Top-quark pair production is observed in lead–lead ($\mathrm{Pb}+\mathrm{Pb}$) collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of $1.9{\mathrm{nb}}^{-1}$. Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross section is ${\sigma }_{t\overline{t}}=3.6{}_{-0.9}^{+1.0}(\mathrm{stat}){}_{-0.5}^{+0.8}(\mathrm{syst})\mathrm{\mu }\mathrm{b}$, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the preequilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early Universe.

The probability per unit area for a radioactive ${}^{88}\mathrm{Zr}$ nucleus (83.4 d half life) to absorb neutrons across resonance energies—its neutron-capture resonance integral—is the largest ever reported [Shusterman et al.,Phys. Rev. C 103, 024614 (2021).] and implies that the neutron-capture cross section is orders of magnitude larger than previously thought at energies relevant to nuclear test diagnostics and nucleosynthesis applications such as the $\nu r$ process [Xiong et al.,Phys. Rev. Lett. 132, 192701 (2024).]. However, these applications require the shape of this cross section across a range of energies, not just this integral quantity. We measured the ${}^{88}\mathrm{Zr}$ neutron total cross section from 0.0253 to 500 eV. Because the same resonances or other nuclear structure responsible for the neutron-capture cross section are reflected in the total cross section, we used these data to extract the former from the latter as a function of energy. Our data reveal a resonance at 0.171 eV and result in a thermal neutron-capture cross section of $771000\pm 31000\mathrm{b}$, in good agreement with the recently published value. In contrast, the neutron-capture resonance integral extracted from our data is $15210\pm 670\mathrm{b}$ and is roughly a factor of 200 smaller than the recently reported value. We discuss the impact of this difference on applications.

This Letter reports on the first cross-section measurements for the ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$ and ${}^{86}\mathrm{Kr}(\alpha ,n){}^{89}\mathrm{Sr}$ reactions. In particular, our measurement of ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$ is the first weak $r$-process reaction cross section obtained using a radioactive ion beam. This experiment was enabled by the use of novel solid helium targets, comprised of silicon thin films with high helium incorporation obtained via a sputtering technique. Yield measurements were performed at center-of-mass energies of 10.4 and 9.0 MeV for the ${}^{86}\mathrm{Kr}(\alpha ,n){}^{89}\mathrm{Sr}$ reaction, and 9.9 MeV for ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$, extending into the respective Gamow energy windows for a temperature of 5 GK. Reactions were uniquely identified by prompt $\gamma$ rays detected in coincidence with heavy ions selected by a recoil mass spectrometer. The obtained cross sections are smaller than predicted for both reactions. In the case of ${}^{94}\mathrm{Sr}(\alpha ,n){}^{97}\mathrm{Zr}$, the reaction rate found here is lowered by an order of magnitude at temperatures below 5 GK, which is expected to impact the predicted abundance of ruthenium, a signature weak $r$-process element.

AMoRE searches for the neutrinoless double beta decay using 100 kg of enriched ${}^{100}\mathrm{Mo}$. Scintillating molybdate crystals coupled with a metallic magnetic calorimeter operate at milli-Kelvin temperatures to measure the energy of electrons emitted in the decay. AMoRE-I is a demonstrator for the full-scale AMoRE, operated at the Yangyang Underground Laboratory for over two years. The exposure was $8.02\mathrm{kg}\mathrm{year}$ (or $3.89{\mathrm{kg}}_{{}^{100}\mathrm{Mo}}\mathrm{year}$), and the total background rate near the $Q$ value was $0.025\pm 0.002\mathrm{counts}/\mathrm{keV}/\mathrm{kg}/\mathrm{year}$. We observed no indication of $0\nu \beta \beta$ decay and report a new lower limit of the half-life of ${}^{100}\mathrm{Mo}$ $0\nu \beta \beta$ decay as $T_{1/2}^{0\nu }>2.9\times {10}^{24}\mathrm{yr}$ at 90% confidence level. The effective Majorana mass limit range is $m_{\beta \beta }<(210–610)\mathrm{meV}$ using nuclear matrix elements estimated in the framework of different models, including the recent shell model calculations.

We report the discovery of the new isotope ${}^{252}\mathrm{Rf}$. With its extremely short half-life of ${60}_{-30}^{+90}\mathrm{ns}$, it expands the range of half-lives of the known superheavy nuclei by about 2 orders of magnitude. This nucleus was synthesized in its high-$K$ isomeric state, for which we measured a half-life of ${13}_{-3}^{+4}\mathrm{\mu }\mathrm{s}$. Our results confirm a smooth onset of decreasing ground-state spontaneous fission half-lives in the neutron-deficient Rf isotopes toward the isotopic border of ${10}^{-14}\mathrm{s}$, which is the time needed to form an atomic shell. Our findings set a new benchmark for further exploration of phenomena associated with high-$K$ states and inverted fission stability in the heaviest nuclei.

Dipole toroidal modes appear in many fields of physics. In nuclei, such a mode was predicted more than 50 years ago, but clear experimental evidence was lacking so far. Using a combination of high-resolution inelastic scattering experiments with photons, electrons, and protons, we identify for the first time candidates for toroidal dipole excitations in the nucleus ${}^{58}\mathrm{Ni}$ and demonstrate that transverse electron scattering form factors represent a relevant experimental observable to prove their nature.

Stable ${}^{205}\mathrm{Tl}$ ions have the lowest known energy threshold for capturing electron neutrinos (${\nu }_e$) of $E_{{\nu }_e}\ge 50.6\mathrm{keV}$. The Lorandite Experiment (LOREX), proposed in the 1980s, aims at obtaining the longtime averaged solar neutrino flux by utilizing natural deposits of Tl-bearing lorandite ores. To determine the ${\nu }_e$ capture cross section, it is required to know the strength of the weak transition connecting the ground state of ${}^{205}\mathrm{Tl}$ and the 2.3 keV first excited state in ${}^{205}\mathrm{Pb}$. The only way to experimentally address this transition is to measure the bound-state beta decay (${\beta }_b$) of fully ionized ${}^{205}{\mathrm{Tl}}^{81+}$ ions. After three decades of meticulous preparation, the half-life of the ${\beta }_b$ decay of ${}^{205}{\mathrm{Tl}}^{81+}$ has been measured to be ${291}_{-27}^{+33}\mathrm{days}$ using the Experimental Storage Ring (ESR) at GSI, Darmstadt. The longer measured half-life compared to theoretical estimates reduces the expected signal-to-noise ratio in the LOREX, thus challenging its feasibility.

The ${}^{244}\mathrm{Pu}({}^{50}\mathrm{Ti},xn){}^{294-x}\mathrm{Lv}$ reaction was investigated at Lawrence Berkeley National Laboratory’s 88-Inch Cyclotron. The experiment was aimed at the production of a superheavy element with $Z\ge 114$ by irradiating an actinide target with a beam heavier than ${}^{48}\mathrm{Ca}$. Produced Lv ions were separated from the unwanted beam and nuclear reaction products using the Berkeley Gas-filled Separator and implanted into a newly commissioned focal-plane detector system. Two decay chains were observed and assigned to the decay of ${}^{290}\mathrm{Lv}$. The production cross section was measured to be ${\sigma }_{\mathrm{prod}}=0.44{(}_{-0.28}^{+0.58})\mathrm{pb}$ at a center-of-target center-of-mass energy of 220(3) MeV. This represents the first published measurement of the production of a superheavy element near the “island of stability,” with a beam of ${}^{50}\mathrm{Ti}$ and is an essential precursor in the pursuit of searching for new elements beyond $Z=118$.

The interaction between light and atomic nuclei is typically weak and confined within the linear and perturbative regime, which limits the achievable nuclear excitation probability and impedes potential applications such as nuclear optical clocks, nuclear lasers, and nuclear energy storage. In this Letter we show that the interaction between hydrogenlike thorium-229 ions (${{}^{229}\mathrm{Th}}^{89+}$) and contemporary intense lasers propels light-nucleus interaction into a highly nonlinear and nonperturbative regime. This interaction unlocks remarkably efficient nuclear excitation: over 10% of the ${}^{229}\mathrm{Th}$ nuclei can be excited to the isomeric state by a single femtosecond laser pulse. Moreover, the laser-driven ${{}^{229}\mathrm{Th}}^{89+}$ ions radiate multiple wavelengths of light that are high-order harmonics of the laser frequency, akin to the high harmonic generation process from laser-driven atoms but with distinct characteristics. These results pioneer a new frontier for exploring light-matter interaction, furnish a powerful method for efficient control over atomic nuclei, and pave a new way of nuclear coherent light emission.

A search for collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged particles using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. Jets are reconstructed with the anti-$k_{\mathrm{T}}$ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $|{\eta }^{\text{jet}}|<1.6$. Two-particle correlations among the charged particles within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations ($\mathrm{\Delta }{\varphi }^{*}$ and $\mathrm{\Delta }{\eta }^{*}$) in a jet coordinate basis, where particles’ ${\eta }^{*}$, ${\varphi }^{*}$ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $N_{\mathrm{ch}}^{\mathrm{j}}\approx 100$. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $|\mathrm{\Delta }{\eta }^{*}|>2$. For low-$N_{\mathrm{ch}}^{\mathrm{j}}$ jets, the long-range elliptic anisotropic harmonic, $v_2^{*}$, is observed to decrease with $N_{\mathrm{ch}}^{\mathrm{j}}$. This trend is well described by Monte Carlo event generators. However, a rising trend for $v_2^{*}$ emerges at $N_{\mathrm{ch}}^{\mathrm{j}}\gtrsim 80$, hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of jet evolution in the vacuum.

Constraints on the quantum decoherence of entangled $\gamma$ quanta at the mega-electron-volt scale, such as those produced following positron annihilation, have remained elusive for many decades. We present the first statistically and kinematically precise experimental data for triple Compton scattering of such entangled $\gamma$. An entanglement witness ($R$), relating to the enhancement of the azimuthal correlation between the final scattering planes, is obtained where one of the $\gamma$ underwent intermediate Compton scattering. The measured $R$, deconvolved from multiple scattering backgrounds, are found to exceed the classical limit for intermediate scatter angles up to $\sim 60°$ and diminish at larger angles. The data are consistent with predictions from a first quantum theory of entangled triple Compton scattering as well as a simple model based approach. The results are crucial to future study and utilization of entangled mega-electron-volt $\gamma$ in fundamental physics and positron emission tomography imaging.

The neutron-rich unbound fluorine isotope ${}^{30}{\mathrm{F}}_{21}$ has been observed for the first time by measuring its neutron decay at the SAMURAI spectrometer (RIBF, RIKEN) in the quasifree proton knockout reaction of ${}^{31}\mathrm{Ne}$ nuclei at $235\mathrm{MeV}/\mathrm{nucleon}$. The mass and thus one-neutron-separation energy of ${}^{30}\mathrm{F}$ has been determined to be $S_n=-472\pm 58(\mathrm{stat})\pm 33(\mathrm{sys})\mathrm{keV}$ from the measurement of its invariant-mass spectrum. The absence of a sharp drop in $S_n({}^{30}\mathrm{F})$ shows that the “magic” $N=20$ shell gap is not restored close to ${}^{28}\mathrm{O}$, which is in agreement with our shell-model calculations that predict a near degeneracy between the neutron $d$ and $fp$ orbitals, with the $1p_{3/2}$ and $1p_{1/2}$ orbitals becoming more bound than the $0f_{7/2}$ one. This degeneracy and reordering of orbitals has two potential consequences: ${}^{28}\mathrm{O}$ behaves like a strongly superfluid nucleus with neutron pairs scattering across shells, and both ${}^{29,31}\mathrm{F}$ appear to be good two-neutron halo-nucleus candidates.

The LHCb Collaboration measures production of the exotic hadron ${\chi }_{c1}(3872)$ in proton-nucleus collisions for the first time. Comparison with the charmonium state $\psi (2S)$ suggests that the exotic ${\chi }_{c1}(3872)$ experiences different dynamics in the nuclear medium than conventional hadrons, and comparison with data from proton-proton collisions indicates that the presence of the nucleus may modify ${\chi }_{c1}(3872)$ production rates. This is the first measurement of the nuclear modification factor of an exotic hadron.

The known $I^{\pi }=8_1^{+}$, $E_x=2129\text{−}\mathrm{keV}$ isomer in the semimagic nucleus ${{}^{130}\mathrm{Cd}}_{82}$ was populated in the projectile fission of a ${}^{238}\mathrm{U}$ beam at the Radioactive Isotope Beam Factory at RIKEN. The high counting statistics of the accumulated data allowed us to determine the excitation energy, $E_x=2001.2(7)\mathrm{keV}$, and half-life, $T_{1/2}=57(3)\mathrm{ns}$, of the $I^{\pi }=6_1^{+}$ state based on $\gamma \gamma$ coincidence information. Furthermore, the half-life of the $8_1^{+}$ state, $T_{1/2}=224(4)\mathrm{ns}$, was remeasured with high precision. The new experimental information, combined with available data for ${}^{134}\mathrm{Sn}$ and large-scale shell model calculations, allowed us to extract proton and neutron effective charges for ${}^{132}\mathrm{Sn}$, a doubly magic nucleus far-off stability. A comparison to analogous information for ${}^{100}\mathrm{Sn}$ provides first reliable information regarding the isospin dependence of the isoscalar and isovector effective charges in heavy nuclei.

New astronomical observations point to a nucleosynthesis picture that goes beyond what was accepted until recently. The intermediate “$i$” process was proposed as a plausible scenario to explain some of the unusual abundance patterns observed in metal-poor stars. The most important nuclear physics properties entering $i$-process calculations are the neutron-capture cross sections and they are almost exclusively not known experimentally. Here we provide the first experimental constraints on the ${}^{139}\mathrm{Ba}(n,\gamma ){}^{140}\mathrm{Ba}$ reaction rate, which is the dominant source of uncertainty for the production of lanthanum, a key indicator of $i$-process conditions. This is an important step towards identifying the exact astrophysical site of stars carrying the $i$-process signature.

We present a new nucleosynthesis process that may take place on neutron-rich ejecta experiencing an intensive neutrino flux. The nucleosynthesis proceeds similarly to the standard $r$ process, a sequence of neutron captures and beta decays with, however, charged-current neutrino absorption reactions on nuclei operating much faster than beta decays. Once neutron-capture reactions freeze out the produced $r$ process, neutron-rich nuclei undergo a fast conversion of neutrons into protons and are pushed even beyond the $\beta$ stability line, producing the neutron-deficient $p$ nuclei. This scenario, which we denote as the $\nu r$ process, provides an alternative channel for the production of $p$ nuclei and the short-lived nucleus ${}^{92}\mathrm{Nb}$. We discuss the necessary conditions posed on the astrophysical site for the $\nu r$ process to be realized in nature. While these conditions are not fulfilled by current neutrino-hydrodynamic models of $r$-process sites, future models, including more complex physics and a larger variety of outflow conditions, may achieve the necessary conditions in some regions of the ejecta.

The 8.4 eV nuclear isomer state in Th-229 is resonantly excited in Th-doped ${\mathrm{CaF}}_2$ crystals using a tabletop tunable laser system. A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance for the ${\mathrm{Th}}^{4+}$ ions in Th:${\mathrm{CaF}}_2$ is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum. These results pave the way toward Th-229 nuclear laser spectroscopy and realizing optical nuclear clocks.

The first measurement of the cross section for incoherent photonuclear production of $J/\psi$ vector mesons as a function of the Mandelstam $|t|$ variable is presented. The measurement was carried out with the ALICE detector at midrapidity, $|y|<0.8$, using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. This rapidity interval corresponds to a Bjorken-$x$ range $(0.3–1.4)\times {10}^{-3}$. Cross sections are given in five $|t|$ intervals in the range $0.04<|t|<1{\mathrm{GeV}}^2$ and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a $|t|$ dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data.

${}^{140}\mathrm{Ce}(\mathrm{n},\gamma )$ is a key reaction for slow neutron-capture ($s$-process) nucleosynthesis due to being a bottleneck in the reaction flow. For this reason, it was measured with high accuracy (uncertainty $\approx 5\%$) at the n_TOF facility, with an unprecedented combination of a high purity sample and low neutron-sensitivity detectors. The measured Maxwellian averaged cross section is up to 40% higher than previously accepted values. Stellar model calculations indicate a reduction around 20% of the $s$-process contribution to the Galactic cerium abundance and smaller sizeable differences for most of the heavier elements. No variations are found in the nucleosynthesis from massive stars.

The structure and decay of the most neutron-rich beryllium isotope, ${}^{16}\mathrm{Be}$, has been investigated following proton knockout from a high-energy ${}^{17}\mathrm{B}$ beam. Two relatively narrow resonances were observed for the first time, with energies of 0.84(3) and 2.15(5) MeV above the two-neutron decay threshold and widths of 0.32(8) and 0.95(15) MeV, respectively. These were assigned to be the ground ($J^{\pi }=0^{+}$) and first excited ($2^{+}$) state, with $E_x=1.31(6)\mathrm{MeV}$. The mass excess of ${}^{16}\mathrm{Be}$ was thus deduced to be 56.93(13) MeV, some 0.5 MeV more bound than the only previous measurement. Both states were observed to decay by direct two-neutron emission. Calculations incorporating the evolution of the wave function during the decay as a genuine three-body process reproduced the principal characteristics of the neutron-neutron energy spectra for both levels, indicating that the ground state exhibits a strong spatially compact dineutron component, while the $2^{+}$ level presents a far more diffuse neutron-neutron distribution.