Nuclear Experiment

Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME) -- an electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ( Δγ ) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy ( v 2 ). We report here the first differential measurements of the correlator as a function of the pair invariant mass ( m inv ) in 20-50\% centrality Au+Au collisions at s NN − − − √ = 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to Δγ are observed. At large m inv where this background is significantly reduced, the Δγ value is found to be also significantly smaller. An event shape engineering technique is deployed to determine the v 2 background shape as a function of m inv . A v 2 -independent signal, possibly indicating a m inv -integrated CME contribution, is extracted to be Δ γ signal = (0.03 ± 0.06 ± 0.08) × 10 −4 , or (2±4±5)% of the inclusive Δγ( m inv >0.4 GeV/ c 2 ) =(1.58±0.02±0.02)× 10 −4 . This presents an upper limit of 0.23× 10 −4 , or 15% of the inclusive result at 95% confidence level.

Background: The evolution of shell structure around doubly magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the `southwest' region of 78 Ni, the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable. Purpose: We intend to measure the isomeric and β decay of 75 Co, with one-proton and two-neutron holes relative to 78 Ni, to access new nuclear structure information in 75 Co and its β -decay daughters 75 Ni and 74 Ni. Methods: The nucleus 75 Co is produced in relativistic in-flight fission reactions of 238 U at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and β decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial β -decay spectra for 75 Ni and 74 Ni, and report a new isomeric transition in 75 Co. The energy [ E γ =1914(2) keV] and half-life [ t 1/2 =13(6) μ s] of the delayed γ ray lend support for the existence of a J ? =(1/ 2 ??) isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in 75 Ni, but the first calculated 1/ 2 ??level in 75 Co, a prolate K=1/2 state, is predicted about 1 MeV below the observed (1/ 2 ??) level. Conclusions: The spherical-like structure of the lowest-lying excited states in 75 Ni is proved. In the case of 75 Co, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below 78 Ni. Further experimental efforts to discern the nature of the J ? =(1/ 2 ??) isomer are necessary.

Photon-induced nuclear excitation (i.e. photo-excitation) can be used for production of nuclear isomers, which have potential applications in astrophysics, energy storing, and medical diagnosis and treatment. This paper presents a feasibility study on production of four nuclear isomers ({99m}^Tc, {103m}^Rh and {113m, 115m}^In) using high-intensity {\gamma}-ray source based on laser-electron Compton scattering (LCS), for use in the medical diagnosis and treatment. The decay properties and the medical applications of these nuclear isomers were reviewed. The cross-section curves, simulated yields and activity of product of each photo-excitation process were calculated. The cutoff energy of LCS {\gamma}-ray beam is optimized by adjusting the electron energy in order to maximize the yields as well as the activities of photo-excitation products. It is found that the achievable activity of above-mentioned isomers can exceed 10 mCi for 6-hour target irradiation at an intensity of the order of 10^{13} {\gamma}/s. Such magnitude of activity satisfies the dose requirement of medical diagnosis. Our simulation results suggest the prospect of producing medically interesting isomers with photo-excitation using the state-of-art LCS {\gamma}-ray beam facility.

The bremsstrahlung flux-averaged cross sections for the photoneutron reactions 93 Nb( γ ,xn;x=1-5) (93-x)m,g Nb were measured in the range of boundary energies of bremsstrahlung γ -quanta E γmax =33-93 MeV with a step ? E γmax ??2 MeV. The isomeric ratios of the average cross-sections of the products of the reactions 93 Nb ( γ ,4n) 89m,g Nb and 93 Nb( γ ,5n) 88m,g Nb were determined in the energy ranges E γmax = 50-93 and 70-93 MeV, respectively. The experiments were carried out on the beam of the linear electron accelerator LU-40 of the Science and Research Establishment (SRE) "Accelerator" at National Science Center Kharkov Institute of Physics and Technology (NSC KIPT) using the method of induced activity. Calculations of the cross sections, average cross sections, and isomeric ratios of the reaction products were performed using the TALYS 1.9 code with default parameters and the GEANT4 code. The tendency of a more successful description of the average cross sections of photoneutron reactions with the formation of final odd-even Nb nuclei than odd-odd Nb nuclei is revealed. The experimental average cross sections for the reactions ( γ ,2n) and ( γ ,4n) are in good agreement with theory, while in the case of reactions ( γ ,n), ( γ ,3n), and ( γ ,5n), some discrepancies are observed. The results obtained for the reactions ( γ ,n), ( γ ,3n) and ( γ ,4n) are in satisfactory agreement with the known literature data. The average cross sections for the reactions ( γ ,2n) and ( γ ,5n) and the isomeric ratios of the reaction products 93 Nb( γ ,5n) 88m,g Nb were measured for the first time.

Ultra-peripheral collisions of heavy ions and protons are the energy frontier for electromagnetic interactions. Both photonuclear and two-photon collisions are studied, at collision energies that are far higher than are available elsewhere. In this review, we will discuss physics topics that can be addressed with UPCs, including nuclear shadowing and nuclear structure and searches for beyond-standard-model physics.

Photoproduction cross sections are reported for the reaction γp→pη using energy-tagged photons and the CLAS spectrometer at Jefferson Laboratory. The η mesons are detected in their dominant charged decay mode, η→ π + π − π 0 , and results on differential cross sections are presented for incident photon energies between 1.2 and 4.7 GeV. These new η photoproduction data are consistent with earlier CLAS results but extend the energy range beyond the nucleon resonance region into the Regge regime. The normalized angular distributions are also compared with the experimental results from several other experiments, and with predictions of η MAID\,2018 and the latest solution of the Bonn-Gatchina coupled-channel analysis. Differential cross sections dσ/dt are presented for incident photon energies E γ >2.9 GeV ( W>2.5 GeV), and compared with predictions which are based on Regge trajectories exchange in the t -channel (Regge models). The data confirm the expected dominance of ρ , ω vector-meson exchange in an analysis by the Joint Physics Analysis Center.

In this paper, the first femtoscopic analysis of pion-kaon correlations at the LHC is reported. The analysis was performed on the Pb-Pb collision data at s NN − − − √ = 2.76 TeV recorded with the ALICE detector. The non-identical particle correlations probe the spatio-temporal separation between sources of different particle species as well as the average source size of the emitting system. The sizes of the pion and kaon sources increase with centrality, and pions are emitted closer to the centre of the system and/or later than kaons. This is naturally expected in a system with strong radial flow and is qualitatively reproduced by hydrodynamic models. ALICE data on pion-kaon emission asymmetry are consistent with (3+1)-dimensional viscous hydrodynamics coupled to a statistical hadronization model, resonance propagation, and decay code THERMINATOR 2 calculation, with an additional time delay between 1 and 2 fm /c for kaons. The delay can be interpreted as evidence for a significant hadronic rescattering phase in heavy-ion collisions at the LHC.

The quark-gluon plasma produced by collisions between ultra-relativistic heavy nuclei is well described in the language of hydrodynamics. Non-central collisions are characterized by very large angular momentum, which in a fluid system manifests as flow vorticity. This rotational structure can lead to a spin polarization of the hadrons that eventually emerge from the plasma, providing experimental access to flow substructure at unprecedented detail. Recently, first observations of Λ hyperon polarization along the direction of collisional angular momentum have been reported. These measurements are in broad agreement with hydrodynamic and transport-based calculations and reveal that the QGP is the most vortical fluid ever observed. However, there remain important tensions between theory and observation which might be fundamental in nature. In the relatively mature field of heavy ion physics, the discovery of global hyperon polarization and three-dimensional simulations of the collision have opened an entirely new direction of research. We discuss the current status of this rapidly developing area and directions for future research.

We report the measurements of the transverse ( P ′ x ) and longitudinal ( P ′ z ) components of the polarization transfer to a bound proton in carbon via the quasi-free 12 C( e ⃗ , e ′ p ⃗ ) reaction, over a wide range of missing momenta. We determine these polarization-transfers separately for protons knocked out from the s - and p -shells. The electron-beam polarization was measured to determine the individual components with systematic uncertainties which allow a detailed comparison with theoretical calculations.

The unprecedented capabilities of state-of-the-art segmented germanium-detector arrays, such as AGATA and GRETA, derive from the possibility of performing pulse-shape analysis. The comparison of the net- and transient-charge signals with databases via grid-search methods allows the identification of the γ -ray interaction points within the segment volume. Their precise determination is crucial for the subsequent reconstruction of the γ -ray paths within the array via tracking algorithms, and hence the performance of the spectrometer. In this paper the position uncertainty of the deduced interaction point is investigated using the bootstrapping technique applied to 60 Co radioactive-source data. General features of the extracted position uncertainty are discussed as well as its dependence on various quantities, e.g. the deposited energy, the number of firing segments and the segment geometry.