Probing short-range correlations in asymmetric nuclei with quasi-free pair knockout reactions

Abstract

Short-range correlations (SRC) in asymmetric nuclei with an unusual neutronto-proton ratio can be studied with quasi-free two-nucleon knockout processes following the collision between accelerated ions and a proton target. We derive an approximate factorized cross section for those SRC-driven p(A, p N 1 N 2 ) reactions. Our reaction model hinges on the factorization properties of SRC-driven A(e, e N 1 N 2 ) reactions for which strong indications are found in theory-experiment comparisons. In order to put our model to the test we compare its predictions with results of 12 C(p, p pn) measurements conducted at Brookhaven National Laboratory (BNL) and find a fair agreement. The model can also reproduce characteristic features of SRC-driven two-nucleon knockout reactions, like back-to-back emission of the correlated nucleons. We study the asymmetry dependence of nuclear SRC by providing predictions for the ratio of proton-proton to proton-neutron knockout cross sections for the carbon isotopes 9−15 C thereby covering neutron excess values (N − Z)/Z between -0.5 and +0.5.

Sequential Nature of (p,3p) Two-Proton Knockout from Neutron-Rich Nuclei

“…The other proton of the pair acts as a spectator and is emitted with its original momentum. This process is similar to the impulse approximation usually assumed in high-energy ðe; e 0 2pÞ and ðp; 3pÞ reactions [18,26,48].…”

“…In particular, one-nucleon knockout reactions at intermediate energies, above ∼50 MeV/ nucleon, have been extensively used for the spectroscopy of unstable isotopes, both with heavy-ion [10,11] and proton targets [12,13]. Given the success of one-nucleon knockout reactions, two-nucleon knockout has naturally garnered much interest, as it allows us to explore further nuclear properties, such as nucleon-nucleon correlations [14][15][16][17][18]. Two-proton knockout from neutron-rich nuclei has been used to populate very exotic species, in a process assumed to be direct, due to the inhibited proton evaporation in these nuclei [19][20][21][22][23].…”

“…The reaction 80 Znðp; 3pÞ has recently been used to populate new 78 Ni states, inaccessible to one-nucleon knockout [24], demonstrating the potential of ðp; 3pÞ reactions as a spectroscopic tool for exotic nuclei. However, to properly interpret the resulting observables, the ðp; 3pÞ reaction mechanism must be well understood, as the removal of spatially correlated proton pairs [18] explores different nuclear properties than the sequential removal of uncorrelated protons [15,25]. For example, the residual nucleus momentum distribution explores its nuclear structure when removing uncorrelated protons [19], while removing a short-range correlated proton pair makes it sensitive to the pair momentum distribution, which is weakly A dependent [26].…”

Isospin composition of the high-momentum fluctuations in nuclei from asymptotic momentum distributions

“…The LCA is a methodology with applications in nuclear reactions and nuclear structure. In LCA one can compute the observables for SRC dominated nucleon knockout reactions [10,[40][41][42][43]. Furthermore, the impact of SRC on nuclear momentum distributions [35,36] can be quantified across the nuclear mass range as even for the heaviest nuclei the numerical calculations are manageable.…”

Coarse graining the Bethe–Goldstone equation: Nucleon–nucleon high-momentum components

“…For example, by measuring the ratio of neutron-proton (np) and proton-proton (pp) pairs in a relative high momentum state, a value of np/pp = 18 ± 5 was reported in [32], providing strong evidence of the crucial impact of the tensor force in the SRC. This suggest a non-trivial dependence of SRC on the N − Z asymmetry, with repercussions on the nuclear equation of state for high densities, which is essential for the understanding of neutron stars [42,43].…”