Nuclear Theory·nucl-th

Tuesday·November 23, 2021

13 papers—5 primary·8 cross-listed

01
Kinetic model of excitation of pairing vibrations in superfluid nuclei
V.I. Abrosimov
Excitation of pairing vibrations in superfluid nuclei is studied by using a kinetic model based on the Vlasov equation with pairing, derived from the time-dependent Hartree-Fock-Bogolyubov theory. The anomalous density response function is used to find the monopole pairing mode and the dynamic variation of the pairing gap associated with this mode. The spectroscopic factor for the excitation of monopole pairing vibrations in two-neutron transfer reaction is estimated. It is found that the pairing correlations give rise to the same coherent contribution to the semiclassical spectroscopic factor as to the corresponding quantum expression, which is essentially determined by the distribution of the neutron levels near the Fermi energy. A numerical evaluation of the reduced spectroscopic factor for the excitation of monopole pairing vibrations in two-neutron transfer reaction in superfluid nuclei shows that it does not exceed several percent of the spectroscopic factor for the transfer of two neutrons to the ground state. This estimate is in agreement with the experimental data obtained for the ratio of the cross section for the excitation of 0+state in the (p,t) reaction in the energy region of double pairing gap to the cross section for the excitation of the ground state for superfluid nuclei of the rare-earth and actinide regions.
nucl-thNucl.Phys.Atom.Energy(2022)·0 citations
02
Dynamical synthesis of 4He in the scission phase of nuclear fission
Z. X. Ren · D. Vretenar · T. Niksic · P. W. Zhao · J. Zhao · J. Meng
In the exothermic process of fission decay, an atomic nucleus splits into two or more independent fragments. Several aspects of nuclear fission are not properly understood, in particular the formation of the neck between the nascent fragments, and the subsequent mechanism of scission into two or more independent fragments. Using an implementation of time-dependent density functional theory, based on a relativistic energy density functional and including pairing correlations, we analyze the final phase of the process of induced fission of $^{240}$ Pu, and show that the time-scale of neck formation coincides with the assembly of two $α$ -like clusters (less than 1 zs = 10 $^{- 21}$ s). Because of its much larger binding energy, the dynamical synthesis of 4He in the neck predominates over other light clusters, e.g., $^{3}$ H and $^{6}$ He. At the instant of scission the neck ruptures exactly between the two $α$ -like clusters, which separate because of the Coulomb repulsion and are eventually absorbed by the two emerging fragments. The newly proposed mechanism of light charged clusters formation at scission provides a natural explanation of ternary fission.
nucl-thnucl-exPRL(2022)·70 citations
03
Partial muon capture rates in $A = 3$ and $A = 6$ nuclei with chiral effective field theory
G. B. King🇺🇸 · S. Pastore🇺🇸 · M. Piarulli🇺🇸 · R. Schiavilla🇺🇸
Searches for neutrinoless-double beta decay rates are crucial in addressing questions within fundamental symmetries and neutrino physics. The rates of these decays depend not only on unknown parameters associated with neutrinos, but also on nuclear properties. In order to reliably extract information about the neutrino, one needs an accurate treatment of the complex many-body dynamics of the nucleus. Neutrinoless-double beta decays take place at momentum transfers on the order of 100 MeV/ $c$ and require both nuclear electroweak vector and axial current matrix elements. Muon capture, a process in the same momentum transfer regime, has readily available experimental data to validate these currents. In this work, we present results of {\it ab initio} calculations of partial muon capture rates for $^{3}$ He and $^{6}$ Li nuclei using variational and Green's Function Monte Carlo computational methods. We estimate the impact of the three-nucleon interactions, the cutoffs used to regularize two-nucleon ( $2 N$ ) interactions, and the energy range of $2 N$ scattering data used to fit these interactions.
nucl-thPRC(2022)·15 citations
04
Transport near the chiral critical point
Alexander Soloviev🇺🇸
The evolution of a heavy ion collision passes close to the O(4) critical point of QCD, where fluctuations of the order parameter are expected to be enhanced. Using the appropriate stochastic hydrodynamic equations in mean field near the the pseudo-critical point, we compute how these enhanced fluctuations modify the transport coefficients of QCD. Finally, we estimate the expected critical enhancement of soft pion yields, which provides a plausible explanation for the excess seen in experiment relative to ordinary hydrodynamic computations.
nucl-thEPJ Web Conf.(2022)·3 citations
05
Anomalous plasma: chiral magnetic effect and all that
Igor A. Shovkovy🇺🇸
Chiral anomalous effects in relativistic plasmas are reviewed. The essence of chiral separation and chiral magnetic effects is explained in simple terms. Qualitative differences between the two phenomena, both of which are triggered by background electromagnetic fields, are highlighted. It is shown how an interplay of the chiral separation and chiral magnetic effects could lead to a new collective plasma mode, called the chiral magnetic wave. It is argued that the chiral magnetic wave is overdamped in weakly interacting plasmas. Its fate in a strongly interacting quark-gluon plasma is less clear, especially in the presence of a superstrong magnetic field. The observational signatures of the chiral anomalous effects are discussed briefly.
nucl-thhep-phChapter 18 in Peter Suranyi 87th Birthday…·9 citations
06
Towards testing the magnetic moment of the tau at one part per million
Andreas Crivellin🇨🇭 · Martin Hoferichter🇨🇭 · J. Michael Roney🇨🇦
If physics beyond the Standard Model (BSM) explains the $4.2 σ$ difference between the Standard-Model and measured muon anomalous magnetic moment, $a_{μ}$ , minimal flavor violation predicts a shift in the analog quantity for the $τ$ lepton, $a_{τ}$ , at the $1 0^{- 6}$ level, and even larger effects are possible in generic BSM scenarios such as leptoquarks. We show that this produces equivalent BSM deviations in the Pauli form factor, $F_{2} (s)$ , at $s = (10 GeV)^{2}$ , and report the first complete two-loop prediction of $Re F_{2}^{eff} (100 GeV^{2}) = - 268.77 (50) \times 1 0^{- 6}$ for resonant $τ$ -pair production in $e^{+} e^{-} \to Υ (n S) \to$ $τ^{+} τ^{-}$ , $n = 1, 2, 3$ . $Re F_{2}^{eff}$ can be measured from $e^{-}$ -helicity-dependent transverse and longitudinal asymmetries in $τ$ -pair events, which requires a longitudinally polarized $e^{-}$ beam. We discuss how Belle II asymmetry measurements could probe $a_{τ}^{BSM}$ at $1 0^{- 6}$ , assuming such a polarization upgrade of the SuperKEKB $e^{+} e^{-}$ collider, and conclude by outlining the next steps to be taken in theory and experiment along this new avenue for exploring realistic BSM effects in $a_{τ}$ .
↳ hep-phhep-exnucl-exnucl-thPRD(2022)·66 citations
07
Evidence for Nonlinear Gluon Effects in QCD and their $A$ Dependence at STAR
STAR Collaboration: M. S. Abdallah · B. E. Aboona · J. Adam · L. Adamczyk · J. R. Adams · J. K. Adkins · G. Agakishiev · I. Aggarwal · M. M. Aggarwal · Z. Ahammed · A. Aitbaev · I. Alekseev and 384 other authors
The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di- $π^{0}$ s produced at forward pseudorapidities ( $2.6 < η < 4.0$ ) in $p$ + $p$ , $p +$ Al, and $p +$ Au collisions at a center-of-mass energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back $π^{0}$ pairs in $p +$ Al and $p +$ Au collisions compared to the $p$ + $p$ data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in $p +$ Au relative to $p +$ Al collisions exhibits a dependence of the saturation scale, $Q_{s}^{2}$ , on the mass number, $A$ . A linear scaling of the suppression with $A^{1/3}$ is observed with a slope of $- 0.09$ $\pm$ $0.01$ .
↳ nucl-exhep-exhep-thnucl-thPRL(2022)·40 citations
08
A New Model of Intranuclear Neutron-Antineutron Transformations in $_{8}^{16}$ O
J. L. Barrow🇺🇸 · A. S. Botvina🇷🇺 · E. S. Golubeva🇷🇺 · J-M. Richard🇫🇷
There has been much work in recent years pertaining to viability studies for the intranuclear observation of neutron-antineutron transformations. These studies begin firstly with the design and implementation of an event generator for the simulation of this rare process, where one hopes to retain as much of the underlying nuclear physics as possible in the initial state, and then studying how these effects may perturb the final state observable particles for detector efficiency studies following simulated reconstruction. There have been several searches for intranuclear neutron-antineutron transformations, primarily utilizing the $_{8}^{16}$ O nucleus, and completed within large underground water Cherenkov detectors such as Super-Kamiokande. The latest iteration of a generator is presented here for use in such an experiment. This generator includes several new features, including a new radial (position) annihilation probability distribution and related intranuclear suppression factor for $_{8}^{16}$ O, as well as a highly general, modern nuclear multifragmentation model with photonic de-excitations. The latter of these may allow for improved identification of the signal using large underground detectors such as Super-Kamiokande and the future Hyper-Kamiokande, potentially increasing the overall signal efficiencies of these rare searches. However, it should be noted that certain fast photonic de-excitations may be washed out by $π^{0}$ decays to photons. These new features implemented in these $n ˉ$ \isotope[15][8]{O} simulations increase the overall physical realism of the model, and are easily portable to other future searches such as to extranuclear $n ˉ_{6}^{12}$ C for the ESS NNBAR experiment, as well as intranuclear $n ˉ_{18}^{39}$ Ar used in DUNE.
↳ hep-exhep-phnucl-thPRC(2022)·5 citations
09
Lattice QCD at nonzero temperature and density
G. Aarts🇬🇧 · C. Allton🇬🇧 · S. Hands🇬🇧 · B. Jäger🇩🇰 · S. Kim🇰🇷 · M.P. Lombardo🇮🇹 · A.A. Nikolaev🇬🇧 · S.M. Ryan🇮🇪 · J.-I. Skullerud🇮🇪
We discuss some selected recent developments in the field of lattice QCD at nonzero temperature and density, describing in particular the transition from the hadronic gas to the quark-gluon plasma, as seen in simulations using Wilson fermions.
↳ hep-latnucl-thJ.Phys.Conf.Ser.(2022)·2 citations
10
Effective Field Theories and Lattice QCD for the X Y Z frontier
Nora Brambilla🇩🇪
Exotic states have been predicted before and after the advent of QCD. In the last decades they have been observed at accelerator experiments in the sector with two heavy quarks, at or above the quarkonium strong decay threshold and called X Y Z states. These states offer a unique possibility for investigating the dynamical properties of strongly correlated systems in QCD. I will show how an alliance of nonrelativistic effective field theories and lattice can allow us to address these states in QCD. In particular I will explain what are the opportunities and challenges of lattice QCD in this respect and which new tools should be developed.
↳ hep-lathep-phhep-thnucl-thPoS(2022)·13 citations
11
Modulation of pulse profile as a signal for phase transitions in a pulsar core
Partha Bagchi🇨🇳 · Biswanath Layek🇮🇳 · Anjishnu Sarkar🇮🇳 · Ajit M. Srivastava🇮🇳
We calculate detailed modification of pulses from a pulsar arising from the effects of phase transition induced density fluctuations on the pulsar moment of inertia. We represent general statistical density fluctuations using a simple model where the initial moment of inertia tensor of the pulsar (taken to be diagonal here) is assumed to get random additional contributions for each of its component which are taken to be Gaussian distributed with certain width characterized by the strength of density fluctuations $ϵ$ . Using sample values of $ϵ$ , (and the pulsar deformation parameter $η$ ) we numerically calculate detailed pulse modifications by solving Euler's equations for the rotational dynamics of the pulsar. We also give analytical estimates which can be used for arbitrary values of $ϵ$ and $η$ . We show that there are very specific patterns in the perturbed pulses which are observable in terms of modulations of pulses over large time periods. In view of the fact that density fluctuations fade away eventually leading to a uniform phase in the interior of pulsar, the off-diagonal components of MI tensor also vanish eventually. Thus, the modification of pulses due to induced wobbling (from the off-diagonal MI components) will also die away eventually. This allows one to distinguish these transient pulse modulations from the effects of any wobbling originally present. Further, the decay of these modulations in time directly relates to relaxation of density fluctuations in the pulsar giving valuable information about the nature of phase transition occurring inside the pulsar.
↳ astro-ph.HEhep-phnucl-exnucl-thMNRAS(2022)·5 citations
12
Upcgen: a Monte Carlo simulation program for dilepton pair production in ultra-peripheral collisions of heavy ions
Nazar Burmasov🇷🇺 · Evgeny Kryshen🇷🇺 · Paul Buehler🇦🇹 · Roman Lavicka🇦🇹
Ultra-peripheral collisions (UPCs) of heavy ions can be used as a clean environment to study two-photon induced interactions such as dilepton pair photoproduction. Recently, precise data on lepton pair production in UPCs were obtained by the ATLAS experiment at the LHC where significant deviations, of up to 20%, from available theoretical predictions were observed. In this work, we present a Monte Carlo event generator, Upcgen, that implements a refined treatment of the photon flux allowing us to improve the agreement with ATLAS data at large dilepton rapidities. Besides, the new generator offers a possibility to study photon polarization effects and set arbitrary values of the lepton anomalous magnetic moment that can be used in the future studies of tau g-2 via ditau production measurements in UPCs.
↳ hep-phnucl-exnucl-thComput.Phys.Commun.(2022)·27 citations
13
Stability of Classical Chromodynamic Fields
Sylwia Bazak🇵🇱 · Stanislaw Mrowczynski🇵🇱
A system of gluon fields generated at the earliest phase of relativistic heavy-ion collisions can be described in terms of classical fields. Numerical simulations show that the system is unstable but a character of the instability is not well understood. With the intention to systematically study the problem, we analyze a stability of classical chromomagnetic and chromoelectric fields which are constant and uniform. We consider the Abelian configurations discussed in the past where the fields are due to the single-color potentials linearly depending on coordinates. However, we mostly focus on the nonAbelian configurations where the fields are generated by the multi-color non-commuting constant uniform potentials. We derive a complete spectrum of small fluctuations around the background fields which obey the linearized Yang-Mills equations. The spectra of Abelian and nonAbelian configurations are similar but different and they both include unstable modes. We briefly discuss the relevance of our results for fields which are uniform only in a limited spatial domain.
↳ hep-phhep-thnucl-thPRD(2022)·8 citations

Affiliations

first authorsco-authorsvia INSPIRE