Previously unknown isotopes ${}^{30}\mathrm{Ar}$ and ${}^{29}\mathrm{Cl}$ have been identified by measurement of the trajectories of their in-flight decay products ${}^{28}\mathrm{S}+p+p$ and ${}^{28}\mathrm{S}+p$, respectively. The analysis of angular correlations of the fragments provided information on decay energies and the structure of the parent states. The ground states of ${}^{30}\mathrm{Ar}$ and ${}^{29}\mathrm{Cl}$ were found at $2.25_{-0.10}^{+0.15}$ and $1.8\pm 0.1\mathrm{MeV}$ above the two- and one-proton thresholds, respectively. The lowest states in ${}^{30}\mathrm{Ar}$ and ${}^{29}\mathrm{Cl}$ point to a violation of isobaric symmetry in the structure of these unbound nuclei. The two-proton decay has been identified in a transition region between simultaneous two-proton and sequential proton emissions from the ${}^{30}\mathrm{Ar}$ ground state, which is characterized by an interplay of three-body and two-body decay mechanisms. The first hint of a fine structure of the two-proton decay of ${}^{30}{\mathrm{Ar}}^{*}(2^{+})$ has been obtained by detecting two decay branches into the ground and first-excited states of the ${}^{28}\mathrm{S}$ fragment.

We describe a spin-echo method for ultracold neutrons (UCNs) confined in a precession chamber and exposed to a $|B_0|=1\mu \mathrm{T}$ magnetic field. We have demonstrated that the analysis of UCN spin-echo resonance signals in combination with knowledge of the ambient magnetic field provides an excellent method by which to reconstruct the energy spectrum of a confined ensemble of neutrons. The method takes advantage of the relative dephasing of spins arising from a gravitationally induced striation of stored UCNs of different energies, and also permits an improved determination of the vertical magnetic-field gradient with an exceptional accuracy of $1.1\mathrm{pT}/\mathrm{cm}$. This novel combination of a well-known nuclear resonance method and gravitationally induced vertical striation is unique in the realm of nuclear and particle physics and should prove to be invaluable for the assessment of systematic effects in precision experiments such as searches for an electric dipole moment of the neutron or the measurement of the neutron lifetime.

We present the first measurement of elliptic ($v_2$) and triangular ($v_3$) flow in high-multiplicity ${}^3\mathrm{He}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in ${}^3\mathrm{He}+\mathrm{Au}$ and in $p+p$ collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the ${}^3\mathrm{He}+\mathrm{Au}$ system. The collective behavior is quantified in terms of elliptic $v_2$ and triangular $v_3$ anisotropy coefficients measured with respect to their corresponding event planes. The $v_2$ values are comparable to those previously measured in $d+\mathrm{Au}$ collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three ${}^3\mathrm{He}$ nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.

Consistent formulations of relativistic viscous hydrodynamics involve short-lived modes, leading to asymptotic rather than convergent gradient expansions. In this Letter we consider the Müller-Israel-Stewart theory applied to a longitudinally expanding quark-gluon plasma system and identify hydrodynamics as a universal attractor without invoking the gradient expansion. We give strong evidence for the existence of this attractor and then show that it can be recovered from the divergent gradient expansion by Borel summation. This requires careful accounting for the short-lived modes which leads to an intricate mathematical structure known from the theory of resurgence.

The second-order azimuthal anisotropy Fourier harmonics, $v_2$, are obtained in $p\text{−}\mathrm{Pb}$ and PbPb collisions over a wide pseudorapidity ($\eta$) range based on correlations among six or more charged particles. The $p$-Pb data, corresponding to an integrated luminosity of $35{\mathrm{nb}}^{-1}$, were collected during the 2013 LHC $p$-Pb run at a nucleon-nucleon center-of-mass energy of $5.02\mathrm{TeV}$ by the CMS experiment. A sample of semiperipheral PbPb collision data at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$, corresponding to an integrated luminosity of $2.5\mu {\mathrm{b}}^{-1}$ and covering a similar range of particle multiplicities as the $p\text{−}\mathrm{Pb}$ data, is also analyzed for comparison. The six- and eight-particle cumulant and the Lee-Yang zeros methods are used to extract the $v_2$ coefficients, extending previous studies of two- and four-particle correlations. For both the $p\text{−}\mathrm{Pb}$ and PbPb systems, the $v_2$ values obtained with correlations among more than four particles are consistent with previously published four-particle results. These data support the interpretation of a collective origin for the previously observed long-range (large $\mathrm{\Delta }\eta$) correlations in both systems. The ratios of $v_2$ values corresponding to correlations including different numbers of particles are compared to theoretical predictions that assume a hydrodynamic behavior of a $p\text{−}\mathrm{Pb}$ system dominated by fluctuations in the positions of participant nucleons. These results provide new insights into the multiparticle dynamics of collision systems with a very small overlapping region.

We present measurements of ${\pi }^{-}$ and ${\pi }^{+}$ elliptic flow, $v_2$, at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200$, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{\mathrm{ch}}$, based on data from the STAR experiment at RHIC. We find that ${\pi }^{-}$ (${\pi }^{+}$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $\sqrt{s_{\mathrm{NN}}}=27\mathrm{GeV}$ and higher. At $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$, the slope of the difference of $v_2$ between ${\pi }^{-}$ and ${\pi }^{+}$ as a function of $A_{\mathrm{ch}}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.

We present azimuthal angular correlations between charged hadrons and energy deposited in calorimeter towers in central $d+\mathrm{Au}$ and minimum bias $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The charged hadron is measured at midrapidity $|\eta |<0.35$, and the energy is measured at large rapidity ($-3.7<\eta <-3.1$, Au-going direction). An enhanced near-side angular correlation across $|\mathrm{\Delta }\eta |>2.75$ is observed in $d+\mathrm{Au}$ collisions. Using the event plane method applied to the Au-going energy distribution, we extract the anisotropy strength $v_2$ for inclusive charged hadrons at midrapidity up to $p_T=4.5\mathrm{GeV}/c$. We also present the measurement of $v_2$ for identified ${\pi }^{\pm }$ and (anti)protons in central $d+\mathrm{Au}$ collisions, and observe a mass-ordering pattern similar to that seen in heavy-ion collisions. These results are compared with viscous hydrodynamic calculations and measurements from $p+\mathrm{Pb}$ at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The magnitude of the mass ordering in $d+\mathrm{Au}$ is found to be smaller than that in $p+\mathrm{Pb}$ collisions, which may indicate smaller radial flow in lower energy $d+\mathrm{Au}$ collisions.

It has been understood since 1897 that accelerating charges must emit electromagnetic radiation. Although first derived in 1904, cyclotron radiation from a single electron orbiting in a magnetic field has never been observed directly. We demonstrate single-electron detection in a novel radio-frequency spectrometer. The relativistic shift in the cyclotron frequency permits a precise electron energy measurement. Precise beta electron spectroscopy from gaseous radiation sources is a key technique in modern efforts to measure the neutrino mass via the tritium decay end point, and this work demonstrates a fundamentally new approach to precision beta spectroscopy for future neutrino mass experiments.

Excitation functions for the Gaussian emission source radii difference ($R_{\text{out}}^2-R_{\text{side}}^2$) obtained from two-pion interferometry measurements in $\mathrm{Au}+\mathrm{Au}$ ($\sqrt{s_{NN}}=7.7–200\mathrm{GeV}$) and $\mathrm{Pb}+\mathrm{Pb}$ ($\sqrt{s_{NN}}=2.76\mathrm{TeV}$) collisions are studied for a broad range of collision centralities. The observed nonmonotonic excitation functions validate the finite-size scaling patterns expected for the deconfinement phase transition and the critical end point (CEP), in the temperature versus baryon chemical potential ($T,{\mu }_B$) plane of the nuclear matter phase diagram. A finite-size scaling (FSS) analysis of these data suggests a second order phase transition with the estimates $T^{\mathrm{cep}}\sim 165\mathrm{MeV}$ and ${\mu }_B^{\mathrm{cep}}\sim 95\mathrm{MeV}$ for the location of the critical end point. The critical exponents ($\nu \approx 0.66$ and $\gamma \approx 1.2$) extracted via the same FSS analysis place this CEP in the 3D Ising model universality class.

We present $\mathrm{\Lambda }\mathrm{\Lambda }$ correlation measurements in heavy-ion collisions for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ using the STAR experiment at the Relativistic Heavy-Ion Collider. The Lednický-Lyuboshitz analytical model has been used to fit the data to obtain a source size, a scattering length and an effective range. Implications of the measurement of the $\mathrm{\Lambda }\mathrm{\Lambda }$ correlation function and interaction parameters for dihyperon searches are discussed.

We present an exact solution of the relativistic Boltzmann equation for a system undergoing boost-invariant longitudinal and azimuthally symmetric transverse flow (“Gubser flow”). The resulting exact nonequilibrium dynamics is compared to first and second order relativistic hydrodynamic approximations for various shear viscosity to entropy density ratios. This novel solution can be used to test the validity and accuracy of different hydrodynamic approximations in conditions similar to those generated in relativistic heavy-ion collisions.

We show that Lorentz invariance is realized nontrivially in the classical action of a massless spin-$\frac{1}{2}$ particle with definite helicity. We find that the ordinary Lorentz transformation is modified by a shift orthogonal to the boost vector and the particle momentum. The shift ensures angular momentum conservation in particle collisions and implies a nonlocality of the collision term in the Lorentz-invariant kinetic theory due to side jumps. We show that $2/3$ of the chiral-vortical effect for a uniformly rotating particle distribution can be attributed to the magnetic moment coupling required by the Lorentz invariance. We also show how the classical action can be obtained by taking the classical limit of the path integral for a Weyl particle.

The production of jets associated to bottom quarks is measured for the first time in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. Jet spectra are reported in the transverse momentum ($p_{\mathrm{T}}$) range of $80–250\mathrm{GeV}/c$, and within pseudorapidity $|\eta |<2$. The nuclear modification factor ($R_{\mathrm{AA}}$) calculated from these spectra shows a strong suppression in the $b$-jet yield in PbPb collisions relative to the yield observed in $pp$ collisions at the same energy. The suppression persists to the largest values of $p_{\mathrm{T}}$ studied, and is centrality dependent. The $R_{\mathrm{AA}}$ is about 0.4 in the most central events, similar to previous observations for inclusive jets. This implies that jet quenching does not have a strong dependence on parton mass and flavor in the jet $p_{\mathrm{T}}$ range studied.

In this Letter we discuss the potential application of antineutrino monitoring to the Iranian heavy water reactor at Arak, the IR-40, as a nonproliferation measure. An above ground detector positioned right outside the IR-40 reactor building could meet IAEA verification goals for reactor plutonium inventories. While detectors with the needed spectral sensitivity have been demonstrated below ground, additional research and development is needed to demonstrate an above-ground detector with this same level of sensitivity. In addition to monitoring the reactor during operation, observing antineutrino emissions from long-lived fission products could also allow monitoring the reactor when it is shut down, provided very low detector backgrounds can be achieved. Antineutrino monitoring could also be used to distinguish different levels of fuel enrichment. Most importantly, these capabilities would not require a complete reactor operational history and could provide a means to reestablish continuity of knowledge in safeguards conclusions should this become necessary.

An approach is presented to experimentally constrain previously unreachable ($p$, $\gamma$) reaction rates on nuclei far from stability in the astrophysical $rp$ process. Energies of all critical resonances in the ${}^{57}\mathrm{Cu}(p,\gamma ){}^{58}\mathrm{Zn}$ reaction are deduced by populating states in ${}^{58}\mathrm{Zn}$ with a ($d$, $n$) reaction in inverse kinematics at $75\mathrm{MeV}/u$, and detecting $\gamma$-ray-recoil coincidences with the state-of-the-art $\gamma$-ray tracking array GRETINA and the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The results reduce the uncertainty in the ${}^{57}\mathrm{Cu}(p,\gamma )$ reaction rate by several orders of magnitude. The effective lifetime of ${}^{56}\mathrm{Ni}$, an important waiting point in the $rp$ process in x-ray bursts, can now be determined entirely from experimentally constrained reaction rates.

We report a measurement of a new high spin $J^{\pi }=5^{-}$ state at 22.4(2) MeV in ${}^{12}\mathrm{C}$ which fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a ${\mathcal{D}}_{3h}$ symmetry characterized by the sequence $0^{+}$, $2^{+}$, $3^{-}$, $4^{\pm }$, $5^{-}$ with almost degenerate $4^{+}$ and $4^{-}$ (parity doublet) states. Such a ${\mathcal{D}}_{3h}$ symmetry was observed in triatomic molecules, and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in ${}^{12}\mathrm{C}$ such as the ($0^{+}$) Hoyle band and the ($1^{-}$) bending mode band and suggest measurements in search of the predicted (“missing”) states that may shed new light on clustering in ${}^{12}\mathrm{C}$ and light nuclei. In particular, the observation (or nonobservation) of the predicted (“missing”) states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.654 MeV in ${}^{12}\mathrm{C}$.

Information on the size and shape of the neutron skin on ${}^{208}\mathrm{Pb}$ is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be $c_n=6.70\pm 0.03(\text{stat}\mathrm{.})\mathrm{fm}$ and $a_n=0.55\pm 0.01(\text{stat}{\mathrm{.})}_{-0.03}^{+0.02}(\text{sys}\mathrm{.})\mathrm{fm}$, respectively, corresponding to a neutron skin thickness $\mathrm{\Delta }{r}_{np}=0.15\pm 0.03(\text{stat}{\mathrm{.})}_{-0.03}^{+0.01}(\text{sys}\mathrm{.})\mathrm{fm}$. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of ${}^{208}\mathrm{Pb}$ has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

The neutron capture cross sections of the main uranium isotopes, ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$, were measured simultaneously for keV energies, for the first time by combining activation technique and atom counting of the reaction products using accelerator mass spectrometry. New data, with a precision of 3%–5%, were obtained from mg-sized natural uranium samples for neutron energies with an equivalent Maxwell-Boltzmann distribution of $kT\sim 25\mathrm{keV}$ and for a broad energy distribution peaking at 426 keV. The cross-section ratio of ${}^{235}\mathrm{U}(n,\gamma )/{}^{238}\mathrm{U}(n,\gamma )$ can be deduced in accelerator mass spectrometry directly from the atom ratio of the reaction products ${}^{236}\mathrm{U}/{}^{239}\mathrm{U}$, independent of any fluence normalization. Our results confirm the values at the lower band of existing data. They serve as important anchor points to resolve present discrepancies in nuclear data libraries as well as for the normalization of cross-section data used in the nuclear astrophysics community for s-process studies.

The interaction between neutron-rich nuclei plays an important role for understanding the reaction mechanism of the fusion process as well as for the energy production through pycnonuclear reactions in the crust of neutron stars. We have performed the first measurements of the total fusion cross sections in the systems ${}^{10,14,15}\mathrm{C}+{}^{12}\mathrm{C}$ using a new active target-detector system. In the energy region accessible with existing radioactive beams, a good agreement between the experimental and theoretical cross sections is observed. This gives confidence in our ability to calculate fusion cross sections for systems which are outside the range of today’s radioactive beam facilities.

The superheavy element with atomic number $Z=117$ was produced as an evaporation residue in the ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their $\alpha$-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-$\mu \mathrm{s}$ and a few days. Two decay chains comprising seven $\alpha$ decays and a spontaneous fission each were identified and are assigned to the isotope ${}^{294}117$ and its decay products. A hitherto unknown $\alpha$-decay branch in ${}^{270}\mathrm{Db}$ ($Z=105$) was observed, which populated the new isotope ${}^{266}\mathrm{Lr}$ ($Z=103$). The identification of the long-lived ($T_{1/2}={1.0}_{-0.4}^{+1.9}\mathrm{h}$) $\alpha$-emitter ${}^{270}\mathrm{Db}$ marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an “island of stability.”