Wed·Apr 8, 2015Wednesday·April 8, 2015

1. 01*

   Observation of Anomalous Internal Pair Creation in ${}^8$Be: A Possible Signature of a Light, Neutral Boson

   A.J. Krasznahorkay🇭🇺 · M. Csatlós🇭🇺 · L. Csige🇭🇺 · Z. Gácsi🇭🇺 · J. Gulyás🇭🇺 · M. Hunyadi🇭🇺 · T.J. Ketel🇳🇱 · A. Krasznahorkay · I. Kuti🇭🇺 · B.M. Nyakó🇭🇺 · L. Stuhl🇭🇺 · J. Timár🇭🇺 · T.G. Tornyi🇭🇺 · Zs. Vajta🇭🇺

   Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^{\pi }=1^{+}$, $T=1$) $\to$ ground state ($J^{\pi }=0^{+}$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^{\pi }=1^{+}$, $T=0$) state $\to$ ground state transitions in ${}^8$Be. Significant deviation from the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $>5\sigma$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$\pm 0.35$ (stat)$\pm 0.5$ (sys) MeV$/c^2$ and $J^{\pi }=1^{+}$ was created.

   nucl-exPRL(2016)·505 citations
2. 02*

   Test of statistical model cross section calculations for $\alpha$-induced reactions on ${}^{107}$Ag at energies of astrophysical interest

   C. Yalcin · Gy. Gyürky · T. Rauscher · G. G. Kiss · N. Özkan · R. T. Güray · Z. Halász · T. Szücs · Zs. Fülöp · Z. Korkulu · E. Somorjai

   Astrophysical reaction rates, which are mostly derived from theoretical cross sections, are necessary input to nuclear reaction network simulations for studying the origin of $p$ nuclei. Past experiments have found a considerable difference between theoretical and experimental cross sections in some cases, especially for ($\alpha$,$\gamma$) reactions at low energy. Therefore, it is important to experimentally test theoretical cross section predictions at low, astrophysically relevant energies. The aim is to measure reaction cross sections of ${}^{107}$Ag($\alpha$,$\gamma$)${}^{111}$In and ${}^{107}$Ag($\alpha$,n)${}^{110}$In at low energies in order to extend the experimental database for astrophysical reactions involving $\alpha$ particles towards lower mass numbers. Reaction rate predictions are very sensitive to the optical model parameters and this introduces a large uncertainty into theoretical rates involving $\alpha$ particles at low energy. We have also used Hauser-Feshbach statistical model calculations to study the origin of possible discrepancies between prediction and data. An activation technique has been used to measure the reaction cross sections at effective center of mass energies between 7.79 MeV and 12.50 MeV. Isomeric and ground state cross sections of the ($\alpha$,n) reaction were determined separately. The measured cross sections were found to be lower than theoretical predictions for the ($\alpha$,$\gamma$) reaction. Varying the calculated averaged widths in the Hauser-Feshbach model, it became evident that the data for the ($\alpha$,$\gamma$) and ($\alpha$,n) reactions can only be simultaneously reproduced when rescaling the ratio of $\gamma$- to neutron width and using an energy-dependent imaginary part in the optical $\alpha$+${}^{107}$Ag potential.......

   nucl-exPRC(2015)·16 citations
3. 03*

   Photon production from gluon mediated quark-anti-quark annihilation at confinement

   Sarah Campbell🇺🇸

   Heavy ion collisions at RHIC produce direct photons at low transverse momentum, $p_T$ from 1-3 GeV/c, in excess of the $p$$+$$p$ spectra scaled by the nuclear overlap factor, $T_{AA}$. These low $p_T$ photons have a large azimuthal anisotropy, $v_2$. Theoretical models, including hydrodynamic models, struggle to quantitatively reproduce the large low $p_T$ direct photon excess and $v_2$ in a self-consistent manner. This paper presents a description of the low $p_T$ photon flow as the result of increased photon production from soft-gluon mediated $q$-$\bar{q}$ interactions as the system becomes color-neutral. This production mechanism will generate photons that follow constituent quark number, $n_q$, scaling of $v_2$ with an $n_q$ value of two for direct photons. ${\chi }^2$ comparisons of the published PHENIX direct photon and identified particle $v_2$ measurements finds that $n_q$-scaling applied to the direct photon $v_2$ data prefers the value $n_q=1.8$ and agrees with $n_q=2$ within errors in most cases. The 0-20% and 20-40% Au$+$Au direct photon data are compared to a coalescence-like Monte Carlo simulation that calculates the direct photon $v_2$ while describing the shape of the direct photon $p_T$ spectra in a consistent manner. The simulation, while systematically low compared to the data, is in agreement with the Au$+$Au measurement at $p_T<3$ GeV/c in both centrality bins. Furthermore, this model predicts that higher order flow harmonics, $v_n$, in direct photons will follow the modified $n_q$-scaling laws seen in identified hadron $v_n$ with an $n_q$ value of two.

   nucl-exPRC(2015)·26 citations
4. 04*

   Evolution of E2 transition strength in deformed hafnium isotopes from new measurements on ${}^{172}$Hf, ${}^{174}$Hf, and ${}^{176}$Hf

   M. Rudigier🇩🇪 · K. Nomura🇫🇷 · M. Dannhoff · R-B. Gerst · J. Jolie · N. Saed-Samii🇩🇪 · S. Stegemann · J-M. Régis🇩🇪 · L. M. Robledo🇪🇸 · R. Rodríguez-Guzmán🇰🇼 · A. Blazhev🇩🇪 · Ch. Fransen🇩🇪 · N. Warr · K. O. Zell🇩🇪

   The available data for E2 transition strengths in the region between neutron-deficient Hf and Pt isotopes are far from complete. More and precise data are needed to enhance the picture of structure evolution in this region and to test state-of-the-art nuclear models. In a simple model, the maximum collectivity is expected at the middle of the major shell. However, for actual nuclei, this picture may no longer be the case, and one should use a more realistic nuclear-structure model. We address this point by studying the spectroscopy of Hf. We remeasure the 2^+_1 half-lives of 172,174,176Hf, for which there is some disagreement in the literature. The main goal is to measure, for the first time, the half-lives of higher-lying states of the rotational band. The new results are compared to a theoretical calculation for absolute transition strengths. The half-lives were measured using \gamma-\gamma and conversion-electron-\gamma delayed coincidences with the fast timing method. For the determination of half-lives in the picosecond region, the generalized centroid difference method was applied. For the theoretical calculation of the spectroscopic properties, the interacting boson model is employed, whose Hamiltonian is determined based on microscopic energy-density functional calculations. The measured 2^+_1 half-lives disagree with results from earlier \gamma-\gamma fast timing measurements, but are in agreement with data from Coulomb excitation experiments and other methods. Half-lives of the 4^+_1 and 6^+_1 states were measured, as well as a lower limit for the 8^+_1 states. We show the importance of the mass-dependence of effective boson charge in the description of E2 transition rates in chains of nuclei. It encourages further studies of the microscopic origin of this mass dependence. New data on transition rates in nuclei from neighboring isotopic chains could support these studies.

   nucl-exnucl-thPRC(2015)·19 citations
5. 05*

   Large amplitude collective dynamic beyond the independent particle/quasiparticle picture

   Denis Lacroix🇫🇷

   In the present note, a summary of selected aspects of time-dependent mean-field theory is first recalled. This approach is optimized to describe one-body degrees of freedom. A special focus is made on how this microscopic theory can be reduced to a macroscopic dynamic for a selected set of collective variables. Important physical phenomena like adiabaticity/diabaticity, one-body dissipation or memory effect are discussed. Special aspects related to the use of a time-dependent density functional instead of a time-dependent Hartree-Fock theory based on a bare hamiltonian are underlined. The absence of proper description of complex internal correlations however strongly impacts the predictive power of mean-field. A brief overview of theories going beyond the independent particles/quasi-particles theory is given. Then, a special attention is paid for finite fermionic systems at low internal excitation. In that case, quantum fluctuations in collective space that are poorly treated at the mean-field level, are important. Several approaches going beyond mean-field, that are anticipated to improve the description of quantum fluctuations, are discussed: the Balian-Vénéroni variational principle, the Time-Dependent Random Phase Approximation and the recently proposed Stochastic Mean-Field theory. Relations between these theories are underlined as well as their advantages and shortcomings.

   ↳ nucl-thcond-mat.str-elnucl-ex7 citations
6. 06*

   Eta photoproduction in a combined analysis of pion- and photon-induced reactions

   D. Rönchen🇩🇪 · M. Döring🇺🇸 · H. Haberzettl🇺🇸 · J. Haidenbauer🇩🇪 · U.-G. Meißner🇩🇪 · K. Nakayama🇺🇸

   The $\eta N$ final state is isospin-selective and thus provides access to the spectrum of excited nucleons without being affected by excited $\Delta$ states. To this end, the world database on eta photoproduction off the proton up to a center-of-mass energy of $E\sim 2.3$ GeV is analyzed, including data on differential cross sections, and single and double polarization observables. The resonance spectrum and its properties are determined in a combined analysis of eta and pion photoproduction off the proton together with the reactions $\pi N\to \pi N$, $\eta N$, $K\Lambda$ and $K\Sigma$. For the analysis, the so-called Jülich coupled-channel framework is used, incorporating unitarity, analyticity, and effective three-body channels. Parameters tied to photoproduction and hadronic interactions are varied simultaneously. The influence of recent MAMI $T$ and $F$ asymmetry data on the eta photoproduction amplitude is discussed in detail.

   ↳ nucl-thhep-phnucl-exEPJA(2015)·100 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.