Baryon production in lepton-nucleon scattering and diquark fragmentation

We study baryon production in deep inelastic scattering using an extended version of the Lund jet model. There are two contributing sources. The first is baryon production in the target fragmentation. In a scheme related to our earlier work on lowp _hu baryon fragmentation we present some details of the fragmentation of a diquark into baryons and mesons. A non-negligible baryon-antibaryon production is observed ine ⁺ e ^? annihilation. In a previous paper we developed a model for this production, and the same mechanism should also give fast baryons in leptoproduction. In this paper we discuss those features of baryon production which can be more easily studied in a leptoproduction experiment.     

Strangeness, charm, and bottom in a chiral quark-meson model

In this paper, we investigate an SU(3) extension of the chiral quark-meson model. The spectra of baryons with strangeness, charm, and bottom are considered within a “rigid oscillator” version of this model. The similarity between the quark sector of the Lagrangian in the model and the Wess-Zumino term in the Skyrme model is noted. The binding energies of baryonic systems with baryon numbers B=2 and 3 possessing strangeness or heavy flavor are also estimated. The results are in good qualitative agreement with those obtained previously in the chiral soliton (Skyrme) model.     

Baryon production in the string fragmentation picture

An improved version of the “pop-corn” model for baryon production in quark and gluon jets is presented. With a reduced number of parameters the model reproduces well both production rates for different baryon species and baryon momentum distributions. Predictions are presented for a set of baryon-antibaryon correlations.     

Intriguing aspects in baryon production at relativistic heavy-ion collider

We review experimental results on baryon production at mid-rapidity in nucleus-nucleus collisions at RHIC. Outstanding physics issues include the mechanism for baryon-anti-baryon production from thermally equilibrated partons, the dynamics of baryon number transport and the evolution dynamics of baryons during hadronic expansion before the final freeze-out. We highlight recent measurements on the production of protons, lambdas and their anti-particles in terms of these physics issues. We propose a physical mechanism of topological baryon formation through gluon junction hadronization and future measurements, which can test this hypothesis experimentally.     

A model of baryons based on duality and unitarity

A model of “planar” baryons with (10 + 8) ? (1 + 8) exchange degeneracy is proposed, based on duality and unitarity. Dynamical mechanisms for deviation from the “planar” baryons are considered and the consequent pattern of broken exchange degenerate trajectories is compared with observed baryon spectra. Our model suggests a string picture of linear molecule type for baryons.     

Spin structure of baryon fragmentation functions

The valence part of the quark fragmentation function resulting in the octet of baryons together with their spin structure are parametrised, using a model for the hadronisation and the available experimental data on baryon products in e⁺e^? annihilation and μp deep inelastic scattering. Spin correlations are predicted.     

Flavored exotic multibaryons and hypernuclei in topological soliton models

The energies of baryon states with positive strangeness, or anticharm (antibeauty), are estimated in the chiral soliton approach, in the “rigid oscillator” version of the bound-state soliton model proposed by Klebanov and Westerberg. Positive strangeness states can appear as relatively narrow nuclear levels (Θ-hypernuclei), and the states with heavy antiflavors can be bound with respect to strong interactions in the original Skyrme variant of the model (SK4 variant). The binding energies of antiflavored states are also estimated in the variant of the model with a sixth-order term in chiral derivatives added to the Lagrangian to stabilize solitons (SK6 variant). This variant is less attractive, and nuclear states with anticharm and antibeauty can be unstable relative to strong interactions. The chances of obtaining bound hypernuclei with heavy antiflavors increase within the “nuclear variant” of the model with a rescaled model parameter (the Skyrme constant e or e′ decreased by a out 30%), which is expected to be valid for baryon numbers greater than B ～ 10. The rational map approximation is used to describe multiskyrmions with a baryon number of up to about 30 and to calculate the quantities necessary for their quantization (moments of inertia, sigma term, etc.).     

String effects on Fermi–Dirac correlation measurements

We investigate some recent measurements of Fermi–Dirac correlations by the LEP collaborations indicating surprisingly small source radii for the production of baryons in e⁺e^- annihilation at the Z⁰ peak. In hadronization models there is besides the Fermi–Dirac correlation effect also a strong dynamical (anti-) correlation. We demonstrate that the extraction of the pure FD effect is highly dependent on a realistic Monte Carlo event generator, both for separation of those dynamical correlations that are not related to Fermi–Dirac statistics, and for corrections of the data and background subtractions. Although the model can be tuned to well reproduce single particle distributions, there are large model uncertainties when it comes to correlations between identical baryons. We therefore, unfortunately, have to conclude that it is at present not possible to draw any firm conclusion about the source radii relevant for baryon production at LEP. PACS 13.60.Rj; 13.66.Bc     

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.     

Supersymmetric three-jets in electron-position annihilation (II)

We consider the production of supersymmetric three-jets in e⁺e^? annihilation via virtual γ and Z⁰ exchange. Finite masses of the squarks and gluinos as well as arbitrary beam polarization are taken into account. We give the full differential cross section and discuss the Dalitz plot distributions (for massive partons) and the angular (beam-event) correlations of the thrust axis. The dependences on thrust, on parton masses and on beam polarization and energy show distinctive features of SUSY jets compared to (normal) QCD jets.     

A QCD model for jet fragmentation including soft gluon interference

A new model for hadronic jet fragmentation in hard processes is presented. It is based on a QCD parton branching mechanism with correct treatment of leading collinear and infra-red singularities (i.e. including soft gluon interference). Hadronization occurs via preconfinement of colour singlet clusters, which decay according to a simple phase-space scheme. Although tightly constrained, the model gives a good account of existing e⁺ e^? annihilation data. It predicts significant differences between quark and gluon jets. Comparisons with datadata on the CERN p?p collider jets suggest that a large fraction of them are gluon jets. The model predicts soft gluon interference effects in hadron distributions which are probably not yet observable for pions but should be clear for kaons and baryons.     

Strange goings on in dense nucleonic matter

It has been suggested that charged pions form a Bose-Einstein condensate in baryonic matter at zero temperature and about twice nuclear density. In this letter it is shown that at somewhat higher densities one finds a charged kaon condensate, driven to a large extent by the “stgma term” interaction with baryons. Using the SU(3) × SU(3) chiral lagrangian to model meson-baryon interactions it is found that baryonic matter acquires a strangeness-per-baryon ratio approaching one at several times nuclear density. The relevance of kaon condensation as a route to strange matter and its role in neutron stars are discussed.     

Properties of gluon jets

We compute the expected properties of gluon jets in a model based on the KUV jet calculus and recombination. Emphasis is placed on a) the production of baryons, and b) the question of whether hadrons produced by the decays of Zweig rule stable quarkonia (e.g. the upsilon) ine ⁺ e ^? have markedly different energy spectra from those produced by the adjacent quark-antiquark continuum.     

Can the Aleph four-jet excess be explained in a supersymmetric model with R-parity violation?

We investigate the possibility that the excess of four-jet events in e ⁺ e ^? collisions at LEP-1.5 which has been reported by the ALEPH Collaboration could be due to the production of charginos or neutralinos, followed by their decay into quark jets through baryon number-violating (λ″) couplings. An estimate at the parton level shows, however, that these events cannot be due to neutralinos because of the low cross-section, and is unlikely to be due to the production of chargino pairs because of the largely different event shapes.     

High transverse momentum Ξ baryon correlations in p+p collisions at √s=200 GeV

Angular correlations between produced high-p _TΞ^? baryons and unidentified charged and neutral particles are observed in high-tower triggered (on a large electromagnetic energy deposit) √s=200 GeV p+p collisions. This trigger favors events with higher average multiplicity than those in minimum bias. These events are likely to contain jets. The average multiplicity of a high-tower triggered event is similar to that of a minimum bias event containing a Ξ baryon, which implies that Ξ baryons are likely to be produced in jets. Ξ^? <p _T> is higher in the triggered data then in the minimum bias sample.     

How to measure quark helicities in e+e− → hadrons

In order to reach completely the vector and axial couplings of the neutral currents to quarks we propose to measure quark helicities from the polarizations of the leading baryons of the jets. Applications are given for e⁺e^? → 2 jets where large degrees of polarization are obtained from γ-Z interference.     

Relationship between the symmetry energy and the single-nucleon potential in isospin-asymmetric nucleonic matter

When skyrmions representing nucleons are put on crystal lattice and compressed to simulate high density, there is a transition above the normal nuclear matter density (n₀) from a matter consisting of skyrmions with integer baryon charge to a state of half-skyrmions with half-integer baryon charge. We exploit this observation in an effective field theory framework to access dense baryonic system. We find that the topology change involved in the transition implies changeover from a Fermi liquid structure to a non-Fermi liquid with the chiral condensate in the “melted-off” nucleon. The ～ 80% of the nucleon mass that remains “unmelted”, invariant under chiral transformation, points to the possible origin of the (bulk of) proton mass that is not encoded in the standard mechanism of spontaneously broken chiral symmetry. The topology change engenders a drastic modification of the nuclear tensor forces, thereby non-trivially affecting the EoS, in particular, the symmetry energy, for compact star matter. It brings in stiffening of the EoS needed to accommodate a neutron star of ～ 2 solar mass. The strong effect on the EoS in general and in the tensor force structure in particular will also have impact on processes that could be measured at RIB-type accelerators.     

QCD jets ine + e −-annihilation and the transition into hadrons

A model has been developed describing QCD jets in the leading logarithmic approximation (LLA) and the subsequent transition into hadrons via a chain decay model. Besides of the production of mesons, the model is also able to describe baryon production. Agreement with recent data frome ⁺ e ^?-annihilation is found. This includes average multiplicities, average transverse momenta and transverse momentum distributions, longitudinal momentum distributions as well as particle production ratios π^α:K ^π : p(0070), charge compensation probabilities and energy flux correlations. The model is compared to similar models developed by other authors.     

Inclusive properties of the ee annihilation into hadrons

The reaction e⁺e⁻ → hadrons is described on the basis of an hadronic structure for the photon. The structure functions of the inclusive reaction e⁺e⁻ → h + anything describe then the parton constitution of the photon in the scaling limit. Various types of couplings and models are considered (global couplings with vector mesons, fundamental photon-parton-antiparton couplings with formation of jets or fans). Precise shapes for the inclusive spectra and multiplicity distributions are deduced. Detailed comparisons with hadronic collisions A + B → h + anything are proposed in the fragmentation, the phase space boundary and the pionisation regions which could determine the nature of the partons and their interactions.     

Baryon production in QCD jets

Recent data from several sources—Υ decays, high energy deep inelastic scattering,e ⁺ e ^? annihilation at PETRA—reveal substantial baryon production in QCD jets. We discuss the likely mechanisms within the context of QCD, and formulate experimental tests to distinguish amongst them.