New articles on Nuclear Theory

[1] 2406.12980

Parity and time-reversal violating nuclear forces with explicit $Δ$-excitations

We emphasize the usefulness of treating delta resonances as explicit degrees of freedom in applications of chiral effective field theory (EFT) to parity-violating and time-reversal-violating (PVTV) nuclear interactions. Compared with the delta-less framework, the explicit inclusion of the delta isobar allows one to resum certain types of contributions to the PVTV two-pion exchange two- and three-nucleon potentials without at the same time introducing any unknown parameters up to next-to-next-to-leading order in the EFT expansion. We provide the corresponding expressions for the delta contributions in momentum and coordinate spaces and compare the convergence of the EFT expansion in both formulations.

[2] 2406.13010

IMSRG with flowing 3 body operators, and approximations thereof

We explore the impact of retaining three-body operators within the in-medium similarity renormalization group (IMSRG), as well as various approximations schemes. After studying two toy problems, idential fermions with a contact interaction and the Lipkin-Meshkov-Glick model, we employ the valence-space formulation of the IMSRG to investigate the even-$A$ carbon isotopes with a chiral two-body potential. We find that retaining only those commutators expressions that scale as $N^7$ provides an excellent approximation of the full three-body treatment.

[3] 2406.13055

Self-consistent strong screening applied to thermonuclear reactions

Self-consistent strong plasma screening around light nuclei is implemented in the Big Bang nucleosynthesis (BBN) epoch to determine the short-range screening potential, $e\phi(r)/T \geq 1$, relevant for thermonuclear reactions. We numerically solve the non-linear Poisson-Boltzmann equation incorporating Fermi-Dirac statistics adopting a generalized screening mass to find the electric potential in the cosmic BBN electron-positron plasma for finite-sized $^4$He nuclei as an example. Although the plasma follows Boltzmann statistics at large distances, Fermi-Dirac statistics is necessary when work performed by ions on electrons is comparable to their rest mass energy. While strong screening effects are generally minor due to the high BBN temperatures, they can enhance the fusion rates of high-$Z>2$ elements while leaving fusion rates of lower-$Z\le 2$ elements relatively unaffected. Our results also reveal a pronounced spatial dependence of the strong screening potential near the nuclear surface. These findings about the electron-positron plasma's role refine BBN theory predictions and offer broader applications for studying weakly coupled plasmas in diverse cosmic and laboratory settings.

[4] 2406.13417

The study of $0νββ$ decay of $^{136}$Xe using nonclosure approach in nuclear shell model

In this investigation, we compute the nuclear matrix elements (NMEs) relevant to the light neutrino-exchange mechanism governing neutrinoless double beta ($0\nu\beta\beta$) decay in $^{136}$Xe. Our method is based on the nonclosure approach within the interacting nuclear shell model framework. This approach considers the genuine effects arising from the excitation energies of two hundred states for each spin-parity of the intermediary nucleus $^{136}$Cs. All computations are performed using the effective shell model Hamiltonian GCN5082. To understand the impact of nuclear structure on $0\nu\beta\beta$ decay, we explore the dependence of the NME on various factors, including the number of intermediate states and their spin-parity characteristics. We identify an optimal closure energy of approximately 3.7 MeV for the $0\nu\beta\beta$ decay of $^{136}$Xe that reproduces the nonclosure NME using the closure approach. The calculated total NME for the light neutrino-exchange $0\nu\beta\beta$ decay of $^{136}$Xe is 2.06 with the CD-Bonn short-range correlation (SRC). These results can be valuable for future experimental investigations into the $0\nu\beta\beta$ decay of $^{136}$Xe.

[5] 2406.13520

Energy dependence of particle production in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 7.7-200 GeV using a multiphase transport model

In this study, we employ a multi-phase transport (AMPT) model to understand the production of $\pi^{\pm}$, $K^{\pm}$, $p$, $\overline{p}$, $K^{0}_{s}$, $\Lambda$, $\bar{\Lambda}$, and $\phi$ in Au + Au collisions at $\sqrt{s_{NN}} = 7.7$, $27$, $39$, $62.4$, and $200$ GeV. We have studied the energy dependence of various bulk properties of the system such as transverse momentum ($p_T$) spectra, particle yields ($dN/dy$), mean transverse mass ($\langle m_T \rangle$), and anti-particle to particle ratios. Model calculations using both default and string melting versions of the AMPT with three distinct sets of initial conditions are compared to the data from the STAR experiment. In the case of $\pi^{\pm}$, $K^{\pm}$, $p$, and $\overline{p}$, we observe that the string melting version shows better agreement with data at higher energies, while the default version performs better at lower collision energies. However, for $K^{0}_{s}$, $\Lambda$, and $\phi$, it is observed that the default version is able to describe the data better at all energies. In addition, we have used the blast-wave model to extract the kinetic freeze-out properties, like the kinetic freeze-out temperature and the radial flow velocity. We observe that these parameters are comparable with the data.

[6] 2406.13717

Beyond modified Urca: the nucleon width approximation for flavor-changing processes in dense matter

Flavor-changing charged current ("Urca") processes are of central importance in the astrophysics of neutron stars. Standard calculations approximate the Urca rate as the sum of two contributions, direct Urca and modified Urca. Attempts to make modified Urca calculations more accurate have been impeded by an unphysical divergence at the direct Urca threshold density. In this paper we describe a systematically improvable approach where, in the simplest approximation, instead of modified Urca we include an imaginary part of the nucleon mass (nucleon width). The total Urca rate is then obtained via a straightforward generalization of the direct Urca calculation, yielding results that agree with both direct and modified Urca at the densities where those approximations are valid. At low densities, we observe an enhancement of the rate by more than an order of magnitude, with important ramifications for neutron star cooling and other transport properties.

[7] 2406.13863

Revealing initial state properties through ultra-central symmetric heavy-ion collisions

Heavy-ion experiments provide a new opportunity to gain a deeper understanding of the structure of nuclei. To achieve this, it is crucial to identify observables under circumstances that are minimally affected by the process that leads to the initial state of heavy-ion collisions from nuclear wavefunction. In this study, we demonstrate that when assuming scale-invariance, the effect of this stage on the initial energy or entropy density moments in ultra-central symmetric collisions is negligible for nucleon sizes of approximately 0.7 fm or larger for large nuclei. By borrowing cluster expansion method from statistical physics and using scale-invariance assumption, we calculate the average ellipticity of initial density at the presence of short-range correlation. We compare our calculations to Monte Carlo studies and assess the accuracy of various methods of short-range correlation sampling. Additionally, we find that the isobar ratio can constrain the initial state parameters, in addition to deformation. Our study indicates that the isobar ratios in ultra-central collisions are especially sensitive to the fluctuation in the weight of the nuclei constituents and the two-body correlation among nucleons. This insight is crucial for drawing conclusions about nuclear deformations based on isobar ratios.

[8] 2406.13899

Faddeev calculations of low-energy $Λ$-deuteron scattering and momentum correlation function

Faddeev calculations of low-energy $\Lambda$-deuteron elastic scattering are performed up to $E_{cm}=20$ MeV across the deuteron threshold. Phase shifts of the $s$-wave with $J=1/2$ and $J=3/2$ are calculated using strangeness $S=-1$ hyperon-nucleon interactions in chiral effective field theory NLO13 and NLO19 parametrized by the J{\"u}lich-Bonn group. Effective range parameters, such as a scattering length and an effective range, are determined through the calculated phase shifts. $\Lambda$-deuteron momentum correlation functions are evaluated using the $\Lambda$-deuteron relative wave function constructed from half-off-shell $t$-matrices. They are compared with those evaluated using an approximate formula.

[9] 2406.14463

Hydrodynamic-like behaviour of glasma

At the earliest stage of ultrarelativistic heavy-ion collisions the produced matter is a highly populated system of gluons called glasma which can be approximately described in terms of classical chromodynamic fields. Although the system's dynamics is governed by Yang-Mills equations, glasma evolution is shown to strongly resemble hydrodynamic behaviour.

[10] 2406.12903

Open-Source Optimization of Hybrid Monte-Carlo Methods for Fast Response Modeling of NaI(Tl) and HPGe Gamma Detectors

Modeling the response of gamma detectors has long been a challenge within the nuclear community. Significant research has been conducted to digitally replicate instruments that can cost over $100,000 and are difficult to operate outside a laboratory setting. Subsequently, there have been multiple attempts to create codes that replicate the response of sodium-iodide and high purity germanium detectors for the purpose of deriving data related to gamma ray interaction with matter. While robust programs do exist, they are often subject to export controls and/or they are not intuitive to use. Through the use of the Hybrid Monte-Carlo methods, MATLAB can be used to produce a fast first-order response of various gamma ray detectors. The combination of a graphics user interface with a numerical based script allows for an open-source and intuitive code. When benchmarked with experimental data from Co-60, Cs-137, and Na-22 the code can numerically calculate a response comparable to experimental and industry standard response codes. Through this code, it is shown that a savings in computational requirements and the inclusion of an intuitive user experience does not heavily compromise data when compared to other standard codes or experimental results. When the application is installed on a computer with 16 cores, the average time to simulate the benchmarked isotopes is 0.26 seconds and 1.63 seconds on a four-core machine. The results indicate that simple gamma detectors can be modeled in an open-source format. The anticipation for the MATLAB application is to be a tool that can be easily accessible and provide datasets for use in an academic setting requiring the gamma ray detectors. Ultimately, providing evidence that Hybrid Monte-Carlo codes in an open-source format can benefit the nuclear community.

[11] 2406.13286

Charm and Bottom Hadrons in Hot Hadronic Matter

Heavy quarks, and the hadrons containing them, are excellent probes of the QCD medium formed in high-energy heavy-ion collisions, as they provide direct information on the transport properties of the medium and how quarks color-neutralize into hadrons. Large theoretical and phenomenological efforts have been dedicated thus far to assess the diffusion of charm and bottom quarks in the quark-gluon plasma and their subsequent hadronization into heavy-flavor (HF) hadrons. However, the fireball formed in heavy-ion collisions also features an extended hadronic phase, and therefore any quantitative analysis of experimental observables needs to account for rescattering of charm and bottom hadrons. This is further reinforced by the presence of a QCD cross-over transition and the notion that the interaction strength is maximal in the vicinity of the pseudo-critical temperature. We review existing approaches for evaluating the interactions of open HF hadrons in a hadronic heat bath and the pertinent results for scattering amplitudes, spectral functions and transport coefficients. While most of the work to date has focused on $D$ mesons, we also discuss excited states as well as HF baryons and the bottom sector. Both the HF hadro-chemistry and bottom observables will play a key role in future experimental measurements. We also conduct a survey of transport calculations in heavy-ion collisions that have included effects of hadronic HF diffusion and assess their sensitivity to various observables.

[12] 2406.13530

Resonances in the quark model

A discussion is presented of the estimates of the energy and width of resonances in constituent models, with focus on the tetraquark states containing heavy quarks.

[13] 2406.13531

LQCD constrained magnetic field dependent coupling constant in an effective model

A magnetic field dependent coupling constant $G(eB)$ is investigated in the two-flavor magnetized NJL model. Based on LQCD results of the neutral (charged) pion mass spectra at vanishing temperature and finite magnetic field, we determine the $G(eB)=G^0(eB)$ ($G(eB)=G^+(eB)$) in the NJL model. $G^0(eB)$ and $G^+(eB)$ are both non-monotonic functions of magnetic fields, but they are different from each other. Furthermore, we calculate the pseudo-critical temperatures $T_{pc}(eB)$ of chiral restoration phase transition with $G^0(eB)$ and $G^+(eB)$ in the magnetized NJL model, respectively. The resulting $T_{pc}(eB)$ are non-monotonic functions of magnetic fields. In previous work, $G(eB)$ in the NJL model fitted from the chiral condensate or pseudo-critical temperature of LQCD simulations is a decreasing function of magnetic field. It can not explain the saturation behavior of mass spectra of neutral pion and decreasing behavior of mass spectra of charged pion with strong magnetic field. We conclude that a magnetic field dependent coupling constant $G(eB)$ in the NJL model can not simultaneously explain the reduction of pseudo-critical temperature of chiral restoration phase transition and the light meson mass spectra under external magnetic field.

[14] 2406.13703

Working group 1 summary: $V_{ud}$, $V_{us}$, $V_{cd}$, $V_{cs}$ and semileptonic/leptonic $D$ decays

We summarize the program of working group 1 at the 12th Workshop on the CKM Unitarity Triangle, whose main subjects covered $V_{ud}$, $V_{us}$, and first-row unitarity as well as $V_{cd}$, $V_{cs}$, and (semi-)leptonic $D$ decays.

[15] 2406.13818

Effective theory tower for $μ\rightarrow e$ conversion

We present theoretical predictions for $\mu \rightarrow e$ conversion rates using a tower of effective field theories connecting the UV to nuclear physics scales. The interactions in nuclei are described using a recently developed nonrelativistic effective theory (NRET) that organizes contributions according to bound nucleon and muon velocities, $\vec{v}_N$ and $\vec{v}_\mu$, with $|\vec{v}_N| > |\vec{v}_\mu|$. To facilitate the top-down matching, we enlarge the set of Lorentz covariant nucleon-level interactions mapped onto the NRET operators to include those mediated by tensor interactions, in addition to the scalar and vector interactions already considered previously, and then match NRET nonperturbatively onto the Weak Effective Theory (WET). At the scale $\mu \approx 2$ GeV WET is formulated in terms of $u$, $d$, $s$ quarks, gluons and photons as the light degrees of freedom, along with the flavor-violating leptonic current. We retain contributions from WET operators up to dimension 7, which requires the full set of 26 NRET operators. The results are encoded in the open-source Python- and Mathematica-based software suite MuonBridge, which we make available to the theoretical and experimental communities interested in $\mu \rightarrow e$ conversion.

[16] 2406.14158

Quantum vortices in fermionic superfluids: from ultracold atoms to neutron stars

Superfluid dilute neutron matter and ultracold gas, close to the unitary regime, exhibit several similarities. Therefore, to a certain extent, fermionic ultracold gases may serve as emulators of dilute neutron matter, which forms the inner crust of neutron stars and is not directly accessed experimentally. Quantum vortices are one of the most significant properties of neutron superfluid, essential for comprehending neutron stars' dynamics. The structure and dynamics of quantum vortices as a function of pairing correlations' strength are being investigated experimentally and theoretically in ultracold gases. Certain aspects of these studies are relevant to neutron stars. We provide an overview of the characteristics of quantum vortices in s-wave-type fermionic and electrically neutral superfluids. The main focus is on the dynamics of fermionic vortices and their intrinsic structure.

[17] 2406.14172

Dynamics of Phase Transition in Quark-Gluon Plasma Droplet Formation under Magnetic Field

Pre-existing density of states for a Quark-Gluon Phase, based on Thomas-Fermi and Bethe mode, is expanded by incorporation of new variables. Results from recent study indicate that perturbations in the form of a finite non-zero chemical potential T, B, dynamic thermal masses M and of course Temperature T are indeed vital to fully comprehend the formation and dynamics of QGP. Simulations depict an overall increase in the stability of QGP in the paradigm of the statistical model. On the top of Free Energy, Entropy and heat capacity are calculated for the phase transition. The overall qualitative behavior, of entropy or Heat Capacity determines the order of phase transition of the QGP. Investigation of order of phase transition is carried out in this study through Monte-Carlo based differential element, which ensures the inclusion of the randomness of the collisions at the particle colliders.

[18] 2406.14466

The Radius of the High Mass Pulsar PSR J0740+6620 With 3.6 Years of NICER Data

We report an updated analysis of the radius, mass, and heated surface regions of the massive pulsar PSR J0740+6620 using NICER data from 2018 September 21 to 2022 April 21, a substantial increase in data set size compared to previous analyses. Using a tight mass prior from radio timing measurements and jointly modeling the new NICER data with XMM-Newton data, the inferred equatorial radius and gravitational mass are $12.49_{-0.88}^{+1.28}$ km and $2.073_{-0.069}^{+0.069}$ $M_\odot$ respectively, each reported as the posterior credible interval bounded by the $16\,\%$ and $84\,\%$ quantiles, with an estimated systematic error $\lesssim 0.1$ km. This result was obtained using the best computationally feasible sampler settings providing a strong radius lower limit but a slightly more uncertain radius upper limit. The inferred radius interval is also close to the $R=12.76_{-1.02}^{+1.49}$ km obtained by Dittmann et al. 2024, when they require the radius to be less than $16$ km as we do. The results continue to disfavor very soft equations of state for dense matter, with $R<11.15$ km for this high mass pulsar excluded at the $95\,\%$ probability. The results do not depend significantly on the assumed cross-calibration uncertainty between NICER and XMM-Newton. Using simulated data that resemble the actual observations, we also show that our pipeline is capable of recovering parameters for the inferred models reported in this paper.

[19] 2406.14467

A More Precise Measurement of the Radius of PSR J0740+6620 Using Updated NICER Data

PSR J0740+6620 is the neutron star with the highest precisely determined mass, inferred from radio observations to be $2.08\pm0.07\,\rm M_\odot$. Measurements of its radius therefore hold promise to constrain the properties of the cold, catalyzed, high-density matter in neutron star cores. Previously, Miller et al. (2021) and Riley et al. (2021) reported measurements of the radius of PSR J0740+6620 based on Neutron Star Interior Composition Explorer (NICER) observations accumulated through 17 April 2020, and an exploratory analysis utilizing NICER background estimates and a data set accumulated through 28 December 2021 was presented in Salmi et al. (2022). Here we report an updated radius measurement, derived by fitting models of X-ray emission from the neutron star surface to NICER data accumulated through 21 April 2022, totaling $\sim1.1$ Ms additional exposure compared to the data set analyzed in Miller et al. (2021) and Riley et al. (2021), and to data from X-ray Multi-Mirror (XMM-Newton) observations. We find that the equatorial circumferential radius of PSR J0740+6620 is $12.92_{-1.13}^{+2.09}$ km (68% credibility), a fractional uncertainty $\sim83\%$ the width of that reported in Miller et al. (2021), in line with statistical expectations given the additional data. If we were to require the radius to be less than 16 km, as was done in Salmi et al. (2024), then our 68% credible region would become $R=12.76^{+1.49}_{-1.02}$ km, which is close to the headline result of Salmi et al. (2024). Our updated measurements, along with other laboratory and astrophysical constraints, imply a slightly softer equation of state than that inferred from our previous measurements.