We report microscopic many-body calculations indicating that rotational bands based on nuclear scissors vibrations exhibit systematic splitting between neighboring spin states ($\mathrm{\Delta }I=2$ bifurcation) in which the magnitude of the moment of inertia oscillates between states having even and odd spins. We show that this unexpected result is caused by self-organization of the deformed proton and neutron bodies in the scissors motion, which is further amplified by the $K^{\pi }=1^{+}$ two-quasiparticle configurations near the scissors states. We propose that the puzzling excited state found above the $1^{+}$ scissors state in ${}^{156}\mathrm{Gd}$ [Phys. Rev. Lett. 118, 212502 (2017)] is the first evidence of this effect, and predict that bifurcation may generally appear in all other scissors rotational bands of deformed nuclei, and possibly in other systems exhibiting collective scissors vibrations.

The rate at which helium (${}^4\mathrm{He}$) and deuterium ($d$) fuse together to produce lithium-6 (${}^6\mathrm{Li}$) and a $\gamma$ ray, ${}^4\mathrm{He}(d,\gamma ){}^6\mathrm{Li}$, is a critical puzzle piece in resolving the discrepancy between big bang predictions and astronomical observations for the primordial abundance of ${}^6\mathrm{Li}$. The accurate determination of this radiative capture rate requires the quantitative and predictive description of the fusion probability across the big bang energy window ($30\mathrm{keV}\lesssim E\lesssim 400\mathrm{keV}$), where measurements are hindered by low counting rates. We present first-principle (or, ab initio) predictions of the ${}^4\mathrm{He}(d,\gamma ){}^6\mathrm{Li}$ astrophysical $S$ factor using validated nucleon-nucleon and three-nucleon interactions derived within the framework of chiral effective field theory. By employing the ab initio no-core shell model with continuum to describe ${}^4\mathrm{He}\text{−}d$ scattering dynamics and bound ${}^6\mathrm{Li}$ product on an equal footing, we accurately and consistently determine the contributions of the main electromagnetic transitions driving the radiative capture process. Our results reveal an enhancement of the capture probability below 100 keV owing to previously neglected magnetic dipole ($M1$) transitions and reduce by an average factor of 7 the uncertainty of the thermonuclear capture rate between 0.002 and 2 GK.

Using 250 neutron star merger simulations with microphysics, we explore for the first time the role of nuclear incompressibility in the prompt collapse threshold for binaries with different mass ratios. We demonstrate that observations of prompt collapse thresholds, either from binaries with two different mass ratios or with one mass ratio but combined with the knowledge of the maximum neutron star mass or compactness, will constrain the incompressibility at the maximum neutron star density $K_{\max }$ to within tens of percent. This otherwise inaccessible measure of $K_{\max }$ can potentially reveal the presence of hyperons or quarks inside neutron stars.

A narrow near-threshold proton-emitting resonance ($E_x=11.4\mathrm{MeV}$, $J^{\pi }=1/2^{+}$, and ${\mathrm{\Gamma }}_p=4.4\mathrm{keV}$) was directly observed in ${}^{11}\mathrm{B}$ via proton resonance scattering. This resonance was previously inferred in the $\beta$-delayed proton emission of the neutron halo nucleus ${}^{11}\mathrm{Be}$. The good agreement between both experimental results serves as a ground to confirm the existence of such exotic decay and the particular behavior of weakly bound nuclei coupled to the continuum. $R$-matrix analysis shows a sizable partial decay width for both, proton and $\alpha$ (${\mathrm{\Gamma }}_{\alpha }=11\mathrm{keV}$) emission channels.

A near-threshold proton resonance in ${}^{11}\mathrm{B}$ at $E_{\mathrm{ex}}=11.44\pm 0.04\mathrm{MeV}$ is observed via the reaction ${}^{10}\mathrm{Be}(d,n){}^{11}\mathrm{B}\to {}^{10}\mathrm{Be}+p$ in inverse kinematics, measured with a beam of the radioactive isotope ${}^{10}\mathrm{Be}$. The resonance energy at $E_{\mathrm{res}}=211(40)\mathrm{keV}$ is consistent with a proton signal observed by Ayyad et al. in the $\beta$-delayed proton decay of ${}^{11}\mathrm{Be}$. By comparison to a distorted wave Born approximation calculation, a 0.27(6) spectroscopic factor is extracted and a tentative ($\ell =0$) character is assigned for this resonance. The significant cross section in the proton-transfer $(d,n)$ reaction, as well as the observation of its proton-decay signal, point to the threshold-resonance character of this state. The position of this state, its structure, and strong coupling to the $s$-wave continuum represent an ideal case to study quantum near-threshold many-body dynamics of unstable states. The presence of this state is an important step toward understanding the excessively large beta-delayed proton-decay branch of ${}^{11}\mathrm{Be}$.

We propose a new theory of second-order viscous relativistic hydrodynamics which does not impose any frame conditions on the choice of the hydrodynamic variables. It differs from Mueller-Israel-Stewart theory by including additional transient degrees of freedom, and its first-order truncation reduces to Bemfica-Disconzi-Noronha-Kovtun theory. Conditions for causality and stability are explicitly given in the conformal regime. As an illustrative example, we consider Bjorken flow solutions to our equations and identify variables which make a hydrodynamic attractor manifest.

The $B_c^{+}$ meson is observed for the first time in heavy ion collisions. Data from the CMS detector are used to study the production of the $B_c^{+}$ meson in lead-lead (Pb-Pb) and proton-proton ($pp$) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, via the $B_c^{+}\to (J/\psi \to {\mu }^{+}{\mu }^{-}){\mu }^{+}{\nu }_{\mu }$ decay. The $B_c^{+}$ nuclear modification factor, derived from the Pb-Pb–to–$pp$ ratio of production cross sections, is measured in two bins of the trimuon transverse momentum and of the Pb-Pb collision centrality. The $B_c^{+}$ meson is shown to be less suppressed than quarkonia and most of the open heavy-flavor mesons, suggesting that effects of the hot and dense nuclear matter created in heavy ion collisions contribute to its production. This measurement sets forth a promising new probe of the interplay of suppression and enhancement mechanisms in the production of heavy-flavor mesons in the quark-gluon plasma.

The electric monopole ($E0$) transition strength ${\rho }^2$ for the transition connecting the third $0^{+}$ level, a “superdeformed” band head, to the “spherical” $0^{+}$ ground state in doubly magic ${}^{40}\mathrm{Ca}$ is determined via $e^{+}{e}^{-}$ pair-conversion spectroscopy. The measured value ${\rho }^2(E0;0_3^{+}\to 0_1^{+})=2.3(5)\times {10}^{-3}$ is the smallest ${\rho }^2(E0;0^{+}\to 0^{+})$ found in $A<50$ nuclei. In contrast, the $E0$ transition strength to the ground state observed from the second $0^{+}$ state, a band head of “normal” deformation, is an order of magnitude larger ${\rho }^2(E0;0_2^{+}\to 0_1^{+})=25.9(16)\times {10}^{-3}$, which shows significant mixing between these two states. Large-scale shell-model (LSSM) calculations are performed to understand the microscopic structure of the excited states and the configuration mixing between them; experimental ${\rho }^2$ values in ${}^{40}\mathrm{Ca}$ and neighboring isotopes are well reproduced by the LSSM calculations. The unusually small ${\rho }^2(E0;0_3^{+}\to 0_1^{+})$ value is due to destructive interference in the mixing of shape-coexisting structures, which are based on several different multiparticle-multihole excitations. This observation goes beyond the usual treatment of $E0$ strengths, where two-state shape mixing cannot result in destructive interference.

The anomaly in lithium abundance is a well-known unresolved problem in nuclear astrophysics. A recent revisit to the problem tried the avenue of resonance enhancement to account for the primordial ${}^7\mathrm{Li}$ abundance in standard big-bang nucleosynthesis. Prior measurements of the ${}^7\mathrm{Be}(d,p){}^8{\mathrm{Be}}^{*}$ reaction could not account for the individual contributions of the different excited states involved, particularly at higher energies close to the $Q$ value of the reaction. We carried out an experiment at HIE-ISOLDE, CERN to study this reaction at $E_{\mathrm{c}.\mathrm{m}.}=7.8\mathrm{MeV}$, populating excitations up to 22 MeV in ${}^8\mathrm{Be}$ for the first time. The angular distributions of the several excited states have been measured and the contributions of the higher excited states in the total cross section at the relevant big-bang energies were obtained by extrapolation to the Gamow window using the talys code. The results show that by including the contribution of the 16.63 MeV state, the maximum value of the total $S$ factor inside the Gamow window comes out to be 167 MeV b as compared to earlier estimate of 100 MeV b. However, this still does not account for the lithium discrepancy.

The isomer depletion of ${}^{93m}\mathrm{Mo}$ was recently reported [Chiara et al., Nature (London) 554, 216 (2018)] as the first direct observation of nuclear excitation by electron capture (NEEC). However, the measured excitation probability of 1.0(3)% is far beyond the theoretical expectation. In order to understand the inconsistency between theory and experiment, we produce the ${}^{93m}\mathrm{Mo}$ nuclei using the ${}^{12}\mathrm{C}({}^{86}\mathrm{Kr},5n)$ reaction at a beam energy of 559 MeV and transport the reaction residues to a detection station far away from the target area employing a secondary beam line. The isomer depletion is expected to occur during the slowdown process of the ions in the stopping material. In such a low $\gamma$-ray background environment, the signature of isomer depletion is not observed, and an upper limit of $2\times {10}^{-5}$ is estimated for the excitation probability. This is consistent with the theoretical expectation. Our findings shed doubt on the previously reported NEEC phenomenon and highlight the necessity and feasibility of further experimental investigations for reexamining the isomer depletion under low $\gamma$-ray background.

While first order perturbation theory is routinely used in quantum Monte Carlo (QMC) calculations, higher-order terms present significant numerical challenges. We present a new approach for computing perturbative corrections in projection QMC calculations. We demonstrate the method by computing nuclear ground state energies up to second order for a realistic chiral interaction. We calculate the binding energies of several light nuclei up to ${}^{16}\mathrm{O}$ by expanding the Hamiltonian around the Wigner SU(4) limit and find good agreement with data. In contrast to the natural ordering of the perturbative series, we find remarkably large second-order energy corrections. This occurs because the perturbing interactions break the symmetries of the unperturbed Hamiltonian. Our method is free from the sign problem and can be applied to QMC calculations for many-body systems in nuclear physics, condensed matter physics, ultracold atoms, and quantum chemistry.

The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos ${\nu }_e$ observed in previous gallium-based radiochemical measurements with high-intensity neutrino sources, commonly referred to as the “gallium anomaly,” which could be interpreted as evidence for oscillations between ${\nu }_e$ and sterile neutrino (${\nu }_s$) states. A 3.414-MCi ${}^{51}\mathrm{Cr}$ ${\nu }_e$ source was placed at the center of two nested Ga volumes and measurements were made of the production of ${}^{71}\mathrm{Ge}$ through the charged current reaction, ${}^{71}\mathrm{Ga}({\nu }_e,e^{-}){}^{71}\mathrm{Ge}$, at two average distances. The measured production rates for the inner and the outer targets, respectively, are [${54.9}_{-2.4}^{+2.5}(\mathrm{stat})\pm 1.4(\mathrm{syst})$] and [${55.6}_{-2.6}^{+2.7}(\mathrm{stat})\pm 1.4(\mathrm{syst})$] atoms of ${}^{71}\mathrm{Ge}/d$. The ratio ($R$) of the measured rate of ${}^{71}\mathrm{Ge}$ production at each distance to the expected rate from the known cross section and experimental efficiencies are $R_{\mathrm{in}}=0.79\pm 0.05$ and $R_{\mathrm{out}}=0.77\pm 0.05$. The ratio of the outer to the inner result is $0.97\pm 0.07$, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20%–24% lower than expected, thus reaffirming the anomaly. These results are consistent with ${\nu }_e\to {\nu }_s$ oscillations with a relatively large $\mathrm{\Delta }{m}^2$ ($>0.5{\mathrm{eV}}^2$) and mixing ${\sin }^{2}2\theta$ ($\approx 0.4$).

A nuclear excitation following the capture of an electron in an empty orbital has been recently observed for the first time. So far, the evaluation of the cross section of the process has been carried out widely using the assumption that the ion is in its electronic ground state prior to the capture. We show that by lifting this restriction new capture channels emerge resulting in a boost of more than three orders of magnitude to the electron capture resonance strength.

An unexplained $>4\sigma$ discrepancy persists between “beam” and “bottle” measurements of the neutron lifetime. A new model proposed that conversions of neutrons $n$ into mirror neutrons $n^{\prime }$, part of a dark mirror sector, can increase the apparent neutron lifetime by 1% via a small mass splitting $\mathrm{\Delta }m$ between $n$ and $n^{\prime }$ inside the 4.6 T magnetic field of the National Institute of Standards and Technology Beam Lifetime experiment. A search for neutron conversions in a 6.6 T magnetic field was performed at the Spallation Neutron Source which excludes this explanation for the neutron lifetime discrepancy.

With the tremendous accomplishments of RHIC and the LHC experiments and the advent of the future electron-ion collider on the horizon, the quest for compelling evidence of the color glass condensate (CGC) has become one of the most aspiring goals in the high energy quantum chromodynamics research. Pursuing this question requires developing the precision test of the CGC formalism. By systematically implementing the threshold resummation, we significantly improve the stability of the next-to-leading-order calculation in CGC for forward rapidity hadron productions in $pp$ and $pA$ collisions, especially in the high $p_T$ region, and obtain reliable descriptions of all existing data measured at RHIC and the LHC across all $p_T$ regions. Consequently, this technique can pave the way for the precision studies of the CGC next-to-leading-order predictions by confronting them with a large amount of precise data.

We report cumulants of the proton multiplicity distribution from dedicated fixed-target $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=3.0\mathrm{GeV}$, measured by the STAR experiment in the kinematic acceptance of rapidity ($y$) and transverse momentum ($p_T$) within $-0.5<y<0$ and $0.4<p_T<2.0\mathrm{GeV}/c$. In the most central 0%–5% collisions, a proton cumulant ratio is measured to be $C_4/C_2=-0.85\pm 0.09(\mathrm{stat})\pm 0.82(\mathrm{syst})$, which is $2\sigma$ below the Poisson baseline with respect to both the statistical and systematic uncertainties. The hadronic transport UrQMD model reproduces our $C_4/C_2$ in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in $C_4/C_2$ is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV.

The electroweak interaction in the standard model is described by a pure vector-axial-vector structure, though any Lorentz-invariant component could contribute. In this Letter, we present the most precise measurement of tensor currents in the low-energy regime by examining the $\beta \text{−}\overline{\nu }$ correlation of trapped ${}^8\mathrm{Li}$ ions with the Beta-decay Paul Trap. We find $a_{\beta \nu }=-0.3325\pm 0.0013_{\mathrm{stat}}\pm 0.0019_{\mathrm{syst}}$ at $1\sigma$ for the case of coupling to right-handed neutrinos ($C_T=-C_T^{\prime }$), which is consistent with the standard model prediction.