This letter presents a measurement of the nuclear modification factor of large-radius jets in $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ $\mathrm{Pb}+\mathrm{Pb}$ collisions by the ATLAS experiment. The measurement is performed using $1.72{\mathrm{nb}}^{-1}$ and $257{\mathrm{pb}}^{-1}$ of $\mathrm{Pb}+\mathrm{Pb}$ and $pp$ data, respectively. The large-radius jets are reconstructed with the anti-$k_t$ algorithm using a radius parameter of $R=1.0$, by reclustering anti-$k_t$ $R=0.2$ jets, and are measured over the transverse momentum ($p_T$) kinematic range of $158<p_T<1000\mathrm{GeV}$ and absolute pseudorapidity $|y|<2.0$. The large-radius jet constituents are further reclustered using the $k_t$ algorithm in order to obtain the splitting parameters, $\sqrt{d_{12}}$ and $\mathrm{\Delta }{R}_{12}$, which characterize the transverse momentum scale and angular separation for the hardest splitting in the jet, respectively. The nuclear modification factor, $R_{AA}$, obtained by comparing the $\mathrm{Pb}+\mathrm{Pb}$ jet yields to those in $pp$ collisions, is measured as a function of jet transverse momentum ($p_T$) and $\sqrt{d_{12}}$ or $\mathrm{\Delta }{R}_{12}$. A significant difference in the quenching of large-radius jets having single subjet and those with more complex substructure is observed. Systematic comparison of jet suppression in terms of $R_{AA}$ for different jet definitions is also provided. Presented results support the hypothesis that jets with hard internal splittings lose more energy through quenching and provide a new perspective for understanding the role of jet structure in jet suppression.

Neutrinoless double beta decay ($0\nu \beta \beta$) is a yet unobserved nuclear process that would demonstrate Lepton number violation, a clear evidence of beyond standard model physics. The process two neutrino double beta decay ($2\nu \beta \beta$) is allowed by the standard model and has been measured in numerous experiments. In this Letter, we report a measurement of $2\nu \beta \beta$ decay half-life of ${}^{100}\mathrm{Mo}$ to the ground state of ${}^{100}\mathrm{Ru}$ of $[7.07\pm 0.02(\mathrm{stat})\pm 0.11(\mathrm{syst})]\times {10}^{18}\mathrm{yr}$ by the CUPID-Mo experiment. With a relative precision of $\pm 1.6\%$ this is the most precise measurement to date of a $2\nu \beta \beta$ decay rate in ${}^{100}\mathrm{Mo}$. In addition, we constrain higher-order corrections to the spectral shape, which provides complementary nuclear structure information. We report a novel measurement of the shape factor ${\xi }_{3,1}=0.45\pm 0.03(\mathrm{stat})\pm 0.05(\mathrm{syst})$ based on a constraint on the ratio of higher-order terms from theory, which can be reliably calculated. This is compared to theoretical predictions for different nuclear models. We also extract the first value for the effective axial vector coupling constant obtained from a spectral shape study of $2\nu \beta \beta$ decay.

A key open question in the study of multiparticle production in high-energy $pp$ collisions is the relationship between the “ridge”—i.e., the observed azimuthal correlations between particles in the underlying event that extend over all rapidities—and hard or semihard scattering processes. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. To address this question, two-particle correlations are measured in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ using data collected by the ATLAS experiment at the LHC, with an integrated luminosity of $15.8{\mathrm{pb}}^{-1}$, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, $v_2$, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge.

The ${}^{12}\mathrm{C}/{}^{13}\mathrm{C}$ ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ and ${}^{13}\mathrm{C}(p,\gamma ){}^{14}\mathrm{N}$ reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date ($E_{\mathrm{c}.\mathrm{m}.}=60\mathrm{keV}$) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt $\gamma$-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%–8%. Compared with most of the literature, our results are systematically lower, by 25% for the ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ reaction and by 30% for ${}^{13}\mathrm{C}(p,\gamma ){}^{14}\mathrm{N}$. We provide the most precise value up to now of $3.6\pm 0.4$ in the 20–140 MK range for the lowest possible ${}^{12}\mathrm{C}/{}^{13}\mathrm{C}$ ratio that can be produced during H burning in giant stars.

${}^{180m}\mathrm{Ta}$ is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known $\beta$ and electron capture decays due to the large K-spin differences and small energy differences between the isomeric and lower-energy states. Detecting its decay presents a significant experimental challenge but could shed light on neutrino-induced nucleosynthesis mechanisms, the nature of dark matter, and K-spin violation. For this study, we repurposed the Majorana Demonstrator, an experimental search for the neutrinoless double-beta decay of ${}^{76}\mathrm{Ge}$ using an array of high-purity germanium detectors, to search for the decay of ${}^{180m}\mathrm{Ta}$. More than 17 kg, the largest amount of tantalum metal ever used for such a search, was installed within the ultralow-background detector array. In this Letter, we present results from the first year of Ta data taking and provide an updated limit for the ${}^{180m}\mathrm{Ta}$ half-life on the different decay channels. With new limits up to $1.5\times {10}^{19}\mathrm{yr}$, we improved existing limits by 1–2 orders of magnitude which are the most sensitive searches for a single $\beta$ and electron capture decay ever achieved. Over all channels, the decay can be excluded for $T_{1/2}<0.29\times {10}^{18}\mathrm{yr}$.

The ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts. We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3–6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ reaction rate is a factor of $\sim 4$ higher than the previous direct measurement of this reaction within temperatures relevant for x-ray bursts, resulting in the ${}^{22}\mathrm{Mg}$ waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the $(\alpha ,p)$ reaction.

We present a microscopic study of chiral plasma instabilities and axial charge transfer in non-Abelian plasmas with a strong gauge-matter coupling $g^2{N}_f=64$, by performing $3+1\mathrm{D}$ real-time classical-statistical lattice simulation with dynamical fermions. We explicitly demonstrate for the first time that—unlike in an Abelian plasma—the transfer of chirality from the matter sector to the gauge fields occurs predominantly due to topological sphaleron transitions. We elaborate on the similiarities and differences of the axial charge dynamics in cold Abelian U(1) and non-Abelian SU(2) plasmas, and comment on the implications of our findings for the study of anomalous transport phenomena, such as the chiral magnetic effect in QCD matter.

The production of prompt $D^0$ mesons in proton-lead collisions in both the forward and backward rapidity regions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=8.16\mathrm{TeV}$ is measured by the LHCb experiment. The nuclear modification factor of prompt $D^0$ mesons is determined as a function of the transverse momentum $p_T$, and the rapidity in the nucleon-nucleon center-of-mass frame $y^{*}$. In the forward rapidity region, significantly suppressed production with respect to $pp$ collisions is measured, which provides significant constraints on models of nuclear parton distributions and hadron production down to the very low Bjorken-$x$ region of $\sim {10}^{-5}$. In the backward rapidity region, a suppression with a significance of 2.0–3.8 standard deviations compared to parton distribution functions in a nuclear environment expectations is found in the kinematic region of $p_T>6\mathrm{GeV}/c$ and $-3.25<y^{*}<-2.5$, corresponding to $x\sim 0.01$.

Charge radii of neutron deficient ${}^{40}\mathrm{Sc}$ and ${}^{41}\mathrm{Sc}$ nuclei were determined using collinear laser spectroscopy. With the new data, the chain of Sc charge radii extends below the neutron magic number $N=20$ and shows a pronounced kink, generally taken as a signature of a shell closure, but one notably absent in the neighboring Ca, K, and Ar isotopic chains. Theoretical models that explain the trend at $N=20$ for the Ca isotopes cannot reproduce this puzzling behavior.

The last proton bound calcium isotope ${}^{35}\mathrm{Ca}$ has been studied for the first time, using the ${}^{37}\mathrm{Ca}(p,t){}^{35}\mathrm{Ca}$ two neutron transfer reaction. The radioactive ${}^{37}\mathrm{Ca}$ nuclei, produced by the LISE spectrometer at GANIL, interacted with the protons of the liquid hydrogen target CRYPTA, to produce tritons $t$ that were detected in the MUST2 detector array, in coincidence with the heavy residues Ca or Ar. The atomic mass of ${}^{35}\mathrm{Ca}$ and the energy of its first $3/2^{+}$ state are reported. A large $N=16$ gap of 4.61(11) MeV is deduced from the mass measurement, which together with other measured properties, makes ${}^{36}\mathrm{Ca}$ a doubly magic nucleus. The $N=16$ shell gaps in ${}^{36}\mathrm{Ca}$ and ${}^{24}\mathrm{O}$ are of similar amplitude, at both edges of the valley of stability. This feature is discussed in terms of nuclear forces involved, within state-of-the-art shell model calculations. Even though the global agreement with data is quite convincing, the calculations underestimate the size of the $N=16$ gap in ${}^{36}\mathrm{Ca}$ by 840 keV.

The nuclear incompressibility is a key parameter of the nuclear equation of state that can be extracted from the measurements of the so-called “breathing mode” of finite nuclei. The most serious discrepancy so far is between values extracted from Pb and Sn, that has provoked the longstanding question “Why is tin so soft?”. To solve this puzzle, a fully self-consistent quasiparticle random-phase approximation plus quasiparticle-vibration coupling approach based on Skyrme-Hartree-Fock-Bogoliubov is developed. We show that the many-body correlations introduced by quasiparticle-vibration coupling, which shift the isoscalar giant monopole resonance energy in Sn isotopes by about 0.4 MeV more than the energy in ${}^{208}\mathrm{Pb}$, play a crucial role in providing a unified description of the isoscalar giant monopole resonance in Sn and Pb isotopes. The best description of the experimental strength functions is given by SV-K226 and KDE0, which are characterized by incompressibility values $K_{\infty }=226\mathrm{MeV}$ and 229 MeV, respectively, at mean field level.

We present an apparatus for detection of cyclotron radiation yielding a frequency-based ${\beta }^{\pm }$ kinetic energy determination in the 5 keV to 2.1 MeV range, characteristic of nuclear $\beta$ decays. The cyclotron frequency of the radiating $\beta$ particles in a magnetic field is used to determine the $\beta$ energy precisely. Our work establishes the foundation to apply the cyclotron radiation emission spectroscopy (CRES) technique, developed by the Project 8 Collaboration, far beyond the 18-keV tritium endpoint region. We report initial measurements of ${\beta }^{-}$’s from ${}^6\mathrm{He}$ and ${\beta }^{+}$’s from ${}^{19}\mathrm{Ne}$ decays to demonstrate the broadband response of our detection system and assess potential systematic uncertainties for $\beta$ spectroscopy over the full (MeV) energy range. To our knowledge, this is the first direct observation of cyclotron radiation from individual highly relativistic $\beta$’s in a waveguide. This work establishes the application of CRES to a variety of nuclei, opening its reach to searches for new physics beyond the TeV scale via precision $\beta$-decay measurements.

Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including $pp$ and $p+\mathrm{Pb}$ collisions. In contrast, while jet quenching is observed in $\mathrm{Pb}+\mathrm{Pb}$ collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in $0.36{\mathrm{nb}}^{-1}$ of $p+\mathrm{Pb}$ and $3.6{\mathrm{pb}}^{-1}$ of $pp$ collisions at 5.02 TeV. The yields of charged hadrons with $p_T^{\mathrm{ch}}>0.5\mathrm{GeV}$ near and opposite in azimuth to jets with $p_T^{\text{jet}}>30$ or 60 GeV, and the ratios of these yields between $p+\mathrm{Pb}$ and $pp$ collisions, $I_{p\mathrm{Pb}}$, are reported. The collision centrality of $p+\mathrm{Pb}$ events is categorized by the energy deposited by forward neutrons from the struck nucleus. The $I_{p\mathrm{Pb}}$ values are consistent with unity within a few percent for hadrons with $p_T^{\mathrm{ch}}>4\mathrm{GeV}$ at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central $p+\mathrm{Pb}$ collisions.

We show within the color glass condensate framework that exclusive vector meson production at high energy is very sensitive to the geometric deformation of the target nucleus at multiple length scales. We show that different multipole deformation parameters affect different regions of transverse momentum transfer. These results have two important consequences: (1) Deformations of nuclear targets need to be taken into account when making predictions for and interpreting certain observables at the EIC. (2) Differential diffractive vector meson production has the potential to become a powerful tool, enabling the most direct measurements of nuclear structure at different length scales, ranging from large scale nuclear deformation at low transverse momentum transfer to fluctuations on nucleon- and subnucleon-size scales at higher transverse momentum transfer.

We used the ${}^{138}\mathrm{Ba}(d,\alpha )$ reaction to carry out an in-depth study of states in ${}^{136}\mathrm{Cs}$, up to around 2.5 MeV. In this Letter, we place emphasis on hitherto unobserved states below the first $1^{+}$ level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon-based experiments. We identify for the first time candidate metastable states in ${}^{136}\mathrm{Cs}$, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for ${}^{136}\mathrm{Xe}$ neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared with the others, dramatically fails to describe the observed low-energy ${}^{136}\mathrm{Cs}$ spectrum, while the other two show reasonably good agreement.