Fri·Dec 4, 2020Friday·December 4, 2020

1. 01*

   A search for a cosmologically-relevant boson in muon decay

   J.I. Collar🇺🇸

   Experiments looking for a lepton flavor-violating decay ${\mu }^{+}\to e^{+}{X}^0$ are reviewed in light of present-day germanium detector technology, with an eye on scenarios where a long-lived, slow-moving massive boson $X^0$ might have a cosmological impact. A broad swath of interesting, unexplored parameter space very close to the kinematic limit of the decay is found to be within the reach of a new proposed search. A number of possible roles for $X^0$ in past and present epochs can be investigated.

   nucl-exastro-ph.COhep-exhep-phPRD(2021)·4 citations
2. 02*

   Charge radii of exotic potassium isotopes challenge nuclear theory and the magic character of $N=32$

   Á. Koszorús🇧🇪 · X. F. Yang🇨🇳 · W. G. Jiang🇺🇸 · S. J. Novario🇺🇸 · S. W. Bai🇨🇳 · J. Billowes🇬🇧 · C. L. Binnersley🇬🇧 · M. L. Bissell🇬🇧 · T. E. Cocolios🇧🇪 · B. S. Cooper🇬🇧 · R. P. de Groote🇫🇮 · A. Ekström🇸🇪 and 17 other authors

   Nuclear charge radii are sensitive probes of different aspects of the nucleon-nucleon interaction and the bulk properties of nuclear matter; thus, they provide a stringent test and challenge for nuclear theory. The calcium region has been of particular interest, as experimental evidence has suggested a new magic number at $N=32$ [1-3], while the unexpectedly large increases in the charge radii [4,5] open new questions about the evolution of nuclear size in neutron-rich systems. By combining the collinear resonance ionization spectroscopy method with $\beta$-decay detection, we were able to extend the charge radii measurement of potassium ($Z=19$) isotopes up to the exotic ${}^{52}$K ($t_{1/2}$ = 110 ms), produced in minute quantities. Our work provides the first charge radii measurement beyond $N=32$ in the region, revealing no signature of the magic character at this neutron number. The results are interpreted with two state-of-the-art nuclear theories. For the first time, a long sequence of isotopes could be calculated with coupled-cluster calculations based on newly developed nuclear interactions. The strong increase in the charge radii beyond $N=28$ is not well captured by these calculations, but is well reproduced by Fayans nuclear density functional theory, which, however, overestimates the odd-even staggering effect. These findings highlight our limited understanding on the nuclear size of neutron-rich systems, and expose pressing problems that are present in some of the best current models of nuclear theory.

   nucl-exnucl-thphysics.atom-phNat.Phys.(2021)·162 citations
3. 03*

   Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

   M. Guttormsen🇳🇴 · Y. Alhassid🇺🇸 · W. Ryssens🇧🇪 · K.O. Ay🇹🇷 · M. Ozgur🇹🇷 · E. Algin · A.C. Larsen🇳🇴 · F.L. Bello Garrote🇳🇴 · L. Crespo Campo🇳🇴 · T. Dahl-Jacobsen · A. Görgen🇳🇴 · T.W. Hagen🇳🇴 and 10 other authors

   Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes ${}^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement.

   nucl-exPLB(2021)·20 citations
4. 04*

   Intrinsic Correlations Among Characteristics of Neutron-rich Matter Imposed by the Unbound Nature of Pure Neutron Matter

   Bao-Jun Cai🇨🇳 · Bao-An Li🇺🇸

   The unbound nature of pure neutron matter (PNM) requires intrinsic correlations between the symmetric nuclear matter (SNM) EOS parameters (incompressibility $K_0$, skewness $J_0$ and kurtosis $I_0$) and those (slope $L$, curvature $K_{\mathrm{sym}}$ and skewness $J_{\mathrm{sym}}$) characterizing the symmetry energy independent of any nuclear many-body theory. We investigate these intrinsic correlations and their applications in better constraining the poorly known high-density behavior of nuclear symmetry energy. Several novel correlations connecting the characteristics of SNM EOS with those of nuclear symmetry energy are found. In particular, at the lowest-order of approximations, the bulk parts of the slope $L$, curvature $K_{\mathrm{sym}}$ and skewness $J_{\mathrm{sym}}$ of the symmetry energy are found to be $L\approx K_0/3,K_{\mathrm{sym}}\approx L{J}_0/2K_0$ and $J_{\mathrm{sym}}\approx I_{0}L/3K_0$, respectively. High-order corrections to these simple relations can be written in terms of the small ratios of high-order EOS parameters. The resulting intrinsic correlations among some of the EOS parameters reproduce very nicely their relations predicted by various microscopic nuclear many-body theories and phenomenological models constrained by available data of terrestrial experiments and astrophysical observations in the literature. The unbound nature of PNM is fundamental and the required intrinsic correlations among the EOS parameters characterizing both the SNM EOS and symmetry energy are universal. These intrinsic correlations provide a novel and model-independent tool not only for consistency checks but also for investigating the poorly known high-density properties of neutron-rich matter by using those with smaller uncertainties.

   ↳ nucl-thastro-ph.HEnucl-exPRC(2021)·10 citations
5. 05*

   The evolution of magnetic dipole strength in ${}^{100-140}$ Sn isotope chain and quenching of nucleon g factors

   G. Kruzic · T. Oishi🇭🇷 · N. Paar🇭🇷

   The evolution of electromagnetic transitions along isotope chains is of particular importance for the nuclear structure and dynamics, as well as for the r-process nucleosynthesis. Recent measurement of inelastic proton scattering on even-even ${}^{112-124}$Sn isotopes provides a novel insight into the isotopic dependence of E1 and M1 strength distributions. We investigate M1 transitions in even-even ${}^{100-140}$Sn isotopes from a theoretical perspective, based on relativistic nuclear energy density functional. The M1 transition strength distribution is characterized by an interplay between single and double-peak structures, that can be understood from the evolution of single-particle states, their occupations governed by the pairing correlations, and two-quasiparticle transitions involved. It is shown that discrepancy between model calculations and experiment on B(M1) transition strength is considerably reduced than previously known, and the quenching of the g-factors for the free nucleons needed to reproduce the experimental data on M1 transition strength amounts $g_{eff}/g_{free}$=0.80-0.93. Since some of the B(M1) strength above the neutron threshold may be missing in the inelastic proton scattering measurement, further experimental studies are required to confirm if only small modifications of the bare g-factors are actually needed when applied in finite nuclei.

   ↳ nucl-thnucl-exPRC(2021)·9 citations
6. 06*

   Constraining the initial stages of ultrarelativistic nuclear collisions

   Jakub Jankowski🇵🇱 · Syo Kamata🇵🇱 · Mauricio Martinez🇺🇸 · Michał Spaliński🇵🇱

   It is frequently supposed that quark-gluon plasma created in heavy-ion collisions undergoes free streaming at early times. We examine this issue based on the assumption that a universal attractor dominates the dynamics already at the earliest stages, which offers a way to connect the initial state with the start of the hydrodynamic expansion in an approximate but conceptually transparent fashion. We demonstrate that the centrality dependence of the measured particle multiplicities can be used to quantitatively constrain the pressure anisotropy and find that it strongly depends on the model of the initial energy deposition. As an illustration, we compare three initial state models and show that they predict rather different early-time values of the pressure anisotropy. This strongly suggests that assuming free streaming prior to hydrodynamization is not necessarily compatible with a generic initial state model and that features of the pre-hydrodynamic flow need to be matched with the model of the initial state.

   ↳ nucl-thhep-phnucl-exPRD(2021)·15 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.