arXiv subjects

Dong Bai

Publications and source records attributed to Dong Bai.

36 records · Page 2Linked to original sources

arXiv2021

$α$-cluster structures above double shell closures via double-folding potentials from chiral effective field theory

$α$-cluster structures above double shell closures are among the cornerstones for nuclear $α$-cluster physics. Semi-microscopic cluster models (SMCMs) are important theoretical models to study their properties. A crucial ingredient of SMCM is the effective potential between the alpha cluster and the doubly magic nucleus. We derive new double-folding potentials between $α$ clusters and doubly magic nuclei from soft local chiral nucleon-nucleon potentials given by chiral effective field theory ($χ$EFT) at the next-to-next-to-leading order. The $α$-cluster structures in ${}^{8}\text{Be}$, ${}^{20}\text{Ne}$, ${}^{44,52}\text{Ti}$, and ${}^{212}\text{Po}$ are explored to validate these new double-folding potentials. The $α$ decay of ${}^{104}\text{Te}$ is also studied in the light of recent experimental results. Our study shows that double-folding potentials from $χ$EFT are the new reliable effective potentials for the SMCM approach to $α$-cluster structures above double shell closures, with both conceptual and phenomenological merits.

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arXiv2021

Generalizing the calculable $R$-matrix theory and eigenvector continuation to the incoming wave boundary condition

The calculable $R$-matrix theory has been formulated successfully for regular boundary conditions with vanishing radial wave functions at the coordinate origins [P. Descouvemont and D. Baye, Rept. Prog. Phys. 73, 036301 (2010)]. We generalize the calculable $R$-matrix theory to the incoming wave boundary condition (IWBC), which is widely used in theoretical studies of low-energy heavy-ion fusion reactions to simulate the strong absorption of incoming flux inside the Coulomb barriers. The generalized calculable $R$-matrix theory also provides a natural starting point to extend eigenvector continuation (EC) [D. Frame et al., Phys. Rev. Lett. 121, 032501 (2018)] to fusion observables. The ${}^{14}\text{N}+{}^{12}\text{C}$ fusion reaction is taken as an example to validate these new theoretical tools. Both local and nonlocal potentials are considered in numerical calculations. Our generalizations of the calculable $R$-matrix theory and EC are found to work well for IWBC.

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arXiv2020

Resonant and Scattering States in the $α+α$ System from the Non-Localized Cluster Model

The non-localized cluster model provides a new perspective on nuclear cluster effects and has been applied successfully to study cluster structures in various bound states and quasi-bound states (i.e., long-lived resonant states). In this work, we extend the application scope of the non-localized cluster model further to resonant and scattering states. Following the $R$-matrix theory, the configuration space is divided into the interior and exterior regions by a large channel radius such that the nuclear forces and the antisymmetrization effects become negligible between clusters in the exterior region. In the interior region, the picture of non-localized clustering is realized mathematically by adopting the Brink-Tohsaki-Horiuchi-Schuck-Röpke (Brink-THSR) wave functions as the bases to construct the interior wave functions. The Bloch-Schrödinger equation is used to match the interior wave functions continuously with the asymptotic boundary conditions of the resonant and scattering states at the channel radius, which leads eventually to solutions of the problem. As a first test of the formalism, the low-lying resonant states of ${}^{8}$Be and the phase shifts of the $α+α$ elastic scattering are studied. The numerical results agree well with the experimental data, which shows the validity of the theoretical framework.

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arXiv2019

Alpha Clustering from the Quartet Model

Alpha clustering in nuclei is considered with the quartet model (QM) where four valence nucleons (the quartet) move on the top of the core (daughter) nucleus. In the QM approach, it is assumed that the intrinsic wave function of the quartet is changed from the pure cluster configuration to the shell-model configuration when it crosses some critical radius and enters into the core nucleus. The QM approach could give not only the level scheme, the electromagnetic transition, the nuclear radius, but also the alpha-cluster formation probability. Numerical results are calculated for ${}^{20}$Ne, ${}^{44}$Ti, and ${}^{212}$Po, where a quartet moves on top of a double magic nucleus. Good agreement with experimental data and previous theoretical studies is obtained. The QM approach is a useful complement to the present phenomenological and microscopic models and could help deepen our understanding of alpha clustering across the nuclide chart.

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arXiv2018

$α$ Clustering slightly above ${}^{100}$Sn in the Light of the New Experimental Data on the Superallowed $α$ Decay

The recently observed $α$-decay chain ${}^{108}\text{Xe}\to{}^{104}\text{Te}\to{}^{100}\text{Sn}$ [K.~Auranen \emph{et al.}, Phys.\ Rev.\ Lett.\ {\bf121}, 182501 (2018)] could provide valuable information on the $α$ clustering in even-even nuclei slightly above the major shells $Z=50$ and $N=50$. In this work, this $α$-decay chain is studied theoretically within the framework of the density-dependent cluster model plus the two-potential approach. We calculate the $α$-decay half-lives of ${}^{104}$Te and ${}^{108}$Xe and study in detail their dependence on the $Q$ value and the density-profile parameters of the core nucleus. Various physical properties of ${}^{104}$Te, the heaviest nucleus with a doubly magic self-conjugate core $+$ $α$, are calculated, with two different assumptions on the renormalization factor of the double-folding potential, which could be a useful reference for future experimental studies.

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arXiv2018

Cluster-Daughter Overlap as a New Probe of Alpha-Cluster Formation in Medium-Mass and Heavy Even-Even Nuclei

We study the possibility to use the cluster-daughter overlap as a new probe of alpha-cluster formation in medium-mass and heavy even-even nuclei. We introduce a dimensionless parameter $\mathfrak{O}$, which is the ratio between the root-mean-square (rms) intercluster separation and the sum of rms point radii of the daughter nucleus and the alpha particle, to measure the degree of the cluster-daughter overlap quantitatively. By using this parameter, a large (small) cluster-daughter overlap between the alpha cluster and daughter nucleus corresponds to a small (large) $\mathfrak{O}$ value. The alpha-cluster formation is shown explicitly, in the framework of the quartetting wave function approach, to be suppressed when the $\mathfrak{O}$ parameter is small, and be favored when the $\mathfrak{O}$ parameter is large. We then use this $\mathfrak{O}$ parameter to explore systematically the landscape of alpha-cluster formation probabilities in medium-mass and heavy even-even nuclei, with $\mathfrak{O}$ being calculated from experimentally measured charge radii. The trends of alpha-cluster formation probabilities are found to be generally consistent with previous studies. The effects of various shell closures on the alpha-cluster formation are identified, along with some hints on a possible subshell structure at $N=106$ along the Hg and Pb isotopic chains. The study here could be a useful complement to the traditional route to probe alpha-cluster formation in medium-mass and heavy even-even nuclei using alpha-decay data. Especially, it would be helpful in the cases where the target nucleus is stable against alpha decay or alpha-decay data are currently not available.

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arXiv2018

$α$ Decays in Superstrong Static Electric Fields

Superstrong static electric fields could deform Coulomb barriers between $α$ clusters and daughter nuclei, and bring up the possibility of speeding up $α$ decays. We adopt a simplified model for the spherical $α$ emitter $^{212}$Po and study its responses to superstrong static electric fields. We find that, superstrong electric fields with field strengths $|\mathbf{E}|\sim0.1$ MV/fm could turn the angular distribution of $α$ emissions from isotropic to strongly anisotropic, and speed up $α$ decays by more than one order of magnitude. We also study the influences of superstrong electric fields along the Po isotope chains, and discuss the implications of our studies on $α$ decays in superstrong monochromatic laser fields. The study here might be helpful for future theoretical studies of $α$ decay in realistic superstrong laser fields.

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arXiv2018

Woods-Saxon-Gaussian Potential and Alpha-Cluster Structures of Alpha+Closed Shell Nuclei

The Woods-Saxon-Gaussian (WSG) potential is proposed as a new phenomenological potential to describe systematically the level scheme, electromagnetic transitions, and alpha-decay half-lives of the alpha-cluster structures in various alpha+closed shell nuclei. It modifies the original Woods-Saxon (WS) potential with a shifted Gaussian factor centered at the nuclear surface. We determine the free parameters in the WSG potential by reproducing the correct level scheme of $^{212}\text{Po}={}^{208}\text{Pb}+α$. It is found that the resulting WSG potential matches with the M3Y double-folding potential at the surface region and makes corrections to the inner part of the cluster-core potential. We also find that the WSG potential with the almost same parameters determined for $^{212}$Po (except for a rescaled radius) could also be used to describe alpha-cluster structures in $^{20}\text{Ne}={}^{16}\text{O}+α$ and $^{44}\text{Ti}={}^{40}\text{Ca}+α$. In all three cases, the calculated values of the level schemes, electromagnetic transitions, and alpha-decay half-lives agree with the experimental data, which shows that the WSG potential could indeed grasp many important features of the alpha-cluster structures in alpha+closed shell nuclei. The study here is a useful complement to the existing cluster-core potentials in literature. The Gaussian form factor centered at the nuclear surface might also help deepen our understanding on the alpha-cluster formation taking place around the same place.

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arXiv2018

Higher Derivative Theories for Interacting Massless Gravitons in Minkowski Spacetime

We study a novel class of higher derivative theories for interacting massless gravitons in Minkowski spacetime. These theories were first discussed by Wald decades ago, and are characterized by scattering amplitudes essentially different from general relativity and many of its modifications. We discuss various aspects of these higher derivative theories, including the Lagrangian construction, violation of asymptotic causality, scattering amplitudes, non-renormalization, and possible implications on emergent gravitons from condensed matter systems.

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arXiv2018

Charged Particle Emissions in High-Frequency Alternative Electric Fields

Proton emission, $α$ decay, and cluster radioactivity play an important role in nuclear physics. We show that high-frequency alternative electric fields could deform Coulomb barriers that trap the charged particle, and raise the possibility of speeding up charged particle emissions. They could also cause anisotropic effects in charged particle emissions, and introduce additional terms in the Geiger-Nuttall laws. Our study may further suggest that, for proton emitters like $^{166}$Ir, when the electric field is strong, the dominant decay mode could be changed from $α$ decay to proton emission. As high-frequency alternative electric fields correspond to high-frequency laser fields in the dipole approximation, our study could be viewed as a benchmark for future theoretical studies of charged particle emissions in realistic laser fields.

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arXiv2018

Repulsive Four-Body Interactions of $α$ Particles and Quasi-Stable Nuclear $α$-Particle Condensates in Heavy Self-Conjugate Nuclei

We study the effects of repulsive four-body interactions of $α$ particles on nuclear $α$-particle condensates in heavy self-conjugate nuclei using a semi-analytic approach, and find that the repulsive four-body interactions could decrease the critical number of $α$ particles, beyond which quasi-stable $α$-particle condensate states can no longer exist, even if these four-body interactions make only tiny contributions to the total energy of the Hoyle-like state of $^{16}$O. Explicitly, we study eight benchmark parameter sets, and find that the critical number $N_\text{cr}$ decreases by $|ΔN_\text{cr}|\sim1-4$ from $N_\text{cr}\sim11$ with vanishing four-body interactions. We also discuss the effects of four-body interactions on energies and radii of $α$-particle condensates. Our study can be useful for future experiments to study $α$-particle condensates in heavy self-conjugate nuclei. Also, the experimental determination of $N_\text{cr}$ will eventually help establish a better understanding on the $α$-particle interactions, especially the four-body interactions.

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arXiv2018

Two-Neutron Halo State of ${}^{15}$B Around $3.48$ MeV By A Three-Body Model

We investigate low-lying bound states of the neutron-rich nucleus ${}^{15}$B by assuming it is a three-body system made of an inert core ${}^{13}$B and two valence neutrons. The three-body wave functions are obtained using the Faddeev formalism. Special attention is paid to the excited state at $3.48(6)$ MeV observed in the ${}^{13}\text{C}({}^{14}\text{C},{}^{12}\text{N}){}^{15}\text{B}$ reaction, whose properties are less clear theoretically. In our three-body model, besides the ground state $3/2_1^-$, a second $3/2_2^-$ state is discovered at around $3.61$ MeV, which might be identified with the excited state observed at $3.48(6)$ MeV. We study this $3/2_2^-$ state in detail, which turns out to be a two-neutron halo state with a large matter radius $r_\text{m}\approx 4.770$ fm.

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arXiv2017

Softness, Polynomial Boundedness and Amplitudes' Positivity

In this note, we study the connections between infrared (IR) and ultraviolet (UV) behaviors of scattering amplitudes of massless channels by exploiting dispersion relations and positivity bounds. Given forward scattering amplitudes, which scale as $\mathcal{A}(s)\sim s^M$ in the IR ($s\to0$) and could be embedded into UV completions satisfying unitarity, analyticity, crossing symmetry and polynomial boundedness $|\mathcal{A}(s)|< c\, |s|^N$ ($|s|\to\infty$), with $M$ and $N$ integers, we show that the inequality $2\ceil*{\frac{N}{2}}\ge M \ge \ceil*{\frac{N}{2}}$ must hold, where $\ceil*{x}$ is the smallest integer greater than or equal to $x$. One immediate consequence of the above inequality is the bound on the UV growth of scattering amplitudes in terms of their IR behaviors. Our results could be useful in studies of massless higher spin particles, as well as the program of UV improvement and weakly-coupled UV completion.

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arXiv2017

Quarkonium decay into photon plus graviton: a golden channel to discriminate General Relativity from Massive Gravity?

After the recent historical discovery of gravitational wave, it is curious to speculate upon the detection prospect of the quantum graviton in the terrestrial accelerator-based experiment. We carefully investigate the "golden" channels, $J/ψ(Υ)\toγ+\text{graviton}$, which can be pursued at \textsf{BESIII} and \textsf{Belle 2} experiments, by searching for single-photon plus missing energy events. Within the effective field theory (EFT) framework of General Relativity (GR) together with Nonrelativistic QCD (NRQCD), we are capable of making solid predictions for the corresponding decay rates. It is found that these extremely suppressed decays are completely swamped by the Standard Model background events $J/ψ(Υ)\to γ+ν\barν$. Meanwhile, we also study these rare decay processes in the context of massive gravity, and find the respective decay rates in the limit of vanishing graviton mass drastically differ from their counterparts in GR. Counterintuitive as the failure of smoothly recovering GR results may look, our finding is reminiscent of the van Dam-Veltman-Zakharov (vDVZ) discontinuity widely known in classical gravity, which can be traced to the finite contribution of the helicity-zero graviton in the massless limit. Nevertheless, at this stage we are not certain about the fate of the discontinuity encountered in this work, whether it is merely a pathology or not. If it could be endowed with some physical significance, the future observation of these rare decay channels, would, in principle, shed important light on the nature of gravitation, whether the graviton is strictly massless, or bears a very small but nonzero mass.

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arXiv2017

Quadratic Contributions of Softly Broken Supersymmetry in the Light of Loop Regularization

Loop regularization (LORE) is a novel regularization scheme in modern quantum field theories. It makes no change to the spacetime structure and respects both gauge symmetries and supersymmetry. As a result, LORE should be useful in calculating loop corrections in supersymmetry phenomenology. To demonstrate further its power, in this article we revisit in the light of LORE the old issue of the absence of quadratic contributions (quadratic divergences) in softly broken supersymmetric field theories. It is shown explicitly by Feynman diagrammatic calculations that up to two loops the Wess-Zumino model with soft supersymmetry breaking terms (WZ' model), one of the simplest models with the explicit supersymmetry breaking, is free of quadratic contributions. All the quadratic contributions cancel with each other perfectly, which is consistent with results dictated by the supergraph techniques.

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arXiv2017

On the uniqueness of ghost-free special gravity

Special gravity refers to interacting theories of massless gravitons in Minkowski space-time which are invariant under the abelian gauge invariance $h_{ab}\rightarrow h_{ab}+\partial_{(a}χ_{b)}$ only. In this article we determine the most general form of special gravity free of Ostrogradski ghosts, meaning its equation of motion is of at most second order. Together with the recent works, this result could be helpful in formulating proofs of General Relativity as the unique physical theory of self-interacting massless gravitons. We also study how to construct gauge invariant couplings to matter fields.

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arXiv2017

More on the Bending of Light in Quantum Gravity

We reconsider the long-range effects of the scattering of massless scalars and photons from a massive scalar object in quantum gravity. At the one-loop level, the relevant quantum mechanical corrections could be sorted into the graviton double-cut contributions, massless-scalar double-cut contributions and photon double-cut contributions. In arXiv:1410.7590 and 1609.07477 N.E.J. Bjerrum-Bohr et al. have considered explicitly the implications of the graviton double-cut contributions on the gravitational bending of light and some classical formulations of the equivalence principle, using the modern double-copy constructions and on-shell unitarity techniques. In this article, instead we consider all three contributions and redo the analysis using the traditional Feynman diagrammatic approach. Our results on the graviton double-cut contributions agree with the aforementioned references, which acts as a nontrivial check of previous computations. Furthermore, it turns out that the massless-scalar double-cut contributions and the photon double-cut contributions do leave non-vanishing quantum effects on the scattering amplitudes and the gravitational bending of light. Yet, we find that the general structure of the gravitational amplitudes and the quantum discrepancy of the equivalence principle suggested in the aforementioned references remain intact.

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arXiv2015

Quantum Electroweak Symmetry Breaking Through Loop Quadratic Contributions

Based on two postulations that (i) the Higgs boson has a large bare mass $m_H \gg m_h \simeq 125 $ GeV at the characteristic energy scale $M_c$ which defines the standard model (SM) in the ultraviolet region, and (ii) quadratic contributions of Feynman loop diagrams in quantum field theories are physically meaningful, we show that the SM electroweak symmetry breaking is induced by the quadratic contributions from loop effects. As the quadratic running of Higgs mass parameter leads to an additive renormalization, which distinguishes from the logarithmic running with a multiplicative renormalization, the symmetry breaking occurs once the sliding energy scale $μ$ moves from $M_c$ down to a transition scale $μ=Λ_{EW}$ at which the additive renormalized Higgs mass parameter $m^2_H(M_c/μ)$ gets to change the sign. With the input of current experimental data, this symmetry breaking energy scale is found to be $Λ_{EW}\simeq 760$ GeV, which provides another basic energy scale for the SM besides $M_c$. Studying such a symmetry breaking mechanism could play an important role in understanding both the hierarchy problem and naturalness problem. It also provides a possible way to explore the experimental implications of the quadratic contributions as $Λ_{EW}$ lies within the probing reach of the LHC and the future Great Collider.

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