Large Angular Momentum Research Articles

Nucleon localization function in rotating nuclei

Background: An electron localization function was originally introduced to visualize bond structures in molecules. It became a useful tool to describe electron configurations in atoms, molecules and solids. In nuclear physics, a nucleon localization function (NLF) has been used to characterize clusters in light nuclei, fragment formation in fission and pasta phases in the inner crust of neutron stars. Purpose: We use the NLF to study the nuclear response to fast rotation. Methods: We generalize the NLF to the case of nuclear rotation. The extended expressions involve both time-even and time-odd local densities. Since current density and density gradient contribute to the NLF primarily at the surface, we propose a simpler spatial measure given by the kinetic-energy density. Illustrative calculations for the superdeformed yrast band of $^{152}$Dy were carried out by using the cranked Skyrme-Hartree-Fock method. We also employed the cranked harmonic-oscillator model to gain insights into patterns revealed by the NLF at high angular momentum. Results: In a deformed rotating nucleus, several NLFs can be introduced, depending on the definition of the spin-quantization axis and self-consistent symmetries of the system. The oscillating pattern of the NLF can be explained by a constructive interference between the kinetic-energy and particle densities. The nodal pattern seen in the NLF along the major axis of a rotating nucleus comes from single-particle orbits with large aligned angular momentum. The variation of the NLF along the minor axis is traced back to deformation-aligned orbits. Conclusions: The NLF allows a simple interpretation of the shell structure evolution in the rotating nucleus in terms of the angular-momentum alignment of individual nucleons. We expect that the NLF will be useful for the characterization of other collective modes and time-dependent processes.

Parametrically narrow tetraquark states containing one heavy quark and one heavy antiquark

This paper focuses on tetraquarks containing one heavy quark and one heavy antiquark in the formal limit where the heavy quark masses go to infinity. It extends the theoretical framework developed for tetraquark containing two heavy quarks based on semi-classical analysis and the Born-Oppenheimer approximation. It is shown that for sufficiently small light quark masses, parametrically narrow $q\bar{q}'Q\bar{Q}$ tetraquarks must exist in the formal limit of arbitrarily large heavy quark masses. The analysis is model-independent; it requires no assumptions about the structure of the state or the mechanism of its decay beyond what can be deduced directly from QCD. The formal analysis here is only directly applicable to tetraquarks with large angular momentum and hence does not directly apply to experimentally observed tetraquark candidates. We also discuss the condition needed for a version of the analysis to apply for low angular momenta states. Whether this condition holds can be explored via lattice calculations. Current lattice calculations suggest that the condition is not met. If these preliminary calculations are confirmed, there are strong implications for the nature of the observed tetraquark resonances containing $\bar{c}c$ or $\bar{b}b$ pairs. It raises the possibility that the observed tetraquarks exist and are narrow not because the charm and bottom quarks are extremely heavy, but because they are sufficiently light.

Generation of Intense Vortex Gamma Rays via Spin-to-Orbital Conversion of Angular Momentum in Relativistic Laser-Plasma Interactions

Vortex $\ensuremath{\gamma}$-ray beams with large orbital angular momentum (OAM) have significant potential applications in material science, nuclear detection, and astrophysics. An all-optical scheme for production of intense vortex $\ensuremath{\gamma}$-ray beams using relativistic laser-plasma interactions is proposed, where the OAM of $\ensuremath{\gamma}$-photons is converted from the spin angular momentum of circularly polarized (CP) lasers. The spin-to-orbital angular momentum conversion is achieved by two processes: resonant acceleration of electrons in CP laser-plasma interactions and nonlinear Compton scattering (NCS) of another linearly polarized laser from the former. Three-dimensional particle-in-cell simulations show that vortex $\ensuremath{\gamma}$-ray beams with a large and controllable OAM of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{18}\ensuremath{\hbar}$ and energy up to hundreds of $\mathrm{MeV}$ can be obtained by using two lasers at intensities of approximately ${10}^{22}\phantom{\rule{0.2em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$, which is much more practical and more efficient than previously proposed NCS of a Laguerre-Gaussian beam.

Crystal electric field excitations in the quantum spin liquid candidate NaErS2

The delafossite family of compounds with a triangular lattice of rare earth ions has been recently proposed as a candidate host for quantum spin liquid (QSL) states. To realize QSLs, the crystal-electric-field (CEF) ground state of the rare earth ions should be composed of a doublet that allows sizable quantum tunneling, but till now the knowledge on CEF states in the delafossite compounds is still limited. Here we employ inelastic neutron scattering (INS) to study the CEF transitions in a powder sample of the delafossite NaErS$_2$, where the large total angular momentum $J = 15/2$ of the Er$^{3+} $ ions and the resulting plethora of CEF transitions enable an accurate fit of the CEF parameters. Our study reveals nearly isotropic spins with large $J_z = \pm 1/2$ components for the Er$^{3+}$ CEF ground states, which might facilitate the development of a QSL state. The scaling of the obtained CEF Hamiltonian to different rare earth ions suggests that sizable $J_z = \pm 1/2$ components are generally present in the CEF ground states, supporting the ternary sulfide delafossites as potential QSL hosts.

Geodesics and periodic orbits around quantum-corrected black holes

Abstract The orbital dynamics in a black hole spacetime can give us a chance to get deep insight into quantum gravity and fundamental physics. In this paper, we mainly focus on a massive particle’s motion around a quantum-corrected Schwarzschild black hole. In order to reveal some signatures of the adiabatic inspiral stage for probing the gravitational wave, the geodesics and periodic orbits around the black hole are investigated. It is found that bound orbits around the quantum-corrected Schwarzschild black hole have larger angular momentum and radius compared to the classic (Schwarzschild) black hole. Furthermore, by taking a taxonomy into account, periodic orbits for the massive particle are studied by a rational number with a triplet of integers. It suggests that the rational number increases with the increased of the energy of the massive particle. Besides, in the adiabatic inspiral stage, the particle’s periodic orbits around the quantum-corrected Schwarzschild black hole have higher energy in comparison to the ones in the classic black hole. These results demonstrated that the dynamical approach for the taxonomy enables us to distinguish the quantum-corrected Schwarzschild black hole from the classic black hole only by a massive particle’s periodic orbits.

Black Hole as a Quantum Field Configuration

We describe 4D evaporating black holes as quantum field configurations by solving the semi-classical Einstein equation G μ ν = 8 π G ⟨ ψ | T μ ν | ψ ⟩ and quantum matter fields in a self-consistent manner. As the matter fields, we consider N massless free scalar fields (N is large). We find a spherically symmetric self-consistent solution of the metric g μ ν and the state | ψ ⟩ . Here, g μ ν is locally A d S 2 × S 2 geometry, and | ψ ⟩ provides ⟨ ψ | T μ ν | ψ ⟩ = ⟨ 0 | T μ ν | 0 ⟩ + T μ ν ( ψ ) , where | 0 ⟩ is the ground state of the matter fields in the metric and T μ ν ( ψ ) consists of the excitation of s-waves that describe the collapsing matter and Hawking radiation with the ingoing negative energy flow. This object is supported by a large tangential pressure ⟨ 0 | T θ θ | 0 ⟩ due to the vacuum fluctuation of the bound modes with large angular momenta l ≫ 1 . This describes the interior of the black hole when the back reaction of the evaporation is taken into account. In this picture, the black hole is a compact object with a surface (instead of horizon) that looks like a conventional black hole from the outside and eventually evaporates without a singularity. If we count the number of configurations { | ψ ⟩ } that satisfy the self-consistent equation, we reproduce the area law of the entropy. This tells that the information is carried by the s-waves inside the black hole. | ψ ⟩ also describes the process that the negative ingoing energy flow created with Hawking radiation is superposed on the collapsing matter to decrease the total energy while the total energy density remains positive. Finally, as a special case, we consider conformal matter fields and show that the interior metric is determined by the matter content of the theory, which leads to a new constraint to the matter contents for the black hole to evaporate.

Laser spectroscopy of pionic helium atoms.

Charged pions1 are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons2-9). Determining the mass of the π- meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino10-13. However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei1. Metastable pionic helium (π4He+) is a hypothetical14-16 three-body atom composed of a helium-4 nucleus, an electron and a π- occupying a Rydberg state of large principal (n≈16) and orbital angular momentum (l≈n-1) quantum numbers. The π4He+ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out17. Its atomic structure is unique owing to the absence of hyperfine interactions18,19 between the spin-0 π- and the 4He nucleus. Here we synthesize π4He+ in a superfluid-helium target and excite the transition (n,l)=(17,16)→(17,15) of the π--occupied π4He+ orbital at anear-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the π- and undergoing fission20,21. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π4He+. This work enables the use of theexperimental techniques of quantum opticsto study ameson.

A universal Schwarzian sector in two-dimensional conformal field theories

We show that an extremely generic class of two-dimensional conformal field theories (CFTs) contains a sector described by the Schwarzian theory. This applies to theories with no additional symmetries and large central charge, but does not require a holographic dual. Specifically, we use bootstrap methods to show that in the grand canonical ensemble, at low temperature with a chemical potential sourcing large angular momentum, the density of states and correlation functions are determined by the Schwarzian theory, up to parametrically small corrections. In particular, we compute out-of-time-order correlators in a controlled approximation. For holographic theories, these results have a gravitational interpretation in terms of large, near-extremal rotating BTZ black holes, which have a near horizon throat with nearly AdS2 × S1 geometry. The Schwarzian describes strongly coupled gravitational dynamics in the throat, which can be reduced to Jackiw-Teitelboim (JT) gravity interacting with a U(1) field associated to transverse rotations, coupled to matter. We match the physics in the throat to observables at the AdS3 boundary, reproducing the CFT results.

Generation of resonant geometric modes from off-axis pumped degenerate cavity Nd:YVO4 lasers with external mode converters.

We propose a theoretical model to generate the resonant geometric modes localized on ray periodic trajectories with orbital angular momentum. Based on the numerical analysis, we realize resonant geometric modes in the off-axis pumped degenerate cavity ${\rm Nd}{:}{{\rm YVO}_4}$Nd:YVO4 lasers with an external mode converter. The experimental results reveal that laser output modes display the planar geometric modes when the off-axis displacement is sufficiently large, and the cavity length is set to satisfy the degenerate conditions. To generate the vortex beams, the planar geometric modes are transformed into circular geometric modes. Finally, the resonant geometric modes with large orbital angular momentum (OAM) can be generated from converting the circular geometric modes with an axicon lens. The experimental results are performed to approve the theoretical analyses.

The Production of the Isotopes with Z = 98–106 in the Multinucleon Transfer Reaction 238U + 251Cf

The multinucleon transfer process in the reaction 238U + 251Cf is investigated by the dinuclear system model with GEMINI++ code. The time evolution of the fragment probability is studied. For the fragments with larger proton number, the time needed to reach the maximum probability is larger. The isotopic distributions become wider at larger evolution time, and more neutron-rich and proton-rich nuclei can be produced. On the neutron-rich side, the ΔU values becomes higher at larger incident angular momentum, resulting in lower cross sections of neutron-rich nuclei. New neutron-rich isotopes with Z = 98–104 are produced with cross sections larger than 1 nb. Three unknown Db isotopes and two unknown Sg isotopes are generated with cross sections larger than 1 pb.

Gravitational waves with orbital angular momentum

Compact orbiting binaries like the black hole binary system observed in GW150914 carry large amount of orbital angular momentum. The post-ringdown compact object formed after merger of such a binary configuration has only spin angular momentum, and this results in a large orbital angular momentum excess. One significant possibility is that the gravitational waves generated by the system carry away this excess orbital angular momentum. An estimate of this excess is made. Arguing that plane gravitational waves cannot possibly carry any orbital angular momentum, a case is made in this paper for gravitational wave beams carrying orbital angular momentum, akin to optical beams. Restricting to certain specific beam-configurations, we predict that such beams may produce a new type of strain, in addition to the longitudinal strains measured at aLIGO for GW150914 and GW170817. Current constraints on post-ringdown spins, derived within the plane-wave approximation of gravitational waves, therefore stand to improve. The minimal modification that might be needed on a laser-interferometer detector (like aLIGO or VIRGO) to detect such additional strains is also briefly discussed.

Fission of Relativistic Nuclei with Fragment Excitation and Reorientation.

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ∼2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2-10 for a large number of individual yields. Here, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the ^{238}U+^{208}Pb reaction at 1 GeV/nucleon (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

Polarization and Vorticity in the Quark–Gluon Plasma

The quark–gluon plasma (QGP) produced by collisions between ultrarelativistic heavy nuclei is well described in the language of hydrodynamics. Noncentral collisions are characterized by very large angular momentum, which in a fluid system manifests as flow vorticity. This rotational structure can lead to a spin polarization of the hadrons that eventually emerge from the plasma, and thus these collisions provide experimental access to flow substructure at unprecedented detail. Recently, the first observations of Λ hyperon polarization along the direction of collisional angular momentum were reported. These measurements are in broad agreement with hydrodynamic and transport-based calculations and reveal that the QGP is the most vortical fluid ever observed. However, there remain important tensions between theory and observation that might be fundamental in nature. In the relatively mature field of heavy-ion physics, the discovery of global hyperon polarization and 3D simulations of the collision have opened an entirely new direction of research. We discuss the current status of this rapidly developing area and directions for future research.

Protostellar disk formation by a nonrotating, nonaxisymmetric collapsing cloud: model and comparison with observations

Context. Planet-forming disks are fundamental objects that are thought to be inherited from large scale rotation through the conservation of angular momentum during the collapse of a prestellar dense core. Aims. We investigate the possibility for a protostellar disk to be formed from a motionless dense core that contains nonaxisymmetric density fluctuations. The rotation is thus generated locally by the asymmetry of the collapse. Methods. We study the evolution of the angular momentum in a nonaxisymmetric collapse of a dense core from an analytical point of view. To test the theory, we performed three-dimensional simulations of a collapsing prestellar dense core using adaptative mesh refinement. We started from a nonaxisymmetrical situation, considering a dense core with random density perturbations that follow a turbulence spectrum. We analyzed the emerging disk by comparing the angular momentum it contains with the one expected from our analytic development. We studied the velocity gradients at different scales in the simulation as is done with observations. Results. We show that the angular momentum in the frame of a stellar object, which is not located at the center of mass of the core, is not conserved due to inertial forces. Our simulations of such nonaxisymmetrical collapse quickly produce accretion disks at the small scales in the core. The analysis of the kinematics at different scales in the simulated core reveals projected velocity gradients of amplitudes similar to the ones observed in protostellar cores and for which directions vary, sometimes even reversing when small and large scales are compared. These complex kinematics patterns appear in recent observations and could be a discriminating feature with models where rotation is inherited from large scales. Our results from simulations without initial rotation are more consistent with these recent observations than when solid-body rotation is initially imprinted. Lastly, we show that the disks that formed in this scenario of nonaxisymmetrical gravitational collapse grow to reach sizes larger than those that are observed, and then fragment. We show that including a magnetic field in these simulations reduces the size of the outcoming disks and it prevents them from fragmenting, as is shown by previous studies. Conclusions. We show that in a nonaxisymmetrical collapse, the formation of a disk can be induced by small perturbations of the initial density field in the core, even in the absence of global large-scale rotation of the core. In this scenario, large disks are generic features that are natural consequences of the hydrodynamical fluid interactions and self-gravity. Since recent observations have shown that most disks are significantly smaller and have a size of a few tens of astronomical units, our study suggests that magnetic braking is the most likely explanation. The kinematics of our model are consistent with typically observed values of velocity gradients and specific angular momentum in protostellar cores. These results open a new avenue in which our understanding of the early phases of disk formation can be explored since they suggest that a fraction of the protostellar disks could be the product of nonaxisymmetrical collapse, rather than directly resulting from the conservation of preexisting large scale angular momentum in rotating cores.

Generation and characterization of focused helical x-ray beams.

The phenomenon of orbital angular momentum (OAM) affects a variety of important applications in visible optics, including optical tweezers, free-space communication, and 3D localization for fluorescence imaging. The lack of suitable wavefront shaping optics such as spatial light modulators has inhibited the ability to impart OAM on x-ray and electron radiation in a controlled way. Here, we report the experimental observation of helical soft x-ray beams generated by holographically designed diffractive optical elements. We demonstrate that these beams rotate as a function of propagation distance and measure their vorticity and coherent mode structure using ptychography. Our results establish an approach for controlling and shaping of complex focused beams for short wavelength scanning microscopy and OAM-driven applications.

Rotating black holes as sources of high energy particles

Ultrahigh energy particles in cosmic rays observed on the Earth can have their origin in the ergosphere of rotating black holes. This paper discusses production of these particles through decay of superheavy dark matter particles, such as multiparticle scattering near horizon, as well as a collision of particles with a large angular momentum and particles on white hole geodesics.

Employing TALYS to deduce angular momentum rootmean-square values, Jrms, in fission fragments

Fission fragments exhibit large angular momenta J, which constitutes a challenge for fission models to fully explain. Systematic measurements of isomeric yield ratios (IYR) are needed for basic nuclear reaction physics and nuclear applications, especially as a function of mass number and excitation energy. One goal is to improve the current understanding of the angular momentum generation and sharing in the fission process. To do so, one needs to improve the modeling of nuclear de-excitation. In this work, we have used the TALYS nuclear-reaction code to relax excited fission fragments and to extract root-mean-square (rms) values of initial spin distributions, after comparison with experimentally determined IYRs. The method was assessed by a comparative study on 252Cf(sf) and 235U(nth,f). The results show a consistent performance of TALYS, both in comparison to reported literature values and to other fission codes. A few discrepant Jrms values were also found. The discrepant literature values could need a second consideration as they could possibly be caused by outdated models. Our TALYS method will be refined to better comply with contemporary sophisticated models and to reexamine older deduced values in literature.

Study of the Shell Evolution Effect on the Nuclei around the 78Ni Core Structure

Study of the Shell Evolution Effect on the Nuclei around the 78Ni Core Structure

Changes in angular momentum during the golf swing and their association with club head speed

ABSTRACTThis study measured changes in angular momentum during a golf swing and determined its associations with club head speed (CHS). Three-dimensional coordinates of anatomical landmarks were obtained during driver shots in 17 male collegiate golfers. A 15-segment model was used to calculate total (body-club system) angular momentum about the whole-body centre of mass and the segmental contributions of the trunk, arms, legs, and club to CHS. Angular momentum of the body-club system and the club increased from the top of the backswing towards ball impact. While angular momentum of the arms reached a maximum before the middle downswing, that of the trunk peaked after the middle downswing. The trunk maximum angular momentum was selected as the sole contributor for CHS (R2 = 0.494, p = 0.002). CHS negatively correlated with the relative difference between maximum angular momentum and angular momentum at impact only in the trunk (r = −0.492, p = 0.045). These results suggest that 1) the club head is accelerated by increasing angular momentum of the body-club system without sequential motion from the proximal (trunk) to distal (arm) segments, and 2) gaining large trunk angular momentum in the later part of the downswing is essential for fast CHS.

Analytic bootstrap for logarithmic CFT

We study logarithmic conformal field theory (LogCFT) in four dimensions using conformal bootstrap techniques in the large spin limit. We focus on the constraints imposed by conformal symmetry on the four point function of certain logarithmic scalar operators and compute the leading correction to the anomalous dimension of double trace operators in the large spin limit. There exist certain holographic duals to such LogCFTs, which involve higher derivative equations of motion. The anomalous dimension is related to the binding energy of a state where two scalars rotate around each other with a large angular momentum. We compute this energy shift and compare it to the anomalous dimension of the large spin double trace operators due to stress tensor exchange in the LogCFT. Our result shows that the cluster decomposition principle is satisfied for LogCFTs as long as the dimensions of the operators are positive.