In relativistic heavy-ion collisions, the analysis of charmonium yields normalized to those in $pp$ collisions has been shown to reveal final-state interactions occurring in the hot and dense strongly interacting matter created in such events. However, until recently, similar analyses of charmonium measured in smaller collision systems—$pA$ versus $pp$—were performed in terms of cold nuclear matter effects. The measurements reported by the PHENIX Collaboration of the production of $J/\mathrm{\Psi }$ and $\mathrm{\Psi }(2S)$ states at forward and backward rapidity, strongly suggest the presence of final-state effects also in small collision systems.

In analogy with the phenomena occurring when two superconductors are placed in proximity, effectively charged neutrons involved in one- and two-nucleon tunneling processes in heavy-ion collisions between superfluid nuclei are expected to emit photons. Based on $T$-matrix calculations, a quantal coherent, or AC Josephson-like, character of the emission is predicted in the case of the Cooper-pair transfer (a two-nucleon channel), whereas a thermally equilibrated (Joule-like) character is obtained for the quasiparticle (one-nucleon channel) transfer. These findings provide a new microscopic insight into superfluidity in finite quantum systems and may stimulate new experimental analyses.

Pumping electrons in atoms via a gateway state to metastable states is a common and important tool with many applications, but the same process has yet to be achieved in atomic nuclei. The authors provide detailed mechanisms to pump a metastable state of ${}^{229}$Th at the extraordinarily low nuclear excitation energy of 8 eV, while also pointing out important gaps in our knowledge of this system. A population inversion to this state could be the basis for a novel “nuclear clock” to rival atomic clocks.

The dimensionless ratio of shear viscosity to entropy density $\eta /s$ typically has a minimum at a phase transition as observed in many atomic and molecular systems. This quantity has been under intense study in high-energy heavy-ion collisions as a sign of a QCD phase transition or rapid crossover between hadronic and quark-gluon degrees of freedom. The authors perform numerical simulations of matter at lower density and temperature associated with a minimum in $\eta /s$ which may shed light on the nuclear liquid-gas phase transition and its connection to the QCD transition.

First results from the BEST Collaboration searching for short-baseline neutrino oscillations to sterile neutrinos with a high-intensity ${}^{51}$Cr monoenergetic neutrino source reaffirm that the so-called gallium anomaly, a deficit in electron neutrinos, persists. $4\sigma$ deficits were observed in the ${}^{71}$Ge production rates at two different distance scales, which could be interpreted as oscillations between an electron neutrino and a hypothetical sterile neutrino. The results are consistent with oscillations with a mass squared difference above about 0.5 eV${}^2$ and a large mixing angle ${\sin }^{2}2\theta \approx 0.4$.

The unbound nucleus ${}^{15}$F was studied in proton-induced reactions on ${}^{14}$O, producing three narrow resonances above the 2$p$ decay threshold in ${}^{15}$F. In comparison to calculations that account for the particle continuum, it was found that the properties of these resonances are determined by the proximity to proton decay channels rather than by carrying the imprint of ${}^{14}$O$(0^{+})+1p$ configurations. Systematic investigations of such narrow resonances in unstable nuclei will open new perspectives in studies of effective interactions in nuclear open quantum systems.

APS has appointed Professor Joseph Kapusta, School of Physics and Astronomy, University of Minnesota as the Lead Editor of Physical Review C. Professor Kapusta takes the helm following the journal’s previous Lead Editor Benjamin F. Gibson.

A look back at Physical Review C’s first half century, and a salute to the talented authors and diligent referees who have made the journal a success.

Researchers look back at key contributions to the field of nuclear physics.

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