Nuclear Experiment

We present recent results on the structure of the one-valence-particle 41 Ca and 49 Ca, and one-valence-hole 47 Ca, nuclei. The isotopes of interest were populated via the cold-neutron capture reactions 40 Ca(n, γ ), 48 Ca(n, γ ) and 46 Ca(n, γ ), respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of HPGe detectors. The γ decay and level schemes of these nuclei were investigated by γ -ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in 41 Ca, 47 Ca, and 49 Ca, respectively. Branching ratios and intensities were extracted for the γ decay from each state, and γ -ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the Hybrid Configuration Mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the 41 Ca, 49 Ca, and 47 Ca nuclei is characterized by the coexistence of either 2p-1h and 1p-2h excitations, or couplings between single-particle/hole degrees of freedom and collective vibrations (phonons) of the doubly-magic "core".

The experimental values of longitudinal and transverse response functions of the 12C nucleus have been obtained at the 3-momentum transfer q = 300 MeV/c. The data are compared with the calculations made with due regard to the dynamics of all the nucleons constituting the 12C nucleus, and also, to the contributions of both the one-body currents only, and their combination with two-body currents.

Nuclear masses play a central role in nuclear astrophysics, significantly impacting the origin of the elements and observables used to constrain ultradense matter. A variety of techniques are available to meet this need, varying in their emphasis on precision and reach from stability. Here I briefly summarize the status of and near-future for the time-of-flight magnetic-rigidity (TOF- Bρ ) mass measurement technique, emphasizing the complementary and interconnectedness with higher-precision mass measurement methods. This includes of recent examples from TOF- Bρ mass measurements that map the evolution of nuclear structure across the nuclear landscape and significantly impact the results and interpretation of astrophysical model calculations. I also forecast expected expansion in the known nuclear mass surface from future measurement at the Facility for Rare Isotope Beams.

By using isochronous mass spectrometry (IMS) at the experimental cooler storage ring CSRe, masses of short-lived 44 Cr, 46 Mn, 48 Fe, 50 Co and 52 Ni were measured for the first time and the precision of the mass of 40 Ti was improved by a factor of about 2. Relative precisions of δm/m=(1−2)× 10 −6 have been achieved. Details of the measurements and data analysis are described. The obtained masses are compared with the Atomic-Mass Evaluation 2016 (AME ′ 16) and with theoretical model predictions. The new mass data enable us to extract the higher order coefficients, d and e , of the quartic form of the isobaric multiplet mass equation (IMME) for the fp -shell isospin quintets. Unexpectedly large d - and e -values for A=44 quintet are found. By re-visiting the previous experimental data on β -delayed protons from 44 Cr decay, it is suggested that the observed anomaly could be due to the misidentification of the T=2 , J π = 0 + isobaric analog state (IAS) in 44 V.

Gamma-ray production cross section excitation functions have been measured for 30 , 42 , 54 and 66 MeV proton beams accelerated onto targets of astrophysical interest, nat C, C + O (Mylar), nat Mg, nat Si and 56 Fe, at the Sector Separated Cyclotron (SSC) of iThemba LABS (near Cape Town, South Africa). The AFRODITE array equipped with 8 Compton suppressed HPGe clover detectors was used to record γ -ray data. For known, intense γ -ray lines the previously reported experimental data measured up to E p ≃ 25 MeV at the Washington and Orsay tandem accelerators were extended to higher proton energies. Our experimental data for the last 3 targets are reported here and discussed with respect to previous data and the Murphy \textit{et al.} compilation [ApJS 183, 142 (2009)], as well as to predictions of the nuclear reaction code TALYS. The overall agreement between theory and experiment obtained in first-approach calculations using default input parameters of TALYS has been appreciably improved by using modified optical model potential (OMP), deformation, and level density parameters. The OMP parameters have been extracted from theoretical fits to available experimental elastic/inelastic nucleon scattering angular distribution data by means of the coupled-channels reaction code OPTMAN. Experimental data for several new γ -ray lines are also reported and discussed. The astrophysical implications of our results are emphasised.

We present the semi-inclusive measurement of charged jets recoiling from direct-photon and π 0 triggers in central Au+Au collisions at s NN − − − √ = 200 GeV, using a dataset with integrated luminosity 13 n b −1 recorded by the STAR experiment in 2014. The photon and π 0 triggers are selected within transverse energy ( E trig T ) between 9 GeV and 20 GeV. Charged jets are reconstructed with the anti- k T algorithm with resolution parameters R = 0.2 and 0.5. A Mixed-Event technique developed previously by STAR is used to correct the recoil jet yield for uncorrelated background, enabling recoil jet measurements over a broad p T,jet range. We report fully corrected charged-jet yields recoiling from direct-photon and π 0 triggers for the above two jet radii and also discuss the jet R dependence of in-medium parton energy loss at the top RHIC energy.

We measure the hyperfine C -constant of the 3d4 s 2 2 D 5/2 atomic state in 45 Sc: C=−0.25(12) \,kHz. High-precision atomic calculations of the hyperfine structure of the 3d4 s 2 2 D 5/2 state and second-order corrections are performed to infer the nuclear magnetic octupole moment Ω=1.6(8) μ N b . With a single valence proton outside of the doubly-magic calcium core, this element is ideally suited for an in-depth study of the many intriguing nuclear structure phenomena observed within the neighboring isotopes of calcium. We compare Ω to shell-model calculations, and find that they cannot reproduce the experimental value of Ω for 45 Sc. We furthermore explore the use of Density Functional Theory for evaluating Ω , and obtain values in line with the shell-model calculations. This work provides a crucial step in guiding future measurements of this fundamental quantity on radioactive scandium isotopes and will hopefully motivate a renewed experimental and theoretical interest.

The cross sections for ({\gamma},xn) reactions at 30 MeV and 40 MeV bremsstrahlung end-point energies on natural rhenium (natRe) targets have been measured by the activation and the off-line {\gamma}-ray spectrometric techniques using a High Purity Germanium detector (HPGe). The measured natRe({\gamma},xn) reaction cross sections were compared to the theoretically calculated cross sections using TALYS 1.9 and EMPIRE 3.2 computer codes. The measurements allowed the determination of the Isomeric Cross section Ratios (ICR) for the 184m,g Re and 182m,gRe isomeric pairs. In addition, we have determined a semi-empirical value for the 186m,gRe isomeric pair. These results for the 30 MeV and 40 MeV end-point bremsstrahlung energies are obtained for the first time.

Azimuthal anisotropies of muons from charm and bottom hadron decays are measured in Pb+Pb collisions at s NN − − − √ =5.02 TeV. The data were collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2018 with integrated luminosities of 0.5 nb −1 and 1.4 n b −1 , respectively. The kinematic selection for heavy-flavor muons requires transverse momentum 4< p T <30 GeV and pseudorapidity |η|<2.0 . The dominant sources of muons in this p T range are semi-leptonic decays of charm and bottom hadrons. These heavy-flavor muons are separated from light-hadron decay muons and punch-through hadrons using the momentum imbalance between the measurements in the tracking detector and in the muon spectrometers. Azimuthal anisotropies, quantified by flow coefficients, are measured via the event-plane method for inclusive heavy-flavor muons as a function of the muon p T and in intervals of Pb+Pb collision centrality. Heavy-flavor muons are separated into contributions from charm and bottom hadron decays using the muon transverse impact parameter with respect to the event primary vertex. Non-zero elliptic ( v 2 ) and triangular ( v 3 ) flow coefficients are extracted for charm and bottom muons, with the charm muon coefficients larger than those for bottom muons for all Pb+Pb collision centralities. The results indicate substantial modification to the charm and bottom quark angular distributions through interactions in the quark-gluon plasma produced in these Pb+Pb collisions, with smaller modifications for the bottom quarks as expected theoretically due to their larger mass.

We report a measurement of the π − photoproduction beam asymmetry for the reaction γ → p→ π − Δ ++ using data from the GlueX experiment in the photon beam energy range 8.2--8.8 GeV. The asymmetry Σ is measured as a function of four-momentum transfer t to the Δ ++ and compared to phenomenological models. We find that Σ varies as a function of t : negative at smaller values and positive at higher values of |t| . The reaction can be described theoretically by t -channel particle exchange requiring pseudoscalar, vector, and tensor intermediaries. In particular, this reaction requires charge exchange, allowing us to probe pion exchange and the significance of higher-order corrections to one-pion exchange at low momentum transfer. Constraining production mechanisms of conventional mesons may aid in the search for and study of unconventional mesons. This is the first measurement of the process at this energy.