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In light of the recent Jefferson Laboratory (JLab) data for the nuclear 12 C (e , e ′ p) transparencies, calculations, obtained in a relativistic multiple scattering Glauber approximation, are discussed. The shell-separated 12 C transparencies are shown and it is concluded that the p-shell nucleons are 75% more transparent than the s-shell ones. The presented comparisons between the calculations made here and the current 12 C (e , e ′ p) data show no clear indication for the onset of color transparency when implemented within the color diffusion model with standard parameters. [ABSTRACT FROM AUTHOR]

We apply variations and extensions of structural balance theory to analyze the dynamics of geopolitical relations using data from the virtual world Eve Online. The highly detailed data enable us to study the interplay of alliance size, power, and geographic proximity on the prevalence and conditional behavior of triads built from empirical political alliances. Through our analysis, we reveal the degree to which the behaviors of players conform to the predictions of structural balance theory and whether our augmentations of the theory improve these predictions. In addition to studying the time series of the proportions of triad types, we investigate the conditional changes in triad types and the formation of polarized political coalitions. We find that player behavior largely conforms to the predictions of a multipolar version of structural balance theory that separates strong and weak configurations of balanced and frustrated triads. The high degree of explanatory power of structural balance theory in this context provides strong support for both the theory and the use of virtual worlds in social science research. [ABSTRACT FROM AUTHOR]

Pseudoscalar-meson photoproduction is characterized by four complex reaction amplitudes. A complete set is a minimum theoretical set of observables that allow to determine these amplitudes unambiguously. It is studied whether complete sets remain complete when experimental uncertainty is involved. To this end, data from the GRAAL Collaboration and simulated data from a realistic model, both for the γp → K+Λ reaction, are analyzed in the transversity representation of the reaction amplitudes. It is found that only the moduli of the transversity amplitudes can be determined without ambiguity but not the relative phases.

We sketch an approximate method to quantify the number of correlated pairs in any nucleus A. It is based on counting independent-particle model (IPM) nucleon-nucleon pairs in a relative S-state with no radial excitation. We show that IPM pairs with those quantum numbers are most prone to short-range correlations and are at the origin of the high-momentum tail of the nuclear momentum distributions. Our method allows to compute the a2 ratios extracted from inclusive electron scattering. Furthermore, our results reproduce the observed linear correlation between the number of correlated pairs and the magnitude of the EMC effect. We show that the width of the pair center-ofmass distribution in exclusive two-nucleon knockout yields information on the quantum numbers of the pairs.

Games involving virtual worlds are popular in several segments of the population and societies. The online environment facilitates that players from different countries interact in a common virtual world. Virtual worlds involving social and economic interactions are particularly useful to test social and economic theories. Using data from EVE Online, a massive online multi-player game simulating a fantasy galaxy, we analyse the relation between the real-world context in which players live and their in-game behaviour at the country level. We find that in-game aggressiveness to non-player characters is positively related to real-world levels of aggressiveness as measured by the Global Peace Index and the Global Terrorist Index at the country level. The opposite is true for in-game aggressiveness towards other players, which seems to work as a safety valve for real-world player aggressiveness. The ability to make in-game friends is also positively related to real-world levels of aggressiveness in much the same way. In-game trading behaviour is dependent on the macro-economic environment where players live. The unemployment rate and exchange rate make players trade more efficiently and cautiously in-game. Overall, we find evidence that the real-world environment affects in-game behaviour, suggesting that virtual worlds can be used to experiment and test social and economic theories, and to infer real-world behaviour at the country level. [ABSTRACT FROM AUTHOR]

According to hedonic pricing theory (HPT) market forces operate on individual characteristics of a good, and the price of a product is the aggregate of the price across those characteristics. The relationship between price and characteristics remains poorly understood because characteristic qualities are hard to quantify, people have varying levels of information about characteristics, and people have heterogeneous preferences over characteristics. By analyzing data from a large, market-driven virtual world we are able to test HPT, while largely avoiding these pitfalls. We find that a linear model with functional characteristics predicts the prices poorly, but a log-linear model performs quite well. Adding social characteristics to this log-linear model improves the predictions substantially. This work strongly supports HPT and demonstrates a “rational” calculus including social value. [ABSTRACT FROM AUTHOR]

Triadic relationships are accepted to play a key role in the dynamics of social and political networks. Building on insights gleaned from balance theory in social network studies and from Boltzmann-Gibbs statistical physics, we propose a model to quantitatively capture the dynamics of the four types of triadic relationships in a network. Central to our model are the triads’ incidence rates and the idea that those can be modeled by assigning a specific triadic energy to each type of triadic relation. We emphasize the role of the degeneracy of the different triads and how it impacts the degree of frustration in the political network. In order to account for a persistent form of disorder in the formation of the triadic relationships, we introduce the systemic variable temperature. In order to learn about the dynamics and motives, we propose a generic Hamiltonian with three terms to model the triadic energies. One term is connected with a three-body interaction that captures balance theory. The other terms take into account the impact of heterogeneity and of negative edges in the triads. The validity of our model is tested on four datasets including the time series of triadic relationships for the standings between two classes of alliances in a massively multiplayer online game (MMOG). We also analyze real-world data for the relationships between the “agents” involved in the Syrian civil war, and in the relations between countries during the Cold War era. We find emerging properties in the triadic relationships in a political network, for example reflecting itself in a persistent hierarchy between the four triadic energies, and in the consistency of the extracted parameters from comparing the model Hamiltonian to the data. [ABSTRACT FROM AUTHOR]

Background: In Π+n and n-p electroproduction, conventional models cannot satisfactorily explain the data above the resonance region, in particular the transverse cross section. Although no high-energy L-T-separated cross-section data are available to date, a similar scenario can be inferred for K+Κ electroproduction. Purpose: Develop a phenomenological model for the p(γ*,K+)Κ reaction at forward angles and high-energies. Propose a universal framework for interpreting charged-kaon and charged-pion electroproduction above the resonance region. Method: Guided by the recent model for charged-pion electroproduction, developed by the authors, a framework for K+Κ electroproduction at high energies and forward angles is constructed. To this end, a Reggeized background model for K+Κ photoproduction is first developed. This model is used as a starting base to set up an electroproduction framework. Results: The few available data of the unseparated p(γ*,K+)Κ cross section are well explained by the model. Predictions for the L-T-separation experiment planned with the 12 GeV upgrade at Jefferson Lab are given. The newly proposed framework predicts an increased magnitude for the transverse structure function, similar to the situation in charged-pion electroproduction. Conclusions: Within a hadronic framework featuring Reggeized background amplitudes, s-channel resonanceparton effects can explain the observed magnitude of the unseparated p{γ*,K+)Κ cross section at high energies and forward angles. Thereby, no hardening of the kaon electromagnetic form factor is required. [ABSTRACT FROM AUTHOR]

We study the properties of the two-dimensional Fermi polaron model in which an impurity attractively interacts with a Fermi sea of particles in the zero-range limit. We use a diagrammatic Monte Carlo (DiagMC) method which allows us to sample a Feynman diagrammatic series to very high order. The convergence properties of the series and the role of multiple particle-hole excitations are discussed. We study the polaron and molecule energy as a function of the coupling strength, revealing a transition from a polaron to a molecule in the ground state. We find a value for the critical interaction strength which complies with the experimentally measured one and predictions from variational methods. For all considered interaction strengths, the polaron Z factor from the full diagrammatic series almost coincides with the one-particle-hole result. We also formally link the DiagMC and the variational approaches for the polaron problem at hand. [ABSTRACT FROM AUTHOR]

Background: Above the nucleon resonance region, the N(e,e'π±)N' data cannot be explained by conventional hadronic models. For example, the observed magnitude of the transverse cross section is significantly underestimated in a framework with Reggeized background amplitudes. Purpose: Develop a phenomenological framework for the N(e,e'π±)N' reaction at high invariant mass W and deep photon virtuality Q². Method: Building on the work of Kaskulov and Mosel [Phys. Rev. C 81, 045202 (2010)], a gauged pion-exchange current is introduced with a running cutoff energy for the proton electromagnetic transition form factor. A new transition form factor is proposed. It respects the correct on-shell limit, has a simple physical interpretation, and reduces the number of free parameters by one. Results: A study of the W dependence of the N(e,e'π±)N' lends support for the newly proposed transition form factor. In addition, an improved description of the separated and unseparated cross sections at -t ≤ 0.5GeV² is obtained. The predictions overshoot the measured unseparated cross sections for -t > 0.5GeV². Introducing a strong hadronic form factor in the Reggeized background amplitudes brings the calculations considerably closer to the high -t data. Conclusions: Hadronic models corrected for resonance-parton duality describe the separated pion electroproduction cross sections above the resonance region reasonably well at low -t. In order to validate the applicability of these models at high -t, separated cross sections are needed. These are expected to provide a more profound insight into the relevant reaction mechanisms. [ABSTRACT FROM AUTHOR]

Background: A complete set is a minimum set of observables which allows one to determine the underlying reaction amplitudes unambiguously. Pseudoscalar-meson photoproduction from the nucléon is characterized by four such amplitudes and complete sets involve single- and double-polarization observables. Purpose: Identify complete sets of observables and study how measurements with finite error bars impact their potential to determine the reaction amplitudes unambiguously. Method: The authors provide arguments to employ the transversity representation in order to determine the amplitudes in pseudoscalar-meson photoproduction. It is studied whether the amplitudes in the transversity basis for the γp → K+Λ reaction can be estimated without ambiguity. To this end, data from the GRAAL collaboration and simulations from a realistic model are analyzed. Results: It is illustrated that the moduli of normalized transversity amplitudes can be determined from precise single-polarization data. Starting from simulations with achievable experimental resolution, it is quite likely to obtain imaginary solutions for the relative phases of the amplitudes. Also the real solutions face a discrete phase ambiguity which makes it impossible to obtain a statistically significant solution for the relative phases at realistic experimental conditions. Conclusions: Single polarization observables are effective in determining the moduli of the amplitudes in a transversity basis. Determining the relative phases of the amplitudes from double-polarization observables is far less evident. The availability of a complete set of observables does not allow one to unambiguously determine the reaction amplitudes with statistical significance. [ABSTRACT FROM AUTHOR]

We report a study of a spin-down impurity strongly coupled to a spin-up Fermi sea (a so-called Fermi polaron) with the diagrammatic Monte Carlo (DiagMC) technique. Conditions of zero temperature and three dimensions are considered for an ultracold atomic gas with resonant interactions in the zero-range limit. A Feynman diagrammatic series is developed for the one-body and two-body propagators providing information about the polaron and molecule channel, respectively. The DiagMC technique allows us to reach diagram orders that are high enough for extrapolation to infinite order. The robustness of the extracted results is examined by checking various resummation techniques and by running the simulations with various choices for the propagators and vertex functions. It turns out that dressing the lines in the diagrams as much as possible is not always the optimal choice. We also identify classes of dominant diagrams for the one-body and two-body self-energy in the region of strong interaction. These dominant diagrams turn out to be the leading processes of the strong-coupling limit. The quasiparticle energies and Z factor are obtained as a function of the interaction strength. We find that the DiagMC results for the molecule and polaron properties are very similar to those obtained with a variational ansatz. Surprisingly, this variational ansatz gives very good predictions for the quasiparticle residue even when this residue is significantly less than 1. [ABSTRACT FROM AUTHOR]

Background: Short-range correlations (SRC) are an important ingredient of the dynamics of nuclei. Purpose: An approximate method to quantify the magnitude of the two-nucleon (2N) and three-nucleon (3N) short-range correlations and their mass dependence is proposed. Method: The proposed method relies on the concept of the "universality" or "local nuclear character" of the SRC. We quantify the SRC by computing the number of independent-particle model (IPM) nucléon pairs and triples which reveal beyond-mean-field behavior. It is argued that those can be identified by counting the number of nucléon pairs and triples in a zero relative orbital momentum state. A method to determine the quantum numbers of pairs and triples in an arbitrary mean-field basis is outlined. Results: The mass dependence of the 2N and 3N SRC is studied. The predictions are compared to measurements. This includes the ratio of the inclusive inelastic electron scattering cross sections of nuclei to ²H and ³He at large values of the Bjorken variable. Corrections stemming from the center-of-mass motion of the pairs are estimated. Conclusions: We find that the relative probability per nucléon for 2N and 3N SRC has a soft dependence with mass number A and that the proton-neutron 2N SRC outnumber the proton-proton (neutron-neutron) 2N SRC. A linear relationship between the magnitude of the EMC effect and the predicted number of proton-neutron SRC pairs is observed. This provides support for the role of local nuclear dynamics on the EMC effect. [ABSTRACT FROM AUTHOR]

We address the issue of unbiased model selection and propose a methodology based on Bayesian inference to extract physical information from kaon photoproduction γ p → K+Λ data. We use the single-channel Regge-plus-resonance (RPR) framework for γ p → K+Λ to illustrate the proposed strategy. The Bayesian evidence Z is a quantitative measure for the model's fitness given the world's data. We present a numerical method for performing the multidimensional integrals in the expression for the Bayesian evidence. We use the γ p → K+Λ data with an invariant energy W > 2.6 GeV in order to constrain the background contributions in the RPR framework with Bayesian inference. Next, the resonance information is extracted from the analysis of differential cross sections and single- and double-polarization observables. This background and resonance content constitutes the basis of a model, which is coined RPR-2011. It is shown that RPR-2011 yields a comprehensive account of the kaon photoproduction data and provides reasonable predictions for e + p → e' + K+ + Λ observables. [ABSTRACT FROM AUTHOR]

We address the issue of whether quasifree single-nucleon knockout measurements carry sufficient information about the nuclear interior. To this end, we present comparisons of the reaction probability densities for A(e,e′p) and A(p,2p) in quasifree kinematics for the target nuclei 4He, 12C, 56Fe, and 208Pb. We adopt a comprehensive framework based on the impulse approximation and on a relativized extension of Glauber multiple-scattering reaction theory in which the medium effects related to short-range correlations (SRCs) are implemented. It is demonstrated that SRCs weaken the effect of attenuation. For light target nuclei, both the quasifree (p,2p) and (e,e′p) can probe average densities of the same order as nuclear saturation density ρ0. For heavy nuclei such as 208Pb, the probed average densities are smaller than 0.1ρ0 and the (e,e′p) reaction is far more efficient in probing the bulk regions than (p,2p). [ABSTRACT FROM AUTHOR]

Background: High-momentum nucleons in a nuclear environment can be associated with short-range correlations (SRC) that primarily occur between nucleon pairs. Observations and theoretical developments have indicated that the SRC properties can be captured by general quantitative principles that are subject to model dependence upon quantification. The variations in the aggregated effect of SRC across nuclei, however, can be quantified in an approximately model-independent fashion in terms of the so-called SRC scaling factors that capture the aggregated effect of SRC for a specific nucleus A relative to the deuteron (A-to-d). Purpose: We aim to provide predictions for the SRC scaling factors across the nuclear periodic table and determine the relative contribution of the different nucleon pair combinations to this quantity. We will also determine the SRC scaling factors for both bound protons and bound neutrons and study how these quantities evolve with the neutron-to-proton (N/Z) ratio in asymmetric nuclei. Methods: We employ the low-order correlation operator approximation (LCA) to compute the SRC contribution to the single-nucleon momentum distribution and ratios of A-to-d momentum distributions. We do this for a sample of fifteen nuclei from He to Pb, thereby gaining access to the evolution of the SRC scaling factor with the nuclear mass 4≤A≤208 and the neutron-to-proton ratio 1.0≤N/Z≤1.54. Results: We provide evidence for approximate A-to-d scaling of the single-nucleon momentum distribution at nucleon momenta exceeding about 4fm-1. For the studied sample of fifteen nuclei, the total SRC scaling factor is in the range 4.05-5.14, of which roughly 3 can be attributed to proton-neutron (pn) correlations. The SRC scaling factors receive sizable contributions from pp and nn correlations. They depend on the (N/Z) ratio reflecting the fact that the minority species (protons) becomes increasingly more short-range correlated with increasing (N/Z). We compare the computed SRC scaling factors in the LCA with those of ab initio calculations and with measured quantities from SRC-sensitive inclusive electron-scattering data. Conclusions: It is shown that the LCA provides predictions for the SRC scaling factors across the nuclear table that are in line with measured values. In asymmetric nuclei there are sizable differences between the SRC scaling factors for protons and neutrons. It is suggested that this phenomenon may impact the variations of the magnitude of the European Muon Collaboration (EMC) effect across nuclei. Our results corroborate the finding that SRC physics can be qualitatively understood by universal principles that build on local modifications of mean-field wave functions of nucleon pairs. [ABSTRACT FROM AUTHOR]

We investigate the factorization properties of the exclusive electroinduced two-nucleon knockout reaction A(e,e' pN). A factorized expression for the cross section is derived and the conditions for factorization are studied. The A(e,e' pN) cross section is shown to be proportional to the conditional center-of-mass (c.m.) momentum distribution for close-proximity pairs in a state with zero relative orbital momentum and zero radial quantum number. The width of this conditional c.m. momentum distribution is larger than the one corresponding with the full c.m. momentum distribution. It is shown that the final-state interactions (FSIs) only moderately affect the shape of the factorization function for the A(e,e' pN) cross sections. Another prediction of the proposed factorization is that the mass dependence of the A(e,e' pp) [A(e,e' pn)] cross sections is much softer than z(z-1)/2 [NZ]. [ABSTRACT FROM AUTHOR]

We address the issue of consistent interactions for off-shell fermion fields of arbitrary spin. These interactions play a crucial role in the quantum hadrodynamical description of high-spin baryon resonances in hadronic processes. The Rarita-Schwinger (R-S) description of high-spin fermion fields involves unphysical degrees of freedom associated with their lower-spin content. These enter the interaction if not eliminated outright. The invariance condition of the interaction under the unconstrained R-S gauge removes the lower-spin content of the fermion propagator and leads to a consistent description of the interaction. We develop the most general consistent interaction structure for high-spin fermions. We find that the power of the momentum dependence of a consistent interaction rises with the spin of the fermion field. This leads to unphysical structures in the energy dependence of the computed tree-level cross sections when the short-distance physics is cut off with standard hadronic form factors. A spin-dependent hadronic form factor is proposed that suppresses the unphysical artifacts. [ABSTRACT FROM AUTHOR]

We present a method that offers perspectives to perform fully antisymmetrized simulations tor inhomogeneous bulk fermion matter. The technique bears resemblance to classical periodic boundary conditions, using localized single-particle states. Such localized states are an ideal tool to discuss phenomena where spatial localization plays an important role. The antisymmetrization is obtained introducing a doubly periodic structure in the many-body fermion wave functions. This results in circulant matrices for the evaluation of expectation values, leading to a computationally tractable formalism to study fully antisymmetrized bulk fermion matter. We show that the proposed technique is able to reproduce essential fermion features in an elegant and computationally advantageous manner. [ABSTRACT FROM AUTHOR]

We suggest that the number of correlated nucleon pairs in an arbitrary nucleus can be estimated by counting the number of proton-neutron, proton-proton, and neutron-neutron pairs residing in a relative S state. We present numerical calculations of those amounts for the nuclei 4He, 9Be, 12C, 27Al, 40Ca, 48Ca, 56Fe, 63Cu, 108Ag, and 197Au. The results are used to predict the values of the ratios of the per-nucleon electron-nucleus inelastic scattering cross section to the deuteron in the kinematic regime where correlations dominate. [ABSTRACT FROM AUTHOR]

Background: Exclusive two-nucleon knockout after electroexcitation of nuclei [A(e,e'NN) in brief] is considered to be a primary source of information about short-range correlations (SRCs) in nuclei. For a proper interpretation of the data, final-state interactions (FSIs) need to be theoretically controlled. Purpose: Our goal is to quantify the role of FSI effects in exclusive A(e,e'pN) reactions for four target nuclei representative of the whole mass region. Our focus is on processes that are SRC driven. We investigate the role of FSIs for two characteristic detector setups corresponding to "small" and "large" coverage of the available phase space. Method: Use is made of a factorized expression for the A(e,e'pN) cross section that is proportional to the two-body center-of-mass (c.m.) momentum distribution of close-proximity pairs. The A(e,e'pp) and A(e,e'pn) reactions for the target nuclei 12C,27Al,56Fe, and 208Pb are investigated. The elastic attenuation mechanisms in the FSIs are included using the relativistic multiple-scattering Glauber approximation (RMSGA). Single-charge exchange (SCX) reactions are also included. We introduce the nuclear transparency TApN, defined as the ratio of exclusive (e,e'pN) cross sections on nuclei to those on "free" nucleon pairs, as a measure for the aggregated effect of FSIs in pN knockout reactions from nucleus A. A toy model is introduced in order to gain a better understanding of the A dependence of TApN. Results: The transparency TApN drops from 0.2-0.3 for 12C to 0.04-0.07 for 208Pb. For all considered kinematics, the mass dependence of TApN can be captured by the power law TApN∝A-λ with 0.4≲λ≲0.5. Apart from an overall reduction factor, we find that FSIs only modestly affect the distinct features of SRC-driven A(e,e'pN) which are dictated by the c.m. distribution of close-proximity pairs. Conclusion: The SCX mechanisms represent a relatively small (order of a few percent) contribution of SRC-driven A(e,e'pN) processes. The mass dependence of FSI effects in exclusive A(e,e'pN) can be captured in a robust power law and is in agreement with the predictions obtained in a toy model. [ABSTRACT FROM AUTHOR]

A Bayesian analysis of the world's p(y, K+)A data is presented. From the proposed selection of 11 resonances, we find that the following nucleón resonances have the highest probability of contributing to the reaction: S11(1535), S11(1650), F15(1680), P13(1720), D13(1900), P13(1900), P11(1900), and F15 (2000). We adopt a Regge-plus-resonance framework featuring consistent couplings for nucleón resonances up to spin J = 5/2. We evaluate all possible combinations of 11 candidate resonances. The best model is selected from the 2048 model variants by calculating the Bayesian evidence values against the world's p{γ, K+)Λ data. [ABSTRACT FROM AUTHOR]

Abstract: We present a novel method to study the dynamics of bulk fermion systems such as the neutron star’s crust. By introducing periodic boundary conditions into Fermionic Molecular Dynamics, it becomes possible to examine the long-range many-body correlations induced by antisymmetrisation in bulk fermion systems. The presented technique treats the spins and the fermionic nature of the nucleons explicitly and permits investigating the dynamics of the system. Despite the increased complexity related to the periodic boundary conditions, the proposed formalism remains computationally feasible. [Copyright &y& Elsevier]

Although artificial neural networks have recently been proven to provide a promising new framework for constructing quantum many-body wave functions, the parametrization of a quantum wave function with non-abelian symmetries in terms of a Boltzmann machine inherently leads to biased results due to the basis dependence. We demonstrate that this problem can be overcome by sampling in the basis of irreducible representations instead of spins, for which the corresponding ansatz respects the non-abelian symmetries of the system. We apply our methodology to find the ground states of the one-dimensional antiferromagnetic Heisenberg (AFH) model with spin-1/2 and spin-1 degrees of freedom, and obtain a substantially higher accuracy than when using the s_{z} basis as an input to the neural network. The proposed ansatz can target excited states, which is illustrated by calculating the energy gap of the AFH model. We also generalize the framework to the case of anyonic spin chains.

A bayesian analysis of the world's p(γ,K^+)Λ data is presented. From the proposed selection of 11 resonances, we find that the following nucleon resonances have the highest probability of contributing to the reaction: S11(1535), S11(1650), F15(1680), P13(1720), D13(1900), P13(1900), P11(1900), and F15(2000). We adopt a Regge-plus-resonance framework featuring consistent couplings for nucleon resonances up to spin J=5/2. We evaluate all possible combinations of 11 candidate resonances. The best model is selected from the 2048 model variants by calculating the bayesian evidence values against the world's p(γ,K+)Λ data.