On the range of the nucleon-antinucleon interaction

Abstract

The range of the NN interaction is re-examined. We find that differences in the Nb7 models do have an influence on the region where annihilation predominantly takes place. A NN model derived consistently within meson theory leads to an annihilation range around 1 fm in spite of the small Compton wavelength of the exchanged baryons usually advocated in range arguments. PACS: 13.75; 21.30 One motivation for the study of the NN system is the occurrence of annihilation effects since here one could hope to learn something about the internal structure of the nucleons. It was argued that from the range of this annihilation one can deduce informations on that domain in which the description of nucleonic forces in terms of meson exchange is losing its validity and where one has to pass over to a theory relying essentially on quark degrees of freedom [-lJ. In this context rather different ranges are usually attributed to the mechanisms themselves leading to annihilation. Within a meson-theoretical approach [-2, 3] annihilation proceeds via the exchange of baryons, the range of which is commonly related to their Compton wavelength of less than 0.2 fm. Alternatively, if we think nucleons and antinucleons being constituted out of quarks then annihilation is accomplished by a rearrangement of these quarks into mesons and/or quark-antiquark fusion I-4, 5J. Such processes can occur as soon as the confinement regions of the quarks overlap, so that the annihilation range is more or less given by twice the bag radius, i.e. between 0.5 and 1.1 fro, depending on the particular model.Recently Shibata [-6] presented an investigation on the N~7 system where he analyzed the range of the interaction in the annihilation channel by means of a simple, phenomenological potential which was fitted to a set of low-energy antiproton data newly obtained at LEAR [7J. The major conclusion drawn from this analysis was that annihilation predominantly takes place at distances of nucleons and antinucleons around 1 fro. On the first glance (and in accordance with widespread belief), this result, being by a factor of five larger than the nucleon Compton wavelength, seems to rule out any meson-theoretical description of annihilation processes and appears to clearly establish the decisive role of explicit quarkgluon dynamics in NN annihilation.The aim of the present paper is to show, however, that this conclusion (which, if true, would have farreaching consequences) is unjustified. We will demonstrate that the annihilation 'range' obtained from such an analysis depends in fact on the particular model applied. Furthermore and most importantly, we will present a baryon-exchange annihilation model which can indeed provide (almost) sufficient annihilation and moreover, in spite of the small nucleon Compton wavelength, leads to an annihilation 'range' comparable to Shibata's model. Our ModelsStarting point for our calculation is the recent Bonn NN potential [8] ; it includes not only traditional oneboson-exchange diagrams but also explicit 2~z-and 7zp-excha...

First experimental detection of antiproton in-flight annihilation on nuclei at ∼ 130 keV

“…In addition both potential models [15,23,27] and phenomenological analyses [28][29][30] state that theNN andNnucleus annihilation mechanisms occur in a thin region placed just outside the nuclear volume. As a consequence, the annihilation process has to be considered a valuable tool for probing the external region of the nucleus where for example the neutron/proton ratio or the extraction energy of the peripheral nucleons can be determined.…”

Limits on the low-energy antinucleon-nucleus annihilations from the Heisenberg principle

“…In other words, as soon as the overlap between the distributions of probability for the antinucleon and for the target nucleons overcomes a certain threshold << 1 the annihilation is supposed to take place, with the practical consequence that any consistent overlap of the projectile and target wavefunctions is forbidden. Most models [12,13,16] or phenomenological optical potential analyses [14,15,6] agree on this property.…”

“…Both in Hydrogen and in heavier nucleus targets, the great bulk of the annihilations is supposed to take place within a region of thickness ∆ ≈ 1 fm placed just out of the nuclear surface [12][13][14][15][16]. Since the realized annihilation implies the statement "N and nucleus at relative distance r ≈ R nucleus defined within uncertainty ∆", we expect strong deviations from semiclassical intuition at k < 1/∆ ∼ 200 MeV/c.…”

Multichannel effective range approach to low energy nucleon-antinucleon interactions

“…Phenomenological analysis of data were made within the various potential models [4][5][6][7][8]; other analysis in the framework of the scattering-length expansion [9, 10 13] incorporate analyticity and unitarity in the M-matrix formalism [11,12]. The nr/channel was taken into account whose threshold is distant from p/5 ones at 98 MeV/c in laboratory system and affect the behavior of p and elastic p/5-amplitude.…”