Nuclear Experiment

We performed a high resolution study of 0 + states in 134 Ba using the 136 Ba( p,t ) two-neutron transfer reaction. Our experiment shows a significant portion of the L=0 pair-transfer strength concentrated at excited 0 + levels in 134 Ba. Potential implications in the context of 136 Xe ??136 Ba neutrinoless double beta decay matrix element calculations are briefly discussed.

A Multi-Phase Transport (AMPT) model is used to study the elliptic flow fluctuations of identified particles using participant and spectator event planes. The elliptic flow measured using the first order spectator event plane is expected to give the elliptic flow relative to the true reaction plane which suppresses the flow fluctuations. However, the elliptic flow measured using the second-order participant plane is expected to capture the elliptic flow fluctuations. Our study shows that the first order spectator event plane could be used to study the elliptic flow fluctuations of the identified particles in the AMPT model. The elliptic flow fluctuations magnitude shows weak particle species dependence and transverse momentum dependence. Such observation will have important implications for understanding the source of the elliptic flow fluctuations.

Flow harmonics ( v n ) of the Fourier expansion for the azimuthal distributions of hadrons are commonly employed to quantify the azimuthal anisotropy of particle production relative to the collision symmetry planes. While lower order Fourier coefficients ( v 2 and v 3 ) are more directly related to the corresponding eccentricities of the initial state, the higher-order flow harmonics ( v n>3 ) can be induced by a mode-coupled response to the lower-order anisotropies, in addition to a linear response to the same-order anisotropies. These higher-order flow harmonics and their linear and mode-coupled contributions can be used to more precisely constrain the initial conditions and the transport properties of the medium in theoretical models. The multiparticle azimuthal cumulant method is used to measure the linear and mode-coupled contributions in the higher-order anisotropic flow, the mode-coupled response coefficients, and the correlations of the event plane angles for charged particles as functions of centrality and transverse momentum in Au+Au collisions at nucleon-nucleon center-of-mass energy s NN − − − − √ = 200 GeV. The results are compared to similar LHC measurements as well as to several viscous hydrodynamic calculations with varying initial conditions.

Background: Neutron transfer measurements for the 18 O + 28 Si system have shown that the experimental one-neutron and two-neutron transfer cross sections are well reproduced with spectroscopic amplitudes from two different shell model interactions for the Si isotopes: \textit{psdmod} for the two-neutron transfer, and \textit{psdmwkpn} for the one-neutron transfer. Purpose: The origin of this ambiguity can be related to a more complex mechanism in the one-neutron transfer that requires the unpairing of neutrons prior to its transfer in the ( 18 O, 17 O) reaction. Studying a nucleus where this characteristic is absent ( 13 C) should help to elucidate this question. Method: One-neutron transfer cross sections were measured for the 13 C + 28 Si at E lab = 30, and 34 MeV, and compared with coupled reaction channel calculations using spectroscopic amplitudes derived from the \textit{psdmod} and \textit{psdmwkpn} shell model interactions. Results: The spectroscopic amplitudes from the \textit{psdmod} interaction for the relevant states in 29 Si provide a good description of the experimental data and the corresponding values agree with previous estimates obtained from the (d,p) reaction. Conclusions: The experimental data for the one-neutron transfer to 28 Si induced by ( 13 C, 12 C) reaction is well reproduced using spectroscopic amplitudes from the \textit{psdmod}.

High energy nuclear collisions manifest a variety of interesting phenomena over a broad range of energy scales. Many of these phenomena are related to the formation of a hot and dense state of deconfined quarks and gluons known as the quark gluon plasma (QGP). Chief among these are the observed near inviscid hydrodynamic expansion of the QGP, as measured through azimuthal anisotropy coefficients of low- p T final state hadrons v n , and the loss of energy of high- p T color charges as they traverse the QGP which is observed as a quenching of strongly-interacting final state objects like jets. Observations of these phenomena in Au+Au and Pb+Pb collisions at RHIC and the LHC provide compelling evidence of QGP formation. Small collision systems, like p +Pb, also show evidence for the creation droplets of QGP through the observation of anisotropic flow; however, these systems show no signs of the energy loss observed in large collision systems. Thus, small systems are an ideal venue to explore the relationship between high- and low- p T QGP phenomena. Furthermore, the low ambient energy of p +Pb compared to A+A collisions allow for the precise determination of perturbative process rates which can be used to understand the nuclear modification of nucleon parton densities. This dissertation explores p +Pb data collected with the ATLAS detector at the LHC. The cross section and nuclear modification factor for prompt, isolated photons are measured and compared to predictions from perturbative QCD calculations and a model of initial state energy loss. Additionally, the charged hadron azimuthal anisotropy coefficients, are measured via two-particle correlations as a function of particle p T and event centrality. Results are shown from minimum bias events and events selected because of the presence of a high- p T jet.

The neutron-proton equilibration process in 48 Ca+ 40 Ca at 35 MeV/nucleon bombarding energy has been experimentally estimated by means of the isospin transport ratio. Experimental data have been collected with a subset of the FAZIA telescope array, which permitted to determine Z and N of detected fragments. For the first time, the QP evaporative channel has been compared with the QP break-up one in a homogeneous and consistent way, pointing out to a comparable n-p equilibration which suggests close interaction time between projectile and target independently of the exit channel. Moreover, in the QP evaporative channel n-p equilibration has been compared with the prediction of the Antisymmetrized Molecular Dynamics (AMD) model coupled to the GEMINI statistical model as an afterburner, showing a larger probability of proton and neutron transfers in the simulation with respect to the experimental data.

Yields of equatorially emitted light isotopes, 1≤Z≤14 , observed in ternary fission in the reaction 241 Pu( n th ,f) are employed to determine apparent chemical equilibrium constants for low-temperature and low-density nuclear matter. The degree of liberation and role of medium modifications are probed through a comparison of experimentally derived reaction quotients with equilibrium constants calculated using a relativistic mean-field model employing a universal medium modification correction for the attractive σ meson coupling. The results of these comparisons indicate that equilibrium is achieved for the lighter ternary fission isotopes. For the heavier isotopes experimental reaction quotients are well below calculated equilibrium constants. This is attributed to a dynamical limitation reflecting insufficient time for full equilibrium to develop. The role of medium effects leading to yield reductions is discussed as is the apparent enhancement of yields for 8 He and other very neutron rich exotic nuclei.

The neutron total cross sections σ tot of 16,18 O, 58,64 Ni, 103 Rh, and 112,124 Sn have been measured at the Los Alamos Neutron Science Center (LANSCE) from low to intermediate energies (3 ≤ E lab ≤ 450 MeV) by leveraging waveform-digitizer technology. The σ tot relative differences between isotopes are presented, revealing additional information about the isovector components needed for an accurate optical-model description away from stability. Digitizer-enabled σ tot -measurement techniques are discussed and a series of uncertainty-quantified dispersive optical model (DOM) analyses using these new data is presented, validating the use of the DOM for modeling light systems ( 16,18 O) and systems with open neutron shells ( 58,64 Ni and 112,124 Sn). The valence-nucleon spectroscopic factors extracted for each isotope reaffirm the usefulness of high-energy proton reaction cross sections for characterizing depletion from the mean-field expectation.

The inclusive J/ ψ elliptic ( v 2 ) and triangular ( v 3 ) flow coefficients measured at forward rapidity (2.5 <y< 4) and the v 2 measured at midrapidity ( |y|< 0.9) in Pb-Pb collisions at s NN − − − √ = 5.02 TeV using the ALICE detector at the LHC are reported. The entire Pb-Pb data sample collected during Run 2 is employed, amounting to an integrated luminosity of 750 μ b −1 at forward rapidity and 93 μ b −1 at midrapidity. The results are obtained using the scalar product method and are reported as a function of transverse momentum p T and collision centrality. At midrapidity, the J/ ψ v 2 is in agreement with the forward rapidity measurement. The centrality averaged results indicate a positive J/ ψ v 3 with a significance of more than 5 σ at forward rapidity in the p T range 2< p T <5 GeV/ c . The forward rapidity v 2 , v 3 , and v 3 / v 2 results at low and intermediate p T ( p T ≲8 GeV/ c ) exhibit a mass hierarchy when compared to pions and D mesons, while converging into a species-independent curve at higher p T . At low and intermediate p T , the results could be interpreted in terms of a later thermalization of charm quarks compared to light quarks, while at high p T , path-length dependent effects seem to dominate. The J/ ψ v 2 measurements are further compared to a microscopic transport model calculation. Using a simplified extension of the quark scaling approach involving both light and charm quark flow components, it is shown that the D-meson v n measurements can be described based on those for charged pions and J/ ψ flow.

Inclusive J/ ψ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair s NN − − − √ = 8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/ ψ mesons are reconstructed at forward (2.03< y cms <3.53) and backward ( −4.46< y cms <−2.96 ) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/ ψ yields at forward and backward rapidity normalized to their respective average values increase with the normalized charged-particle pseudorapidity density, the former showing a weaker increase than the latter. The normalized average transverse momenta at forward and backward rapidity manifest a steady increase from low to high charged-particle pseudorapidity density with a saturation beyond the average value.