Nuclear Theory·nucl-th

Tuesday·February 9, 2021

16 papers—7 primary·9 cross-listed

01
Determining the leading-order contact term in neutrinoless double $β$ decay
Vincenzo Cirigliano🇺🇸 · Wouter Dekens🇺🇸 · Jordy de Vries🇳🇱 · Martin Hoferichter🇨🇭 · Emanuele Mereghetti🇺🇸
We present a method to determine the leading-order (LO) contact term contributing to the $nn \to pp e^{-} e^{-}$ amplitude through the exchange of light Majorana neutrinos. Our approach is based on the representation of the amplitude as the momentum integral of a known kernel (proportional to the neutrino propagator) times the generalized forward Compton scattering amplitude $n (p_{1}) n (p_{2}) W^{+} (k) \to p (p_{1}^{'}) p (p_{2}^{'}) W^{-} (k)$ , in analogy to the Cottingham formula for the electromagnetic contribution to hadron masses. We construct model-independent representations of the integrand in the low- and high-momentum regions, through chiral EFT and the operator product expansion, respectively. We then construct a model for the full amplitude by interpolating between these two regions, using appropriate nucleon factors for the weak currents and information on nucleon-nucleon ( $N N$ ) scattering in the $^{1} S_{0}$ channel away from threshold. By matching the amplitude obtained in this way to the LO chiral EFT amplitude we obtain the relevant LO contact term and discuss various sources of uncertainty. We validate the approach by computing the analog $I = 2$ $N N$ contact term and by reproducing, within uncertainties, the charge-independence-breaking contribution to the $^{1} S_{0}$ $N N$ scattering lengths. While our analysis is performed in the $MS$ scheme, we express our final result in terms of the scheme-independent renormalized amplitude $A_{ν} (∣ p ∣, ∣ p^{'} ∣)$ at a set of kinematic points near threshold. We illustrate for two cutoff schemes how, using our synthetic data for $A_{ν}$ , one can determine the contact-term contribution in any regularization scheme, in particular the ones employed in nuclear-structure calculations for isotopes of experimental interest.
nucl-thhep-lathep-phJHEP(2021)·81 citations
02
Cross sections for neutron-induced reactions from surrogate data: revisiting the Weisskopf-Ewing approximation for $(n, n^{'})$ and $(n, 2 n)$ reactions
Oliver C. Gorton · Jutta E. Escher
Earlier work has demonstrated that cross sections for neutron-induced fission and radiative neutron capture can be determined from a combination of surrogate reaction data and theory. For the fission case, it was shown that Weisskopf-Ewing approximation, which significantly simplifies the implementation of the surrogate method, can be employed. Capture cross sections cannot be obtained, and require a detailed description of the surrogate reaction process. In this paper we examine the validity of the Weisskopf-Ewing approximation for determining unknown $(n, n^{'})$ and $(n, 2 n)$ cross sections from surrogate data. We find that peak cross sections can be estimated using the Weisskopf-Ewing approximation, but the shape of the $(n, n^{'})$ and $(n, 2 n)$ cross sections, especially for low neutron energies, cannot be reliably determined without accounting for the angular-momentum differences between the neutron-induced and surrogate reaction. To obtain reliable $(n, n^{'})$ and $(n, 2 n)$ cross sections from surrogate reaction data, a detailed description of the surrogate reaction mechanisms is required. To do so for the compound-nucleus energies and decay channels relevant to these reactions, it becomes necessary to extend current modeling capabilities.
nucl-thnucl-exPRC(2023)·5 citations
03
Fission fragment distributions and their impact on the r-process nucleosynthesis in neutron star mergers
J.-F. Lemaître🇧🇪 · S. Goriely🇧🇪 · A. Bauswein🇩🇪 · H.-T. Janka🇩🇪
Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A >140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties. Here, we introduce new improvements to our scission-point model, called SPY, to derive the fission fragment distribution for all neutron-rich fissioning nuclei of relevance in r-process calculations. These improvements include a phenomenological modification of the scission distance and a smoothing procedure of the distribution. Such corrections lead to a much better agreement with experimental fission yields. Those yields are also used to estimate the number of neutrons emitted by the excited fragments on the basis of different neutron evaporation models. Our new fission yields are extensively compared to those predicted by the so-called GEF model. The impact of fission on the r-process nucleosynthesis in binary neutron mergers is also reanalyzed. Two scenarios are considered, the first one with low initial electron fraction is subject to intense fission recycling, in contrast to the second one which includes weak interactions on nucleons. The various regions of the nuclear chart responsible for fission recycling during the neutron irradiation as well as after freeze-out are discussed. The contribution fission processes may have to the final abundance distribution is also studied in detail in the light of newly defined quantitative indicators describing the fission recycling, the fission seeds and the fission progenitors. In particular, those allow us to estimate the contribution of fission to the final abundance distribution stemming from specific heavy nuclei. Calculations obtained with SPY and GEF fission fragment distributions are compared for both r-process scenarios.
nucl-thPRC(2021)·40 citations
04
Variation after projection calculations for high-spin states
Zao-Chun Gao
In this paper, an improvement has been made on the variation after projection (VAP) method, which is crucial in the calculations of high-spin states. It turns out that, the form of the trial VAP wave function with spin $J$ can be simplified by adopting just one projected state rather than previously adopting all $(2 J + 1)$ angular momentum projected states for each selected reference state, $∣Φ ⟩$ . The present calculations show that such simplification still minimizes the angular momentum projected energy up to a very good approximation. In this simplified VAP scheme, one can obtain almost equivalent VAP wave functions starting from different sets of the projected basis states. This clearly shows that a nuclear state can not be identified with a single intrinsic state, while in the traditional nuclear collective models, an intrinsic state is usually assigned to a well deformed rotational band.
nucl-thPLB(2022)·14 citations
05
Rotating neutron stars with quark cores
I. A. Rather🇮🇳 · Usuf Rahaman🇮🇳 · M. Imran🇮🇳 · H. C. Das🇮🇳 · A. A. Usmani🇮🇳 · S. K. Patra🇮🇳
The rotating neutron star properties are studied with a phase transition to quark matter. The density-dependent relativistic mean-field model (DD-RMF) is employed to study the hadron matter, while the Vector-Enhanced Bag model (vBag) model is used to study the quark matter. The star matter properties like mass, radius,the moment of inertia, rotational frequency, Kerr parameter, and other important quantities are studied to see the effect on quark matter. The maximum mass of rotating neutron star with DD-LZ1 and DD-MEX parameter sets is found to be around 3 $M_{⊙}$ for pure hadronic phase and decreases to a value around 2.6 $M_{⊙}$ with phase transition to quark matter, which satisfies the recent GW190814 constraints. For DDV, DDVT, and DDVTD parameter sets, the maximum mass decreases to satisfy the 2 $M_{⊙}$ . The moment of inertia calculated for various DD-RMF parameter sets decreases with the increasing mass satisfying constraints from various measurements. Other important quantities calculated also vary with the bag constant and hence show that the presence of quarks inside neutron stars can also allow us to constraint these quantities to determine a proper EoS. Also, the theoretical study along with the accurate measurement of uniformly rotating neutron star properties may offer some valuable information concerning the high-density part of the equation of state.
nucl-thastro-ph.HEPRC(2021)·39 citations
06
Few-nucleon matrix elements in pionless effective field theory in a finite volume
W. Detmold🇺🇸 · P. E. Shanahan🇺🇸
Pionless effective field theory in a finite volume (FVEFT $_{π /}$ ) is investigated as a framework for the analysis of multi-nucleon spectra and matrix elements calculated in lattice QCD (LQCD). By combining FVEFT $_{π /}$ with the stochastic variational method, the spectra of nuclei with atomic number $A \in {2, 3}$ are matched to existing finite-volume LQCD calculations at heavier-than-physical quark masses corresponding to a pion mass $m_{π} = 806$ MeV, thereby enabling infinite-volume binding energies to be determined using infinite-volume variational calculations. Based on the variational wavefunctions that are constructed in this approach, the finite-volume matrix elements of various local operators are computed in FVEFT $_{π /}$ and matched to LQCD calculations of the corresponding QCD operators in the same volume, thereby determining the relevant one and two-body EFT counterterms and enabling an extrapolation of the LQCD matrix elements to infinite volume. As examples, the scalar, tensor, and axial matrix elements are considered, as well as the magnetic moments and the isovector longitudinal momentum fraction.
nucl-thhep-lathep-phPRD(2021)·35 citations
07
Why nuclear forces favor the highest weight irreducible representations of the fermionic SU(3) symmetry
Andriana Martinou🇬🇷 · Dennis Bonatsos🇬🇷 · K.E. Karakatsanis🇬🇷 · S. Sarantopoulou🇬🇷 · I.E. Assimakis🇬🇷 · S.K. Peroulis🇬🇷 · N. Minkov🇧🇬
The consequences of the attractive, short-range nucleon-nucleon (NN) interaction on the wave functions of the Elliott SU(3) and the proxy-SU(3) symmetry are discussed. The NN interaction favors the most symmetric spatial SU(3) irreducible representation, which corresponds to the maximal spatial overlap among the fermions. The percentage of the symmetric components out of the total in an SU(3) wave function is introduced, through which it is found, that no SU(3) irrep is more symmetric than the highest weight irrep for a certain number of valence particles in a three dimensional, isotropic, harmonic oscillator shell. The consideration of the highest weight irreps in nuclei and in alkali metal clusters, leads to the prediction of a prolate to oblate shape transition beyond the mid-shell region.
nucl-thEPJA(2021)·28 citations
08
Radiative corrections to nucleon weak charges and Beyond Standard Model impact
Leendert Hayen🇺🇸
The nucleon axial charge is a central ingredient in nuclear and particle physics, and a key observable in precision tests of the electroweak Standard Model sector and beyond. We report on the first complete calculation of its electroweak quantum corrections up to $O (α)$ , using a combination of current algebra techniques and QCD sum rules. We find a substantial enhancement due to the weak magnetism contribution in the elastic channel, and include higher-twist and target mass corrections at low $Q^{2}$ to find $Δ_{R}^{A} = 0.02881 (22)$ . Using analogous methods, we determine a new value for the vector charge renormalization, $Δ_{R}^{V} = 0.02474 (27)$ , and show how the two most recent calculations can be brought into agreement. This allows us to determine a corrected experimental $g_{A}^{0} = 1.2730 (13)$ , which is a $> 2 σ$ shift away from the commonly quoted value. We use this new result to set constraints on exotic right-handed currents by comparing to lattice QCD results, and resolve a double-counting issue in the $∣ V_{u d} ∣$ extraction from mirror decays.
↳ hep-phnucl-exnucl-th11 citations
09
Fermion propagator in a rotating environment
Alejandro Ayala🇲🇽 · L. A. Hernández🇿🇦 · K. Raya🇲🇽 · R. Zamora🇨🇱
We apply the exponential operator method to derive the propagator for a fermion immersed within a rigidly rotating environment with cylindrical geometry. Given that the rotation axis provides a preferred direction, Lorentz symmetry is lost and the general solution is not translationally invariant in the radial coordinate. However, under the approximation that the fermion is completely dragged by the vortical motion, valid for large angular velocities, translation invariance is recovered. The propagator can then be written in momentum space. The result is suited to be used applying ordinary Feynman rules for perturbative calculations in momentum space.
↳ hep-phhep-thnucl-thPRD(2021)·23 citations
10
Critical behaviour of hydrodynamic series
M. Asadi (1)🇮🇷 · H. Soltanpanahi (2 and 3)🇨🇳 · Farid Taghinavaz (1) ((1) IPM, Tehran, (2) South China Normal University, Guangzhou, (3) Jagiellonian University, Krakow)🇮🇷
We investigate the time-dependent perturbations of strongly coupled $N = 4$ SYM theory at finite temperature and finite chemical potential with a second order phase transition. This theory is modelled by a top-down Einstein-Maxwell-dilaton description which is a consistent truncation of the dimensional reduction of type IIB string theory on AdS $_{5} \times$ S $^{5}$ . We focus on spin-1 and spin-2 sectors of perturbations and compute the linearized hydrodynamic transport coefficients up to the third order in gradient expansion. We also determine the radius of convergence of the hydrodynamic mode in spin-1 sector and the lowest non-hydrodynamic modes in spin-2 sector. Analytically, we find that all the hydrodynamic quantities have the same critical exponent near the critical point $θ = 1/2$ . Moreover, we establish a relation between symmetry enhancement of the underlying theory and vanishing the only third order hydrodynamic transport coefficient $θ_{1}$ , which appears in the shear dispersion relation of a conformal theory on a flat background.
↳ hep-thcond-mat.str-elhep-phmath-ph+2JHEP(2021)·16 citations
11
High Energy Behaviour of Light Meson Photoproduction
Moskov J. Amaryan (ODU)🇺🇸 · William J. Briscoe (GWU)🇺🇸 · Michael G. Ryskin (PNPI)🇷🇺 · Igor I. Strakovsky (GWU)🇺🇸
We evaluated recent CLAS Collaboration measurements for the $9 0^{\circ}$ meson photoproduction off the nucleon using a tagged photon beam spanning the energy interval $s = 3 - 11$ GeV $^{2}$ . The results are compared with the "Quark Counting Rules" predictions.
↳ hep-phhep-exnucl-exnucl-thPRC(2021)·6 citations
12
Condensation of Cooper Triples
Sora Akagami🇯🇵 · Hiroyuki Tajima🇯🇵 · Kei Iida🇯🇵
The condensation of Cooper pairs, originating from the Fermi-surface instability due to a weakly attractive interaction between two fermions, opened a new frontier for exploring many-body physics in interdisciplinary contexts. In this work, we discuss the possible condensation of Cooper triples, which are three-body counterparts of Cooper pairs for three-component fermions with a three-body attraction. Although each composite trimer-like state obeys the Fermi-Dirac statistics, its aggregate can form a condensate at zero center-of-mass momentum in the presence of the internal degrees of freedom associated with the relative momenta of constituent particles of momenta close to the Fermi surface. Such condensation can be regarded as bosonization in infinite-component fermions. We propose a variational wave function for the condensate of Cooper triples in analogy with the Bardeen-Cooper-Schrieffer ground state and obtain the ground-state energy.
↳ cond-mat.quant-gascond-mat.supr-conhep-phnucl-thPRA(2021)·16 citations
13
Electronic width of the $ψ (3770)$ resonance interfering with the background
N.N. Achasov🇷🇺 · G.N. Shestakov🇷🇺
Methods for extracting the $ψ (3770) \to e^{+} e^{-}$ decay width from the data on the reaction cross section $e^{+} e^{-} \to D D ˉ$ are discussed. Attention is drawn to the absence of the generally accepted method for determining $Γ_{ψ (3770) e^{+} e^{-}}$ in the presence of interference between the contributions of the $ψ (3770)$ resonance and background. It is shown that the model for the experimentally measured $D$ meson form factor, which satisfies the requirement of the Watson theorem and takes into account the contribution of the complex of the mixed $ψ (3770)$ and $ψ (2 S)$ resonances, allows uniquely determine the value of $Γ_{ψ (3770) e^{+} e^{-}}$ by fitting. The $Γ_{ψ (3770) e^{+} e^{-}}$ values found from the data processing are compared with the estimates in the potential models.
↳ hep-phhep-exnucl-thPRD(2021)·5 citations
14
Relativistic corrections to decays of heavy baryons in the quark model
Ahmad Jafar Arifi🇯🇵 · Daiki Suenaga🇯🇵 · Atsushi Hosaka🇯🇵
We investigate relativistic corrections of order $1/ m^{2}$ , where $m$ is the constituent quark mass, to heavy baryon decays by emitting one pseudoscalar meson in the quark model. This work is motivated by shortcomings in the previous studies in the nonrelativistic quark model for decays of the Roper-like states such as $Λ_{c} (2765)$ . We find that the relativistic corrections due to the internal motion of quarks are essential ingredients in improving their decay properties such that the decay widths are significantly increased. In addition, such corrections can explain a phenomenological suppression of the quark axial-vector coupling constant $g_{A}^{q}$ for the $Σ_{c} (2455)$ and $Σ_{c} (2520)$ decays.
↳ hep-phnucl-thPRD(2021)·36 citations
15
The axial charge of the triton from lattice QCD
Assumpta Parreño🇪🇸 · Phiala E. Shanahan🇺🇸 · Michael L. Wagman🇺🇸 · Frank Winter🇺🇸 · Emmanuel Chang · William Detmold🇺🇸 · Marc Illa🇪🇸
The axial charge of the triton is investigated using lattice quantum chromodynamics (QCD). Extending previous work at heavier quark masses, calculations are performed using three ensembles of gauge field configurations generated with quark masses corresponding to a pion mass of 450 MeV. Finite-volume energy levels for the triton, as well as for the deuteron and diproton systems, are extracted from analysis of correlation functions computed on these ensembles, and the corresponding energies are extrapolated to infinite volume using finite-volume pionless effective field theory (FVEFT). It is found with high likelihood that there is a compact bound state with the quantum numbers of the triton at these quark masses. The axial current matrix elements are computed using background field techniques on one of the ensembles and FVEFT is again used to determine the axial charge of the proton and triton. A simple quark mass extrapolation of these results and earlier calculations at heavier quark masses leads to a value of the ratio of the triton to proton axial charges at the physical quark masses of $g_{A}^{^{3} H} / g_{A}^{p} = 0.91 + 0.07 - 0.09$ . This result is consistent with the ratio determined from experiment and prefers values less than unity (in which case the triton axial charge would be unmodified from that of the proton), thereby demonstrating that QCD can explain the modification of the axial charge of the triton.
↳ hep-lathep-phnucl-thPRD(2021)·23 citations
16
Isochronic evolution and the radioactive decay of r-process nuclei
T. M. Sprouse · G. Wendell Misch · M. R. Mumpower
We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased and nuclear decays are the dominant transmutation channels. We find that the decay from short-lived to long-lived species naturally leads to an isochronic evolution in which nuclei with similar half-lives are populated at the same time. We consider random perturbations along each isobaric chain to initial solar-like r-process compositions to demonstrate the isochronic nature of the late-time phase of the r-process. Our analysis shows that detailed knowledge of the final isotopic composition allows for the prediction of late-time evolution with a high degree of confidence despite uncertainties that exist in astrophysical conditions and the nuclear physics properties of the most neutron-rich nuclei. We provide the time-dependent nuclear composition in the Appendix as supplemental material.
↳ astro-ph.HEnucl-thApJ(2022)·12 citations

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