New articles on Nuclear Theory

[1] 2106.09029

Polarization tensor of magnetized quark-gluon plasma at nonzero baryon density

We derive a general expression for the absorptive part of the one-loop photon polarization tensor in a strongly magnetized quark-gluon plasma at nonzero baryon chemical potential. To demonstrate the application of the main result in the context of heavy-ion collisions, we study the effect of a nonzero baryon chemical potential on the photon emission rate. The rate and the ellipticity of photon emission are studied numerically as a function the transverse momentum (energy) for several values of temperature and chemical potential. When the chemical potential is small compared to the temperature, the rates of the quark and antiquark splitting processes (i.e., $q\rightarrow q +\gamma$ and $\bar{q}\rightarrow \bar{q} +\gamma$, respectively) are approximately the same. However, the quark splitting gradually becomes the dominant process with increasing the chemical potential. We also find that increasing the chemical potential leads to a growing total photon production rate but has only a small effect on the ellipticity of photon emission. The quark-antiquark annihilation ($q+\bar{q}\rightarrow \gamma$) also contributes to the photon production, but its contribution remains relatively small for a wide range of temperatures and chemical potentials investigated.

[2] 2106.09088

Level Densities from 0-30 MeV

Photon strength, $f(E_{\gamma})$, measured in photonuclear reactions, is the product of the average level density per MeV, $\rho(E_x)$, and the average reduced level width, $\Gamma_{\gamma}/E_{\gamma}^3$ for levels populated primarily by E1 transitions at an excitation energy $E_x=E_{\gamma}$. It can be calculated with the Brink-Axel (BA) formulation modified to include contributions from the Giant Dipole Resonance (GDR) and higher lying resonances. Level densities and reduced widths have been calculated for 17 nuclei with atomic numbers between Z=14-92. Level densities below the GDR energy were calculated with the CT-JPI model and combined with the BA photon strength to determine the associated reduced widths. The reduced widths varied exponentially with level energy and could be extrapolated up to higher energies. The extrapolated widths were then combined with the BA photon strength to determine the level densities at higher energies. The level densities are found to increase exponentially at low energies, peak near the GDR energy due to the appearance of new states at the $2\hbar\omega$ shell closure, and continue to increase less rapidly up to at least 30 MeV. The average level densities have been compared with the Fermi Gas Level Density (FGLD), Back-Shifted Fermi Gas (BSFG), and Hartree-Fock-Bogoliubov (HFB) models. Good agreement is found with the nearly identical FGLD and BDFG models, while the HFB models gives substantially lower level densities. A universal set of FGLD model parameters were determined as a function of mass and temperature that are applicable to all nuclei.

[3] 2106.09124

Continuous evolution of electromagnetic field in heavy-ion collisions

In heavy-ion collisions the electromagnetic field exists before the hot nuclear matter emergence. Requiring the field continuity we compute it in the central rapidity region by taking into account the electromagnetic response of the Quark Gluon Plasma. We show that the electromagnetic field is nearly time-independent from about 1~fm/c after the collision until the freezeout.

[4] 2106.09152

Semi-microscopic construction of multi-$α$ cluster spaces

An approximate but straight forward projection method to molecular many alpha-particle states is proposed and the overlap to the shell model space is determined. The resulting space is in accordance with the shell model, but still contains states which are not completely symmetric under permutations of the alpha-particles, which is one reason to call the construction semi-microscopic. A new contribution is the construction of the 6- and 7-$\alpha$-particle spaces. The errors of the method propagate toward larger number of alpha-particles and larger shell excitations. In order to show the effectiveness of the construction proposed, the so obtained spaces are applied, within an algebraic cluster model, to $^{20}$Ne, $^{24}$Mg and $^{28}$Si, each treated as a many-alpha-particle system. Former results on $^{12}$C and $^{16}$O are resumed.

[5] 2106.09443

Global spin polarization of multistrange hyperons and feed-down effect in heavy-ion collisions

Global spin polarization of hyperons is an important observable to probe the vorticity of the quark-gluon plasma produced in heavy-ion collisions. We calculate the global polarizations of $\Lambda$, $\Xi^-$, and $\Omega^-$ in Au+Au collisions at energies $\sqrt{s_\text{NN}}=$ 7.7--200 GeV based on a multiphase transport model. Our calculations suggest that their primary global polarizations fulfill $P_{\Omega^-}\simeq 5/3P_{\Xi^-}\simeq 5/3P_\Lambda$. We also estimate the feed-down effect of particle decay on the global polarization. With the feed-down effect taken into account, the global polarizations of $\Lambda$ and $\Xi^-$ are clearly separated, and the final global polarizations are in the ordering: $P_{\Omega^-}>P_{\Xi^-}>P_\Lambda$. Such a relation can be tested in experiments, which will provide us more information about the global polarization mechanism.

[6] 2106.09484

Investigations of the linear and non-linear flow harmonics using the A Multi-Phase Transport model

The higher-order flow harmonics of the Fourier expansion for the azimuthal distributions of particles are anticipated to be produced by a non-linear response from the lower-order anisotropies, in addition to a linear response from the same-order anisotropies. Detailed study of these higher-order flow harmonics and their non-linear and linear components can be used to constrain the heavy-ion collisions' initial conditions and the system transport properties. The multiparticle azimuthal correlation technique is used within the A Multi-Phase Transport (AMPT) model framework to study the linear and non-linear response to the higher-order flow harmonics, the non-linear response coefficients, and the correlations between different order flow symmetry planes for Au--Au collisions at 200~GeV. The current study shows that the AMPT model can to a good degree describe the experimental measurements and also suggest that conducting detailed measurements over a broad range of system size and beam-energy can serve as an additional constraint for accurate $\eta / \textit{s}$ extraction.

[7] 2106.09031

An effective field theory of the Delta-resonance

We present an effective field theory of the $\Delta$-resonance as an interacting Weinberg's $(3/2,0)\oplus (0,3/2)$ field in the multi-spinor formalism. We derive its interactions with nucleons $N$, pions $\pi$ and photons $\gamma$, and compute the $\Delta$-resonance cross-sections in pion-nucleon scattering and pion photo-production. The theory contains only the physical spin-3/2 degrees of freedom. Thus, it is intrinsically consistent at the Hamiltonian level and, unlike the commonly used Rarita-Schwinger framework, does not require any additional ad hoc manipulation of couplings or propagators. The symmetries of hadronic physics select a unique operator for each coupling $N\pi\Delta$ and $\gamma\pi\Delta$. The proposed framework can be extended to also describe other higher-spin hadronic resonances.

[8] 2106.09058

Quantum Monte Carlo studies with microscopic two- and three-body interactions of a trimer scaling function

We present an energy scaling function to predict, in a specific range, the energy of bosonic trimers with large scattering lengths and finite range interactions, which is validated by quantum Monte Carlo calculations using microscopic Hamiltonians with two- and three-body potentials. The proposed scaling function depends on the scattering length, effective range, and a reference energy, which we chose as the trimer energy at unitarity. We obtained the scaling function as a limit cycle from the solution of the renormalized zero-range model with effective range corrections. We proposed a simple parameterization of the energy scaling function. Besides the intrinsic interest in theoretical and experimental investigations, this scaling function allows one to probe Efimov physics with only the trimer ground-states, which may open opportunities to identify Efimov trimers whenever access to excited states is limited.

[9] 2106.09080

Diquark properties from full QCD lattice simulations

We study diquarks on the lattice in the background of a static quark, in a gauge-invariant formalism with quark masses down to almost physical $m_\pi$. We determine mass differences between diquark channels as well as diquark-quark mass differences. The lightest and next-to-lightest diquarks have ''good'' scalar, $\bar{3}_F$, $\bar{3}_c$, $J^P=0^+$, and ''bad'' axial vector, $6_F$, $\bar{3}_c$, $J^P=1^+$, quantum numbers, and a bad-good mass difference for $ud$ flavors, $198(4)~\rm{MeV}$, in excellent agreement with phenomenological determinations. Quark-quark attraction is found only in the ''good'' diquark channel. We extract a corresponding diquark size of $\sim 0.6~\rm{fm}$ and perform a first exploration of the ''good'' diquark shape, which is shown to be spherical. Our results provide quantitative support for modeling the low-lying baryon spectrum using good light diquark effective degrees of freedom.

[10] 2106.09185

Dispersion relation analysis of the radiative corrections to $g_A$ in the neutron $β$-decay

We present the first and complete dispersion relation analysis of the inner radiative corrections to the axial coupling constant $g_A$ in the neutron $\beta$-decay. Using experimental inputs from the elastic form factors and the spin-dependent structure function $g_1$, we determine the contribution from the $\gamma W$-box diagram to a precision better than $10^{-4}$. Our calculation indicates that the inner radiative corrections to the Fermi and the Gamow-Teller matrix element in the neutron $\beta$-decay are almost identical, i.e. the ratio $\lambda=g_A/g_V$ is almost unrenormalized. With this result, we predict the bare axial coupling constant to be {$\mathring{g}_A=-1.2754(13)_\mathrm{exp}(2)_\mathrm{RC}$} based on the PDG average $\lambda=-1.2756(13)$

[11] 2106.09243

Search for the chiral magnetic effect via charge-dependent azimuthal correlations relative to spectator and participant planes in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen with a significance of 1--3 standard deviations in mid-central (intermediate impact parameter) collisions. Significant residual background effects may, however, still be present.

[12] 2106.09417

Inclusive Production of Heavy Quarkonia in pNRQCD

We develop a formalism for computing inclusive production cross sections of heavy quarkonia based on the nonrelativistic QCD and the potential nonrelativistic QCD effective field theories. Our formalism applies to strongly coupled quarkonia, which include excited charmonium and bottomonium states. Analogously to heavy quarkonium decay processes, we express nonrelativistic QCD long-distance matrix elements in terms of quarkonium wavefunctions at the origin and universal gluonic correlators. Our expressions for the long-distance matrix elements are valid up to corrections of order $1/N_c^2$. These expressions enhance the predictive power of the nonrelativistic effective field theory approach to inclusive production processes by reducing the number of nonperturbative unknowns, and make possible first-principle determinations of long-distance matrix elements once the gluonic correlators are known. Based on this formalism, we compute the production cross sections of $P$-wave charmonia and bottomonia at the LHC, and find good agreement with measurements.

[13] 2106.09515

A Neutron Star is born

A neutron star was first detected as a pulsar in 1967. It is one of the most mysterious compact objects in the universe, with a radius of the order of 10 km and masses that can reach two solar masses. In fact, neutron stars are star remnants, a kind of stellar zombies (they die, but do not disappear). In the last decades, astronomical observations yielded various contraints for the neutron star masses and finally, in 2017, a gravitational wave was detected (GW170817). Its source was identified as the merger of two neutron stars coming from NGC 4993, a galaxy 140 million light years away from us. The very same event was detected in $\gamma$-ray, x-ray, UV, IR, radio frequency and even in the optical region of the electromagnetic spectrum, starting the new era of multi-messenger astronomy. To understand and describe neutron stars, an appropriate equation of state that satisfies bulk nuclear matter properties is necessary. GW170817 detection contributed with extra constraints to determine it. On the other hand, magnetars are the same sort of compact objects, but bearing much stronger magnetic fields that can reach up to 10$^{15}$ G on the surface as compared with the usual 10$^{12}$ G present in ordinary pulsars. While the description of ordinary pulsars is not completely established, describing magnetars poses extra challenges. In this paper, I give an overview on the history of neutron stars and on the development of nuclear models and show how the description of the tiny world of the nuclear physics can help the understanding of the cosmos, especially of the neutron stars.