Nuclear Theory·nucl-th

Thursday·August 30, 2018

14 papers—7 primary·7 cross-listed

01
Mass-dependence of pseudoscalar meson elastic form factors
Muyang Chen🇨🇳 · Minghui Ding🇨🇳 · Lei Chang🇨🇳 · Craig D. Roberts🇺🇸
A continuum approach to quark-antiquark bound-states is used to determine the electromagnetic form factors of pion-like mesons with masses $m_{0^{-}} / GeV = 0.14$ , $0.47$ , $0.69$ , $0.83$ on a spacelike domain that extends to $Q^{2} ≲ 10$ GeV $^{2}$ . The results enable direct comparisons with contemporary lattice-QCD calculations of heavy-pion form factors at large values of momentum transfer and aid in understanding them. They also reveal, inter alia, that the form factor of the physical pion provides the best opportunity for verification of the factorised hard-scattering formula relevant to this class of exclusive processes and that this capacity diminishes steadily as the meson mass increases.
nucl-thhep-lathep-phnucl-exPRD(2018)·56 citations
02
Two-neutron correlations in a Borromean $^{20} C + n + n$ system: Sensitivity of unbound subsystems
Jagjit Singh🇯🇵 · W. Horiuchi🇯🇵 · L. Fortunato🇮🇹 · A. Vitturi🇮🇹
The structure of $^{22}$ C plays a vital role in the new physics at subshell closure of $N = 16$ in the neutron-rich region. We study the two-neutron correlations in the ground state of the weakly-bound Borromean nucleus $^{22}$ C sitting at the edge of the neutron-drip line and its sensitivity to $core$ - $n$ potential. For the present study, we employ a three-body ( $core + n + n$ ) structure model designed for describing the Borromean system by explicit coupling of unbound continuum states of the subsystem ( $core + n$ ). We use a density-independent contact-delta interaction to describe the neutron-neutron interaction and its strength is varied to fix the binding energy. Along with the ground-state properties of $^{22}$ C, we investigate its electric-dipole and monopole responses, discussing the contribution of various configurations. Our results indicate more configuration mixing as compared to the previous studies in the ground state of $^{22}$ C. However, they strongly depend upon the choice of the $^{20} C$ - $n$ potential as well as the binding energy of $^{22}$ C, which call for new precise measurements for the low-lying continuum structure of the binary system ( $^{20} C + n$ ) and the mass of $^{22}$ C. These measurements will be essential to understand the Borromean three-body system $^{22}$ C with more accuracy.
nucl-thnucl-exFew Body Syst.(2019)·13 citations
03
Bound and continuum state $β^{-}$ decay of bare atoms: enhancement of decay rate and changes in $β^{-}$ decay branching
Arkabrata Gupta🇮🇳 · Chirashree Lahiri🇮🇳 · S. Sarkar🇮🇳
We have calculated rates of $β^{-}$ decay to both continuum and bound states separately for some fully ionized (bare) atoms in the mass range A $\approx$ 60-240. One of the motivations of this work is that the previous theoretical calculations were very old and/or informatically incomplete. Probably no theoretical study on this subject has been done in the last three decades. For the calculation, we have derived a framework from the usual $β^{-}$ decay theory used by previous authors. Dependence of the calculated rates on the nuclear radius and neutral atom Q-value have been examined. We have used the latest experimental data for nuclear and atomic observables, such as $β^{-}$ decay Q-value, ionization energy, neutral atom $β^{-}$ decay branchings, neutral atom half-lives etc. Results of $β^{-}$ decay rates for decay to continuum and bound states and the enhancement factor due to the bound state decay for a number of nuclei have been tabulated and compared with the previously calculated values, if available. The effective rate or half-life calculated for bare atom might be helpful to set a limit for the maximum enhancement due to bound state decay. Finally, $β^{-}$ decay branching for bare atom has been calculated. The changes in branching in bare atom compared to that in the neutral atom and for the first time branching flip for a few cases have been obtained. Reason for this branching change has been understood in terms of Q-values of the transitions in the neutral and bare atoms. Verification of this branching change / flip phenomenon in bare atom decay might be of interest for future experiments.
nucl-thPRC(2019)·10 citations
04
Weak sensitivity of three-body ( $d, p$ ) reactions to $n p$ force models
A. Deltuva
Adiabatic distorted-wave approximation (ADWA) study of three-body $(d, p)$ transfer reactions [G.W. Bailey, N.K. Timofeyuk, and J.A. Tostevin, Phys. Rev. Lett. 117, 162502 (2016)] reported strong sensitivity of cross sections to the neutron-proton $(n p)$ interaction model when the nucleon-nucleus optical potential is nonlocal. The verification of this unusual finding using more reliable methods is aimed for in the present work. A rigorous Faddeev-type three-body scattering theory is applied to the study of $(d, p)$ transfer reactions. The equations for transition operators are solved in the momentum-space partial-wave framework. Differential cross sections for $^{26}$ Al $(d, p)^{27}$ Al reactions are calculated using nonlocal nuclear optical potentials and a number of realistic $n p$ potentials. Only a weak dependence on the $n p$ force model is observed, typically one order of magnitude lower than in the previous ADWA study. The shape of the angular distribution of the experimental data is well reproduced. Cross sections of $(d, p)$ transfer reactions calculated using a rigorous three-body method show little sensitivity to the $n p$ interaction model. This indicates a failure of the ADWA in the context of nonlocal potentials. Some evident shortcomings of the ADWA are pointed out.
nucl-thnucl-exPRC(2018)·9 citations
05
Elliptic flow in ultra-relativistic collisions with polarized deuterons
Piotr Bozek🇵🇱 · Wojciech Broniowski🇵🇱
Predictions are made for elliptic flow in collisions of polarized deuterons with a heavy nucleus. It is shown that the eccentricity of the initial fireball, evaluated with respect to the deuteron polarization axis perpendicular to the beam direction, has a substantial magnitude for collisions of highest multiplicity. Within the Glauber approach we obtain $\sim 7%$ for the deuteron states with spin projection 0, and $\sim - 3%$ for spin projection $\pm 1$ . We propose to measure the elliptic flow coefficient as the second order harmonic coefficient in the azimuthal distribution of produced charged hadrons with respect to the fixed polarization axis. Collective expansion yields a value of the order of $1%$ for this quantity, as compared to zero in the absence of polarization and/or collectivity. Such a vivid rotational symmetry breaking could be measured with the current experimental accuracy of the relativistic heavy-ion experiments. The effect has a fundamental significance for understanding the nature of dynamics in small systems, as its experimental confirmation would prove the presence of the shape-flow transmutation mechanism, typical of hydrodynamic expansion or rescattering in the later stages of the fireball evolution.
nucl-thhep-phnucl-exPRL(2018)·17 citations
06
The isospin and neutron-to-proton excess dependence of short-range correlations
Jan Ryckebusch🇧🇪 · Wim Cosyn🇧🇪 · Sam Stevens🇧🇪 · Corneel Casert🇧🇪 · Jannes Nys🇧🇪
We provide a systematic study of the isospin composition and neutron-to-proton $(\frac{N}{Z})$ ratio dependence of nuclear short-range correlations (SRC) across the nuclear mass table. We use the low-order correlation operator approximation (LCA) to compute the SRC contribution to the single-nucleon momentum distributions for 14 different nuclei from $A = 4$ to $A = 208$ . Ten asymmetric nuclei are included for which the neutrons outnumber the protons by a factor of up to 1.54. The computed momentum distributions are used to extract the pair composition of the SRC. We find that there is a comprehensive picture for the isospin composition of SRC and their evolution with nucleon momentum. We also compute the non-relativistic kinetic energy of neutrons and protons and its evolution with nuclear mass $A$ and $\frac{N}{Z}$ . Confirming the conclusions from alternate studies it is shown that the minority species (protons) become increasingly more short-range correlated as the neutron-to-proton ratio increases. We forge connections between measured nucleon-knockout quantities sensitive to SRC and single-nucleon momentum distributions. It is shown that the LCA can account for the observed trends in the data, like the fact that in neutron-rich nuclei the protons are responsible for an unexpectedly large fraction of the high-momentum components.
nucl-thnucl-exPLB(2019)·27 citations
07
The influence of the neutron skin and the asymmetry energy on the $π^{-} / π^{+}$ ratio
C. Hartnack🇫🇷 · A. Le Fevre🇩🇪 · Y. Leifels🇩🇪 · J. Aichelin🇫🇷
We use the Isospin Quantum Molecular Dynamics model (IQMD) to analyze the centrality dependence of the isospin ratio of pions, $π^{-} / π^{+}$ . We find that the density dependence of the asymmetry potential, the Pauli blocking of the $Δ$ -decay and the thickness of the neutron skin influence in different ways this observable. Using the centrality dependence of this ratio at different beam energies we can disentangle the different contributions and open the way for their experimental determination.
nucl-th8 citations

Affiliations

first authorsco-authorsvia INSPIRE

Mass-dependence of pseudoscalar meson elastic form factors

Two-neutron correlations in a Borromean 20C+n+n system: Sensitivity of unbound subsystems

Bound and continuum state β− decay of bare atoms: enhancement of decay rate and changes in β− decay branching

Weak sensitivity of three-body (d,p) reactions to np force models

Elliptic flow in ultra-relativistic collisions with polarized deuterons

The isospin and neutron-to-proton excess dependence of short-range correlations

The influence of the neutron skin and the asymmetry energy on the π−/π+ ratio

Affiliations

Two-neutron correlations in a Borromean $^{20} C + n + n$ system: Sensitivity of unbound subsystems

Bound and continuum state $β^{-}$ decay of bare atoms: enhancement of decay rate and changes in $β^{-}$ decay branching

Weak sensitivity of three-body ( $d, p$ ) reactions to $n p$ force models

The influence of the neutron skin and the asymmetry energy on the $π^{-} / π^{+}$ ratio