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Nuclear Experiment (nucl-ex) updates on the arXiv.org e-print archive



Published: 2017-11-20T20:30:00-05:00

 



Scaling within the Spectral Function approach. (arXiv:1711.06697v1 [nucl-th])

Scaling features of the nuclear electromagnetic response functions unveil aspects of nuclear dynamics that are crucial for interpretating neutrino- and electron-scattering data. In the large momentum-transfer regime, the nucleon-density response function defines a universal scaling function, which is independent of the nature of the probe. In this work, we analyze the nucleon-density response function of $^{12}$C, neglecting collective excitations. We employ particle and hole spectral functions obtained within two distinct many-body methods, both widely used to describe electroweak reactions in nuclei. We show that the two approaches provide compatible nucleon-density scaling functions that for large momentum transfers satisfy first-kind scaling. Both methods yield scaling functions characterized by an asymmetric shape, although less pronounced than that of experimental scaling functions. This asymmetry, only mildly affected by final state interactions, is mostly due to nucleon-nucleon correlations, encoded in the continuum component of the hole SF.




Mechanisms of Coulomb dissociation processes. (arXiv:1711.06741v1 [nucl-ex])

The Coulomb dissociation is studied of the $^8L$i nuclei on Pb target at energies 40.3 and 69.5 MeV/nucleon, in the experiment NSCL #03038. The $^{6,7}$Li, $^{4,6}$He, and $^2$H fragments were identified. The resonance decay and the direct breakup reactions were observed. The data give experimental evidence that the Coulomb dissociation is a two-step process. The projectile in the approaching phase is braked down and the valence neutron gets forced oscillation. The increasing Coulonb force holds the projectile and brings through the excited states up to the closest approach point. There released, the projectile is trapped into a primary, highly excited state in the continuum. The lifetime of the primary state depends on the multipolarity of the deformed projectile. At intermediate energy the collision is a sudden reaction, the valence neutron may stay --- during the impact --- in the forward or backward hemisphere of the nucleus, and the orbit of the valence neutron gets prolate or oblate, dipole or multipole deformation and the nucleus gets single-particle or collective excitation. The primary excited states, developing will decay prompt or delayed into the same reaction channel, resulting in the two decay mechanisms.




Measurements and Monte-Carlo simulations of the particle self-shielding effect of B4C grains in neutron shielding concrete. (arXiv:1711.06742v1 [physics.ins-det])

A combined measurement and Monte-Carlo simulation study was carried out in order to characterize the particle self-shielding effect of B4C grains in neutron shielding concrete. Several batches of a specialized neutron shielding concrete, with varying B4C grain sizes, were exposed to a 2 {\AA} neutron beam at the R2D2 test beamline at the Institute for Energy Technology located in Kjeller, Norway. The direct and scattered neutrons were detected with a neutron detector placed behind the concrete blocks and the results were compared to Geant4 simulations. The particle self-shielding effect was included in the Geant4 simulations by calculating effective neutron cross-sections during the Monte-Carlo simulation process. It is shown that this method well reproduces the measured results. Our results imply that shielding calculations using such materials would lead to an underestimate of the shielding required for a certain design scenario if the particle self-shielding effect is not included in the calculations.




Energy dependence of non-local potentials. (arXiv:1711.06747v1 [nucl-th])

Recently a variety of studies have shown the importance of including non-locality in the description of reactions. The goal of this work is to revisit the phenomenological approach to determining non-local optical potentials from elastic scattering. We perform a $\chi^2$ analysis of neutron elastic scattering data off $^{40}$Ca, $^{90}$Zr and $^{208}$Pb at energies $E \approx 5-40$ MeV, assuming a Perey and Buck or Tian, Pang, and Ma non-local form for the optical potential. We introduce energy and asymmetry dependencies in the imaginary part of the potential and refit the data to obtain a global parameterization. Independently of the starting point in the minimization procedure, an energy dependence in the imaginary depth is required for a good description of the data across the included energy range. We present two parameterizations, both of which represent an improvement over the original potentials for the fitted nuclei as well as for other nuclei not included in our fit. Our results show that, even when including the standard Gaussian non-locality in optical potentials, a significant energy dependence is required to describe elastic-scattering data.




Container evolution for cluster structures in $^{16}{\rm O}$. (arXiv:1711.06884v1 [nucl-th])

Background: $\alpha+{^{12}{\rm C}}$ clustering in $^{16}{\rm O}$ has been of historical importance in nuclear clustering. In the last 15 years the $4\alpha$ condensate state has been proposed as a new-type cluster state.

Purpose: The aim is to reveal a dynamical process of the formation of different kinds of cluster states, in terms of a "container" aspect of clusters, in $^{16}{\rm O}$.

Method: The so-called THSR wave function for the $4\alpha$ clusters is extended to inclusion of two different containers occupied independently by the $^{12}{\rm C}$ $(3\alpha)$ and $\alpha$ clusters.

Results: The five $J^\pi=0^+$ states with $4\alpha$ tetrahedral shape, $\alpha+{^{12}{\rm C}}$ cluster structures, and the $4\alpha$ condensate character, are found to be represented, to good approximation, by single configurations of the extended THSR wave function with containers of appropriate shape and size.

Conclusions: It is demonstrated in $^{16}{\rm O}$ that the dynamical evolution of cluster structures can be caused by size and shape evolution of a container occupied with clusters. The $\alpha$ condensate with gaslike $4\alpha$ configuration appears as a limit of the cluster formation.




L\'evy analysis of HBT correlation functions in $\sqrt{s_{\rm NN}}=$ 62 GeV and 39 GeV Au+Au collisions at PHENIX. (arXiv:1711.06891v1 [nucl-ex])

The phase diagram of strongly interacting matter can be explored by analyzing data of heavy-ion collisions at different center of mass collision energies. For investigating the space-time structure of the hadron emission source, HBT correlation measurements are among the best tools. In this paper we present the latest results from the RHIC PHENIX experiment on such measurements in $\sqrt{s_{\rm NN}}=$ 39 GeV and 62 GeV Au+Au collisions.




Measurement of $^{144}\rm{Pr}$ beta-spectrum with Si(Li) detectors for the purpose of determining the spectrum of electron antineutrinos. (arXiv:1711.06985v1 [physics.ins-det])

Here we present the specifications of the newly developed beta-spectrometer based on thick full absorption Si(Li) detector. The spectrometer can be used for precision measurements of various beta-spectra, namely for the beta-spectrum shape study of $^{144}$Pr, which is considered to be the most promising anti-neutrino source for sterile neutrino searches.




New quasibound states of the compound nucleus in $\alpha$-particle capture by the nucleus. (arXiv:1711.07012v1 [nucl-th])

We generalize the theory of nuclear decay and capture of Gamow that is based on tunneling through the barrier and internal oscillations inside the nucleus. In our formalism an additional factor is obtained, which describes distribution of the wave function of the $\alpha$ particle inside the nuclear region. We discover new most stable states (called quasibound states) of the compound nucleus (CN) formed during the capture of $\alpha$ particle by the nucleus. With a simple example, we explain why these states cannot appear in traditional calculations of the $\alpha$ capture cross sections based on monotonic penetrabilities of a barrier, but they appear in a complete description of the evolution of the CN. Our result is obtained by a complete description of the CN evolution, which has the advantages of (1) a clear picture of the formation of the CN and its disintegration, (2) a detailed quantum description of the CN, (3) tests of the calculated amplitudes based on quantum mechanics (not realized in other approaches), and (4) high accuracy of calculations (not achieved in other approaches). These peculiarities are shown with the capture reaction of $\alpha + ^{44}{\rm Ca}$. We predict quasibound energy levels and determine fusion probabilities for this reaction. The difference between our approach and theory of quasistationary states with complex energies applied for the $\alpha$ capture is also discussed. We show (1) that theory does not provide calculations for the cross section of $\alpha$ capture (according to modern models of the $\alpha$ capture), in contrast with our formalism, and (2) these two approaches describe different states of the $\alpha$ capture (for the same $\alpha$-nucleus potential).




Critical point of nuclear matter and beam energy dependence of net proton number fluctuations. (arXiv:1711.07260v1 [nucl-th])

The beam energy dependence of net baryon number susceptibilities is studied in the framework of the hadron resonance gas model with the attractive and repulsive van der Waals interactions between baryons. The collision energy dependences for the skewness $S\sigma$ and kurtosis $\kappa\sigma^2$ deviate significantly from the Poisson baseline and demonstrate the existence of rich structures at moderate collision energies. This behavior may result from the critical end point of the nuclear liquid-gas first order phase transition. In particular, $\kappa\sigma^2$ shows a non-monotonic energy dependence, and, in contrast to the standard scenario for the QCD critical point, it does not decrease at low collision energies. It is also found that the measurable net proton fluctuations differ significantly from the net baryon fluctuations when interactions between baryons cannot be neglected. The results are compared with the experimental net proton number fluctuations measured by the STAR collaboration.




First extraction of the proton dynamical polarizabilities from real Compton scattering data. (arXiv:1711.07401v1 [hep-ph])

We present the first attempt to extract the dipole dynamical polarizabilities from proton real Compton scattering data below pion production threshold. The theoretical framework combines dispersion relations technique, low energy expansion and multipoles decomposition of the scattering amplitudes. The results are obtained with statistical tools that have never been applied so far to Compton scattering data and are crucial to overcome problems inherent to the analysis of the available data set.




Charged-particle multiplicity and transverse-energy distribution using the Weibull-Glauber approach in heavy-ion collisions. (arXiv:1610.02419v3 [nucl-th] UPDATED)

The charged-particle multiplicity distribution and the transverse-energy distribution measured in heavy-ion collisions at top RHIC and LHC energies are described using the two-component model approach based on a convolution of the Monte Carlo Glauber model with the Weibull model for particle production. The model successfully describes the multiplicity and transverse-energy distribution of minimum-bias collision data for a wide range of energies. The Weibull-Glauber model can be used to determine the centrality classes in heavy-ion collisions as an alternative to the conventional negative binomial distribution (NBD)-Glauber approach.




Revisit of directed flow in relativistic heavy-ion collisions from a multiphase transport model. (arXiv:1701.01805v2 [nucl-th] UPDATED)

We have revisited several interesting questions on how the rapidity-odd directed flow is developed in relativistic $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}$ = 200 and 39 GeV based on a multiphase transport model. As the partonic phase evolves with time, the slope of the parton directed flow at midrapidity region changes from negative to positive as a result of the later dynamics at 200 GeV, while it remains negative at 39 GeV due to the shorter life time of the partonic phase. The directed flow splitting for various quark species due to their different initial eccentricities is observed at 39 GeV, while the splitting is very small at 200 GeV. From a dynamical coalescence algorithm with Wigner functions, we found that the directed flow of hadrons is a result of competition between the coalescence in momentum and coordinate space as well as further modifications by the hadronic rescatterings.




The nucleon spin and momentum decomposition using lattice QCD simulations. (arXiv:1706.02973v3 [hep-lat] UPDATED)

We determine within lattice QCD, the nucleon spin carried by valence and sea quarks, and gluons. The calculation is performed using an ensemble of gauge configurations with two degenerate light quarks with mass fixed to approximately reproduce the physical pion mass. We find that the total angular momentum carried by the quarks in the nucleon is $J_{u+d+s}{=}0.408(61)_{\rm stat.}(48)_{\rm syst.}$ and the gluon contribution is $J_g {=}0.133(11)_{\rm stat.}(14)_{\rm syst.}$ giving a total of $J_N{=}0.54(6)_{\rm stat.}(5)_{\rm syst.}$ consistent with the spin sum. For the quark intrinsic spin contribution we obtain $\frac{1}{2}\Delta \Sigma_{u+d+s}{=}0.201(17)_{\rm stat.}(5)_{\rm syst.}$. All quantities are given in the $\overline{\textrm{MS}}$ scheme at 2~GeV. The quark and gluon momentum fractions are also computed and add up to $\langle x\rangle_{u+d+s}+\langle x\rangle_g{=}0.804(121)_{\rm stat.}(95)_{\rm syst.}+0.267(12)_{\rm stat.}(10)_{\rm syst.}{=}1.07(12)_{\rm stat.}(10)_{\rm syst.}$ satisfying the momentum sum.




A Magnetic Field Cloak For Charged Particle Beams. (arXiv:1707.02361v2 [physics.acc-ph] UPDATED)

Shielding charged particle beams from transverse magnetic fields is a common challenge for particle accelerators and experiments. We demonstrate that a magnetic field cloak is a viable solution. It allows for the use of dipole magnets in the forward regions of experiments at an Electron Ion Collider (EIC) and other facilities without interfering with the incoming beams. The dipoles can improve the momentum measurements of charged final state particles at angles close to the beam line and therefore increase the physics reach of these experiments. In contrast to other magnetic shielding options (such as active coils), a cloak requires no external powering. We discuss the design parameters, fabrication, and limitations of a magnetic field cloak and demonstrate that cylinders made from 45 layers of YBCO high-temperature superconductor, combined with a ferromagnetic shell made from epoxy and stainless steel powder, shield more than 99% of a transverse magnetic field of up to 0.45 T (95 % shielding at 0.5 T) at liquid nitrogen temperature. The ferromagnetic shell reduces field distortions caused by the superconductor alone by 90% at 0.45 T.




Polarization Transfer Observables in Elastic Electron Proton Scattering at $Q^2 = $2.5, 5.2, 6.8, and 8.5 GeV$^2$. (arXiv:1707.08587v2 [nucl-ex] UPDATED)

The GEp-III and GEp-2$\gamma$ experiments were carried out in Jefferson Lab's (JLab's) Hall C from 2007-2008, to extend the knowledge of $G_E^p/G_M^p$ to the highest practically achievable $Q^2$ and to search for effects beyond the Born approximation in polarization transfer observables of elastic $\vec{e}p$ scattering. This article reports an expanded description of the common experimental apparatus and data analysis procedure, and the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance data of the GEp-2$\gamma$ experiment. The Hall C High Momentum Spectrometer detected and measured the polarization of protons recoiling elastically from collisions of JLab's polarized electron beam with a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scattered electrons in coincidence to suppress inelastic backgrounds. The final GEp-III data are largely unchanged relative to the originally published results. The statistical uncertainties of the final GEp-2$\gamma$ data are significantly reduced at $\epsilon = 0.632$ and $0.783$ relative to the original publication. The decrease with $Q^2$ of $G_E^p/G_M^p$ continues to $Q^2 = 8.5$ GeV$^2$, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At $Q^2 = 2.5$ GeV$^2$, the proton form factor ratio $G_E^p/G_M^p$ shows no statistically significant $\epsilon$-dependence, as expected in the Born approximation. The ratio $P_\ell/P_\ell^{Born}$ of the longitudinal polarization transfer component to its Born value shows an enhancement of roughly 1.7\% at $\epsilon = 0.783$ relative to $\epsilon = 0.149$, with $\approx 2.2\sigma$ significance based on the total uncertainty, implying a similar effect in the transverse component $P_t$ that cancels in the ratio $R$.