Fri·Jul 31, 2020Friday·July 31, 2020

1. 01*

   An experimental program with high duty-cycle polarized and unpolarized positron beams at Jefferson Lab

   A. Accardi🇺🇸 · A. Afanasev🇺🇸 · I. Albayrak🇹🇷 · S.F. Ali🇺🇸 · M. Amaryan🇺🇸 · J.R.M. Annand🇬🇧 · J. Arrington🇺🇸 · A. Asaturyan🇦🇲 · H. Atac🇺🇸 · H. Avakian🇺🇸 · T. Averett🇺🇸 · C. Ayerbe Gayoso🇺🇸 and 218 other authors

   Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.

   nucl-exhep-exhep-phEPJA(2021)·64 citations
2. 02*

   Parity-transfer $({}^{16}\mathrm{O},{}^{16}\mathrm{F}(0^{-},g.s.))$ reaction as a selective probe of isovector $0^{-}$ states in nuclei

   M. Dozono🇯🇵 · M. Ichimura · S. Michimasa🇯🇵 · M. Takaki🇯🇵 · M. Kobayashi🇯🇵 · M. Matsushita🇯🇵 · S. Ota🇯🇵 · H. Tokieda · N. Fukuda🇯🇵 · N. Inabe · S. Kawase🇯🇵 · K. Kisamori🇯🇵 and 25 other authors

   We demonstrate that the parity-transfer $({}^{16}\mathrm{O},{}^{16}\mathrm{F}(0^{-},g.s.))$ reaction provides a selective probe of isovector $0^{-}$ excitations in nuclei. This reaction selectively populates unnatural-parity states through a $0^{+}\to 0^{-}$ transition in the projectile. The excitation-energy spectrum of ${}^{12}\mathrm{B}$ was reconstructed via the ${}^{12}\mathrm{C}({}^{16}\mathrm{O},{}^{16}\mathrm{F}(0^{-},g.s.))$ reaction at 247~MeV/u from the coincident detection of the ${}^{15}\mathrm{O}+p$ decay products of ${}^{16}\mathrm{F}$. The known $0^{-}$ state at $E_x=9.3\mathrm{MeV}$ was clearly observed with an enhanced forward cross section, confirming the selectivity of the reaction. Structures observed at $E_x=6.6\pm 0.4$ and $14.8\pm 0.3\mathrm{MeV}$ exhibit forward-peaked angular distributions and are suggested to contain significant $0^{-}$ strength. These results demonstrate that the parity-transfer reaction provides a powerful probe of $0^{-}$ excitations and highlight its potential for systematic studies of spin-isospin modes, including pion-related dynamics, in nuclei.

   nucl-exnucl-thPTEP(2026)·2 citations
3. 03*

   Improved precision on the experimental E0 decay branching ratio of the Hoyle state

   T.K. Eriksen🇦🇺 · T. Kibédi🇦🇺 · M.W. Reed🇦🇺 · A.E. Stuchbery🇦🇺 · K.J. Cook🇦🇺 · A. Akber🇦🇺 · B. Alshahrani · A.A. Avaa · K. Banerjee🇮🇳 · A.C. Berriman · L.T. Bezzina · L. Bignell🇦🇺 and 26 other authors

   Stellar carbon synthesis occurs exclusively via the $3\alpha$ process, in which three $\alpha$ particles fuse to form ${}^{12}$C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $\alpha$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10$\% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0_2^{+}$ Hoyle state and $2_1^{+}$ state in ${}^{12}$C, respectively. The excited states were populated by the ${}^{12}$C$(p,p^{\prime })$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2_1^{+}$ state in the reaction. For this purpose, proton scattering and $\gamma$-ray angular distribution experiments were also performed. An $E0$ branching ratio of ${\Gamma }_{\pi }^{E0}/\Gamma =8.2(5)\times 10^{-6}$ was deduced in the current work, and an adopted value of ${\Gamma }_{\pi }^{E0}/\Gamma =7.6(4)\times 10^{-6}$ is recommended based on a weighted average of previous literature values and the new result. The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of ${\Gamma }_{\pi }^{E0}/\Gamma =6.7(6)\times 10^{-6}$, while the uncertainty has been reduced from 9% to 5%.

   nucl-exPRC(2020)·20 citations
4. 04*

   Search for the $\eta$ mesic ${}^3$He in the $pd\to dp{\pi }^0$ reaction with the WASA-at-COSY facility

   P. Adlarson🇸🇪 · W. Augustyniak🇵🇱 · M. Bashkanov🇬🇧 · S. D. Bass🇵🇱 · F. S. Bergmann🇩🇪 · M. Berlowski🇵🇱 · A. Bondar🇷🇺 · M. Buscher🇩🇪 · H. Calen🇸🇪 · I. Ciepal🇵🇱 · H. Clement🇩🇪 · E. Czerwinski🇵🇱 and 71 other authors

   The excitation function for the $pd\to$ $dp{\pi }^0$ reaction has been measured by WASA-at-COSY experiment with the aim of searching for ${}^3\text{mbox}He$-$\eta$ mesic nuclei. The measurement in the vicinity of $\eta$ meson production was performed using a ramped proton beam. The data analysis and interpretation was carried out with the assumption that the $\eta$-mesic Helium decays via the formation of an intermediate N${}^{\ast }$(1535) resonance. No direct signal of the $\eta$-mesic nucleus is observed in the excitation function. We determine a new improved upper limit for the total cross section for the bound state production and decay in the $pd\to$ ${(}^3\text{mbox}He$-$\eta {)}_{bound}\to$ $dp{\pi }^0$ process. It varies between 13 nb to 24 nb for the bound state with width in the range $\Gamma \in (5,50)$ MeV.

   nucl-exPRC(2020)·10 citations
5. 05*

   Measurement of $\gamma$ rays from ${}^6$LiF tile as an inner wall of a neutron-decay detector

   J. Koga🇯🇵 · S. Ieki🇯🇵 · A. Kimura🇯🇵 · M. Kitaguchi🇯🇵 · R. Kitahara🇯🇵 · K. Mishima🇯🇵 · N. Nagakura🇯🇵 · T. Okudaira🇯🇵 · H. Otono🇯🇵 · H. M. Shimizu🇯🇵 · N. Sumi🇯🇵 · S. Takada🇯🇵 and 3 other authors

   A neutron lifetime measurement conducted at the Japan Proton Accelerator Research Complex (J-PARC) is counting the number of electrons from neutron decays with a time projection chamber (TPC). The $\gamma$ rays produced in the TPC cause irreducible background events. To achieve the precise measurement, the inner walls of the TPC consist of ${}^6$Li-enriched lithium-fluoride (${}^6$LiF) tiles to suppress the amount of $\gamma$ rays. In order to estimate the amount of $\gamma$ rays from the ${}^6\mathrm{LiF}$ tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured $\gamma$-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of $\gamma$ rays emitted from the${}^6\mathrm{LiF}$ tile was $(2.3_{-0.3}^{+0.7})\times 10^{-4}$ per incident neutron. This is $1.4_{-0.2}^{+0.5}$ times the value assumed for a mole fraction of the ${}^6\mathrm{LiF}$ tile. We concluded that the amount of $\gamma$ rays produced from the ${}^6\mathrm{LiF}$ tile is not more twice the originally assumed value.

   ↳ physics.ins-dethep-exnucl-exJINST(2021)·2 citations
6. 06*

   A Bayesian-Neural-Network Prediction for Fragment Production in Proton Induced Spallation Reaction

   Chun-Wang Ma🇨🇳 · Dan Peng · Hui-Ling Wei · Yu-Ting Wang🇨🇳 · Jie Pu🇨🇳

   Fragments productions in spallation reactions are key infrastructure data for various applications. Based on the empirical parameterizations {\sc spacs}, a Bayesian-neural-network (BNN) approach is established to predict the fragment cross sections in the proton induced spallation reactions. A systematic investigation have been performed for the measured proton induced spallation reactions of systems ranging from the intermediate to the heavy nuclei and the incident energy ranging from 168 MeV/u to 1500 MeV/u. By learning the residuals between the experimental measurements and the {\sc spacs} predictions, the BNN predicted results are in good agreement with the measured results. The established method is suggested to benefit the related researches in the nuclear astrophysics, nuclear radioactive beam source, accelerator driven systems, and proton therapy, etc.

   ↳ nucl-thnucl-exCPC(2020)·17 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.