Tue·May 15, 2018Tuesday·May 15, 2018

1. 01*

   Beam-Target Helicity Asymmetry $E$ in $K^0\Lambda$ and $K^0{\Sigma }^0$ Photoproduction on the Neutron

   CLAS Collaboration: D. H. Ho · R. A. Schumacher · A. D'Angelo · A. Deur · J. Fleming · C. Hanretty · T. Kageya · F. J. Klein · E. Klempt · M. M. Lowry · H. Lu · V. A. Nikonov and 136 other authors

   We report the first measurements of the $E$ beam-target helicity asymmetry for the $\vec{\gamma }\vec{n}\to K^0\Lambda$, and $K^0{\Sigma }^0$ channels in the energy range 1.70$\le W\le$2.34 GeV. The CLAS system at Jefferson Lab uses a circularly polarized photon beam and a target consisting of longitudinally polarized solid molecular hydrogen deuteride with low background contamination for the measurements. The multivariate analysis method boosted decision trees was used to isolate the reactions of interest. Comparisons with predictions from the KaonMAID, SAID, and Bonn-Gatchina models are presented. These results will help separate the isospin $I=0$ and $I=1$ photo-coupling transition amplitudes in pseudoscalar meson photoproduction.

   nucl-exPRC(2018)·16 citations
2. 02*

   Towards an energy measurement of the internal conversion electron in the de-excitation of the Th-229 isomer

   Simon Stellmer · Yudai Shigekawa · Veronika Rosecker · Georgy A. Kazakov · Yoshitaka Kasamatsu · Yuki Yasuda🇯🇵 · Atsushi Shinohara · Thorsten Schumm

   The first excited isomeric state of Th-229 has an exceptionally low energy of only a few eV and could form the gateway to high-precision laser spectroscopy of nuclei. The excitation energy of the isomeric state has been inferred from precision gamma spectroscopy, but its uncertainty is still too large to commence laser spectroscopy. Reducing this uncertainty is one of the most pressing challenges in the field. Here we present an approach to infer the energy of the isomer from spectroscopy of the electron which is emitted when the isomer de-excites through internal conversion (IC). The experiment builds on U-233, which decays to Th-229 and populates the isomeric state with a 2% fraction. A film of U-233 is covered by a stopping layer of few-nm thickness and placed between an alpha detector and an electron detector, such that the alpha particle and the IC electron can be detected in coincidence. Retarding field electrodes allow for an energy measurement. In the present design, the signal of the Th-229m IC electrons is masked by low-energy electrons emitted from the surface of the metallic stopping layer. We perform reference measurements with U-232 and U-234 to study systematic effects, and we study various means to reduce the background of low-energy electrons. Our study gives guidelines to the design of an experiment that is capable of detecting the IC electrons and measuring the isomer energy.

   nucl-exPRC(2018)·8 citations
3. 03*

   Analysis of the apparent nuclear modification in peripheral Pb-Pb collisions at 5.02 TeV

   ALICE Collaboration

   Charged-particle spectra at midrapidity are measured in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV and presented in centrality classes ranging from most central (0-5%) to most peripheral (95-100%) collisions. Possible medium effects are quantified using the nuclear modification factor ($R_{\mathrm{AA}}$) by comparing the measured spectra with those from proton-proton collisions, scaled by the number of independent nucleon-nucleon collisions obtained from a Glauber model. At large transverse momenta ($8<p_{\mathrm{T}}<20$ GeV/$c$), the average $R_{\mathrm{AA}}$ is found to increase from about $0.15$ in 0-5% central to a maximum value of about $0.8$ in 75-85% peripheral collisions, beyond which it falls off strongly to below $0.2$ for the most peripheral collisions. Furthermore, $R_{\mathrm{AA}}$ initially exhibits a positive slope as a function of $p_{\mathrm{T}}$ in the $8$-$20$ GeV/$c$ interval, while for collisions beyond the 80% class the slope is negative. To reduce uncertainties related to event selection and normalization, we also provide the ratio of $R_{\mathrm{AA}}$ in adjacent centrality intervals. Our results in peripheral collisions are consistent with a PYTHIA-based model without nuclear modification, demonstrating that biases caused by the event selection and collision geometry can lead to the apparent suppression in peripheral collisions. This explains the unintuitive observation that $R_{\mathrm{AA}}$ is below unity in peripheral Pb-Pb, but equal to unity in minimum-bias p-Pb collisions despite similar charged-particle multiplicities.

   nucl-exhep-exPLB(2019)·67 citations
4. 04*

   Measurement of the suppression and azimuthal anisotropy of muons from heavy-flavor decays in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with the ATLAS detector

   ATLAS Collaboration

   ATLAS measurements of the production of muons from heavy-flavor decays in $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV Pb+Pb collisions and $\sqrt{s}=2.76$ TeV $pp$ collisions at the LHC are presented. Integrated luminosities of 0.14 ${\mathrm{nb}}^{-1}$ and 570 ${\mathrm{nb}}^{-1}$ are used for the Pb+Pb and $pp$ measurements, respectively, which are performed over the muon transverse momentum range $4<p_{\mathrm{T}}<14$ GeV and for five Pb+Pb centrality intervals. Backgrounds arising from in-flight pion and kaon decays, hadronic showers, and mis-reconstructed muons are statistically removed using a template-fitting procedure. The heavy-flavor muon differential cross-sections and per-event yields are measured in $pp$ and Pb+Pb collisions, respectively. The nuclear modification factor $R_{\mathrm{AA}}$ obtained from these is observed to be independent of $p_{\mathrm{T}}$, within uncertainties, and to be less than unity, which indicates suppressed production of heavy-flavor muons in Pb+Pb collisions. For the 10% most central Pb+Pb events, the measured $R_{\mathrm{AA}}$ is approximately 0.35. The azimuthal modulation of the heavy-flavor muon yields is also measured and the associated Fourier coefficients $v_n$ for $n=$ 2, 3 and 4 are given as a function of $p_{\mathrm{T}}$ and centrality. They vary slowly with $p_{\mathrm{T}}$ and show a systematic variation with centrality which is characteristic of other anisotropy measurements, such as that observed for inclusive hadrons. The measured $R_{\mathrm{AA}}$ and $v_n$ values are also compared with theoretical calculations.

   nucl-exhep-exPRC(2018)·48 citations
5. 05*

   Estimate of the CME signal in heavy-ion collisions from measurements relative to the participant and spectator flow planes

   Sergei A. Voloshin🇺🇸

   An interpretation of the charge dependent correlations sensitive to the Chiral Magnetic Effect (CME) -- the separation of the electric charges along the system magnetic field (across the reaction plane) -- is ambiguous due to a possible large background (non-CME) effects. The background contribution is proportional to the elliptic flow $v_2$; it is the largest in measurements relative to the participant plane, and is smaller in measurements relative to the flow plane determined by spectators, where the CME signal, on opposite, is likely larger. In this note I discuss a possible strategy for corresponding experimental measurements, and list and evaluate different assumptions related to this approach.

   nucl-exnucl-thPRC(2018)·40 citations
6. 06*

   r-Process Nucleosynthesis: Connecting Rare-Isotope Beam Facilities with the Cosmos

   C. J. Horowitz🇺🇸 · A. Arcones🇩🇪 · B. Côté🇺🇸 · I. Dillmann🇨🇦 · W. Nazarewicz🇺🇸 · I. U. Roederer🇺🇸 · H. Schatz🇺🇸 · A. Aprahamian🇺🇸 · D. Atanasov🇩🇪 · A. Bauswein🇩🇪 · J. Bliss🇩🇪 · M. Brodeur🇺🇸 and 26 other authors

   This is an exciting time for the study of r-process nucleosynthesis. Recently, a neutron star merger GW170817 was observed in extraordinary detail with gravitational waves and electromagnetic radiation from radio to gamma rays. The very red color of the associated kilonova suggests that neutron star mergers are an important r-process site. Astrophysical simulations of neutron star mergers and core collapse supernovae are making rapid progress. Detection of both, electron neutrinos and antineutrinos from the next galactic supernova will constrain the composition of neutrino-driven winds and provide unique nucleosynthesis information. Finally FRIB and other rare-isotope beam facilities will soon have dramatic new capabilities to synthesize many neutron-rich nuclei that are involved in the r-process. The new capabilities can significantly improve our understanding of the r-process and likely resolve one of the main outstanding problems in classical nuclear astrophysics. However, to make best use of the new experimental capabilities and to fully interpret the results, a great deal of infrastructure is needed in many related areas of astrophysics, astronomy, and nuclear theory. We will place these experiments in context by discussing astrophysical simulations and observations of r-process sites, observations of stellar abundances, galactic chemical evolution, and nuclear theory for the structure and reactions of very neutron-rich nuclei. This review paper was initiated at a three-week International Collaborations in Nuclear Theory program in June 2016 where we explored promising r-process experiments and discussed their likely impact, and their astrophysical, astronomical, and nuclear theory context.

   ↳ astro-ph.SRnucl-exnucl-thJ.Phys.G(2019)·175 citations
7. 07*

   Electron scattering from a deeply bound nucleon on the light-front

   Frank Vera🇺🇸 · Misak M. Sargsian🇺🇸

   We calculate the cross section of the electron scattering from a bound nucleon within light-front approximation. The advantage of this approximation is the possibility of systematic account for the off-shell effects which become essential in high energy electro-nuclear processes aimed at probing the nuclear structure at small distances. We derive a new dynamical parameter which allows to control the extent of the "off-shellness" of electron - bound-nucleon electromagnetic current for different regions of momentum transfer and initial light-cone momenta of the bound nucleon. The derived cross section is compared with the results of other approaches in treating the off-shell effects in electron-nucleon scattering.

   ↳ nucl-thhep-phnucl-exPRC(2018)·3 citations
8. 08*

   Examination of the ${}^{22}$C radius determination with interaction cross sections

   T. Nagahisa · W. Horiuchi🇯🇵

   A nuclear radius of ${}^{22}$C is investigated with the total reaction cross sections at medium- to high-incident energies in order to resolve the radius puzzle in which two recent interaction cross section measurements using ${}^1$H and ${}^{12}$C targets show the quite different radii. The cross sections of ${}^{22}$C are calculated consistently for these target nuclei within a reliable microscopic framework, the Glauber theory. To describe appropriately such a reaction involving a spatially extended nucleus, the multiple scattering processes within the Glauber theory are fully taken into account, that is, the multi-dimensional integration in the Glauber amplitude is evaluated using a Monte Carlo technique without recourse to the optical-limit approximation. We discuss the sensitivity of the spatially extended halo tail to the total reaction cross sections. The root-mean-square matter radius obtained in this study is consistent with that extracted from the recent cross section measurement on ${}^{12}$C target. We show that the simultaneous reproduction of the two recent measured cross sections is not feasible within this framework.

   ↳ nucl-thnucl-exPRC(2018)·43 citations
9. 09*

   Non-Bessel-Gaussianity and Flow Harmonic Fine-Splitting

   Hadi Mehrabpour🇮🇷 · Seyed Farid Taghavi🇮🇷

   Both collision geometry and event-by-event fluctuations are encoded in the experimentally observed flow harmonic distribution $p(v_n)$ and $2k$-particle cumulants $c_n\{2k\}$. In the present study, we systematically connect these observables to each other by employing Gram-Charlier A series. We quantify the deviation of $p(v_n)$ from Bessel-Gaussianity in terms of flow harmonic fine-splitting. Subsequently, we show that the corrected Bessel-Gaussian distribution can fit the simulated data better than the Bessel-Gaussian distribution in the more peripheral collisions. Inspired by Gram-Charlier A series, we introduce a new set of cumulants $q_n\{2k\}$ that are more natural to study distributions near Bessel-Gaussian. These new cumulants are obtained from $c_n\{2k\}$ where the collision geometry effect is extracted from it. By exploiting $q_2\{2k\}$, we introduce a new set of estimators for averaged ellipticity ${\bar{v}}_2$ which are more accurate compared to $v_2\{2k\}$ for $k>1$. As another application of $q_2\{2k\}$, we show we are able to restrict the phase space of $v_2\{4\}$, $v_2\{6\}$ and $v_2\{8\}$ by demanding the consistency of ${\bar{v}}_2$ and $v_2\{2k\}$ with $q_2\{2k\}$ equation. The allowed phase space is a region such that $v_2\{4\}-v_2\{6\}\gtrsim 0$ and $12v_2\{6\}-11v_2\{8\}-v_2\{4\}\gtrsim 0$, which is compatible with the experimental observations.

   ↳ nucl-thhep-phnucl-exEPJC(2019)·16 citations
10. 10*

    Constraining gluon distributions in nuclei using dijets in proton-proton and proton-lead collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=$ 5.02 TeV

    CMS Collaboration

    The pseudorapidity distributions of dijets as a function of their average transverse momentum ($p_{\mathrm{T}}^{\text{ave}}$) are measured in proton-lead (pPb) and proton-proton (pp) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the pPb spectra with respect to the pp spectra is observed in all $p_{\mathrm{T}}^{\text{ave}}$ intervals investigated. The ratios of the pPb and pp distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.

    ↳ hep-exnucl-exPRL(2018)·73 citations
11. 11*

    Nuclear Astrophysics in the New Era of Multimessenger Astronomy

    J. Piekarewicz🇺🇸

    Neutron stars are unique cosmic laboratories for the exploration of matter under extreme conditions of density and neutron-proton asymmetry. Due to their enormous dynamic range, neutron stars display a myriad of exotic states of matter that are impossible to recreate under normal laboratory conditions. In these three lectures I will discuss how the strong synergy that has developed between nuclear physics and astrophysics will uncover some of the deepest secrets behind these fascinating objects. In particular, I will highlight the enormous impact that the very first detection of gravitational waves from the binary neutron-star merger GW170817 is having in constraining the composition, structure, and dynamics of neutron stars.

    ↳ nucl-thastro-ph.SRnucl-ex6 citations
12. 12*

    Measurement of the groomed jet mass in PbPb and pp collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=$ 5.02 TeV

    CMS Collaboration

    A measurement of the groomed jet mass in PbPb and pp collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV with the CMS detector at the LHC is presented. Jet grooming is a recursive procedure which sequentially removes soft constituents of a jet until a pair of hard subjets is found. The resulting groomed jets can be used to study modifications to the parton shower evolution in the presence of the hot and dense medium created in heavy ion collisions. Predictions of groomed jet properties from the PYTHIA and HERWIG++ event generators agree with the measurements in pp collisions. When comparing the results from the most central PbPb collisions to pp data, a hint of an increase of jets with large jet mass is observed, which could originate from additional medium-induced radiation at a large angle from the jet axis. However, no modification of the groomed mass of the core of the jet is observed for all PbPb centrality classes. The PbPb results are also compared to predictions from the JEWEL and Q-PYTHIA event generators, which predict a large modification of the groomed mass not observed in the data.

    ↳ hep-exnucl-exJHEP(2018)·117 citations
13. 13*

    Experiments with ultracold neutrons - first 50 years

    Yu.N. Pokotilovski🇷🇺

    This year is 50 anniversary of the first experimental observation of ultracold neutrons (UCN). I present my reminiscences of the first experiment in Dubna, first encounter with F.L. Shapiro -- pioneer of the UCN investigations, and the early stage of the UCN experiments. Present status of investigations with UCN is shortly reviewed.

    ↳ physics.hist-phnucl-ex4 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.