Featured Researches

Nuclear Theory

Analysis of the 3 He(α,γ ) 7 Be and 3 H(α,γ ) 7 Li astrophysical direct capture reactions in a modified potential-model approach

Astrophysical S factors and reaction rates of the direct radiative capture processes 3 He(α,γ ) 7 Be and 3 H(α,γ ) 7 Li , as well as the primordial abundance of the 7 Li element, are estimated in the framework of a modified two-body potential model. It is shown that suitable modification of phase-equivalent α − 3 He potentials in the d waves can improve the description of the astrophysical S factor for the direct 3 He(α,γ ) 7 Be radiative capture reaction at energies above 0.5 MeV. An estimated 7 Li/H abundance ratio of (4.89±0.18)× 10 −10 is in very good agreement with the recent measurement of (5.0±0.3)× 10 −10 by the LUNA collaboration.

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Nuclear Theory

Analytical Results for the Classical and Quantum Tsallis Hadron Transverse Momentum Spectra: the Zeroth Order Approximation and beyond

We derive the analytical expressions for the first and second order terms in the hadronic transverse momentum spectra obtained from the Tsallis normalized (Tsallis-1) statistics. We revisit the zeroth order quantum Tsallis distributions and obtain the corresponding analytical closed form expressions. It is observed that unlike the classical case, the analytical closed forms of the zeroth order quantum spectra do not resemble the phenomenological distributions used in the literature after q→ q −1 substitution, where q is the Tsallis entropic parameter. However, the factorization approximation increases the extent of similarity.

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Nuclear Theory

Angular Momentum of Fission Fragments from Microscopic Theory

During nuclear fission, a heavy nucleus splits into two rotating fragments. The associated angular momentum is large, yet the mechanism of its generation and its dependence on the mass of fragments remain poorly understood. In this Letter, we provide the first microscopic calculations of angular momentum distributions in fission fragments for a wide range of fragment masses. For the benchmark case of 239 Pu( n th ,f), we find that the angular momentum of the fragments is largely determined by the nuclear shell structure and deformation, and that the heavy fragments therefore typically carry less angular momentum than their light partners. We use the fission model FREYA to simulate the emission of neutrons and photons from the fragments. The dependence of the angular momenta on fragment mass after the emission of neutrons and statistical photons is linear for the heavy fragments and either constant or weakly linear for the light fragments, consistent with the universal sawtooth pattern suggested by recent experimental data. Finally, we observe that using microscopic angular momentum distributions modifies the number of emitted photons significantly.

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Nuclear Theory

Anisotropic fluid dynamical simulations of heavy-ion collisions

We present VAH, a (3+1)-dimensional simulation that evolves the far-from-equilibrium quark-gluon plasma produced in ultrarelativistic heavy-ion collisions with anisotropic fluid dynamics. We solve the hydrodynamic equations on an Eulerian grid using the Kurganov-Tadmor algorithm in combination with a new adaptive Runge-Kutta method. Our numerical scheme allows us to start the simulation soon after the nuclear collision, largely avoiding the need to integrate it with a separate pre-equilibrium dynamics module. We test the code's performance by simulating on the Eulerian grid conformal and non-conformal Bjorken flow as well as conformal Gubser flow, whose (0+1)-dimensional solutions are precisely known. Finally, we compare non-conformal anisotropic hydrodynamics to second-order viscous hydrodynamics in central Pb+Pb collisions and find that the former's longitudinal flow profile responds more consistently to the fluid's gradients along the spacetime rapidity direction.

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Nuclear Theory

Antikaon-induced K 1 (1270 ) + meson production on nuclei near threshold

We study the inclusive strange axial-vector meson K 1 (1270 ) + production in K ??A reactions at near-threshold laboratory incident antikaon momenta within a nuclear spectral function approach, which describes incoherent direct K 1 (1270 ) + meson production in K ??meson--proton K ??p??K 1 (1270 ) + ? ??production processes and accounts for three different options for its in-medium mass shift (or for its effective scalar potential) at central density ? 0 . We calculate the absolute differential and total cross sections for the production of K 1 (1270 ) + mesons on 12 C and 184 W target nuclei at laboratory angles of 0 ??--45 ??by K ??mesons with momenta of 2.5, 2.8 and 3.5 GeV/c, which are close to the threshold momentum ( ??2.95 GeV/c) for K 1 (1270 ) + meson production off the free target proton at rest. The intrinsic properties of carbon and tungsten target nuclei have been described in terms of their spectral functions, which take into account the momenta of target protons and the energies of their separation from the considered nuclei. We show that the differential and total (absolute and relative) K 1 (1270 ) + antikaon-induced production cross sections at initial momenta not far from threshold -- at momenta ??2.8--3.5 GeV/c, at which there is yet no strong drop in their strength, reveal a distinct sensitivity to changes in the in-medium shift of the K 1 (1270 ) + mass, studied in the paper, both in the K 1 (1270 ) + meson low-momentum (0.1--1.0 GeV/c) and in its full-momentum ranges. This would permit evaluating this shift. Experimental data necessary for this aim can be obtained in a dedicated experiment at the J-PARC Hadron Experimental Facility.

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Nuclear Theory

Application of artificial intelligence in the determination of impact parameter in heavy-ion collisions at intermediate energies

The impact parameter is one of the crucial physical quantities of heavy-ion collisions (HICs), and can affect obviously many observables at the final state, such as the multifragmentation and the collective flow. Usually, it cannot be measured directly in experiments but might be inferred from observables at the final state. Artificial intelligence has had great success in learning complex representations of data, which enables novel modeling and data processing approaches in physical sciences. In this article, we employ two of commonly used algorithms in the field of artificial intelligence, the Convolutional Neural Networks (CNN) and Light Gradient Boosting Machine (LightGBM), to improve the accuracy of determining impact parameter by analyzing the proton spectra in transverse momentum and rapidity on the event-by-event basis. Au+Au collisions with the impact parameter of 0 ≤ b ≤ 10 fm at intermediate energies ( E lab = 0.2 - 1.0 GeV / nucleon) are simulated with the ultrarelativistic quantum molecular dynamics (UrQMD) model to generate the proton spectra data. It is found that the average difference between the true impact parameter and the estimated one can be smaller than 0.1 fm. The LightGBM algorithm shows an improved performance with respect to the CNN on the task in this work. By using the LightGBM's visualization algorithm, one can obtain the important feature map of the distribution of transverse momentum and rapidity, which may be helpful in inferring the impact parameter or centrality in heavy-ion experiments.

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Nuclear Theory

Applications of the chiral potential with the semi-local regularization in momentum space to the disintegration processes

We apply the chiral potential with the momentum space semi-local regularization to the 2 H and 3 He photodisintegration processes and to the (anti)neutrino induced deuteron breakup reactions. Specifically, the differential cross section, the photon analyzing power and the final proton polarization have been calculated for the deuteron photodisintegration at the photon energies 30 MeV and 100 MeV. For the 3 He photodisintegration predictions for the semi-inclusive and exclusive differential cross sections are presented for the photon energies up to 120 MeV. The total cross section is calculated for the (anti)neutrino disintegrations of the deuteron for the (anti)neutrino energies below 200 MeV. The predictions based on the Argonne V18 potential or on the older chiral force with regularization applied in coordinate space are used for comparison. Using the fifth order chiral nucleon-nucleon potential supplemented with dominant contributions from the sixth order allows us to obtain converged predictions for the regarded reactions and observables. Our results based on the newest semi-local chiral potentials show even smaller cutoff dependence for the considered electroweak observables than the previously reported ones with a coordinate-space regulator. However, some of the studied polarization observables in the deuteron photodisintegration process reveal more sensitivity to the regulator value than the unpolarized cross section. The chiral potential regularized semi-locally in momentum space yields also fast convergence of results with the chiral order. These features make the used potential a high quality tool to study electroweak processes.

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Nuclear Theory

Astrophysical Constraints on the Symmetry Energy and the Neutron Skin of 208 Pb with Minimal Modeling Assumptions

The symmetry energy and its density dependence are crucial inputs for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. Recently, PREX-II reported a value of 0.29±0.07 fm for the neutron-skin thickness of 208 Pb, implying a slope parameter L=110±37 MeV, larger than most ranges obtained from microscopic calculations and other nuclear experiments. We use a nonparametric equation of state representation based on Gaussian processes to constrain the symmetry energy S 0 , L , and R 208 Pb skin directly from observations of neutron stars with minimal modeling assumptions. The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER clearly favor smaller values of the neutron skin and L , as well as negative symmetry incompressibilities. Combining astrophysical data with PREX-II and chiral effective field theory constraints yields S 0 = 34 +3 ?? MeV, L= 58 +19 ??9 MeV, and R 208 Pb skin = 0.19 +0.03 ??.04 fm.

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Nuclear Theory

Asymptotic normalization coefficient method for two-proton radiative capture

The method of asymptotic normalization coefficients is a standard approach for studies of two-body non-resonant radiative capture processes in nuclear astrophysics. This method suggests a fully analytical description of the radiative capture cross section in the low-energy region of the astrophysical interest. We demonstrate how this method can be generalized to the case of three-body 2p radiative captures. It was found that an essential feature of this process is the highly correlated nature of the capture. This reflects the complexity of three-body Coulomb continuum problem. Radiative capture 15 O+ p + p → 17 Ne+ γ is considered as an illustration.

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Nuclear Theory

Baryon stopping as a relativistic Markov process in phase space

We reconsider baryon stopping in relativistic heavy-ion collisions in a nonequilibrium-statistical framework. The approach combines earlier formulations based on quantum chromodynamics with a relativistic diffusion model through a suitably derived fluctuation-dissipation relation, thus allowing for a fully time-dependent theory that is consistent with QCD. We use an existing framework for relativistic stochastic processes in spacetime that are Markovian in phase space, and adapt it to derive a Fokker-Planck equation in rapidity space, which is solved numerically. The time evolution of the net-proton distribution function in rapidity space agrees with stopping data from the CERN Super Proton Synchrotron and the BNL Relativistic Heavy Ion Collider.

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