The European Physical Journal A最新文献

Several non-perturbative results for hot QCD are challenging some aspects of the phase diagram and its associated degrees of freedom which were previously believed to be well understood. With increasing temperature, the chiral crossover is followed by an intermediate region with an approximate chiral spin symmetry larger than chiral symmetry, in which pseudo-scalar mesons continue to exist as hadron-like excitations, before at some higher temperature the expected chiral symmetry is recovered. By testing general formal considerations against lattice data, it can be shown that thermally modified versions of stable vacuum particles, so-called thermoparticles, form the constituents of thermal quantum field theories, with properties quite different from what is expected perturbatively. This “viewpoint” aims to raise broader and, in particular, phenomenological interest in these directions.

In this Review article, a brief description of the stochastic mean-field (SMF) theory for describing reaction dynamics in low-energy heavy-ion collisions at bombarding energies in the vicinity of the Coulomb barrier is presented. In these collisions, as a result of strong Pauli blocking, binary nucleon collisions do not have a significant effect on the dissipation and fluctuations. At low energies, the mean-field fluctuations, due to initial correlations, have a dominant effect on fluctuations of macroscopic variables. The SMF theory proposes the determination of an ensemble of single-particle density matrices by specifying random initial fluctuations according to a distribution law. Employing an ensemble of single-particle density matrices, not only the mean values but also the distribution functions of the one-body observables can be determined. If the di-nuclear structure is maintained in heavy-ion collisions, such as deep inelastic collisions and fast quasi-fission reactions, a much simpler description of the reaction mechanism can be derived in terms of several macroscopic variables such as mass and charge asymmetry, and relative linear and relative angular momentum. In this case, by geometric projection of the SMF equations, it is possible to derive the quantal Langevin equations for macroscopic variables. As an application of quantal transport description, an analysis of multinucleon transfers and kinetic energy dissipation and fluctuations is presented for selected quasi-fission reactions.

The merger of a He white dwarf (WD) and a CO WD is the favored formation channel for R Coronae Borealis (RCB) stars. These stars exhibit (^{16}textrm{O}/^{18}textrm{O}) ratios that are orders of magnitude lower than the solar value. However, it is not fully understood whether such low (^{16}textrm{O}/^{18}textrm{O}) ratios can be achieved in WD merger remnants for the predicted lifetime of RCB stars of around (10^4,textrm{years}). In this work, we perform detailed nucleosynthesis calculations of a 3D magnetohydrodynamical simulation of a merger of a (0.3,M_odot ) He WD and a (0.6,M_odot ) CO WD for (4000,textrm{s}) at which point a steady state in temperature and density is reached. From this point, we follow several radial zones to study the long-term production of (^{18}textrm{O}) and its variability throughout the burning region. We find that the asymmetric merger process leaves an imprint on the distribution of the abundances at the end of our hydrodynamic simulation. During the long-term evolution up to (100,textrm{years}), we observe (^{16}textrm{O}/^{18}textrm{O}) ratios of order of unity, although the timescale on which (^{18}textrm{O}) is destroyed again is highly location dependent. Importantly, our calculations suggest that in the outer layers of the burning shell, the dominant production channel is (^{14}textrm{C}(alpha ,gamma )^{18}textrm{O}) instead of the commonly considered (^{14}textrm{N}(alpha ,gamma )^{18}textrm{F}(beta ^+)^{18}textrm{O}) reaction, whereby the former can be sustained for longer periods of time. Furthermore, these outer regions do not reach the conditions necessary for fast (alpha )-captures in (^{18}textrm{O}) to (^{22}textrm{Ne}), thus being favorable to maintaining a low (^{16}textrm{O}/^{18}textrm{O}) ratio.

This study delves into the deficient s-process nucleosynthesis of krypton (Kr) isotopes within a 15 solar mass star, aiming to elucidate the impact of metallicity and helium (He) mass fraction on the formation and distribution of these isotopes. Krypton, a rare element with 37 known isotopes ranging from ⁶⁹ to ¹⁰⁵Kr, includes several stable isotopes such as ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³Kr, ⁸⁴Kr, and ⁸⁶Kr, alongside the long-lived radioactive isotope ⁸⁵Kr, predominantly produced via the s-process. To investigate the synthesis of Kr isotopes from A = 80 to A = 87 within this stellar model, we employed an assessed reaction rate for the ⁸³Kr(n,γ)⁸⁴Kr reaction, computed using the TALYS v1.96 nuclear reaction code. This calculated reaction rate aligns well with data from the Reaclib database across a temperature range of 0.05 GK to 5 GK, ensuring reliability in our simulations. Employing the MESA stellar evolution simulator, we performed simulations across a spectrum of metallicities and helium mass fractions, including solar analogs. Our results were meticulously cross-referenced with existing observational and theoretical data, demonstrating a robust concordance. Given krypton’s scarcity in the cosmos, mapping the abundance of its isotopes under varying conditions of metallicity and helium mass fraction provides pivotal insights into their synthesis and cosmic dissemination. These findings hold significant implications for our understanding of heavy element nucleosynthesis and the chemical evolution of galaxies, illuminating the underlying processes that have shaped the universe’s elemental composition.

Accurate neutron capture cross sections are essential for constraining nuclear reaction models and for applications in reactor technology and astrophysical nucleosynthesis. Among potential reference isotopes, holmium-165 exhibits favorable nuclear characteristics but lacks high-precision experimental data in the resolved resonance region. In this work, the neutron capture yield of ¹⁶⁵Ho was measured using the 4(pi ) (hbox {BaF}_2) Gamma Total Absorption Facility (GTAF) at the Back-streaming White Neutron Beamline (Back-n) of the China Spallation Neutron Source (CSNS). Resonance parameters in the energy range from 1 to 1.0 keV were extracted through Bayesian R-matrix analyses performed with the code SAMMY. For 18 s-wave resonances below 100 eV, the resonance energy (E_R), neutron width (varGamma _n), and radiative width (varGamma _gamma ) were determined. The distribution of mean level spacings follows the Wigner–Dyson form with (langle D_0rangle ) = 4.53(3) eV, indicating chaotic compound-nucleus behavior, while the reduced neutron widths obey the Porter–Thomas (chi ^2) distribution with one degree of freedom, consistent with a single entrance channel. The mean radiative width for s-wave resonances was (langle varGamma _gamma rangle ) = 88.10(89) meV, and the s-wave neutron strength function was determined to be (10^{4}S_0) = 2.01(1), in excellent agreement with evaluated values in the Atlas of Neutron Resonances and the ENDF/B-VIII.0 library. These results provide an improved experimental foundation for neutron-capture modeling and for refining nuclear data evaluations involving odd-odd systems.

We investigate the transverse momentum distribution spectra of (phi ) meson production from proton–proton (p+p), proton–nucleus (p+A) and nucleus–nucleus (A+A) collisions at various Large Hadron Collider center-of-mass collision energies ranging from (sqrt{s_{NN}}) = 2.76–13 TeV for most central and peripheral collisions within a unified statistical thermal freeze-out model (USTFM). The comparison to experimental data on transverse momentum ((p_T)) spectra from the above mentioned collisions enhances the understanding of freeze-out conditions. The freeze-out characteristics, including the kinetic freeze-out temperature (T), transverse flow velocity parameter ((beta _T^0)), and velocity flow profile parameter (n), are extracted from various fits and analyzed within the context of thermal and statistical framework. We anchor the decoupling scale to statistical-hadronization systematics and fix the temperature at (T = 152.0 pm 4.0~textrm{MeV}) across centralities at midrapidity, while fitting the collective sector via the transverse flow magnitude (beta _{T}^0) and its profile index n. USTFM consistently couples longitudinal and transverse dynamics and contains the Blast-Wave (BW) limit, we perform a system- and energy-spanning analysis of (phi ) spectra and quantify the evolution of (beta _{T}^0) and n with system size and centrality. The collision energy ((sqrt{s_{NN}})) dependence of the freeze-out temperature is taken from the previous works where it is extracted by fitting the relative hadronic yields. The transverse flow velocity parameter decreases from the most central to peripheral collisions. The model calculation shows an excellent match with the experimental transverse momentum spectra of the (phi ) meson up to approximately 4 GeV/c. The proposed model accounts for the influence of both longitudinal and transverse hydrodynamic flow, highlighting their roles in shaping the observed spectra and particle yields.

We have analyzed molybdenum, ruthenium, and barium isotopes simultaneously in 21 individual high-density presolar graphite grains from the Murchison CM2 meteorite using the Chicago Instrument for Laser Ionization (CHILI). While all grains clearly suffered from contamination with molybdenum of solar composition, probably from the parent asteroid of the host meteorite, six grains showed s-process enrichments in at least one of the elements analyzed, pointing toward an AGB star origin of these grains. For one of the grains, we found almost pure s-process ruthenium, challenging existing AGB star models that assume full homogenization of freshly produced s-process material in the convective AGB star envelope prior to grain condensation. Furthermore, our results indicate that these grains condensed before about half of the ⁹⁹Tc decayed to ⁹⁹Ru and that technetium did not condense into the grains.

The Anomalous Viscous Fluid Dynamics (AVFD) framework is utilized to generate (^{197}_{79}textrm{Au}+{}^{197}_{79}textrm{Au}), (^{96}_{44}textrm{Ru}+{}^{96}_{44}textrm{Ru}), and (^{96}_{40}textrm{Zr}+{}^{96}_{40}textrm{Zr}) collision events at (sqrt{s_{textrm{NN}}}) = 200 GeV to investigate the Chiral Magnetic Effect (CME). The CME signal is modulated through the axial charge per entropy density ((n_5/s)) in each event to produce data sets with varying CME signal strengths. Additionally, a 33(%) local charge conservation (LCC) is implemented in each event. These data sets are analyzed using CME-sensitive two- and three-particle correlators. Furthermore, the Sliding Dumbbell Method (SDM) is employed to identify potential CME-like events within each data set. The identified events selected using the SDM exhibit characteristics consistent with CME driven charge separation. The CME fraction in these events is quantified while accounting for background contributions. It is found that a (33%) LCC contribution can mimic the CME signal in both (textrm{Au} + textrm{Au}) and isobar collisions.