Nuclear Experiment

Detailed measurements of five-fold differential cross sections and a rich set of vector and tensor analyzing powers of the 2H(d; dp)n break-up process using polarized deuteron-beam energy of 65 MeV/nucleon with a liquid-deuterium target are presented. The experiment was conducted at the AGOR facility at KVI using the BINA 4Pi-detection system. We discuss the analysis procedure including a thorough study of the systematic uncertainties. The results can be used to examine upcoming state-of-the-art calculations in the four-nucleon scattering domain, and will, thereby, provide further insights into the dynamics of three- and four-nucleon forces in few-nucleon systems. The results of coplanar configurations are compared with the results of recent theoretical calculations based on the Single-Scattering Approximation (SSA). Through these comparisons, the validity of SSA approximation is investigated in the Quasi-Free (QF) region.

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The γ -ray strength functions (GSFs) extracted from primary γ -decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide the most comprehensive test of the generalized Brink-Axel hypothesis in heavy nuclei so far, demonstrating its applicability in the energy region of the pygmy dipole resonance.

"Why are the tin isotopes soft?" has remained, for the past decade, an open problem in nuclear structure physics: models which reproduce the isoscalar giant monopole resonance (ISGMR) in the "doubly-closed shell" nuclei, 90 Zr and 208 Pb, overestimate the ISGMR energies of the open-shell tin and cadmium nuclei, by as much as 1 MeV. In an effort to shed some light onto this problem, we present results of detailed studies of the ISGMR in the molybdenum nuclei, with the goal of elucidating where--and how--the softness manifests itself between 90 Zr and the cadmium and tin isotopes. The experiment was conducted using the 94,96,98,100 Mo( α, α ′ ) reaction at E α =386 MeV. A comparison of the results with relativistic, self-consistent Random-Phase Approximation calculations indicates that the ISGMR response begins to show softness in the molybdenum isotopes beginning with A=92 .

The expected connections between signatures in the soft and hard sectors of small collision systems, and the status of experimental attempts to identify them, are discussed. These proceedings summarize the talk as given at the International Symposium on Multi-Particle Dynamics in September 2019 in Santa Fe, NM (ISMD19). As such, the choice of content and focus are selective and not intended to be comprehensive.

Experimental results related to charged particle and π 0 suppression obtained at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven for Au-Au (Cu-Cu) collisions and at the Large Hadron Collider (LHC) at CERN for Pb-Pb (Xe-Xe) collisions are compiled in terms of the usual nuclear modification factors, R AA and R CP , and of the newly introduced R N AA and R N CP as a function of ⟨ N part ⟩ and ⟨d N ch /dη⟩ . The studies are focused on a p T range in the region of maximum suppression evidenced in the experiments. The core contribution to R AA is presented. Considerations on the missing suppression in high charged particle multiplicity events for pp collisions at 7 TeV are presented. The trends of R CP and R N CP for charged particles and R π 0 AA and ( R N AA ) π 0 as a function of s NN − − − − √ , measured at RHIC in Au-Au collisions and at LHC in Pb-Pb collisions, show a suppression that becomes larger from s NN − − − − √ = 39 GeV up to s NN − − − − √ =200 GeV, followed by a saturation up to the highest energy of s NN − − − − √ =5.02 TeV in Pb-Pb collisions. A clear change in the dependence of (1− R π 0 AA )/⟨dN/dy⟩ for the most central collisions as a function of collision energy is evidenced in the region of s NN − − − − √ =62.4 - 130 GeV.

Nucleon-transfer sum rules have been assessed via a consistent reanalysis of cross-section data from neutron-adding ( d , p ) and -removing ( d , t ) reactions on well-deformed isotopes of Gd, Dy, Er, Yb, and W, with 92≤N≤108 , studied at the Niels Bohr Institute in the 1960s and 1970s. These are complemented by new measurements of cross sections using the ( d , p ), ( d , t ), and ( p , d ) reactions on a subset of these nuclei. The sum rules, defined in a Nilsson-model framework, are remarkably consistent. A single overall normalization is used in the analysis, which appears to be sensitive to assumptions about the reaction mechanism, and in the case of sums using the ( d , t ) reaction, differs from values determined from reactions on spherical systems.

The 22 Ne( α , γ ) 26 Mg and 22 Ne( α ,n) 25 Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial α -widths of resonances in 26 Mg located in the Gamow window for the 22 Ne+ α capture. These resonances were populated using 22 Ne( 6 Li,d) 26 Mg and 22 Ne( 7 Li,t) 26 Mg reactions at energies near the Coulomb barrier. At these low energies α -transfer reactions favor population of low spin states and the extracted partial α -widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the 22 Ne( α , γ ) 26 Mg and the 22 Ne( α ,n) 25 Mg reactions are shown to be significantly different than the previously suggested values.

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at s NN − − − √ = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity ( η ) and transverse momentum ( p T ) ranges |η|<0.8 and 0.2< p T <5 GeV/ c . A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators γ 1,1 =⟨cos( φ α + φ β −2 Ψ 2 )⟩ and γ 1,−3 =⟨cos( φ α −3 φ β +2 Ψ 2 )⟩ . The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured γ 1,1 . Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form γ 1,2 =⟨cos( φ α +2 φ β −3 Ψ 3 )⟩ , which are insensitive to the CME signal, verify again that background contributions dominate the measurement of γ 1,1 .

40K plays a significant role in the radiogenic heating of earth-like exoplanets, which can affect the development of a habitable environment on their surfaces. The initial amount of 40K in the interior of these planets depends on the composition of the interstellar clouds from which they formed. Within this context, nuclear reactions that regulate the production of 40K during stellar evolution can play a critical role. In this study, we constrain for the first time the astrophysical reaction rate of 40K(n,p)40Ar, which is responsible for the destruction of 40K during stellar nucleosynthesis. We performed differential cross-section measurements on the 40Ar(p,n)40K reaction, for six energies in the center-of-mass between 3.2 and 4.0 MeV and various angles between 0-deg and 135-deg. The experiment took place at the Edwards Accelerator Laboratory at Ohio University using the beam swinger target location and a standard neutron time-of-flight technique. The total and partial cross-sections varied with energy due to the contribution from isobaric analog states and Ericson type fluctuations. The energy-averaged neutron angular distributions were symmetrical relative to 90-deg and consistent with the theoretical predictions of the statistical model. Based on the experimental data, local transmission coefficients were extracted and were used to calculate the astrophysical reaction rates of 40Ar(p,n)40K and 40K(n,p)40Ar reactions. Our results support that the destruction rate of 40K in massive stars via the 40K(n,p)40Ar reaction is larger compared to previous estimates. This result directly affects the predicted stellar yields of 40K from nucleosynthesis, which is a critical input parameter for the galactic chemical evolution models that are currently employed for the study of significant properties of exoplanets.

In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at \sqrt{s_{NN}} = 2.42 GeV collected with HADES. In this energy regime the population of \Delta(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dM{\pi}pdpTdy distributions of correlated {\pi}p pairs have been determined by subtracting the {\pi}p combinatorial background using an iterative method. The invariant-mass distributions in the \Delta(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1 < Minv (GeV/c2) < 1.4, the yield is found to be similar for {\pi}+p and {\pi}-p pairs, and to follow a power law <Apart>^{\alpha}, where <Apart> is the mean number of participating nucleons. The exponent {\alpha} depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is {\alpha} = 1.5 \pm 0.08stat \pm 0.2sys.