Exchange Degeneracy Breaking Research Articles

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.

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 1 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.

Multiquark states and the breaking of exchange degeneracy

In a dual approach, exotic states emerge on daughter trajectories as a consequence of exchange degeneracy.breaking. Breaking parameters and violation of the FWR rule are simply thereby related. The exotic meson or baryonium spectrum and couplings are determined.

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 ϱ and A 2 trajectories. Implications on different f-IP schemes are discussed.

Implications of duality, unitarity and SU 6 symmetry for baryons and baryonium

It is shown that there exists a limit, involving both an exactly SU 6 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 SU 6 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.

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 ϱ and A 2 amplitudes is confirmed, and it is suggested that this may be due to an exotic P 01 KN resonance. Regge trajectories are extracted from the integrals and are in good agreement with those found from direct fits to high-energy data.

Hypercharge-exchange reactions, exchange degeneracy breaking and Regge cuts

Presents an extensive study of hypercharge-exchange reactions within a dual Regge-cut model. Without any free parameter, a very satisfactory description of experimental data is obtained and predictions for the double charge-exchange reactions are presented. The scattering amplitudes obtained for the K* and K** quantum number exchanges agree fairly well with a universal behavior for vector and tensor exchanges.

S-channel discontinuities of reggeon diagrams with planar vertices

The mutual relations between s-channel discontinuities of reggeon diagrams with planar triple-Regge vertices are discussed. It is shown that, in general, contributions coming from different cuttings are not simply related to each other as certain vertices acquire non-trivial phase factors depending on the nature of the cutting. For diagrams with only triple-Regge vertices this phase may be related to the triple-Regge phase. The complete four-reggeon vertices are not known in general. An approximation is considered which amounts to preserving only the absorptive part of triple- or four-reggeon amplitudes in certain parts of the diagrams. This approximation turns out to be equivalent to a simple absorbed multiperipheral model. The problem of exchange degeneracy breaking within the dual unitarisation Scheme is briefly discussed as an example.

J-dependence of the twist parameter in dual unitarity

It is pointed out that the twist parameter which governs the breaking of exchange degeneracy, Zweig rule violation, exotic exchange, and the relative production of vector and tensor mesons, is J-dependent. A simple model for this dependence reconciles the smallness of exotic exchange with the large ratio of vector to tensor production, and successfully predicts the sign and magnitude of α ϱ(0)−α A 2(0).

The valence-core exchange model

We consider a Reggeized quark model in which the hadrons contain a quark-antiquark core in addition to the valence quarks. Reggeons and pomerons are allowed to couple to both valence and core quarks. The core leads to a term in the cross-section which is proportional to the product of nonstrange quark number times the total quark number. It is able to explain Lipkin’s third component in the total crosssections and breaks additivity at the 10% level. At the present level of accuracy, the model allows the exclusive use of strongly exchangedegenerate Regge trajectories. The observed breaking of exchange degeneracy in the data is due to the presence of a nonstrange core. A comparison with the data shows that, at high energies, this model accounts quite satisfactorily for all the qualitative and quantitative features of the well-measured hadronic total cross-sections and for the ratios of the real to imaginary parts of the forward scattering amplitudes. Predictions for other cross-sections of theoretical interest are given.

Non-planar corrections to the pomeron and reggeons in the dual unitarity scheme

We study a special class of non-planar diagrams, those in which the produced clusters are crossed. Insertion of these diagrams in the reggeon propagators accounts for exchange degeneracy breaking. The even-signature reggeons get their intercept promoted to a larger value, while the odd-signature ones are left unchanged.

Exchange degeneracy breaking in the scattering and particle regions

We perform the analytic continuation, from the scattering ( t < 0) to the particle ( t > 0) region, of the non-planar diagrams (having torus topology) which generate a splitting between planar trajectories with non-zero iso-spin. Using heavily our previous work on the planar and cylinder amplitudes, we derive “asymptotic planarity” and estimate the rate of approach to it, also for the torus under rather general assumption on the Reggeon-Reggeon amplitude fixed pole residue.

The breaking of exchange degeneracy in dual unitarisation

The splitting of trajectories with non-vacuum quantum numbers is discussed within the dual unitarisation scheme. The estimated difference in intercepts between vector-tensor trajectories of opposite signatures, such as ϱ − A 2 and K ∗ − K ∗∗ , agrees both in sign and magnitude with that deduced from phenomenological analyses.

Dynamical realization of breaking of exchange degeneracy and violation of the Okubo-Zweig-Iizuka rule

A multiperipheral model that implements part of the Veneziano topological expansion and in which the cylinder correction breaks exchange degeneracy and produces violation of the Okubo-Zweig-Iizuka (OZI) rule as well as a Pomeron above one is presented. In the negative charge conjugation channel the model is identical to the O meson model of Freund and Nambu if continued to a spin-one pole on the daughter of the Pomeron. (AIP)

A study of the line reversed hypercharge exchange reactions π+p → K +Σ+(1385) and K −p → π−Σ+(1385) at 10 GeV/c

We present results on the differential cross-sections for the reactions π +p → K + Σ +(1385) and K −p → π − Σ +(1385) at 10 GeV/ c. For the first time, the same equipment has been used in measuring both reactions, in order to obtain good relative normalization. In the region of low t ( t min to −0.3 (GeV/ c) 2) the two differential cross-sections have similar shape, and show a sharp forward dip indicating a dominant helicity flip contribution. However, the magnitudes of the cross-sections are significantly different, indicating substantial exchange degeneracy breaking. We find the ratio of the integrated cross-sections for the reactions K −p → π − Σ +(1385) and π +p → K + Σ +(1385) over the range −0.3 < t′ < 0.0 (GeV/ c) 2 to be 2.0 ± 0.2.

Elastic scattering in perturbative reggeon calculus

A perturbative approach to the reggeon calculus relevant at present energies is developed and applied to πN, KN and NN elastic scattering. In order to cover low and intermediate energies one has to introduce secondary Regge trajectories with a very large exchange degeneracy breaking. We describe correctly the total cross sections, the ratios of real over imaginary parts at t=0 and the slopes of the differential cross sections from 10 GeV/ c up to the highest ISR energies. The intercept of the pomeron turns out to be α(0)∼1.13.

Amplitudes for ϱ and A 2 Regge exchanges in δ production processes

In δ production processes like π + p → π 0δ ++, K + p → K 0δ ++, K − n → K 0δ − , exchange degeneracy breaking between ϱ and A 2 Regge poles is shown to be present in all s-channel helicity amplitudes including the double-flip one. Once. the dominance of two among four amplitudes and SU(3) invariance are assumed, the amplitudes are obtained in a rather model-indenpendent way. The peculiar feature of these amplitudes are analyzed and in particular some evidence for universality in the impact parameter representation is pointed out. Possible explanations are proposed.

Study of NN → ΔN and its line-reversed reaction

Experimental data on the lined reversed reactions pp → Δ ++ n and p n → Δ −− p are summarized. An attempt is made to understand the unusually large difference between σ(pp → Δn) and σ( p n → Δ p) in terms of the breaking of exchange degeneracy and complex Regge poles.

Broken exchange degeneracies and secondary trajectories in charge and hypercharge exchange reactions

A simple model is developed for the difference of K and K nucleon charge exchange cross-sections. In addition to a breaking of exchange degeneracy, the model requires secondary trajectories which are identified with the scalar meson trajectories. Hypercharge exchange reactions are also discussed.