We determine the magnetization of quantum chromodynamics for several temperatures around and above the transition between the hadronic and the quark-gluon phases of strongly interacting matter. We obtain a paramagnetic response that increases in strength with the temperature. We argue that due to this paramagnetism, chunks of quark-gluon plasma produced in noncentral heavy ion collisions should become squeezed perpendicular to the magnetic field. This anisotropy will then contribute to the elliptic flow $v_2$ observed in such collisions, in addition to the pressure gradient that is usually taken into account. We present a simple estimate for the magnitude of this new effect and a rough comparison to the effect due to the initial collision geometry. We conclude that the paramagnetic effect might have a significant impact on the value of $v_2$.

We discuss the consequences of the approximate conservation of the vector and axial currents for the hadronic matrix elements appearing in $\beta$ decay if nonstandard interactions are present. In particular, the isovector (pseudo)scalar charge $g_{S(P)}$ of the nucleon can be related to the difference (sum) of the nucleon masses in the absence of electromagnetic effects. Using recent determinations of these quantities from phenomenological and lattice QCD studies we obtain the accurate values $g_S=1.02(11)$ and $g_P=349(9)$ in the modified minimal subtraction scheme at $\mu =2\mathrm{GeV}$. The consequences for searches of nonstandard scalar interactions in nuclear $\beta$ decays are studied, finding for the corresponding Wilson coefficient ${\epsilon }_S=0.0012(24)$ at 90% C.L., which is significantly more stringent than current LHC bounds and previous low-energy bounds using less precise $g_S$ values. We argue that our results could be rapidly improved with updated computations and the direct calculation of certain ratios in lattice QCD. Finally, we discuss the pion-pole enhancement of $g_P$, which makes $\beta$ decays much more sensitive to nonstandard pseudoscalar interactions than previously thought.

We report the beam energy ($\sqrt{s_{NN}}=7.7–200\mathrm{GeV}$) and collision centrality dependence of the mean ($M$), standard deviation ($\sigma$), skewness ($S$), and kurtosis ($\kappa$) of the net-proton multiplicity distributions in $\mathrm{Au}+\mathrm{Au}$ collisions. The measurements are carried out by the STAR experiment at midrapidity ($|y|<0.5$) and within the transverse momentum range $0.4<p_T<0.8\mathrm{GeV}/c$ in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the quantum chromodynamic phase diagram. The products of the moments, $S\sigma$ and $\kappa {\sigma }^2$, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and antiproton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation and also to a hadron resonance gas model.

We study modification of the photon-tagged and inclusive jets in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ due to mini-quark-gluon plasma which can be produced in high multiplicity events. We show that for underlying events with $d{N}_{\mathrm{ch}}/d\eta \sim 20–60$ the medium effects lead to a considerable modification of the photon-tagged and inclusive jet fragmentation functions. For inclusive jets, the magnitude of the effect is surprisingly large. The effect is quite strong even for typical underlying events. We find that the spectrum of charged hadrons is suppressed by $\sim 35\%–40\%$ at $p_T\sim 5–10\mathrm{GeV}$.

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of ${}^{1}H(\mathrm{\gamma ⃗},\mathrm{p⃗}){\pi }^0$ in the range $0.4<E_{\gamma }<1.4\mathrm{GeV}$ validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

We present results from the first next-to-leading-order (NLO) numerical analysis of forward hadron production in $pA$ and $dA$ collisions in the small-$x$ saturation formalism. Using parton distributions and fragmentation functions at NLO, as well as the dipole amplitude from the solution to the Balitsky-Kovchegov equation with running coupling, together with the NLO corrections to the hard coefficients, we obtain a good description of the available RHIC data in $d\mathrm{Au}$ collisions. In the large $p_{\perp }$ region beyond the saturation scale, we find that the NLO correction becomes dominant and negative, which indicates that other physics beyond NLO becomes important and should also be taken into account. Furthermore, we make predictions for forward hadron production in $p\mathrm{Pb}$ collisions at the LHC. This analysis not only incorporates the important NLO corrections for all partonic channels, but also reduces the renormalization scale dependence and helps to significantly reduce the theoretical uncertainties. It therefore provides a precise test of saturation physics beyond the leading logarithmic approximation.

Using a partonic transport model based on the 3-flavor Nambu–Jona-Lasinio model and a relativistic hadronic transport model to describe, respectively, the evolution of the initial partonic and the final hadronic phase of heavy-ion collisions at energies carried out in the beam-energy scan program of the BNL Relativistic Heavy Ion collider, we study the effects of both the partonic and hadronic mean-field potentials on the elliptic flow of particles relative to that of their antiparticles. We find that to reproduce the measured relative elliptic flow differences between nucleons and antinucleons as well as between kaons and antikaons requires a vector coupling constant as large as 0.5–1.1 times the scalar coupling constant in the Nambu–Jona-Lasinio model. Implications of our results in understanding the QCD phase structure at finite baryon chemical potential are discussed.

Study of ${\beta }^{+}$ decay of the exotic $T_z=-3/2$ nucleus ${}^{55}\mathrm{Cu}$, via delayed $\gamma$ rays, has revealed a strongly isospin mixed doublet (4599–4579 keV) in ${}^{55}\mathrm{Ni}$, which represents the fragmented and previously unknown isobaric analog of the ground state of ${}^{55}\mathrm{Cu}$. The observed small $\log ft$ values to both states in the doublet confirm the superallowed Fermi $\beta$ decay. The near degeneracy of a pair of $3/2^{-}$ levels in ${}^{55}\mathrm{Ni}$ results in the strong isospin mixing. The isospin mixing matrix element between the $T=3/2$ and $T=1/2$ levels is inferred from the experiment to be 9(1) keV, which agrees well with the matrix element of the charge symmetry breaking shell model Hamiltonian of Ormand and Brown. A precise value of the half-life of ${}^{55}\mathrm{Cu}$ at 57(3) ms was also obtained.

Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of ${}^6\mathrm{Li}$, ${}^8\mathrm{Be}$, and ${}^6\mathrm{He}$ are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape.

The $\gamma$-ray strength function of ${}^{56}\mathrm{Fe}$ has been measured from proton-$\gamma$ coincidences for excitation energies up to $\approx 11\mathrm{MeV}$. The low-energy enhancement in the $\gamma$-ray strength function, which was first discovered in the $({}^3\mathrm{He},\alpha \gamma ){}^{56}\mathrm{Fe}$ reaction, is confirmed with the $(p,p^{\prime }\gamma ){}^{56}\mathrm{Fe}$ experiment reported here. Angular distributions of the $\gamma$ rays give for the first time evidence that the enhancement is dominated by dipole transitions.

The $E1$ strength distribution in ${}^{68}\mathrm{Ni}$ has been investigated using Coulomb excitation in inverse kinematics at the $\mathrm{R}{}^3\mathrm{B}\mathrm{\text{−}}\mathrm{LAND}$ setup and by measuring the invariant mass in the one- and two-neutron decay channels. The giant dipole resonance and a low-lying peak (pygmy dipole resonance) have been observed at 17.1(2) and 9.55(17) MeV, respectively. The measured dipole polarizability is compared to relativistic random phase approximation calculations yielding a neutron-skin thickness of 0.17(2) fm. A method and analysis applicable to neutron-rich nuclei has been developed, allowing for a precise determination of neutron skins in nuclei as a function of neutron excess.

The coupling between bound quantum states and those in the continuum is of high theoretical interest. Experimental studies of bound drip-line nuclei provide ideal testing grounds for such investigations since they, due to the feeble binding energy of their valence particles, are easy to excite into the continuum. In this Letter, continuum states in the heaviest particle-stable Be isotope, ${}^{14}\mathrm{Be}$, are studied by employing the method of inelastic proton scattering in inverse kinematics. New continuum states are found at excitation energies $E^{*}=3.54(16)\mathrm{MeV}$ and $E^{*}=5.25(19)\mathrm{MeV}$. The structure of the earlier known $2_1^{+}$ state at 1.54(13) MeV was confirmed with a predominantly $(0d_{5/2}{)}^2$ configuration while there is very clear evidence that the $2_2^{+}$ state has a predominant ($1s_{1/2}$, $0d_{5/2}$) structure with a preferential three-body decay mechanism. The region at about 7 MeV excitation shows distinct features of sequential neutron decay via intermediate states in ${}^{13}\mathrm{Be}$. This demonstrates that the increasing availability of energetic beams of exotic nuclei opens up new vistas for experiments leading towards a new understanding of the interplay between bound and continuum states.

Neutron star (NS) merger ejecta offer a viable site for the production of heavy $r$-process elements with nuclear mass numbers $A\gtrsim 140$. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties, but calculations sensitively depend on nuclear physics. In particular, the fission fragment yields determine the creation of $110\lesssim A\lesssim 170$ nuclei. Here, we apply a new scission-point model, called SPY, to derive the fission fragment distribution (FFD) of all relevant neutron-rich, fissioning nuclei. The model predicts a doubly asymmetric FFD in the abundant $A\simeq 278$ mass region that is responsible for the final recycling of the fissioning material. Using ejecta conditions based on relativistic NS merger calculations, we show that this specific FFD leads to a production of the $A\simeq 165$ rare-earth peak that is nicely compatible with the abundance patterns in the Sun and metal-poor stars. This new finding further strengthens the case of NS mergers as possible dominant origin of $r$ nuclei with $A\gtrsim 140$.

The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ relative to the collision symmetry plane defined by the spectator nucleons. A negative slope of the rapidity-odd directed flow component with approximately 3 times smaller magnitude than found at the highest RHIC energy is observed. This suggests a smaller longitudinal tilt of the initial system and disfavors the strong fireball rotation predicted for the LHC energies. The rapidity-even directed flow component is measured for the first time with spectators and found to be independent of pseudorapidity with a sign change at transverse momenta $p_T$ between 1.2 and $1.7\mathrm{GeV}/c$. Combined with the observation of a vanishing rapidity-even $p_T$ shift along the spectator deflection this is strong evidence for dipolelike initial density fluctuations in the overlap zone of the nuclei. Similar trends in the rapidity-even directed flow and the estimate from two-particle correlations at midrapidity, which is larger by about a factor of 40, indicate a weak correlation between fluctuating participant and spectator symmetry planes. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.

Magnetic dipole strength functions are deduced from averages of a large number of $M1$ transition strengths calculated within the shell model for the nuclides ${}^{90}\mathrm{Zr}$, ${}^{94}\mathrm{Mo}$, ${}^{95}\mathrm{Mo}$, and ${}^{96}\mathrm{Mo}$. An enhancement of $M1$ strength toward low transition energy is found for all nuclides considered. Large $M1$ strengths appear for transitions between close-lying states with configurations including proton as well as neutron high-$j$ orbits that recouple their spins and add up their magnetic moments coherently. The $M1$ strength function deduced from the calculated $M1$ transition strengths is compatible with the low-energy enhancement found in (${}^3\mathrm{He}$, ${}^3\mathrm{He}^{\prime }$) and ($d$, $p$) experiments. The Letter presents an explanation of the experimental findings.

Classical novae are expected to contribute to the 1809-keV Galactic $\gamma$-ray emission by producing its precursor ${}^{26}\mathrm{Al}$, but the yield depends on the thermonuclear rate of the unmeasured ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction. Using the $\beta$ decay of ${}^{26}\mathrm{P}$ to populate the key $J^{\pi }=3^{+}$ resonance in this reaction, we report the first evidence for the observation of its exit channel via a $1741.6\pm 0.6(\mathrm{stat})\pm 0.3(\mathrm{syst})\mathrm{keV}$ primary $\gamma$ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured $\gamma$-ray energy and intensity with other experimental data on ${}^{26}\mathrm{Si}$, we find the center-of-mass energy and strength of the resonance to be $E_r=414.9\pm 0.6(\mathrm{stat})\pm 0.3(\mathrm{syst})\pm 0.6(\mathrm{lit}.)\mathrm{keV}$ and $\omega \gamma =23\pm 6(\mathrm{stat}{)}_{-10}^{+11}(\mathrm{lit}.)\mathrm{meV}$, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model ${}^{26}\mathrm{Al}$ production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic ${}^{26}\mathrm{Al}$.

The early stages of high energy heavy ion collisions are studied in the color glass condensate framework, with a real-time classical lattice simulation. When increasing the coupling constant, we observe a rapid increase of the ratio of longitudinal to transverse pressure. The transient regime that precedes this behavior is of the order of $1\mathrm{fm}/c$.

I use properties of doubly magic nuclei to constrain symmetric nuclear matter and neutron matter equations of state. I conclude that these data determine the value of the neutron equation of state at a density of ${\rho }_{on}=0.10\mathrm{nucleons}/{\mathrm{fm}}^3$ to be 11.4(10) MeV. The slope at that point is constrained by the value of the neutron skin. Analytical equations are given that show the dependence of the Skyrme equations of state on the neutron skin.

We use the classical Bogomol’nyi-Prasad-Sommerfield (BPS) soliton solutions of the BPS Skyrme model together with corrections from the collective coordinate quantization of spin and isospin, the electrostatic Coulomb energies, and a small explicit breaking of the isospin symmetry—accounting for the proton-neutron mass difference—to calculate nuclear binding energies. We find that the resulting binding energies are already in excellent agreement with their physical values for heavier nuclei, demonstrating thereby that the BPS Skyrme model is a distinguished starting point for a detailed quantitative investigation of nuclear and low-energy strong interaction physics.

The most precise determination of the neutron lifetime using the beam method was completed in 2005 and reported a result of ${\tau }_n=(886.3\pm 1.2[\mathrm{stat}]\pm 3.2[\mathrm{syst}])\mathrm{s}$. The dominant uncertainties were attributed to the absolute determination of the fluence of the neutron beam (2.7 s). The fluence was measured with a neutron monitor that counted the neutron-induced charged particles from absorption in a thin, well-characterized ${}^6\mathrm{Li}$ deposit. The detection efficiency of the monitor was calculated from the areal density of the deposit, the detector solid angle, and the evaluated nuclear data file, ENDF/B-VI ${}^6\mathrm{Li}(n,t){}^4\mathrm{He}$ thermal neutron cross section. In the current work, we measure the detection efficiency of the same monitor used in the neutron lifetime measurement with a second, totally absorbing neutron detector. This direct approach does not rely on the ${}^6\mathrm{Li}(n,t){}^4\mathrm{He}$ cross section or any other nuclear data. The detection efficiency is consistent with the value used in 2005 but is measured with a precision of 0.057%, which represents a fivefold improvement in the uncertainty. We verify the temporal stability of the neutron monitor through ancillary measurements, allowing us to apply the measured neutron monitor efficiency to the lifetime result from the 2005 experiment. The updated lifetime is ${\tau }_n=(887.7\pm 1.2[\mathrm{stat}]\pm 1.9[\mathrm{syst}])\mathrm{s}$.