Nuclear Experiment

The nuclear astrophysics setup at the Institute for Nuclear Physics, University of Cologne, Germany is dedicated to measurements of total and partial cross sections of charged-particle induced reactions at astrophysically relevant energies. These observables are key ingredients for reaction network calculations of various stellar scenarios, and crucial for the understanding of the nucleosynthesis of elements. The experiments utilize the high-efficiency γ -ray spectrometer HORUS, and the 10 MV FN-Tandem accelerator. An updated target chamber as well as further experimental methods established in the last years will be presented which allow to measure cross sections down to the nb region. The reliability of the measured cross sections is proven by a 89 Y(p, γ ) 90 Zr commissioning experiment. Additionally, an application for nuclear astrophysics will be presented. The results of a 93 Nb(p, γ ) 94 Mo experiment will be discussed as well as their deviations compared to formerly reported results.

This contribution presents a brief summary of the recent past efforts to experimentally explore the QCD phase diagram at high baryon chemical potentials through heavy-ion collisions. A few measurements are highlighted to present the current status in the search for a first-order phase transition, for a possible critical endpoint, and for evidence of chiral symmetry restoration. Finally, the outlook for the ongoing beam energy scan II program and future experiments at the FAIR complex are discussed.

Exploration of the rich structure of the QCD phase diagram is an important topic in the RHIC heavy ion program. One of the ultimate goals of this program is to search for the critical endpoint. Investigation of the space-time structure of hadron emissions at various phase transition points using Bose-Einstein correlations of identical bosons may provide insight on the location of the critical endpoint. PHENIX has performed comprehensive measurements of the Bose-Einstein correlation in Au+Au collisions at sqrt(sNN) = 15, 19, 27, 39, 62.4, and 200 GeV, where we incorporated Levy-type source functions to describe the measured correlation functions. We put particular focus on one of the parameters of the Levy-type source functions, the index of stability alpha, which is related to one of the critical exponents (the so-called correlation exponent eta). We have measured its collision energy and centrality dependence. We have also extended our analysis from two-particle to three-particle correlations to characterize the nature of the hadron emission source. The three particle correlations confirmed the findings of the two-particle correlations, and also provide insight on the pion production mechanism beyond the core-halo model.

Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne/ 29 F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the 27 F (∗) +n and 26 F (∗) +2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with S n ( 28 F )=−199(6) keV, while analysis of the 2n decay channel allowed a considerably improved S n ( 27 F )=1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F. Importantly, in the case of the ground state, the reconstructed 27 F +n momentum distribution following neutron removal from 29 F indicates that it arises mainly from the 1 p 3/2 neutron intruder configuration. This demonstrates that the island of inversion around N=20 includes 28 F, and most probably 29 F, and suggests that 28 O is not doubly magic.

The understanding of the antineutrino production in fission and the theoretical calculation of their energy spectra in different types of fission reactors rely on the application of the summation method, where the individual contributions from the different radioactive nuclides that undergo a beta decay are estimated and summed up. The most accurate estimation of the independent fission-product yields is essential to this calculation. We have coupled for the first time the predictions of the general fission model GEF for the fission yields to fission-product beta-decay data in a summation calculation of reactor antineutrino energy spectra. The first comparisons performed between the spectra from GEF and those obtained with the evaluated nuclear databases exhibited large discrepancies that highlighted the exigency of the modelisation of the antineutrino spectra and showing their usefulness in the evaluation of nuclear data. Additional constraints for the GEF model were thus needed in order to reach the level of accuracy required. The quality of different sources of information on the fission yields has been investigated with GEF, and the benefit of a combined analysis is demonstrated. The quality of fission yields emerging from different experimental techniques has been analyzed and indications for shortcomings of mass yields for some of the studied fissioning systems were provided. The combination of a careful study of the independent isotopic yields and the adjunction of the LOHENGRIN fission-yield data as additional constraints led to a substantially improved agreement between the antineutrino spectra computed with GEF and with the evaluated data. The comparison of inverse beta-decay yields computed with GEF with those measured by the Daya Bay experiment shows the excellent level of predictiveness of the GEF model for the fundamental or applied antineutrino physics.

The dependence of fusion dynamics on neutron excess for light nuclei is extracted. This is accomplished by comparing the average fusion cross-section at energies just above the fusion barrier for 12−15 C + 12 C with measurements of the interaction cross-section from high evergy collisions. The experimental results indicate that the fusion cross-section associated with dynamics increases with increasing neutron excess. Calculations with a time-dependent Hartree-Fock model fail to describe the observed trend.

We have measured beam-spin asymmetries to extract the sinϕ moment A sinϕ LU from the hard exclusive e → p→ e ′ n π + reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center-of-mass. The A sinϕ LU moment has been measured up to 6.6 GeV 2 in −t , covering the kinematic regimes of Generalized Parton Distributions (GPD) and baryon-to-meson Transition Distribution Amplitudes (TDA) at the same time. The experimental results in very forward kinematics demonstrate the sensitivity to chiral-odd and chiral-even GPDs. In very backward kinematics where the TDA framework is applicable, we found A sinϕ LU to be negative, while a sign change was observed near 90 ∘ in the center-of-mass. The unique results presented in this paper will provide critical constraints to establish reaction mechanisms that can help to further develop the GPD and TDA frameworks.

The specific shear viscosity, η/s , of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC is estimated based on the progressive longitudinal broadening of transverse momentum two-particle correlators, G 2 , reported as a function of collision centrality by the STAR and ALICE experiments. Estimates are computed as a function of collision centrality using the Gavin ansatz which relates the G 2 longitudinal broadening to the specific shear viscosity. Freeze out times required for the use of the ansatz are computed using a linear fit of freeze out times reported as a function of the cubic root of the charged particle pseudorapidity density ( d N ch /d η ) 1/3 . Estimates of η/s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that η/s might be a function of collision centrality at top RHIC energy.

The β − decay of 81 Zn to the neutron magic N=50 nucleus 81 Ga, with only three valence protons with respect to 78 Ni, was investigated. The study was performed at the ISOLDE facility at CERN by means of γ spectroscopy. The 81 Zn half-life was determined to be T 1/2 =290(4) ms while the β -delayed neutron emission probability was measured as P n =23(4)% . The analysis of the β -gated γ -ray singles and γ - γ coincidences from the decay of 81 Zn provides 47 new levels and 70 new transitions in 81 Ga. The β − n decay of 81 Zn was observed and a new decay scheme into the odd-odd 80 Ga nucleus was established. The half-lives of the first and second excited states of 81 Ga were measured via the fast-timing method using LaBr 3 (Ce) detectors. The level scheme and transition rates are compared to large-scale shell-model calculations. The low-lying structure of 81 Ga is interpreted in terms of the coupling of the three valence protons outside the doubly-magic 78 Ni core.

A hypothesis is proposed herein, suggesting that a pion-nuclear resonance may be observed in the α+d??6 Li(3.563)+ ? 0 reaction. The resonance has a ?NNα structure, containing αNN and ?NN subsystems. The former corresponds to the A=6 isotriplet ( 6 He g.s. , 6 Li(3.563) , 6 Be g.s. ), whereas the latter is a hypothetical NN -decoupled dibaryon. We propose an experiment to search for this resonance using the 7 Li(p,d) reaction.