Precise Measurement of Short-Range Correlations in Nuclei from Bremsstrahlung Gamma Ray Emission in Low-Energy Heavy-Ion Collisions

Abstract

Atomic nuclei and dense nucleonic matter in neutron stars exhibit short-range correlations (SRCs), where nucleons form temporally correlated pairs in proximity beyond mean-field approximation. It is essential to make precision measurement of the fraction of SRC since it carries the signature of underlying quark dynamics in nuclear medium. In this letter, we present the first high-precision measurement of neutron-proton bremsstrahlung $γ$-ray emission from the symmetric $\rm ^{124}Sn$+$\rm ^{124}Sn$ reactions at 25 MeV/u. From the observed spectral hardening, the precise SRC fraction in the $\rm ^{124}Sn$ nucleus is extracted to be $(20 \pm 3)\%$. This result provides a novel, direct and unambiguous evidence of SRCs, and demonstrates that low-energy heavy-ion collisions offers a new approach to studying nuclear structure in connection with quark-level dynamics.

Figures (4)

• Figure 1: An illustration of the CSHINE detector setup. A schematic view of an $\rm ^{124}Sn$+$\rm ^{124}Sn$ event is plotted, involving the $np$ collision that emits a bremsstrahlung $\gamma$-ray, subsequently detected by the CsI(Tl) hodoscope with three surrounding plastic scintillators.
• Figure 2: Event-wise vertical spreading $\delta_y$ as a function of the total reconstructed energy $E_{\rm tot}$ for beam-on (a) and beam-off (b) data. Correlation between the energy deposited in the event core $E_{\rm core}$ and $E_{\rm tot}$ for beam-on (c) and beam-off (d) data. A typical display of a high energy $\gamma$ event from the reaction (e), and of a cosmic muon penetration event through the $\gamma$-hodoscope (f).
• Figure 3: The experimental $\gamma$ energy spectrum in laboratory measured with beam-on (black), compared to the background spectrum with beam-off (red), normalized to the counts above $110~\rm MeV$. The inset displays the background-subtracted $\gamma$ spectrum in laboratory.
• Figure 4: (a) Comparison of the rebinned experimental $\gamma$ spectrum (black dots) in CM reference with various detector-filtered theoretical curves. Statistical (systematic) uncertainties are denoted by vertical bars (gray bands). (b) Likelihood distribution as a function of $R_{\rm HMT}$ with quadratic fit (dashed curve). The error bars represent the standard deviation of likelihood values by varying the calibration and detector response data sets. Several main confidence levels are indicated.