Transverse-momentum distribution of <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> mesons produced in proton-nucleus and lead-lead interactions

The transverse-momentum and transverse-mass distributions of J/ψ mesons produced in proton-nucleus and lead-lead interactions at 400 GeV and 158 GeV/nucleon, respectively, were measured in the NA50 experiment. The change in these distributions with increasing centrality is studied for various interaction energies. Data obtained in the NA38 experiment for collisions of light nuclei at 200 GeV per nucleon is also used in the present analysis. Investigation of the transverse-momentum and transverse-mass distributions of J/ψ particles can provide additional information about the properties of the phase transition of ordinary nuclear matter to quark-gluon plasma.     

Flow effects in high-energy nucleus collisions with Ag(Br) in emulsion

Various flow phenomena observed by a unique emulsion method are reviewed. The experimental data of the emission of projectile and target fragments and relativistic particles in collisions of 1–160 A GeV/c ¹⁶O, ²²Ne, ²⁸Si, ³²S, ⁸⁴Kr, ¹⁹⁷Au, and ²⁰⁸Pb nuclei with ¹⁰⁸Ag (⁸⁰Br) targets are investigated. The transverse-momentum approach, the flow-angle analysis using principal vectors, the azimuthal correlation functions, the method of azimuthal correlations between charged secondaries, and the method of Fourier expansion of the azimuthal angle distributions are applied. Evidence of the directed flow of spectators has been obtained in the medium-impact nuclear interactions. In azimuthal distributions, with respect to the reaction plane, the signal of the elliptic flow of participants has been observed.     

Q-PYTHIA: a medium-modified implementation of final state radiation

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse-momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-p _T partons and a decrease at large p _T, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.     

Light-like noncommutativity,light-front quantization and new light on UV/IR mixing

We revisit the problem of quantizing field theories on noncommutative Moyal space–time with light-like   noncommutativity. To tackle the issues arising from noncommuting and hence nonlocal time, we argue that for this case light-front quantization procedure should be employed. In this appropriate quantization scheme we perform the non-planar loop analysis for the light-like noncommutative field theories. One of the important and peculiar features of light-front quantization is that the UV cutoff of the light-cone Hamiltonian manifests itself as an IR cutoff for the light-cone momentum, p⁺$p^{+}$. Due to this feature, the naive results of covariant quantization for the light-like case allude to the absence of the UV/IR mixing in the light-front quantization. However, by a careful analysis of non-planar loop integrals we show that this is not the case and the UV/IR mixing persists. In addition, we argue in favour of the perturbative unitarity of light-like noncommutative field theories in the light-front quantization scheme.     

Some special features of the fragmentation of a relativistic nucleus 11B in photoemulsion

Events where two doubly charged fragments are directly formed from a fragmenting nucleus ¹¹B of momentum 2.75 GeV/c per nucleon are separated, the cascade production of such fragments via the fragmentation channel ⁸Be → 2α being avoided. Where possible, the mass numbers of these doubly charged fragments are determined by using a signal from Coulomb scattering in photoemulsion. It is found that the measured fraction of the isotope ⁶He is (9.6 ± 1.5)%, while its calculated probability is about 12%. The transverse-momentum distributions obtained experimentally for the isotopes ³He and ⁴He are compatible with a Rayleigh distribution characterized by constant values of 110.4 ± 6.0 and 127.5 ± 6.0 MeV/c, respectively. These features of the distributions agree with a purely statistical mechanism of the fragmentation of ¹¹B nuclei.     

Implementation of a medium-modified parton shower algorithm

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse-momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-p _T partons and a decrease at large p _T, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.     

QCD on the Light-Front

Light-front Hamiltonian theory, derived from the quantization of the QCD Lagrangian at fixed light-front time x ⁺ = x ⁰ + x ³, provides a rigorous frame-independent framework for solving nonperturbative QCD. The eigenvalues of the light-front QCD Hamiltonian H _LF predict the hadronic mass spectrum, and the corresponding eigensolutions provide the light-front wavefunctions which describe hadron structure, providing a direct connection to the QCD Lagrangian. In the semiclassical approximation the valence Fock-state wavefunctions of the light-front QCD Hamiltonian satisfy a single-variable relativistic equation of motion, analogous to the nonrelativistic radial Schrödinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. Remarkably, the potential U has a unique form of a harmonic oscillator potential if one requires that the chiral QCD action remains conformally invariant. A mass gap and the color confinement scale also arises when one extends the formalism of de Alfaro, Fubini and Furlan to light-front Hamiltonian theory. In the case of mesons, the valence Fock-state wavefunctions of H _LF for zero quark mass satisfy a single-variable relativistic equation of motion in the invariant variable \({\zeta^2=b^2_\perp x(1-x)}\) , which is conjugate to the invariant mass squared \({{M^2_{q\bar q}}}\) . The result is a nonperturbative relativistic light-front quantum mechanical wave equation which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics, including a massless pion for zero quark mass and linear Regge trajectories \({M^2(n, L, S) = 4\kappa^2( n+L +S/2)}\) with the same slope in the radial quantum number n and orbital angular momentum L. Only one mass parameter \({\kappa}\) appears. The corresponding light-front Dirac equation provides a dynamical and spectroscopic model of nucleons. The same light-front equations arise from the holographic mapping of the soft-wall model modification of AdS₅ space with a unique dilaton profile to QCD (3 + 1) at fixed light-front time. Light-front holography thus provides a precise relation between the bound-state amplitudes in the fifth dimension of AdS space and the boost-invariant light-front wavefunctions describing the internal structure of hadrons in physical space-time. We also discuss the implications of the underlying conformal template of QCD for renormalization scale-setting and the implications of light-front quantization for the value of the cosmological constant.     

Parton physics from large-momentum effective field theory

Parton physics,when formulated as light-front correlations,are difficult to study non-perturbatively,despite the promise of lightfront quantization.Recently an alternative approach to partons have been proposed by re-visiting original Feynman picture of a hadron moving at asymptotically large momentum.Here I formulate the approach in the language of an effective field theory for a large hadron momentum P in lattice QCD,LaMET for short.I show that using this new effective theory,parton properties,including light-front parton wave functions,can be extracted from lattice observables in a systematic expansion of 1/P,much like that the parton distributions can be extracted from the hard scattering data at momentum scales of a few GeV.     

Finite-Temperature Field Theory on the Light Front

The formulation of statistical physics using light-front quantization, instead of conventional equal-time boundary conditions, has important advantages for describing relativistic statistical systems, such as heavy ion collisions. We develop light-front field theory at finite temperature and density with special attention to quantum chromodynamics. First, we construct the most general form of the statistical operator allowed by the Poincaré algebra. In light-front quantization, the Green’s functions of a quark in a medium can be defined in terms of just two-component spinors and do not lead to doublers in the transverse directions. Since the theory is non-local along the light cone, we use causality arguments to construct a solution to the related zero-mode problem. A seminal property of light-front Green’s functions is that they are related to parton densities in coordinate space. Namely, the diagonal and off-diagonal parton distributions measured in hard scattering experiments can be interpreted as light-front density matrices.     

Probing gluonic spin-orbit correlations in photon pair production

We consider photon pair production in hadronic collisions at large mass and small transverse momentum of the pair, assuming that factorization in terms of transverse-momentum dependent parton distributions applies. The unpolarized cross section is found to have azimuthal angular dependencies that are generated by a gluonic version of the Boer-Mulders function. In addition, the single transversely polarized cross section is sensitive to the gluon Sivers function. We present simple numerical estimates for the Boer-Mulders and Sivers effects in diphoton production at RHIC and find that the process would offer unique opportunities for exploring transverse-momentum dependent gluon distributions.     

Low-mass Drell-Yan pairs at RHIC and LHC

We summarize the calculation of Drell-Yan transverse-momentum distributions using QCD perturbation theory. In particular, the transversemomentum spectrum of low-mass Drell-Yan pairs is calculated with all-order resummation. We demonstrate that the transverse-momentum distribution of low-mass Drell-Yan pairs is an advantageous source of constraints on the gluon distribution and its nuclear dependence. We argue that low-mass Drell-Yan pairs in the forward region provide a good and clean probe of small-x gluons at RHIC and LHC.     

Measurement of open beauty production at HERA in the D*μ final state

The production of beauty quarks with a and a muon in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb^-1. Low transverse-momentum thresholds for the muon and D^* meson allow for a measurement of beauty production closer to the production threshold than previous measurements. The beauty signal was extracted using the charge correlations and angular distributions of the muon with respect to the D^* meson. Cross sections for photoproduction and deep inelastic scattering are somewhat higher than, but compatible with, next-to-leading-order QCD predictions, and compatible with other measurements.     

Inclusive cross sections for gluon production in collision of two projectiles on two targets in the BFKL approach

We describe the Resummino package, a C++ and Fortran program dedicated to precision calculations in the framework of gaugino and slepton pair production at hadron colliders. This code allows to calculate transverse-momentum and invariant-mass distributions as well as total cross sections by combining the next-to-leading order predictions obtained by means of perturbative QCD with the resummation of the large logarithmic contributions arising in the small transverse-momentum region and close to the production threshold. The results computed in this way benefit from reduced theoretical uncertainties, compared to a pure next-to-leading order approach as currently employed in the experimental analyses searching for sleptons and gauginos at hadron colliders. This is illustrated by using Resummino in the context of a typical supersymmetric benchmark point dedicated to superpartner searches at the Large Hadron Collider.     

Covariance of light-front models: pair current

We compute the + component, i.e., j⁺=j⁰+j³, of the electromagnetic current of a composite spin-one two-fermion system for vanishing momentum transfer component q⁺=q⁰+q³. In particular, we extract the nonvanishing pair production amplitude on the light-front. It is a consequence of the longitudinal zero momentum mode, contributing to the light-front current in the Breit-frame. The covariance of the current is violated, if such pair terms are not included in its matrix elements. We illustrate our discussion with some numerical examples.     

Production of π 0 and η mesons in carbon-induced relativistic heavy-ion collisions

The production of π ⁰ and η mesons has been investigated in the system ¹²C+¹²C at 0.8A GeV, 1.0A GeV, and 2.0A GeV using the TAPS photon detector. The production cross sections and transverse-momentum distributions measured around midrapidity extend the existing systematics of neutral-meson production to light systems and the highest energy available at the GSI accelerator facility. At all beam energies scaling of the differential production cross section with the transverse mass of the emitted meson is observed. In addition π⁰ production has been studied in the mass-asymmetric systems ¹²C+¹⁹⁷Au and ¹⁹⁷Au+¹²C at 0.8A GeV.     

Explicitly covariant light-front dynamics and relativistic few-body systems

The wave function of a composite system is defined in relativity on a space–time surface. In the explicitly covariant light-front dynamics, reviewed in the present article, the wave functions are defined on the plane ω·x=0, where ω is an arbitrary four-vector with ω²=0. The standard non-covariant approach is recovered as a particular case for ω=(1, 0, 0,−1). Using the light-front plane is of crucial importance, while the explicit covariance gives strong advantages emphasized through all the review.The properties of the relativistic few-body wave functions are discussed in detail and are illustrated by examples in a solvable model. The three-dimensional graph technique for the calculation of amplitudes in the covariant light-front perturbation theory is presented.The structure of the electromagnetic amplitudes is studied. We investigate the ambiguities which arise in any approximate light-front calculations, and which lead to a non-physical dependence of the electromagnetic amplitude on the orientation of the light-front plane. The elastic and transition form factors free from these ambiguities are found for spin 0, and 1 systems.The formalism is applied to the calculation of the relativistic wave functions of two-nucleon systems (deuteron and scattering state), with particular attention to the role of their new components in the deuteron elastic and electrodisintegration form factors and to their connection with meson exchange currents. Straightforward applications to the pion and nucleon form factors and the ρ−π transition are also made.     

Wigner Distributions in Light-Front Quark Models

We discuss the quark Wigner distributions which represent the quantum-mechanical analogues of the classical phase-space distributions. These functions can be obtained through a Fourier transform in the transverse space of the generalized transverse momentum dependent parton distributions, which encode the most general one-body information of partons in momentum space. In particular, we present a study within light-front quark models. The quark orbital angular momentum is also obtained from the phase-space average of the orbital angular momentum operator weighted with the Wigner distribution of unpolarized quark in a longitudinally polarized nucleon. The corresponding results calculated within different light-front quark models are compared with alternative definitions of the quark orbital angular momentum as given in terms of generalized parton distributions and transverse momentum dependent parton distributions.     

Peripheral fragmentation of 8B nuclei in nuclear emulsion at an energy of 1.2 GeV per nucleon

The results obtained by studying the charge topology of fragments produced in the peripheral dissociation of relativistic ⁸B nuclei in emulsion are presented. Fifty-five events of the peripheral dissociation of a ⁸B nucleus in events where there was no production of target-nucleus fragments and mesons (“white stars”) were selected. A leading contribution of the ⁸B → ⁷Be + p mode, which has the lowest energy threshold, was revealed on the basis of these events. Information about the branching ratios for dissociation modes characterized by a higher multiplicity was obtained. The dissociation of the ⁷Be core in ⁸B bears resemblance to the dissociation of a free ⁷Be nucleus. The transverse-momentum distributions of fragments originating from the ⁸B → ⁷Be + p dissociation mode were obtained. For these distributions, a small mean value of 〈P*_T〉 = 52 ± 5 MeV/c in the c.m. frame suggests a low binding energy of the outer proton in the ⁸B nucleus. An indication of a strong azimuthal correlation of the fragments ⁷Be and p was found.