Baryon trajectories revisited

Various properties of baryon trajectories are re-examined in the light of the latest data: in particular we study trajectory assignments of the 1982 baryon resonance data and evaluate effective trajectories for backward πN scattering incorporating FNAL data. We find that all of the information is consistent with trajectories linear inu, withF/D ratios constant along the trajectories and parity doublets absent. We find that exchange degeneracy (EXD) is good for Λ and Σ trajectories and residues and shed some qualitative light on the systematics of EXD breaking.     

Implications of duality,unitarity and SU6 symmetry for baryons and baryonium

It is shown that there exists a limit, involving both an exactly SU₆ symmetric spectrum and exactly exchange degenerate Regge trajectories, in which the OZI like selection rule for baryonium is exact. Deviations from the rule occur when SU₆ is broken and when unitarity forces the breaking of exchange degeneracy. By combining both of these latter effects we can qualitatively account for the observed pattern of broken exchange degeneracy among baryons. The strength of the baryonium coupling to mesons is related to the breaking of exchange degeneracy.     

Flavouring the Gribov process

The exchange of flavour carrying trajectories is studied in the non-covariant parton interpretation of reggeon field theory. While pomeron exchange is described by wee partondensities, i.e. diagonal elements of the density matrix of a fast hadron, meson exchange is described by density matrix elements which are diagonal in parton number but off-diagonal in flavour. The reggeon field theory “hamiltonian” describes a markoffian evolution of this block-diagonal density matrix during a boost. This interpretation is possible both if there are two distinctf and ? trajectories and in case of ? identity. The meson trajectories are superpositions of odd and even signature trajectories.     

Effects of Coulomb interactions on spin states in vertical semiconductor quantum dots

The effects of direct Coulomb and exchange interactions on spin states are studied for quantum dots contained in circular and rectangular mesas. For a circular mesa a spin-triplet favored by these interactions is observed at zero and nonzero magnetic fields. We tune and measure the relative strengths of these interactions as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states. For a rectangular mesa we observe no signatures of the spin-triplet at zero magnetic field. Due to the anisotropy in the lateral confinement single-particle state degeneracy present in the circular mesa is lifted, and Coulomb interactions become weak. We evaluate the degree of the anisotropy by measuring the magnetic field dependence of the energy spectrum for the ground and excited states, and find that at zero magnetic field the spin-singlet is more significantly favored by the lifting of level degeneracy than by the reduction in the Coulomb interaction. We also find that the spin-triplet is recovered by adjusting the level degeneracy with magnetic field. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Supersymmetric Higgs boson pair production: Discovery prospects at hadron colliders

We study the potential of hadron colliders in the search for the pair production of neutral Higgs bosons in the framework of the minimal supersymmetric standard model. We perform a detailed signal and background analysis, working out efficient kinematical cuts for the extraction of the signal. The important role of squark loop contributions to the signal is re-emphasized. If the signal is sufficiently enhanced by these contributions, it could even be observable at the next run of the upgraded Tevatron collider in the near future. At the LHC the pair production of light and heavy Higgs bosons might be detectable simultaneously. Received: 23 February 2000 / Revised version: 18 April 2000 / Published online: 31 August 2000     

Single-spin asymmetries for small-angle pion production in high-energy hadron collisions

Within the framework of a simple model, we study single-spin asymmetries for pion production in hadron–hadron collisions at high energies with one hadron polarized. The asymmetries are generated via a mechanism of final- (initial-) state interactions. For peripheral kinematics, we find nonzero asymmetries at the high-energy limit when the pion belongs to the fragmentation region of the polarized proton. Numerical results and comparison with existing experimental data are presented. We also discuss the relationship with Odderon exchange phenomenology. Received: 11 May 1999 / Revised version: 22 July 1999 / Published online: 3 November 1999     

Hadron masses in cavity quantum chromodynamics to order \alpha_{\rm s}^2

The non-divergent diagrams describing two-gluon exchange and annihilation between quarks and antiquarks are calculated in the Feynman gauge, based on quantum chromodynamics in a spherical cavity. Using the experimental N, , , and masses to fit the free parameters of the M.I.T. bag model, the predicted states agree very well with the observed low-lying hadrons. As expected, the two-gluon annihilation graphs lift the degeneracy of the and , while the and remain degenerate. Diagonalizing the subspace Hamiltonian yields a very good value for the mass of the meson. Received: 28 October 1999 / Revised version: 3 April 2000 / Published online: 8 June 2000     

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.     

Flavour and spin structure of linear baryons

Based on the string picture, we construct a phenomenological model for baryons and study their flavour symmetry, exchange degeneracy pattern and spin structure. Baryons on leading trajectories are assumed to have the configuration of two quarks being attached to the ends of a linear string and the third sitting in the middle, called linear baryons. For such linear baryons, a unitarization scheme can be constructed in a manner similar to the dual unitarity scheme for mesons but without recourse to the $\text{1}\text{N}$ expansion. We find that the interchange interaction of the middle quark with one of the other two quarks at the ends of the string can give rise to a large exchange degeneracy breaking of the baryon spectrum. With this non-planar correction, the model of linear baryons can account for the observed pattern of leading baryon states.     

Exchange Degeneracy of Relativistic Two-Particle Quantum States

The phenomenon of exchange degeneracy of 2-particle quantum states is studied in detail within the framework of Relativistic Schrödinger Theory (RST). In conventional quantum theory this kind of degeneracy refers to the circumstance that, under neglection of the interparticle interactions, symmetric and anti-symmetric 2-particle states have identical energy eigenvalues. However the analogous effect of RST degeneracy is rather related to the emergence of two types of mixtures (positive and negative) in connection with the vanishing or non-vanishing of certain components of the Hamiltonian (exchange fields). As a consequence, there arise two subcases of RST degeneracy: (i) mixture degeneracy through neglection of the exchange fields and (ii) exchange degeneracy through neglection of the mixture character of matter. The latter RST exchange degeneracy consists in the fact that the RST dynamics admits a certain set of pure-state solutions, as borderline case between positive and negative mixtures, and all these different solutions are generating the same physical situation, e.g., concerning mass eigenvalues and physical densities (of current and energy-momentum). The general results are exemplified by considering the 2-particle states for (scalar) Helium. Analogously as the conventional exchange degeneracy is broken (ortho- and para-Helium) by taking into account the interparticle interactions (e.g., Coulomb forces), the RST degeneracy is broken by simultaneously taking into account the mixture character of matter together with non-zero exchange fields.     

Kaon-nucleon finite energy sum rules revisited

Kaon-nucleon finite energy and continuous moment sum rule integrals are evaluated using recent phase-shift solutions to extract the t-channel exchange amplitudes and the results are compared with Regge pole models. The exchange degeneracy breaking previously observed in the ? and A₂ amplitudes is confirmed, and it is suggested that this may be due to an exotic P₀₁ KN resonance. Regge trajectories are extracted from the integrals and are in good agreement with those found from direct fits to high-energy data.     

Analytic model of Regge trajectories

A model for a Regge trajectory compatible with the threshold behavior required by unitarity and asymptotics in agreement with analyticity constraints is given in explicit form. The model is confronted in the time-like region with widths and masses of the mesonic resonances and, in the space-like region, the ρ trajectory is compared with predictions derived from the π-N charge-exchange reaction. Breaking of the exchange degeneracy is studied in the model and its effect on both the masses and widths is determined. Received: 23 January 2001 / Accepted: 9 February 2001     

s-channel discontinuities of the torus insertion in the dual unitarisation scheme

The refined analysis of the s-channel discontinuities of the torus insertion in the topological expansion is performed. Assuming the dependence on reggeon masses of the fixed-pole residue similar to that given by the Veneziano model we obtain almost exact cancellations of the discontinuities which in the one-dimensional approximation were argued to be responsible for the exchange degeneracy breaking between ? and A₂ trajectories. Implications on different f-IP schemes are discussed.     

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     

Symmetries of Regge-Regge cuts

Regge-Regge cuts are shown to satisfy strong symmetry constraints in any model which incorporates Finkelstein's cut selection rule, SU(3) symmetry of Regge residues, and exchange degeneracy of trajectories and residues. These imply, amongst other things, the strong suppression of Regge-Regge cuts in pion-nucleon charge exchange, arising from a cancellation between ?-P′ and A₂-ω cuts. Therefore if Finkelstein's selection rule is valid, the πN change amplitudes cannot be understood by adding Regge-Regge cuts to an absorbed ? pole amplitude.     

Semihard hadron processes and the quark-gluon string model

A new approach to the analysis of soft and semihard hadron processes is suggested. In the framework of the Quark-Gluon String Model, the interaction of quarks from colliding hadrons is calculated as the exchange by nonperturbative gluons, for which the cut-off parameter in the gluonic propagator is included. This modification has allowed us to analyse the inclusive hadron spectra in hadron collisions at transverse momenta up to 3–4 GeV/c.     

A Regge-pole overview of kaon-nucleon scattering

Regge-pole residues and trajectories have been extracted at fixed-t values in the range $\text{0 ? t ? ?1.0 (}\text{GeV}\text{/c)}{}^{\mathrm{<mtext>2</mtext>}}$ by simultaneously fitting CMSR and measured data on all two-body K(K)N → K(K)N processes for laboratory momenta between 1.5 GeV/c and 280 GeV/c. A good fit to all the data and CMSR is possible provided that an f′ is introduced in the vacuum quantum number exchange amplitude. This explains the possible violation of duality previously seen in this amplitude. Particular attention is paid to the problem of $\text{?-}\text{A}{}_{\mathrm{<mtext>2</mtext>}}$ exchange degeneracy and the role of their secondary trajectories, both in the CMSR and for the line-reversal breaking observed in KN charge exchange.     

Flavor asymmetry of polarized antiquark distributions and semi-inclusive DIS

The –expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large– limit (chiral quark–soliton model), which give sizable results for and . We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive deep–inelastic scattering. We show that the large flavor asymmetries predicted by the chiral quark–soliton model are consistent with the recent HERMES data for spin asymmetries in charged hadron production. Received: 30 November 1999 / Published online: 17 March 2000     

Photon-exchange processes at hadron colliders as a probe of the dynamics of diffraction

The rich structure of photon-exchange processes at hadron colliders is studied. We discuss central vector meson production , W production and production. Each process has distinct, and large, soft pp rescattering effects, which can be directly observed by detecting the outgoing protons. This allows a probe of the optical density of the proton, which plays a crucial role in the evaluation of the rapidity gap survival probabilities in diffractive-like processes at hadron colliders. We note that an alternative mechanism for production is odderon, instead of photon, exchange; and that the ratio of odderon to photon contributions is enhanced (suppressed) for vector meson production. Received: 1 February 2002 / Revised version: 4 April 2002 / Published online: 22 May 2002     

Soft hadron production in pp interactions up to ISR energies

Soft hadron production is described as a two-step process, where the interaction of the partonic constituents of the colliding hadrons leads to the production of intermediate subsystems (fireballs), which decay subsequently into hadrons. The weights of the various final states are derived from the corresponding phase-space factors modified by empirical transition elements. The results compare well with data at energies between particle production thresholds and ISR energies. Special emphasis is put on correlation data, which offer the opportunity to shed some light on the question whether particle production proceeds via fireballs or strings. Received: 20 July 2000 / Revised version: 26 October 2000 / Published online: 21 December 2000