We present a model of electron-positron pair production in pion-nucleon collisions in the exclusive reaction $\pi N\to N{e}^{+}{e}^{-}$. The model is based on an effective field theory approach, incorporating 16 baryon resonances below 2 GeV. Parameters of the model are fitted to pion photoproduction data. We present the resulting dilepton invariant mass spectra for ${\pi }^{-}p$ collisions up to $\sqrt{s}=1.9$ GeV center-of-mass collision energy. These results are meant to give predictions for the planned experiments at the HADES spectrometer in GSI, Darmstadt.

We calculate the electromagnetic form factors of the nucleon to third chiral order in manifestly Lorentz-invariant effective field theory. The $\rho$ and $\omega$ mesons as well as the $\Delta \left(1232\right)$ resonance are included as explicit dynamical degrees of freedom. To obtain a self-consistent theory with respect to constraints we consider the proper relations among the couplings of the effective Lagrangian. For the purpose of generating a systematic power counting, the extended on-mass-shell renormalization scheme is applied in combination with the small-scale expansion. The results for the electric and magnetic Sachs form factors are analyzed in terms of experimental data and compared to previous findings in the framework of chiral perturbation theory. The pion-mass dependence of the form factors is briefly discussed.

Background: The density of the nucleus has been important in explaining the nuclear dependence of the quark distributions, also known as the EMC effect, as well as the presence of high-momentum nucleons arising from short-range correlations (SRCs). Recent measurements of both of these effects on light nuclei have shown a clear deviation from simple density-dependent models.

Purpose: A better understanding of the nuclear quark distributions and short-range correlations requires a careful examination of the experimental data on these effects to constrain models that attempt to describe these phenomena.

Methods: We present a detailed analysis of the nuclear dependence of the EMC effect and the contribution of SRCs in nuclei, comparing to predictions and simple scaling models based on different pictures of the underlying physics. We also make a direct, quantitative comparison of the two effects to further examine the connection between these two observables related to nuclear structure.

Results: We find that, with the inclusion of the new data on light nuclei, neither of these observables can be well explained by common assumptions for the nuclear dependence. The anomalous behavior of both effects in light nuclei is consistent with the idea that the EMC effect is driven by either the presence of high-density configurations in nuclei or the large virtuality of the high-momentum nucleons associated with these configurations.

Conclusions: The unexpected nuclear dependence in the measurements of the EMC effect and SRC contributions appear to suggest that the local environment of the struck nucleon is the most relevant quantity for explaining these results. The common behavior suggests a connection between the two seemingly disparate phenomena, but the data do not yet allow for a clear preference between models which aim to explain this connection.

Neutron star crusts are studied using a classical molecular dynamics model developed for heavy-ion reactions. After the model is shown to produce a plethora of the so-called pasta shapes, a series of techniques borrowed from nuclear physics, condensed matter physics, and topology is used to craft a method that can be used to characterize the shape of the pasta structures in an unequivocal way.

Background: Superallowed $\beta$-decay rates provide stringent constraints on physics beyond the standard model of particle physics. To extract crucial information about the electroweak force, small isospin-breaking corrections to the Fermi matrix element of superallowed transitions must be applied.

Purpose: We perform systematic calculations of isospin-breaking corrections to superallowed $\beta$ decays and estimate theoretical uncertainties related to the basis truncation, to time-odd polarization effects related to the intrinsic symmetry of the underlying Slater determinants, and to the functional parametrization.

Methods: We use the self-consistent isospin- and angular-momentum-projected nuclear density functional theory employing two density functionals derived from the density-independent Skyrme interaction. Pairing correlations are ignored. Our framework can simultaneously describe various effects that impact matrix elements of the Fermi decay: symmetry breaking, configuration mixing, and long-range Coulomb polarization.

Results: Isospin-breaking corrections to the $I=0^{+}$, $T=1\to I=0^{+}$, $T=1$ pure Fermi transitions are computed for nuclei from $A=10$ to $A=98$ and, for the first time, to the Fermi branch of the $I,T=1/2\to I$, $T=1/2$ transitions in mirror nuclei from $A=11$ to $A=49$. We carefully analyze various model assumptions impacting theoretical uncertainties of our calculations and provide theoretical error bars on our predictions.

Conclusions: The overall agreement with empirical isospin-breaking corrections is very satisfactory. Using computed isospin-breaking corrections we show that the unitarity of the CKM matrix is satisfied with a precision of better than 0.1$\%$.

High-$p_T$ hadrons produced in hard collisions and detected inclusively bear peculiar features: (i) they originate from jets whose initial virtuality and energy are of the same order and (ii) such jets are rare and have a very biased energy sharing among the particles, namely the detected hadron carries the main fraction of the jet energy. The former feature leads to an extremely intensive gluon radiation and energy dissipation at the early stage of hadronization, either in vacuum or in a medium. As a result, a leading hadron must be produced on a short length scale. Evaluation within a model of perturbative fragmentation confirms the shortness of the production length. This result is at variance with the unjustified assumption of long production length, made within the popular energy-loss scenario. Thus, we conclude that the main reason of suppression of high-$p_T$ hadrons in heavy-ion collisions is the controlled-by-color-transparency attenuation of a high-$p_T$ dipole propagating through the hot medium. Adjusting a single parameter, the transport coefficient, we describe quite well the data from the Large Hadron Collider and the Relativistic Heavy Ion Collider for the suppression factor $R_{AA}$ as function of $p_T$, collision energy, and centrality. We observe that the complementary effect of the initial-state interaction causes a flattening and even fall of $R_{AA}$ at large $p_T$. The azimuthal anisotropy of hadron production, calculated with no further adjustment, also agrees well with data at different energies and centralities.

Background: There has been much recent investigation into dijets at the Large Hadron Collider, but predictions for dijets at Relativistic Heavy Ion Collider (RHIC) scales are lacking.

Purpose: We present a systematic study of the dijet suppression at RHIC using the VNI/Bass-Müller-Srivastava parton cascade.

Methods: We examine the modification of the dijet asymmetry $A_j$ and the within-cone transverse energy distribution (jet shape) along with partonic fragmentation distributions $z$ and $j_t$ in terms of $\widehat{q}$, the path length of leading and subleading jets, cuts on the jet energy distributions, the jet cone angle, and the jet-medium interaction mechanism.

Results: We find that $A_j$ is most sensitive to $\widehat{q}$ and relatively insensitive to the nature of the jet-medium interaction mechanism. The jet profile is dominated by $\widehat{q}$ and the nature of the interaction mechanism. The partonic fragmentation distributions clearly show the jet modification and differentiate between elastic and radiative plus elastic modes.

Conclusions: Dijets at RHIC scales are strongly modified by a quark-gluon plasma medium; measurements of their suppression will provide a vital additional data point for understanding the modification of hard probes.

Background: There is a renewed experimental interest in antiproton-proton scattering with an intense, possibly polarized antiproton beam. On the theoretical side, significant progress has been made in the understanding of the nuclear force from chiral effective field theory.

Purpose: We provide a high-quality model-independent description of the available low-energy antiproton-proton scattering data.

Method: We perform a new energy-dependent partial-wave analysis of all antiproton-proton elastic ($\overline{p}p\to \overline{p}p$) and charge-exchange ($\overline{p}p\to \overline{n}n$) scattering data below 925 MeV/$c$ antiproton laboratory momentum. The Schrödinger equation is solved for the coupled $\overline{p}p$ and $\overline{n}n$ channels. The relevant long-range parts of the electromagnetic and the one- and two-pion exchange interactions are included exactly. The short-range interactions, including the coupling to the mesonic annihilation channels, are parametrized by a complex boundary condition at a radius of $r=1.2$ fm.

Results: The updated database, which includes significantly more high-quality charge-exchange data, contains 3749 scattering data. The fit gives ${\chi }_{\min }^2/N_{\mathrm{df}}=1.048$, where $N_{\mathrm{df}}=3578$ is the number of degrees of freedom. We discuss the description of the experimental data and we present the antiproton-proton phase-shift parameters.

Conclusions: Chiral effective field theory provides an excellent long-range antinucleon-nucleon interaction. The results of the PWA should serve as the starting point for future theoretical and experimental investigations of low-energy antiproton-proton scattering.

The Gamow-Teller (GT) strength distribution has been investigated in a high-resolution (${}^3$He,$t$) charge-exchange experiment on the double-beta ($\beta \beta$) decaying nucleus ${}^{100}$Mo. The experiment was carried out at the Osaka University Research Center for Nuclear Physics (RCNP) with a 420 MeV incident ${}^3$He beam and the Grand Raiden magnetic spectrometer. A final-state energy resolution of 33 keV was achieved. It was found that nearly the entire low-energy (up to $\approx 4$ MeV) ${\mathrm{GT}}^{-}$ strength is concentrated in the ground-state transition to ${}^{100}$Tc, which is interpreted as a result of the particular neutron and proton sub-shell occupancies. The impact on the matrix elements for the two-neutrino double-beta ($2\nu \beta \beta$) decay and the use of ${}^{100}$Mo as a neutrino detection material is discussed.

Background: The $\beta$ decays of ${}^3$H and ${}^6$He can play an important role in testing nuclear wave-function calculations and fixing low-energy constants in effective-field theory approaches. However, there exists a large discrepancy between previous measurements of the ${}^6$He half-life.

Purpose: Our measurement aims at resolving this long-standing discrepancy in the ${}^6$He half-life and providing a reliable $ft$ value and Gamow-Teller matrix element for comparison with theoretical ab initio calculations.

Method: We measured the ${}^6$He half-life by counting the $\beta$-decay electrons with two scintillator detectors operating in coincidence.

Results: The measured ${}^6$He half-life is $806.89\pm 0.{11}_{\mathrm{stat}}{}_{-0.19}^{+0.23}{}_{\mathrm{syst}}$ ms corresponding to a relative precision of $3\times {10}^{-4}$. Calculating the statistical rate function we determined the $ft$ value to be $803.{04}_{-0.23}^{+0.26}$ s.

Conclusions: Our result resolves the previous discrepancy by providing a higher-precision result with careful analysis of potential systematic uncertainties. The result provides a reliable basis for future precision comparisons with ab initio calculations.

Rotational bands in ${}^{158}$Er at ultrahigh spin have been studied in the framework of relativistic and nonrelativistic nuclear density-functional theories. Consistent results are obtained across the theoretical models used but some puzzles remain when confronted with experiment. Namely, the many-body configurations which provide good description of experimental transition quadrupole moments and dynamic moments of inertia require substantial increase of the spins of observed bands as compared with experimental estimates, which are still subject to large uncertainties. If, however, the theoretical spin assignments turned out to be correct, experimental band 1 in ${}^{158}$Er would be the highest spin structure ever observed.

Background: Gamow-Teller (GT) transition strength distributions in stable and unstable $pf$-shell isotopes are key inputs for estimating electron-capture rates important for stellar evolution. Charge-exchange experiments at intermediate beam energies have long been used to test theoretical predictions for GT strengths, but previous experiments were largely restricted to stable nuclei. Since a large fraction of the nuclei relevant for astrophysical applications (including key nuclei such as ${}^{56}$Ni) are unstable, new methods are needed to perform charge-exchange experiments in inverse kinematics with unstable isotopes.

Purpose: The ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were measured in inverse kinematics in order to extract GT strengths for transitions to ${}^{56}$Cu and ${}^{55}$Ni, respectively. The extracted strength distributions were compared with shell-model predictions in the $pf$ shell using the KB3G and GXPF1J interactions. By invoking isospin symmetry, these strength distributions are relevant for electron captures on the ground states of ${}^{56}$Ni and ${}^{55}$Ni to final states in ${}^{56}$Co and ${}^{55}$Co, respectively.

Method: Differential cross sections and excitation energy spectra for the ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were determined by measuring neutrons recoiling from a liquid hydrogen target into the Low Energy Neutron Detector Array. GT contributions to the spectra were extracted by using a multipole decomposition analysis and were converted to strengths by employing the proportionality between GT strength and differential cross section at zero linear momentum transfer.

Results: GT strengths from ${}^{56}$Ni and ${}^{55}$Co were extracted up to excitation energies of 8 and 15 MeV, respectively. Shell-model calculations performed in the $pf$ shell with the GXPF1J interaction reproduced the experimental GT strength distributions better than calculations with the KB3G interaction.

Conclusions: A new technique for measuring ($p,n$) charge-exchange reactions on unstable nuclei was successfully developed. It can be used to study the isovector response of unstable nuclei in any mass region and for excitation energies beyond the particle decay threshold. In the first experiment, ${}^{56}$Ni($p,n$) and ${}^{55}$Co($p,n$) reactions were studied and GT transition strengths were extracted for the purpose of testing shell-model calculations used to estimate electron-capture rates in simulations of late stellar evolution. The calculation using the GXPF1J interaction was found to best reproduce the experimental strength distribution.

Background: Time-reversal-invariance violation, or equivalently $CP$ violation, may explain the observed cosmological baryon asymmetry as well as indicate physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation $D\langle {\vec{J}}_n\rangle /J_n·({\vec{\beta }}_e\times {\widehat{p}}_{\nu })$ is a parity-even, time-reversal-odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects contribute to $D$ at the level of 10${}^{-5}$ and can be calculated with a precision of 1$\%$ or better.

Purpose: We have improved the sensitivity to $T$-odd, $P$-even interactions in nuclear $\beta$ decay.

Methods: We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract $D$ and study systematic effects in a blind analysis.

Results: We find $D=[-0.94\pm 1.89\left(\mathrm{stat}\right)\pm 0.97\left(\mathrm{sys}\right)]\times {10}^{-4}$. This differs from the result of our recent paper [Phys. Rev. Lett. 107, 102301 (2011)] due to refinement of corrections for background and backscattering.

Conclusions: This is the most sensitive measurement of $D$ in nuclear $\beta$ decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants ($C_A/C_V=\left|\lambda \right|e^{i{\phi }_{AV}}$) with ${\phi }_{AV}=180.{012}^{\circ }\pm 0.{028}^{\circ }$ (68$\%$ confidence level). This result can also be used to constrain time-reversal-violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks.

Background: Neutrino-nucleus quasi-elastic scattering is crucial to interpret the neutrino oscillation results in long baseline neutrino experiments. There are rather large uncertainties in the cross section, due to insufficient knowledge on the role of two-body weak currents.

Purpose: Determine the role of two-body weak currents in neutrino-deuteron quasi-elastic scattering up to GeV energies.

Methods: Calculate cross sections for inclusive neutrino scattering off deuteron induced by neutral and charge-changing weak currents, from threshold up to GeV energies, using the Argonne $v_{18}$ potential and consistent nuclear electroweak currents with one- and two-body terms.

Results: Two-body contributions are found to be small, and increase the cross sections obtained with one-body currents by less than 10$\%$ over the whole range of energies. Total cross sections obtained by describing the final two-nucleon states with plane waves differ negligibly, for neutrino energies $\gtrsim$ MeV, from those in which interaction effects in these states are fully accounted for. The sensitivity of the calculated cross sections to different models for the two-nucleon potential and/or two-body terms in the weak current is found to be weak. Comparing cross sections to those obtained in a naive model in which the deuteron is taken to consist of a free proton and neutron at rest, nuclear structure effects are illustrated to be non-negligible.

Conclusion: Contributions of two-body currents in neutrino-deuteron quasi-elastic scattering up to GeV are found to be smaller than 10$\%$. Finally, it should be stressed that the results reported in this work do not include pion production channels.

Background: Quadrupole moments of excited nuclear states are important observables for geometrically interpreting nuclear structure in terms of deformed shapes, although data are scarce and sometimes ambiguous, in particular, in neutron-rich nuclides.

Purpose: A measurement was performed for determining the spectroscopic quadrupole moment of the $2_1^{+}$ state of ${}^{140}$Ba in order to clarify the character of quadrupole deformation (prolate or oblate) of the state in its yrast sequence of levels.

Method: We have utilized a new combined technique of lifetime measurement at REX-ISOLDE and MINIBALL using the Doppler-shift attenuation method (DSAM) and a reorientation analysis of Coulomb-excitation yields.

Results: On the basis of the new lifetime of $\tau \left(2_1^{+}\right)=10.4_{-0.8}^{+2.2}$ ps the electric quadrupole moment was determined to be $Q\left(2_1^{+}\right)=-0.52\left(34\right)$ $e$b, indicating a predominant prolate deformation.

Conclusions: This finding is in agreement with beyond-mean-field calculations using the Gogny D1S force and with results from the Monte Carlo shell-model approach.

Neutrino-nucleus coherent elastic scattering provides a theoretically appealing way to measure the neutron part of nuclear form factors. Using an expansion of form factors into moments, we show that neutrinos from stopped pions can probe not only the second moment of the form factor (the neutron radius) but also the fourth moment. Using simple Monte Carlo techniques for argon, germanium, and xenon detectors of 3.5 tonnes, 1.5 tonnes, and 300 kg, respectively, we show that the neutron radii can be found with an uncertainty of a few percent when near a neutrino flux of $3\times {10}^7$ neutrinos/(cm${}^2$ s). If the normalization of the neutrino flux is known independently, one can determine the moments accurately enough to discriminate among the predictions of various nuclear energy functionals.

Recent data on the nuclear modification factor $R_{AA}$ of jet fragments in $2.76$ ATeV Pb+Pb collisions at the Large Hadron Collider (LHC) indicate that the jet-medium coupling in a quark-gluon plasma (QGP) is reduced at LHC energies and not compatible with the coupling deduced from data at the Relativistic Hadron Collider (RHIC). We estimate the reduction factor from a combined fit to the available data on $R_{AA}(\sqrt{s},p_T,b)$ and the elliptic flow $v_2(\sqrt{s},p_T,b)$ at $\sqrt{s}=0.2,2.76$ ATeV over a transverse momentum range $p_T\sim 10–100$ GeV and a broad impact parameter, $b$, range. We use a simple analytic “polytrope” model ($dE/dx=-\kappa E^a{x}^z{T}^c$) to investigate the dynamical jet-energy loss model dependence. Varying $a=0–1$ interpolates between weakly coupled and strongly coupled models of jet-energy dependence while $z=0–2$ covers a wide range of possible jet-path dependencies from elastic and radiative to holographic string mechanisms. Our fit to LHC data indicates an approximate 40$\%$ reduction of the coupling $\kappa$ from RHIC to LHC and excludes energy-loss models characterized by a jet-energy exponent with $a>1/3$. In particular, the rapid rise of $R_{AA}$ with $p_T\ge 10$ GeV combined with the slow variation of the asymptotic $v_2\left(p_T\right)$ at the LHC rules out popular exponential geometric optics models ($a=1$). The LHC data are compatible with $0\le a\le 1/3$ pQCD-like energy-loss models where the jet-medium coupling is reduced by approximately 10$\%$ between RHIC and LHC.

Background: Theoretical calculations of the four-body scattering above the four-body breakup threshold are technically very difficult owing to complicated singularities in the momentum space or boundary conditions in the coordinate space.

Purpose: We aim at calculating the neutron-${}^3$H scattering observables above the four-nucleon breakup threshold.

Methods: We employ Alt, Grassberger, and Sandhas (AGS) integral equations for the four-nucleon transition operators and solve them in the momentum-space framework using the complex-energy method. We significantly improve its accuracy and practical applicability by introducing the numerical integration method with the special weights.

Results: Using realistic nuclear interaction models we obtain fully converged results for the neutron-${}^3$H scattering. Elastic differential cross section and neutron analyzing power as well as the total cross section are calculated at 14.1, 18.0, and 22.1 MeV neutron energy.

Conclusions: Realistic four-nucleon scattering calculations above the four-nucleon breakup threshold are feasible. There is quite good agreement between the theoretical predictions and experimental data for the neutron-${}^3$H scattering in the considered energy regime.

We study the influence of a temperature-dependent shear viscosity over entropy density ratio $\eta /s$, different shear relaxation times ${\tau }_{\pi }$, as well as different initial conditions on the transverse momentum spectra of charged hadrons and identified particles. We investigate the azimuthal flow asymmetries as a function of both collision energy and centrality. The elliptic flow coefficient turns out to be dominated by the hadronic viscosity at RHIC energies. Only at higher collision energies the impact of the viscosity in the QGP phase is visible in the flow asymmetries. Nevertheless, the shear viscosity near the QCD transition region has the largest impact on the collective flow of the system. We also find that the centrality dependence of the elliptic flow is sensitive to the temperature dependence of $\eta /s$.

Following neutron knockout from an ${}^{13}$O beam, ${}^{12}$O fragments were created and the three decay products following two-proton decay were detected. A new ground-state mass was determined by the invariant mass method implying a decay kinetic energy of 1.638(24) MeV, and the width was found to be less than 72 keV. The latter is inconsistent with previous measurements with lower experimental resolutions but consistent with theoretical estimates. The isobaric analog of ${}^{12}$O in ${}^{12}$N was produced from proton knockout reactions with the same beam and decayed by two-proton emission to the isobaric analog state in ${}^{10}$B with a decay kinetic energy of 1.165(29) MeV. It represents only the second case of an analog state where two-proton decay is the only isospin- and energy-conserving particle decay mode. With our measurements of the mass excesses of ${}^{12}$O and its analog, the quadratic form of the isobaric multiplet mass equation was found to fit the $A=12$ quintet and any deviations are less than the magnitude found for the $A=8$ quintet and $A=7$ and 9 quartets.