Nuclear Theory·nucl-th

Monday·August 29, 2016

10 papers—6 primary·4 cross-listed

01
Pion transition form factor through Dyson-Schwinger equations
Khépani Raya🇲🇽
In the framework of Dyson-Schwinger equations (DSE), we compute the $γ^{*} γ \to π^{0}$ transition form factor, $G (Q^{2})$ . For the first time, in a continuum approach to quantun chromodynamics (QCD), it was possible to compute $G (Q^{2})$ on the whole domain of space-like momenta. Our result agrees with CELLO, CLEO and Belle collaborations and, with the well-known asymptotic QCD limit, $2 f_{π}$ . Our analysis unifies this prediction with that of the pion's valence-quark parton distribution amplitude (PDA) and elastic electromagnetic form factor.
nucl-thhep-phJ.Phys.Conf.Ser.(2016)·0 citations
02
A Euclidean bridge to the relativistic constituent quark model
T. J. Hobbs🇺🇸 · Mary Alberg🇺🇸 · Gerald A. Miller🇺🇸
$Background$ Knowledge of nucleon structure is today ever more of a precision science, with heightened theoretical and experimental activity expected in coming years. At the same time, a persistent gap lingers between theoretical approaches grounded in Euclidean methods (e.g., lattice QCD, Dyson-Schwinger Equations [DSEs]) as opposed to traditional Minkowski field theories (such as light-front constituent quark models). $Purpose$ Seeking to bridge these complementary worldviews, we explore the potential of a Euclidean constituent quark model (ECQM). This formalism enables us to study the gluonic dressing of the quark-level axial-vector vertex, which we undertake as a test of the framework. $Method$ To access its indispensable elements with a minimum of inessential detail, we develop our ECQM using the simplified quark $+$ scalar diquark picture of the nucleon. We construct a hyperspherical formalism involving polynomial expansions of diquark propagators to marry our ECQM with the results of Bethe-Salpeter Equation (BSE) analyses, and constrain model parameters by fitting electromagnetic form factor data. $Results$ From this formalism, we define and compute a new quantity --- the Euclidean density function (EDF) --- an object that characterizes the nucleon's various charge distributions as functions of the quark's Euclidean momentum. Applying this technology and incorporating information from BSE analyses, we find the dressing effect on the proton's axial-singlet charge to be small in magnitude and consistent with zero. $Conclusions$ The scalar quark $+$ diquark ECQM is a step toward a realistic quark model in Euclidean space, and urges additional refinements. The small size we obtain for the impact of the dressed vertex on the axial-singlet charge suggests that models without this effect are on firm ground to neglect it.
nucl-thhep-phPRC(2017)·14 citations
03
Hadronic weak charges and parity-violating forward Compton scattering
Mikhail Gorchtein🇩🇪 · Hubert Spiesberger🇩🇪
Parity-violating elastic electron-nucleon scattering at low momentum transfer allows one to access the nucleon's weak charge, the vector coupling of the $Z$ -boson to the nucleon. In the Standard Model and at tree level, the weak charge of the proton is related to the weak mixing angle and accidentally suppressed, $Q_{W}^{p, tree} = 1 - 4 sin^{2} θ_{W} \approx 0.07$ . Modern experiments aim at extracting $Q_{W}^{p}$ at $\sim 1%$ accuracy. Similarly, parity non-conservation in atoms allows to access the weak charge of atomic nuclei. We consider a novel class of radiative corrections, an exchange of two photons with parity violation in the hadronic/nuclear system. These corrections may affect the extraction of $sin^{2} θ_{W}$ from the experimental data at the relevant level of precision because they are affected by long-range interactions similar to other parity-violating radiative corrections, such as, e.g., the $γ Z$ -exchange, which has obtained much attention recently. We show that the significance of this new correction increases with the beam energy in parity-violating electron scattering, but the general properties of the parity-violating forward Compton amplitude protect the formal definition of the weak charge as a limit at zero-momentum transfer and zero-energy. We also discuss the relevance of the new correction for upcoming experiments.
nucl-thhep-phnucl-exPRC(2016)·6 citations
04
The Algebraic Cluster Model: Structure of 16O
R. Bijker · F. Iachello
We discuss an algebraic treatment of four-body clusters which includes both continuous and discrete symmetries. In particular, tetrahedral configurations with T(d) symmetry are analyzed with respect to the energy spectrum, transition form factors and B(EL) values. It is concluded that the low-lying spectrum of 16O can be described by four alpha-particles at the vertices of a regular tetrahedron, not as a rigid structure but rather a more floppy structure with relatively large rotation-vibration interactions and Coriolis forces.
nucl-thnucl-exNPA(2017)·54 citations
05
Systematic study of actinide and pre-actinide fission modes
E. Andrade-II · G. S. Karapetyan · A. Deppman · J. L. Bernal-Castillo · A. R. Balabekyan · N. A. Demekhina · J. Adam · F. Garcia · F. Guzmán
In this work, we present new experimental data on mass distribution of fission fragments from $^{241}$ Am proton-induced fission at $660$ MeV measured at the LNR Phasotron (JINR). The systematic analysis of several measured fragment mass distributions from different fission reactions available in the literature is also presented. The proton-induced fission of $^{241}$ Am, $^{237}$ Np and $^{238}$ U at 26.5, 62.9 and 660 MeV was studied. The proton-induced fission of $^{232}$ Th was studied at 26.5, 62.9 and 190 MeV. The fission of $^{208}$ Pb also by a proton was investigated at 190, 500 and 1000 MeV. The fission of $^{197}$ Au was studied for 190 and 800 MeV protons. Bremsstrahlung reactions with maximum photon energies of 50 and 3500 MeV were studied for $^{232}$ Th and $^{238}$ U. The framework of the Random Neck Rupture Model was applied in the analysis. The roles of the neutron excess and of the so called fissility parameter were also investigated.
nucl-th0 citations
06
Gradient expansion for anisotropic hydrodynamics
Wojciech Florkowski🇵🇱 · Radoslaw Ryblewski🇵🇱 · Michał Spaliński🇵🇱
We compute the gradient expansion for anisotropic hydrodynamics. The results are compared with the corresponding expansion of the underlying kinetic-theory model with the collision term treated in the relaxation time approximation. We find that a recent formulation of anisotropic hydrodynamics based on an anisotropic matching principle yields the first three terms of the gradient expansion in agreement with those obtained for the kinetic theory. This gives further support for this particular hydrodynamic model as a good approximation of the kinetic-theory approach. We further find that the gradient expansion of anisotropic hydrodynamics is an asymptotic series, and the singularities of the analytic continuation of its Borel transform indicate the presence of non-hydrodynamic modes.
nucl-thhep-phPRD(2016)·43 citations
07
Probing the hardest branching of jets in heavy ion collisions
Yang-Ting Chien🇺🇸 · Ivan Vitev🇺🇸
We present the first calculation of the momentum sharing and angular separation distributions between the leading subjets inside a reconstructed jet in heavy ion collisions. These observables are directly sensitive to the hardest branching in the process of jet formation and are, therefore, ideal for studying the early stage of the in-medium parton shower evolution. The modification of the momentum sharing and angular separation distributions in lead-lead relative to proton-proton collisions is evaluated using the leading-order medium-induced splitting functions obtained in the framework of soft-collinear effective theory with Glauber gluon interactions. Qualitative and in most cases quantitative agreement between theory and preliminary CMS measurements suggests that the parton shower in heavy ion collisions can be dramatically modified early in the branching history. We propose a new measurement which will illuminate the angular distribution of the hardest branching within jets in heavy ion collisions.
↳ hep-phhep-exnucl-exnucl-thPRL(2017)·129 citations
08
Medium Induced Transverse Momentum Broadening in Hard Processes
A. H. Mueller🇺🇸 · Bin Wu🇺🇸 · Bo-Wen Xiao🇨🇳 · Feng Yuan🇺🇸
Using deep inelastic scattering on a large nucleus as an example, we consider the transverse momentum broadening of partons in hard processes in the presence of medium. We find that one can factorize the vacuum radiation contribution and medium related $P_{T}$ broadening effects into the Sudakov factor and medium dependent distributions, respectively. Our derivations can be generalized to other hard processes, such as dijet productions, which can be used as a probe to measure the medium $P_{T}$ broadening effects in heavy ion collisions when Sudakov effects are not overwhelming.
↳ hep-phnucl-thPRD(2017)·54 citations
09
Directed flow in heavy-ion collisions at NICA: what is interesting to measure?
L.V. Bravina🇳🇴 · E.E. Zabrodin🇳🇴
We study the formation of the directed flow of hadrons in nuclear collisions at energies between AGS and SPS in Monte Carlo cascade model. The slope of the proton flow at midrapidity tends to zero (softening) with increasing impact parameter of the collision. For very peripheral topologies this slope becomes negative (antiflow). The effect is caused by rescattering of hadrons in remnants of the colliding nuclei. Since the softening of the proton flow can be misinterpreted as indication of the presence of quark-gluon plasma, we propose several measurements at NICA facility which can help one to distinguish between the cases with and without the plasma formation.
↳ hep-phnucl-exnucl-thEPJA(2016)·7 citations
10
A lower limit on the heat capacity of the neutron star core
Andrew Cumming · Edward F. Brown · Farrukh J. Fattoyev · C. J. Horowitz · Dany Page · Sanjay Reddy
We show that observations of the core temperature of transiently-accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass X-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked (CFL) phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.
↳ astro-ph.HEnucl-thPRC(2017)·71 citations

Affiliations

first authorsco-authorsvia INSPIRE