The rapid shape change in Zr isotopes near neutron number $N=60$ is identified to be caused by type II shell evolution associated with massive proton excitations to its $0g_{9/2}$ orbit, and is shown to be a quantum phase transition. Monte Carlo shell-model calculations are carried out for Zr isotopes of $N=50–70$ with many configurations spanned by eight proton orbits and eight neutron orbits. Energy levels and $B(E2)$ values are obtained within a single framework in good agreement with experiment, depicting various shapes in going from $N=50$ to 70. The novel coexistence of prolate and triaxial shapes is suggested.

Background: Type II shell evolution has recently been identified as a microscopic cause for nuclear shape coexistence. Purpose: Establish a low-lying rotational band in ${}^{96}\mathrm{Zr}$. Methods: High-resolution inelastic electron scattering and a relative analysis of transition strengths are used. Results: The $B(E2;0_1^{+}\to 2_2^{+})$ value is measured and electromagnetic decay strengths of the $2_2^{+}$ state are deduced. Conclusions: Shape coexistence is established for ${}^{96}\mathrm{Zr}$. Type II shell evolution provides a systematic and quantitative mechanism to understand deformation at low excitation energies.

In an experiment with the BigRIPS separator at the RIKEN Nishina Center, we observed two-proton ($2p$) emission from ${}^{67}\mathrm{Kr}$. At the same time, no evidence for $2p$ emission of ${}^{59}\mathrm{Ge}$ and ${}^{63}\mathrm{Se}$, two other potential candidates for this exotic radioactivity, could be observed. This observation is in line with $Q$ value predictions which pointed to ${}^{67}\mathrm{Kr}$ as being the best new candidate among the three for two-proton radioactivity. ${}^{67}\mathrm{Kr}$ is only the fourth $2p$ ground-state emitter to be observed with a half-life of the order of a few milliseconds. The decay energy was determined to be 1690(17) keV, the $2p$ emission branching ratio is 37(14)%, and the half-life of ${}^{67}\mathrm{Kr}$ is 7.4(30) ms.

The energy-dependent cross section of the ${}^7\mathrm{Be}(n,\alpha ){}^4\mathrm{He}$ reaction, of interest for the so-called cosmological lithium problem in big bang nucleosynthesis, has been measured for the first time from 10 meV to 10 keV neutron energy. The challenges posed by the short half-life of ${}^7\mathrm{Be}$ and by the low reaction cross section have been overcome at n_TOF thanks to an unprecedented combination of the extremely high luminosity and good resolution of the neutron beam in the new experimental area (EAR2) of the n_TOF facility at CERN, the availability of a sufficient amount of chemically pure ${}^7\mathrm{Be}$, and a specifically designed experimental setup. Coincidences between the two alpha particles have been recorded in two $\mathrm{Si}\text{−}{}^7\mathrm{Be}\text{−}\mathrm{Si}$ arrays placed directly in the neutron beam. The present results are consistent, at thermal neutron energy, with the only previous measurement performed in the 1960s at a nuclear reactor. The energy dependence reported here clearly indicates the inadequacy of the cross section estimates currently used in BBN calculations. Although new measurements at higher neutron energy may still be needed, the n_TOF results hint at a minor role of this reaction in BBN, leaving the long-standing cosmological lithium problem unsolved.

How do protons and neutrons bind to form nuclei? This is the central question of ab initio nuclear structure theory. While the answer may seem as simple as the fact that nuclear forces are attractive, the full story is more complex and interesting. In this work we present numerical evidence from ab initio lattice simulations showing that nature is near a quantum phase transition, a zero-temperature transition driven by quantum fluctuations. Using lattice effective field theory, we perform Monte Carlo simulations for systems with up to twenty nucleons. For even and equal numbers of protons and neutrons, we discover a first-order transition at zero temperature from a Bose-condensed gas of alpha particles (${}^4\mathrm{He}$ nuclei) to a nuclear liquid. Whether one has an alpha-particle gas or nuclear liquid is determined by the strength of the alpha-alpha interactions, and we show that the alpha-alpha interactions depend on the strength and locality of the nucleon-nucleon interactions. This insight should be useful in improving calculations of nuclear structure and important astrophysical reactions involving alpha capture on nuclei. Our findings also provide a tool to probe the structure of alpha cluster states such as the Hoyle state responsible for the production of carbon in red giant stars and point to a connection between nuclear states and the universal physics of bosons at large scattering length.

We present an improved search for neutrinoless double-beta ($0\nu \beta \beta$) decay of ${}^{136}\mathrm{Xe}$ in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ${}^{110m}\mathrm{Ag}$ contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the $0\nu \beta \beta$ decay half-life of $T_{1/2}^{0\nu }>1.07\times 10^{26}\mathrm{yr}$ at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61–165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.

We present a systematic study of both nuclear radii and binding energies in (even) oxygen isotopes from the valley of stability to the neutron drip line. Both charge and matter radii are compared to state-of-the-art ab initio calculations along with binding energy systematics. Experimental matter radii are obtained through a complete evaluation of the available elastic proton scattering data of oxygen isotopes. We show that, in spite of a good reproduction of binding energies, ab initio calculations with conventional nuclear interactions derived within chiral effective field theory fail to provide a realistic description of charge and matter radii. A novel version of two- and three-nucleon forces leads to considerable improvement of the simultaneous description of the three observables for stable isotopes but shows deficiencies for the most neutron-rich systems. Thus, crucial challenges related to the development of nuclear interactions remain.

We show within the saturation framework that measurements of exclusive vector meson production at high energy provide evidence for strong geometric fluctuations of the proton. In comparison, the effect of saturation scale and color charge fluctuations is weak. This knowledge will allow detailed future measurements of the incoherent cross section to tightly constrain the fluctuating geometry of the proton as a function of the parton momentum fraction $x$.

We generalize the state-of-the-art perturbative equation of state of cold quark matter to nonzero temperatures, needed in the description of neutron star mergers and core collapse processes. The new result is accurate to $\mathcal{O}(g^5)$ in the gauge coupling, and is based on a novel framework for dealing with the infrared sensitive soft field modes of the theory. The zero Matsubara mode sector is treated via a dimensionally reduced effective theory, while the soft nonzero modes are resummed using the hard thermal loop approximation. This combination of known effective descriptions offers unprecedented access to small but nonzero temperatures, both in and out of beta equilibrium.

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on ${}^{69}\mathrm{Ni}$, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented.

The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the $\beta$ decay of the free neutron. We report on the RDK II experiment which measured the photon spectrum using two different detector arrays. An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782 keV. The spectral shape was consistent with theory, and we determined a branching ratio of $0.00335\pm 0.00005[\mathrm{stat}]\pm 0.00015[\mathrm{syst}]$. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14 keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be $0.00582\pm 0.00023[\mathrm{stat}]\pm 0.00062[\mathrm{syst}]$. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies.

The $N=48$ ${}^{80}\mathrm{Ge}$ nucleus is studied by means of $\beta$-delayed electron-conversion spectroscopy at ALTO. The radioactive ${}^{80}\mathrm{Ga}$ beam is produced through the isotope separation on line photofission technique and collected on a movable tape for the measurement of $\gamma$ and $e^{-}$ emission following $\beta$ decay. An electric monopole $E0$ transition, which points to a 639(1) keV intruder $0_2^{+}$ state, is observed for the first time. This new state is lower than the $2_1^{+}$ level in ${}^{80}\mathrm{Ge}$, and provides evidence of shape coexistence close to one of the most neutron-rich doubly magic nuclei discovered so far, ${}^{78}\mathrm{Ni}$. This result is compared with theoretical estimates, helping to explain the role of monopole and quadrupole forces in the weakening of the $N=50$ gap at $Z=32$. The evolution of intruder $0_2^{+}$ states towards ${}^{78}\mathrm{Ni}$ is discussed.

Collinear laser spectroscopy is performed on the ${}_{30}{}^{79}{\mathrm{Zn}}_{49}$ isotope at ISOLDE-CERN. The existence of a long-lived isomer with a few hundred milliseconds half-life is confirmed, and the nuclear spins and moments of the ground and isomeric states in ${}^{79}\mathrm{Zn}$ as well as the isomer shift are measured. From the observed hyperfine structures, spins $I=9/2$ and $I=1/2$ are firmly assigned to the ground and isomeric states. The magnetic moment $\mu$ $({}^{79}\mathrm{Zn})=-1.1866(10){\mu }_N$, confirms the spin-parity $9/2^{+}$ with a $\nu g_{9/2}^{-1}$ shell-model configuration, in excellent agreement with the prediction from large scale shell-model theories. The magnetic moment $\mu$ $({}^{79m}\mathrm{Zn})=-1.0180(12){\mu }_N$ supports a positive parity for the isomer, with a wave function dominated by a $2h\text{−}1p$ neutron excitation across the $N=50$ shell gap. The large isomer shift reveals an increase of the intruder isomer mean square charge radius with respect to that of the ground state, $\delta ⟨r_c^2{⟩}^{79,79m}=+0.204(6){\mathrm{fm}}^2$, providing first evidence of shape coexistence.

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, $\mathrm{\Delta }\varphi$, and pseudorapidity, $\mathrm{\Delta }\eta$, in $\sqrt{s}=13$ and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|\eta |<2.5$. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $\mathrm{\Delta }\varphi \sim 0$ that extends over a wide range of $\mathrm{\Delta }\eta$, which has been referred to as the “ridge.” Per-trigger-particle yields, $Y(\mathrm{\Delta }\varphi )$, are measured over $2<|\mathrm{\Delta }\eta |<5$. For both collision energies, the $Y(\mathrm{\Delta }\varphi )$ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by $\cos (2\mathrm{\Delta }\varphi )$. The fitted Fourier coefficient, $v_{2,2}$, exhibits factorization, suggesting that the ridge results from per-event $\cos (2\varphi )$ modulation of the single-particle distribution with Fourier coefficients $v_2$. The $v_2$ values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a $p_{\mathrm{T}}$ dependence similar to that measured in $p+\mathrm{Pb}$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions. The $v_2$ values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arises from the same or similar underlying physics as observed in $p+\mathrm{Pb}$ collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.

We present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at $\sqrt{s}=500\mathrm{GeV}$ by the STAR experiment at RHIC. The measured observable is sensitive to the Sivers function, one of the transverse-momentum-dependent parton distribution functions, which is predicted to have the opposite sign in proton-proton collisions from that observed in deep inelastic lepton-proton scattering. These data provide the first experimental investigation of the nonuniversality of the Sivers function, fundamental to our understanding of QCD.

We describe the fissioning dynamics of ${}^{240}\mathrm{Pu}$ from a configuration in the proximity of the outer fission barrier to full scission and the formation of the fragments within an implementation of density functional theory extended to superfluid systems and real-time dynamics. The fission fragments emerge with properties similar to those determined experimentally, while the fission dynamics appears to be quite complex, with many excited shape and pairing modes. The evolution is found to be much slower than previously expected, and the ultimate role of the collective inertia is found to be negligible in this fully nonadiabatic treatment of nuclear dynamics, where all collective degrees of freedom (CDOF) are included (unlike adiabatic treatments with a small number of CDOF).

The neutron-rich nucleus ${}^{144}\mathrm{Ba}$ ($t_{1/2}=11.5\mathrm{s}$) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers $A$ less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced $E1$ transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV ${}^{144}\mathrm{Ba}$ beam on a $1.0\text{−}\mathrm{mg}/{\mathrm{cm}}^2$ ${}^{208}\mathrm{Pb}$ target. The measured value of the matrix element, $⟨{3_1}^{-}\parallel \mathcal{M}(E3)\parallel {0_1}^{+}⟩=0.65\left(\genfrac{}{}{0ex}{}{+17}{-23}\right)$ $e{b}^{3/2}$, corresponds to a reduced $B(E3)$ transition probability of $48\left(\genfrac{}{}{0ex}{}{+25}{-34}\right)\mathrm{W}.\mathrm{u}.$ This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

The thermonuclear ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key ${}^{31}\mathrm{S}$ proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the $\beta$ decay of ${}^{31}\mathrm{Cl}$, we have observed the $\beta$-delayed $\gamma$ decay of a ${}^{31}\mathrm{S}$ state at $E_x=6390.2(7)\mathrm{keV}$, with a ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ resonance energy of $E_r=259.3(8)\mathrm{keV}$, in the middle of the ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state at $E_x=6279.0(6)\mathrm{keV}$, giving it an unambiguous spin and parity of $3/2^{+}$ and making it an important $l=0$ resonance for proton capture on ${}^{30}\mathrm{P}$.

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ${}^4\mathrm{He}({}^8\mathrm{He},{}^8\mathrm{Be})$ at $186\mathrm{MeV}/\mathrm{u}$. The energy of the state is $0.83\pm 0.65(\mathrm{stat})\pm 1.25(\mathrm{syst})\mathrm{MeV}$ above the threshold of four-neutron decay with a significance level of $4.9\sigma$. Utilizing the large positive $Q$ value of the $({}^8\mathrm{He},{}^8\mathrm{Be})$ reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

The ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle affects the synthesis of the elements between ${}^{20}\mathrm{Ne}$ and ${}^{27}\mathrm{Al}$ in asymptotic giant branch stars and novae. The ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ reaction rate is very uncertain because of a large number of unobserved resonances lying in the Gamow window. At proton energies below 400 keV, only upper limits exist in the literature for the resonance strengths. Previous reaction rate evaluations differ by large factors. In the present work, the first direct observations of the ${}^{22}\mathrm{Ne}(p,\gamma ){}^{23}\mathrm{Na}$ resonances at 156.2, 189.5, and 259.7 keV are reported. Their resonance strengths are derived with 2%–7% uncertainty. In addition, upper limits for three other resonances are greatly reduced. Data are taken using a windowless ${}^{22}\mathrm{Ne}$ gas target and high-purity germanium detectors at the Laboratory for Underground Nuclear Astrophysics in the Gran Sasso laboratory of the National Institute for Nuclear Physics, Italy, taking advantage of the ultralow background observed deep underground. The new reaction rate is a factor of 20 higher than the recent evaluation at a temperature of 0.1 GK, relevant to nucleosynthesis in asymptotic giant branch stars.