of the multiplicity in one hemisphere, one arrives at samples with strongly reduced long range correlations. At very high energies, these samples represent elementary bremsstrahlung components whose factual existence corroborates the OIE model.     

Hard photons and neutral pions as probes of hot and dense nuclear matter

The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems ⁸⁶Kr+⁵⁸Ni at 60A MeV, ¹⁸¹Ta+¹⁹⁷Au at 40A MeV and ²⁰⁸Pb+¹⁹⁷Au at 30A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by π⁰ and Δ decay but predominantly by radiative capture of pions. The π⁰ absorption in the nuclear medium is further analyzed by examining their emission pattern.     

Multihadron production in a cascade model for e+e− annihilation: I. Total cross section and multiplicity distribution

We consider a simple dynamical model for e⁺e^? annihilation in which the production of pions is the result of a kind of bremsstrahlung process, each step of the cascade being characterized by the decay of a (massive) spin-one system into a pion plus a similar spin-one system. We study the high-energy behaviour of the n-particle and total cross sections as well as the multiplicity distribution for a large class of transition form factors. The model is found to include features familiar from the uncorrelated jet, the multiperipheral and the thermodynamical models. In particular, it is able to accomodate a logarithmically increasing multiplicity as well as a constant total cross section.     

Low-energy neutral current phenomenology and grand unified theories

We derive necessary and sufficient conditions to be satisfied by any expanded electroweak gauge model in order to reproduce the standard model low-energy neutral current predictions. These conditions imply several constraints on the neutral gauge boson masses and quantum number assignments for the ordinary fermions. Using these conditions, we prove that the popular grand unified theories based on the gauge groups SO(10) and E₆ can only accomodate trivial extensions of the standard model. As a consequences, if any of these grand unified models works at some energy scale, present low-energy neutral current phenomenology implies that the Z-boson must be produced with the expected mass and couplings to the ordinary fermions. Any additional neutral gauge boson (with the possible exception of very heavy ones) could only be produced in hadronic collisions and it would not decay in e⁺e^?.     

Lovelock theory and the AdS/CFT correspondence

Lovelock theory is the natural extension of general relativity to higher dimensions. It can be also thought of as a toy model for ghost-free higher curvature corrections in gravitational theories. It admits a family of AdS vacua, which provides an appealing arena to explore different holographic aspects in a broader setup within the context of the AdS/CFT correspondence. We will elaborate on these features and review previous work concerning the constraints that Lovelock theory entails on the CFT parameters when imposing conditions like unitarity, positivity of the energy or causality.     

Multiple soft pion production in nonlinear chiral sigma model

Multiple soft pion production in the baryon scattering reactions is considered in the framework of chiral nonlinear sigma model neglecting the pion mass. Treating baryons in the eikonal approximation as classical sources, a set of analytical solutions for the pion field is found. A tree S-matrix is constructed on the basis of these solutions describing the emission (or absorption) of any number of soft pions. Then the contribution of soft virtual pions is taken into account in a closed form. It is shown that the loop corrections strongly suppress the pion radiation, and for the two limiting cases of nonrelativistic and ultra-relativistic baryon scatterings there is no pion emission from the “ends”. Thus, the mechanism similar to the soft photon bremsstrahlung in the quantum electrodynamics seems to be unable to create a state with a large number of the soft pions.     

Model Independent QED Corrections to the Process ep → eX

We give an exhaustive presentation of the semi-analytical approach to the model independent leptonic QED corrections to deep inelastic neutral current lepton-nucleon scattering. These corrections include photonic bremsstrahlung from and vertex corrections to the lepton current of the order φ(α) with soft photon exponentiation. A common treatment of these radiative corrections in several variables — leptonic, hadronic, mixed, Jaquet-Blondel variables — has been developed and double differential cross sections are calculated. In all sets of variables we use some structure functions, which depend on the hadronic variables and which do not have to be defined in the quark parton model. The remaining numerical integrations are twofold (for leptonic variables) or onefold (for all other variables). For the case of hadronic variables, all phase space integrals have been performed analytically. Numerical results are presented for a large kinematical range, covering fixed target as well as collider experiments at HERA or LEP⊗LHC, with a special emphasis on HERA physics.     

Partonic effects in heavy ion collisions at RHIC

Effects of partonic interactions in heavy ion collisions at RHIC are studied in a multiphase transport model (AMPT) that includes both initial partonic and final hadronic interactions. It is found that a large parton scattering cross section is needed to understand the measured elliptic flow of pions and two-pion correlation function.     

Unitarity,asymptotic constraints and hadronic slopes

The connection between multiparticle unitarity and the structure of hadronic slopes is investigated within a simple model for the nondiffractive production amplitudes in which the (energy dependent) normalization in the popular form of the negative binomial distribution is taken as input. It is shown that the model can be made consistent with all known high energy constraints following from asymptotic theorems and experimental information in the sense that simple parametrizations of the (three) energy dependent parameters of the model satisfying all these constraints allow us to account for all thepp and \(\bar pp\) slope data from “low” to the highest available energies.     

Troubles with Quantum Anisotropic Cosmological Models: Loss of Unitarity

The anisotropic Bianchi I cosmological model coupled with perfect fluid is quantized in the minisuperspace. The perfect fluid is described by using the Schutz formalism which allows to attribute dynamical degrees of freedom to matter. A Schrödinger-type equation is obtained where the matter variables play the role of time. However, the signature of the kinetic term is hyperbolic. This Schrödinger-like equation is solved and a wave packet is constructed. The norm of the resulting wave function comes out to be time dependent, indicating the loss of unitarity in this model. The loss of unitarity is due to the fact that the effective Hamiltonian is hermitian but not self-adjoint. The expectation value and the bohmian trajectories are evaluated leading to different cosmological scenarios, what is a consequence of the absence of a unitary quantum structure. The consistency of this quantum model is discussed as well as the generality of the absence of unitarity in anisotropic quantum models.