Based on the factorization in perturbative QCD, a jet cross section in heavy-ion collisions can be expressed as a convolution of the jet cross section in $p+p$ collisions and a jet energy loss distribution. Using this simple expression and the Markov Chain Monte Carlo method, we carry out Bayesian analyses of experimental data on jet spectra to extract energy loss distributions for both single inclusive and $\gamma$-triggered jets in $\mathrm{Pb}+\mathrm{Pb}$ collisions with different centralities at two colliding energies at the Large Hadron Collider. The average jet energy loss has a dependence on the initial jet energy that is slightly stronger than a logarithmic form and decreases from central to peripheral collisions. The extracted jet energy loss distributions with a scaling behavior in $x=\mathrm{\Delta }{p}_T/⟨\mathrm{\Delta }{p}_T⟩$ have a large width. These are consistent with the linear Boltzmann transport model simulations, in which the observed jet quenching is caused on the average by only a few out-of-cone scatterings.

We propose a new phenomenological approach to establish QCD factorization of jet cross sections in the heavy-ion environment. Starting from a factorization formalism in proton-proton collisions, we introduce medium modified jet functions to capture the leading interaction of jets with the hot and dense QCD medium. A global analysis using a Monte Carlo sampling approach is performed in order to reliably determine the new jet functions from the nuclear modification factor of inclusive jets at the LHC. We find that gluon jets are significantly more suppressed due to the presence of the medium than quark jets. In addition, we observe that the jet radius dependence is directly related to the relative suppression of quark and gluon jets. Our approach may help to improve the extraction of medium properties from data.

Until recently, uncertainty quantification in low energy nuclear theory was typically performed using frequentist approaches. However in the last few years, the field has shifted toward Bayesian statistics for evaluating confidence intervals. Although there are statistical arguments to prefer the Bayesian approach, no direct comparison is available. In this work, we compare, directly and systematically, the frequentist and Bayesian approaches to quantifying uncertainties in direct nuclear reactions. Starting from identical initial assumptions, we determine confidence intervals associated with the elastic and the transfer process for both methods, which are evaluated against data via a comparison of the empirical coverage probabilities. Expectedly, the frequentist approach is not as flexible as the Bayesian approach in exploring parameter space and often ends up in a different minimum. We also show that the two methods produce significantly different correlations. In the end, the frequentist approach produces significantly narrower uncertainties on the considered observables than the Bayesian. Our study demonstrates that the uncertainties on the reaction observables considered here within the Bayesian approach represent reality more accurately than the much narrower uncertainties obtained using the standard frequentist approach.

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of ${}^{48}\mathrm{Ca}$, ${}^{50}\mathrm{Ti}$, and ${}^{54}\mathrm{Cr}$ with ${}^{208}\mathrm{Pb}$. Moving from ${}^{48}\mathrm{Ca}$ to ${}^{54}\mathrm{Cr}$ leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity $P_{CN}$ (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ${}^{50}\mathrm{Ti}$ and 15 for ${}^{54}\mathrm{Cr}$ in comparison to ${}^{48}\mathrm{Ca}$. The energy dependence of $P_{\mathrm{CN}}$ indicates that cold fusion reactions (involving ${}^{208}\mathrm{Pb}$) are not driven by a diffusion process.

We report a fuel-dependent reactor electron antineutrino (${\overline{\nu }}_e$) yield using six $2.8{\mathrm{GW}}_{\mathrm{th}}$ reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses 850 666 ${\overline{\nu }}_e$ candidate events with a background fraction of 2.0% acquired through inverse beta decay (IBD) interactions in the near detector for 1807.9 live days from August 2011 to February 2018. Based on multiple fuel cycles, we observe a fuel ${}^{235}\mathrm{U}$ dependent variation of measured IBD yields with a slope of $(1.51\pm 0.23)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ and measure a total average IBD yield of $(5.84\pm 0.13)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$. The hypothesis of no fuel-dependent IBD yield is ruled out at $6.6\sigma$. The observed IBD yield variation over ${}^{235}\mathrm{U}$ isotope fraction does not show significant deviation from the Huber-Mueller (HM) prediction at $1.3\sigma$. The measured fuel-dependent variation determines IBD yields of $(6.15\pm 0.19)\times {10}^{-43}$ and $(4.18\pm 0.26)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ for two dominant fuel isotopes ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$, respectively. The measured IBD yield per ${}^{235}\mathrm{U}$ fission shows the largest deficit relative to the HM prediction. Reevaluation of the ${}^{235}\mathrm{U}$ IBD yield per fission may mostly solve the reactor antineutrino anomaly (RAA) while ${}^{239}\mathrm{Pu}$ is not completely ruled out as a possible contributor to the anomaly. We also report a $2.9\sigma$ correlation between the fractional change of the 5 MeV excess and the reactor fuel isotope fraction of ${}^{235}\mathrm{U}$.

The ${}^{23}\mathrm{Al}(p,\gamma ){}^{24}\mathrm{Si}$ reaction is among the most important reactions driving the energy generation in type-I x-ray bursts. However, the present reaction-rate uncertainty limits constraints on neutron star properties that can be achieved with burst model-observation comparisons. Here, we present a novel technique for constraining this important reaction by combining the GRETINA array with the neutron detector LENDA coupled to the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The ${}^{23}\mathrm{Al}(d,n)$ reaction was used to populate the astrophysically important states in ${}^{24}\mathrm{Si}$. This enables a measurement in complete kinematics for extracting all relevant inputs necessary to calculate the reaction rate. For the first time, a predicted close-lying doublet of a $2_2^{+}$ and $(4_1^{+},0_2^{+})$ state in ${}^{24}\mathrm{Si}$ was disentangled, finally resolving conflicting results from two previous measurements. Moreover, it was possible to extract spectroscopic factors using GRETINA and LENDA simultaneously. This new technique may be used to constrain other important reaction rates for various astrophysical scenarios.

A record number of ${}^{100}\mathrm{Sn}$ nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the $\beta$-decay end point energy of ${}^{100}\mathrm{Sn}$ were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure $0^{+}\to 1^{+}$ Gamow-Teller decay was improved to ${\mathcal{B}}_{\mathrm{GT}}={4.4}_{-0.7}^{+0.9}$. A discrimination between different model calculations was possible for the first time, and the level scheme of ${}^{100}\mathrm{In}$ is investigated further.

Discrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the $4\sigma$ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being $n\to \chi +e^{+}{e}^{-}$. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle.

We introduce a new “quantile” analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum ($p_T$). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower $p_T$, making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet $p_T$. In this strategy, jets in heavy-ion collisions ordered by $p_T$ are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet $p_T$ is a natural observable ($Q_{AA}$). Furthermore, despite nonmonotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the $p_T$ of the parton that initiates a heavy-ion jet and the $p_T$ of the vacuum jet which corresponds to it via the quantile procedure ($p_T^{\text{quant}}$). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of $p_T$ migration in heavy-ion collisions.

A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus ${}^{42}\mathrm{Si}$—going beyond earlier comparisons of excited-state energies—is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying ${}^{42}\mathrm{Si}(2_1^{+})$ energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich ${}^{42}\mathrm{Si}$ with a one-proton removal reaction from ${}^{43}\mathrm{P}$ projectiles at $81\mathrm{MeV}/\mathrm{nucleon}$. The measured cross sections to the individual ${}^{42}\mathrm{Si}$ final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited $0^{+}$ states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in ${}^{42}\mathrm{Si}$ is proposed to be the ($0_2^{+}$) level.

A recent nuclear physics experiment [C. J. Chiara et al., Nature (London) 554, 216 (2018)] reports the first direct observation of nuclear excitation by electron capture (NEEC) in the depletion of the ${}^{93m}\mathrm{Mo}$ isomer. The experiment used a beam-based setup in which Mo highly charged ions with nuclei in the isomeric state ${}^{93m}\mathrm{Mo}$ at 2.4 MeV excitation energy were slowed down in a solid-state target. In this process, nuclear excitation to a higher triggering level led to isomer depletion. The reported excitation probability $P_{\text{exc}}=0.01$ was solely attributed to the so-far unobserved process of NEEC in lack of a different known channel of comparable efficiency. In this work, we investigate theoretically the beam-based setup and calculate excitation rates via NEEC using state-of-the-art atomic structure and ion stopping-power models. For all scenarios, our results disagree with the experimental data by approximately 9 orders of magnitude. This stands in conflict with the conclusion that NEEC was the excitation mechanism behind the observed depletion rate.

The $\beta$-delayed $\gamma$-ray spectroscopy of neutron-rich ${}^{123,125}\mathrm{Ag}$ isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted $1/2^{-}$ $\beta$-emitting isomers in ${}^{123,125}\mathrm{Ag}$ are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest $9/2^{+}$ and $1/2^{-}$ levels is extended in Ag isotopes up to $N=78$, providing a clear signal for the reduction of the $Z=40$ subshell gap in Ag towards $N=82$. Shell-model calculations with the state-of-the-art $V_{\mathrm{MU}}$ plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in ${}^{123,125}\mathrm{Ag}$. The tensor force is found to play a crucial role in the evolution of the size of the $Z=40$ subshell gap. The observed inversion of the single-particle levels around ${}^{123}\mathrm{Ag}$ can be well interpreted in terms of the monopole shift of the $\pi 1g_{9/2}$ orbitals mainly caused by the increasing occupation of $\nu 1h_{11/2}$ orbitals.

We present a precision analysis of the ${}^{136}\mathrm{Xe}$ two-neutrino $\beta \beta$ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino $\beta \beta$ rate allows us to measure the ratio of the leading and subleading $2\nu \beta \beta$ nuclear matrix elements (NMEs), ${\xi }_{31}^{2\nu }=-0.26_{-0.25}^{+0.31}$. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ${\xi }_{31}^{2\nu }$ range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ${\xi }_{31}^{2\nu }$ values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless $\beta \beta$ decay, ${\xi }_{31}^{2\nu }$ provides new insights toward reliable neutrinoless $\beta \beta$ NMEs.

The first laser spectroscopic determination of the change in the nuclear charge radius for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-accuracy measurement of the isotope shift in the $2s^{2}2p{{}^{2}P}_{1/2}\to 2s^{2}3s{{}^{2}S}_{1/2}$ ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes ${}^{10,11}\mathrm{B}$ and the results are used to extract their difference in the mean-square charge radius $⟨r_c^2{⟩}^{11}-⟨r_c^2{⟩}^{10}=-0.49(12){\mathrm{fm}}^2$. The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used to benchmark new ab initio nuclear structure calculations using the no-core shell model and Green’s function Monte Carlo approaches. These results are the foundation for a laser spectroscopic determination of the charge radius of the proton-halo candidate ${}^8\mathrm{B}$.

The cross sections of nuclear reactions between the radioisotope ${}^7\mathrm{Be}$ and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in big-bang nucleosynthesis, were measured at center-of-mass energies between 0.2 and 1.5 MeV. The measured cross sections are dominated by the $(d,\alpha )$ reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5) MeV with a strength of $\omega \gamma =1.7(5)\mathrm{keV}$ was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial ${}^7\mathrm{Li}$, but not sufficiently to solve the primordial lithium problem.

Using the dilute-dense factorization in the color glass condensate framework, we investigate the azimuthal angular correlation between a heavy quarkonium and a charged light hadron in proton-nucleus collisions. We extract the second harmonic $v_2$, commonly known as the elliptic flow, with the light hadron as the reference. This particular azimuthal angular correlation between a heavy meson and a light hadron was first measured at the LHC recently. The experimental results indicate that the elliptic flows for heavy flavor mesons ($J/\psi$ and $D^0$) are almost as large as those for light hadrons. Our calculation demonstrates that this result can be naturally interpreted as an initial state effect due to the interaction between the incoming partons from the proton and the dense gluons inside the target nucleus. Since the heavy quarkonium $v_2$ exhibits a weak mass dependence according to our calculation, we predict that the heavy quarkonium $\mathrm{\Upsilon }$ should have a similar elliptic flow as compared to that of the $J/\psi$, which can be tested in future measurements.

We measured the triple coincidence $A(e,e^{\prime }np)$ and $A(e,e^{\prime }pp)$ reactions on carbon, aluminum, iron, and lead targets at $Q^2>1.5(\mathrm{GeV}/c{)}^2$, $x_B>1.1$ and missing momentum $>400\mathrm{MeV}/c$. This was the first direct measurement of both proton-proton ($pp$) and neutron-proton ($np$) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton ($pp$) to neutron-proton ($np$) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for single-charge exchange effects decreased the SRC pairs ratio to $\sim 3\%$, which is lower than previous results. Comparisons to theoretical generalized contact formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower $pp$ to $np$ pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale and scheme dependence in cross-section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.