Experimental Evidence of “In Medio” Effects in Heavy-Ion Collisions at Intermediate Energies
Heavyion collisions at bombarding energies ranging from about 100 MeV/nucleon up to a few GeV/nucleon represent a unique tool to study the excitation of non-nucleonic degrees of freedom like baryonic resonances in excited nuclear matter far from ground-state conditions, i. e. outside the usual domain of existing nuclear structure information. Indeed, in a recent paper1 we have demonstrated the existence of the elementary indirect process N N → N Δ → N Nπ0 in 36Ar+27Al collisions at around 100 MeV/nucleon and we have deduced from experimental data the relative cross section. Notwithstanding Δ → Nπ is by far the most favoured decay channel (B.R.~ 100% 2), it is not however the best-suited one to study the signals of excitation and propagation of Δ(1232)-resonance in nuclear matter because of the high distortion introduced by the final-state interactions of pions with the surrounding medium. In this context, the electromagnetic decay Δ →Nγ would be, on the contrary, much more appropriate due to the almost complete absence of interaction of photons with nuclear matter. The free branching ratio of that decay channel is, however, only 6 · 10−3 2, and the successful realization of an experiment aimed to the detection of γ’s coming from Δ decay has to reckon with the existence of several serious drawbacks: i) in order not to have contamination from other mechanisms (such as statistical photon emission and/or giant-resonance de-excitation) a lower energy cut-off of at least 25–30 MeV must be imposed on the data and this strongly reduces the yields, ii) it is well known that high-energy photons are mostly emitted in the elementary direct process N N →N Nγ so that one has to identify a reasonable ensemble of conditions on the available observables apt to disentangle the indirect mechanism from the direct one, iii) in order to reduce as much as possible the strong background due to photons coming from π0 decays, the bombarding energy should not be much larger than 100 MeV/nucleon and, at the same time, it should not be much smaller than that value because of the consequent reduction of the phase space available for the excitation of the Δ resonance.
KeywordsInvariant Mass Nuclear Matter Heavyion Collision Photon Energy Spectrum Electromagnetic Decay
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