Results of a new series of experiments on the study of production cross sections and decay properties of the isotopes of element 115 in the reaction ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ are presented. Twenty-one new decay chains originating from ${}^{288}115$ were established as the product of the $3n$-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus ${}^{291}115$. The decay properties of all newly observed nuclei are in full agreement with those we measured in 2003. At the lowest excitation energy $E^{*}=33\mathrm{MeV}$, for the first time we registered the product of the $2n$-evaporation channel, ${}^{289}115$, which was also observed previously in the reaction ${}^{249}\mathrm{Bk}+{}^{48}\mathrm{Ca}$ as the daughter nucleus of the decay of ${}^{293}117$. The maximum cross section for the production of ${}^{288}115$ is found to be 8.5 pb at $E^{*}\approx 36\mathrm{MeV}$.

Penning trap mass measurements of short-lived nuclides have been performed for the first time with highly charged ions, using the TITAN facility at TRIUMF. Compared to singly charged ions, this provides an improvement in experimental precision that scales with the charge state $q$. Neutron-deficient Rb isotopes have been charge bred in an electron beam ion trap to $q=8-12+$ prior to injection into the Penning trap. In combination with the Ramsey excitation scheme, this unique setup creating low energy, highly charged ions at a radioactive beam facility opens the door to unrivaled precision with gains of 1–2 orders of magnitude. The method is particularly suited for short-lived nuclides such as the superallowed $\beta$ emitter ${}^{74}\mathrm{Rb}$ ($T_{1/2}=65\mathrm{ms}$). The determination of its atomic mass and an improved $Q_{EC}$ value are presented.

The intense, nearly monoenergetic, 100% polarized $\overrightarrow{\gamma }$-ray beams available at the $\mathrm{HI}\overrightarrow{\gamma }\mathrm{S}$ facility, along with the realization that the $E1\mathrm{\text{−}}E2$ interference term that appears in the Compton scattering polarization observable has opposite signs in the forward and backward angles, make it possible to obtain an order-of-magnitude improvement in the determination of the parameters of the isovector giant quadrupole resonance (IVGQR). Accurate IVGQR parameters will lead to a more detailed knowledge of the symmetry energy in the nuclear equation of state which is important for understanding nuclear matter under extreme conditions such as those present in neutron stars. Our new method is demonstrated for the case of ${}^{209}\mathrm{Bi}$.

We report the observation of two-neutrino double-beta decay in ${}^{136}\mathrm{Xe}$ with $T_{1/2}=2.11\pm 0.04(\mathrm{stat})\pm 0.21(\mathrm{syst})\times {10}^{21}\mathrm{yr}$. This second-order process, predicted by the standard model, has been observed for several nuclei but not for ${}^{136}\mathrm{Xe}$. The observed decay rate provides new input to matrix element calculations and to the search for the more interesting neutrinoless double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale.

A new technique to measure $(p,n)$ charge-exchange reactions in inverse kinematics at intermediate energies on unstable isotopes was successfully developed and used to study the ${}^{56}\mathrm{Ni}(p,n)$ reaction at $110\mathrm{MeV}/\mathrm{u}$. Gamow-Teller transition strengths from ${}^{56}\mathrm{Ni}$ leading to ${}^{56}\mathrm{Cu}$ were obtained and compared with shell-model predictions in the $pf$ shell using the KB3G and GXPF1A interactions. The calculations with the GXPF1A interaction reproduce the experimental strength distribution much better than the calculations that employed the KB3G interaction, indicating deficiencies in the spin-orbit and proton-neutron residual potentials for the latter. The results are important for improving the description of electron-capture rates on nuclei in the iron region, which are important for modeling the late evolution of core-collapse and thermonuclear supernovae.

A precision measurement of the $\gamma$ yields following the $\beta$ decay of ${}^{32}\mathrm{Cl}$ has determined its isobaric-analogue branch to be $({22.47}_{-0.18}^{+0.21})\%$. Since it is an almost pure-Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, ${\delta }_C=5.3(9)\%$, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly calculated, yet smaller, corrections that are currently applied to $0^{+}\to 0^{+}$ transitions for tests of the standard model.

For the first time the differential cross section for the elastic neutron-triton ($n\mathrm{\text{−}}{}^3\mathrm{H}$) and neutron-deuteron ($n\mathrm{\text{−}}{}^2\mathrm{H}$) scattering at 14.1 MeV has been measured by using an inertial confinement fusion facility. In these experiments, which were carried out by simultaneously measuring elastically scattered ${}^3\mathrm{H}$ and ${}^2\mathrm{H}$ ions from a deuterium-tritium gas-filled inertial confinement fusion capsule implosion, the differential cross section for the elastic $n\mathrm{\text{−}}{}^3\mathrm{H}$ scattering was obtained with significantly higher accuracy than achieved in previous accelerator experiments. The results compare well with calculations that combine the resonating-group method with an ab initio no-core shell model, which demonstrate that recent advances in ab initio theory can provide an accurate description of light-ion reactions.

We investigate the linear chain configurations of four-$\alpha$ clusters in ${}^{16}\mathrm{O}$ using a Skyrme cranked Hartree-Fock method and discuss the relationship between the stability of such states and angular momentum. We show the existence of a region of angular momentum ($13–18\hslash$) where the linear chain configuration is stabilized. For the first time we demonstrate that stable exotic states with a large moment of inertia (${\hslash }^2/2\Theta \sim 0.06–0.08\mathrm{MeV}$) can exist.

A comparison of the relative yields of ${\rm Y}$ resonances in the ${\mu }^{+}{\mu }^{-}$ decay channel in Pb-Pb and $pp$ collisions at a center-of-mass energy per nucleon pair of 2.76 TeV is performed with data collected with the CMS detector at the LHC. Using muons of transverse momentum above $4\mathrm{GeV}/c$ and pseudorapidity below 2.4, the double ratio of the ${\rm Y}(2S)$ and ${\rm Y}(3S)$ excited states to the ${\rm Y}(1S)$ ground state in Pb-Pb and $pp$ collisions, $[{\rm Y}(2S+3S)/{\rm Y}(1S){]}_{\mathrm{Pb}\mathrm{\text{−}}\mathrm{Pb}}/[{\rm Y}(2S+3S)/{\rm Y}(1S){]}_{pp}$, is found to be ${0.31}_{-0.15}^{+0.19}(\mathrm{stat})\pm 0.03(\mathrm{syst})$. The probability to obtain the measured value, or lower, if the true double ratio is unity, is calculated to be less than 1%.

Charge-changing cross sections ${\sigma }_{\mathrm{cc}}$ of stable and unstable nuclei (${}^{9–11}\mathrm{Be}$, ${}^{14–16}\mathrm{C}$, and ${}^{16–18}\mathrm{O}$) on a carbon target were investigated at $300\mathrm{MeV}/\mathrm{nucleon}$. A phenomenological analysis based on the Glauber theory indicates an approximate, but universal, scaling of ${\sigma }_{\mathrm{cc}}$ over a wide range of $A/Z$. This allows the determination of the density distributions of protons tightly bound in the nuclei. An application to ${}^{16}\mathrm{C}$, which is considered to be an anomalously deformed nucleus, indicates a systematic evolution of proton root-mean-square radii and has revealed for the first time a neutron skin effect in carbon isotopes. Being complementary to isotope-shift and electron-scattering experiments, the present method can open up a new approach to explore the structure of exotic nuclei.

We report on the first direct measurement of the proton separation energy for the proton-unbound nucleus ${}^{69}\mathrm{Br}$. Bypassing the ${}^{68}\mathrm{Se}$ waiting point in the rp process is directly related to the $2p$-capture rate through ${}^{69}\mathrm{Br}$, which depends exponentially on the proton separation energy. We find a proton separation energy for ${}^{69}\mathrm{Br}$ of $S_p({}^{69}\mathrm{Br})=-{785}_{-40}^{+34}\mathrm{keV}$; this is less bound compared to previous predictions which have relied on uncertain theoretical calculations. The influence of the extracted proton separation energy on the rp process occurring in type I x-ray bursts is examined within the context of a one-zone burst model.

We show that the Bogoliubov–de Gennes equations for nuclear ground-state wave functions support solutions in which the condensate has a mixture of spin-singlet and spin-triplet pairing. We find that such mixed-spin condensates do not occur when there are equal numbers of neutrons and protons, but only when there is an isospin imbalance. Using a phenomenological Hamiltonian, we predict that such nuclei may occur in the physical region within the proton dripline. We also solve the Bogoliubov–de Gennes equations with variable constraints on the spin-singlet and spin-triplet pairing amplitudes. For nuclei that exhibit this new pairing behavior, the resulting energy surface can be rather soft, suggesting that there may be low-lying excitations associated with the spin mixing.

The Hoyle state plays a crucial role in the helium burning of stars heavier than our Sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago nuclear theorists have not yet uncovered the nature of this state from first principles. In this Letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at $-85(3)\mathrm{MeV}$ with all of the properties of the Hoyle state and in agreement with the experimentally observed energy.

A possibility of the amplification of the 7.6 eV $\gamma$ radiation by the stimulated $\gamma$ emission of the ensemble of the ${}^{229m}\mathrm{Th}$ isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of (1) the excitation of a large number of ${}^{229m}\mathrm{Th}$ isomers by laser radiation, (2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field, (3) the emission or absorption of the optical photons by thorium nuclei in the crystal without recoil, and (4) the nuclear spin relaxation through the conduction electrons of the metallic covering.

The masses of ten proton-rich nuclides, including the $N=Z+1$ nuclides ${}^{85}\mathrm{Mo}$ and ${}^{87}\mathrm{Tc}$, were measured with the Penning trap mass spectrometer SHIPTRAP. Compared to the Atomic Mass Evaluation 2003 a systematic shift of the mass surface by up to 1.6 MeV is observed causing significant abundance changes of the ashes of astrophysical x-ray bursts. Surprisingly low $\alpha$ separation energies for neutron-deficient Mo and Tc are found, making the formation of a ZrNb cycle in the $rp$ process possible. Such a cycle would impose an upper temperature limit for the synthesis of elements beyond Nb in the $rp$ process.

A very exotic process of $\beta$-delayed fission of ${}^{180}\mathrm{Tl}$ is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the fission-fragment mass distribution of the post-$\beta$-decay daughter nucleus ${}^{180}\mathrm{Hg}$ ($N/Z=1.25$) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two ${}^{90}\mathrm{Zr}$ fragments, with magic $N=50$ and semimagic $Z=40$. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for $\beta$-delayed fission of ${}^{180}\mathrm{Tl}$ is $3.6(7)\times {10}^{-3}\%$, approximately 2 orders of magnitude larger than in an earlier study.

The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is $1584\pm 4(\mathrm{stat})\pm 76(\mathrm{syst})$, which corresponds to $8.3\pm 0.4(\mathrm{syst})$ per participating nucleon pair. This represents an increase of about a factor 1.9 relative to $pp$ collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at $\sqrt{s_{NN}}=0.2\mathrm{TeV}$. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.

We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region ($|\eta |<0.8$) and transverse momentum range $0.2<p_t<5.0\mathrm{GeV}/c$. The elliptic flow signal $v_2$, measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is $0.087\pm 0.002(\mathrm{stat})\pm 0.003(\mathrm{syst})$ in the 40%–50% centrality class. The differential elliptic flow $v_2(p_t)$ reaches a maximum of 0.2 near $p_t=3\mathrm{GeV}/c$. Compared to RHIC Au-Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.

By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

The “island of inversion” nucleus ${}^{32}\mathrm{Mg}$ has been studied by a ($t$, $p$) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited $0^{+}$ state in ${}^{32}\mathrm{Mg}$ has been identified by the characteristic angular distribution of the protons of the $\Delta L=0$ transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low $\gamma$-ray intensity observed for the decay of this $0^{+}$ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations.