Nuclear Theory·nucl-th

Tuesday·January 16, 2018

14 papers—9 primary·5 cross-listed

01
Two-photon production of dilepton pairs in peripheral heavy ion collisions
Spencer R. Klein🇺🇸
The STAR collaboration has observed an excess production of $e^{+} e^{-}$ pairs in relativistic heavy ion collisions, over the expectations from hadronic production models. The excess pairs have transverse momenta $p_{T} < 150 MeV / c$ and are most prominent in peripheral gold-gold and uranium-uranium collisions. The pairs exhibit a peak at the $J / ψ$ mass, but include a wide continuum, with pair invariant masses from 400 MeV/c $^{2}$ up to 2.6 GeV/c $^{2}$ . The ALICE Collaboration observes a similar excess in peripheral lead-lead collisions, but only at the $J / ψ$ mass, without a corresponding continuum. This paper presents a calculation of the cross-section and kinematic for two-photon production of $e^{+} e^{-}$ pairs, and find general agreement with the STAR data. The calculation is based on the STARlight simulation code, which is based on the Weizsäcker-Williams virtual photon approach. The STAR continuum observations are compatible with two-photon production of $e^{+} e^{-}$ pairs. The ALICE analysis required individual muon $p_{T}$ be greater than 1 GeV/c; this eliminated almost all of the pairs from two-photon interactions, while leaving most of the $J / ψ$ decays.
nucl-thhep-phPRC(2018)·51 citations
02
Nuclear mass predictions based on Bayesian neural network approach with pairing and shell effects
Z. M. Niu · H. Z. Liang
Bayesian neural network (BNN) approach is employed to improve the nuclear mass predictions of various models. It is found that the noise error in the likelihood function plays an important role in the predictive performance of the BNN approach. By including a distribution for the noise error, an appropriate value can be found automatically in the sampling process, which optimizes the nuclear mass predictions. Furthermore, two quantities related to nuclear pairing and shell effects are added to the input layer in addition to the proton and mass numbers. As a result, the theoretical accuracies are significantly improved not only for nuclear masses but also for single-nucleon separation energies. Due to the inclusion of the shell effect, in the unknown region, the BNN approach predicts a similar shell-correction structure to that in the known region, e.g., the predictions of underestimation of nuclear mass around the magic numbers in the relativistic mean-field model. This manifests that better predictive performance can be achieved if more physical features are included in the BNN approach.
nucl-thastro-ph.SRnucl-exPLB(2018)·216 citations
03
Chiral NNLO $_{sat}$ descriptions of nuclear multipole resonances within the random phase approximation
Q. Wu · B. S. Hu · F. R. Xu · Y. Z. Ma · S. J. Dai · Z.H. Sun · G. R. Jansen
We study nuclear multipole resonances in the framework of the random phase approximation using the chiral potential NNLO $_{sat}$ . This potential includes two- and three-body terms that has been simultaneously optimized to low-energy nucleon-nucleon scattering data and selected nuclear structure data. Our main foci have been the isoscalar monopole, isovector dipole, and isoscalar quadrupole resonances of the closed-shell nuclei, $^{4}$ He, $^{16, 22, 24}$ O, and $^{40, 48}$ Ca. These resonance modes have been widely observed in experiment. In addition, we use a renormalized chiral potential $V_{low- k}$ , based on the N $^{3}$ LO two-body potential by Entem and Machleidt [Phys. Rev. \textbf{C68}, 041001 (2011)]. This introduces a dependency on the cutoff parameter used in the normalization procedure as reported in previous works by other groups. While NNLO $_{sat}$ can reasonably reproduce observed multipole resonances, it is not possible to find a single cutoff parameter for the $V_{low- k}$ potential that simultaneously describe the different types of resonance modes. The sensitivity to the cutoff parameter can be explained by missing induced three-body forces in the calculations. Our results for neutron-rich $^{22, 24}$ O show a mixing nature of isoscalar and isovector resonances in the dipole channel at low energies. We predict that $^{22}$ O and $^{24}$ O have low-energy isoscalar quadrupole resonances at energies lower than 5 MeV.
nucl-thPRC(2018)·9 citations
04
Shell model study on the possibility of using an effective field theory for disentangling several contributions to the neutrinoless double-beta decay
Andrei Neacsu🇺🇸 · Mihai Horoi🇺🇸
Neutrinoless double-beta decay represents the most promising approach for revealing some of the most important, yet-unknown, properties of neutrinos related to their absolute masses and their nature. This transition involves beyond standard model theories that predict the violation of the lepton number conservation by two units. There is no experimental confirmation yet for this decay, but new experiments have set lower-limits for the associated half-lives in the case of several isotopes. Using an effective field theory that describes this transition, we calculate half-life ratios for five experimentally interesting isotopes in the case of 12 lepton number violating couplings. These half-life ratios can be used to probe the sensitivity of the five isotopes in relation to their respective mechanisms, to predict the half-life limits needed to match the different experimental results, and in the case of experimental confirmation, these ratios could possibly indicate the dominant mechanism of the transition. We provide an analysis that could reveal valuable information regarding the dominant neutrinoless double-beta decay mechanism, if experimental half-life data becomes available for different isotopes.
nucl-thhep-phhep-thPRC(2018)·34 citations
05
The Nuclear Contact Formalism - the Deuteron Channel
Ronen Weiss · Nir Barnea
The contact formalism, devised to elucidate the important role of short-range correlations, was recently generalized for systems with coupled channels, such as the deuteron channel, where s and d waves are coupled into a $J = 1$ state. The coupling of the two channels implies two independent asymptotic solutions and results a 2x2 contact matrix. For a strong coupling, with appropriate boundary conditions, such as in nuclear physics, the two solutions degenerate into a single asymptotic wave function. Here, we explore the asymptotic behavior of the correlated neutron-proton pairs in the deuteron channel using both schematic models and realistic nuclear forces.
nucl-thcond-mat.quant-gasnucl-exphysics.atm-clus+11 citation
06
Imprints of the nuclear symmetry energy on the tidal deformability of neutron stars
Plamen G. Krastev · Bao-An Li
Applying an equation of state (EOS) with its symmetric nuclear matter (SNM) contribution and low-density symmetry energy $E_{sy m} (ρ)$ constrained by heavy-ion reaction data, we calculate the dimensionless tidal deformability $Λ$ of neutron stars in coalescing binary systems. Corresponding to the partially constrained EOS that previously predicted a radius of 11.5 km $\leq R_{1.4} \leq$ 13.6 km for canonical neutron-star configurations, $Λ$ is found to be in the range of 292 $\leq Λ_{1.4} \leq$ 680, consistent with the very recent observation of the GW170817 event. We investigate the effect of the high-density behavior of $E_{sy m} (ρ)$ on the tidal properties of neutron stars and find that while $Λ$ depends strongly on the details of the symmetry energy, different trends of $E_{sy m} (ρ)$ lead to very similar values of $Λ$ . In particular, the transition from stiff/soft to soft/stiff $E_{sy m} (ρ)$ could yield the same $Λ$ . Thus, measuring $Λ$ alone may not determine completely the density dependence of the symmetry energy. Coherent analyses of the dense neutron-rich nuclear matter EOS underlying both nuclear laboratory experiments and astrophysical observations are therefore necessary to break this degeneracy and determine precisely the details of the $E_{sy m} (ρ)$ .
nucl-thastro-ph.HEnucl-exJ.Phys.G(2019)·76 citations
07
Nucleon viewed as a Borromean Bound-State
Jorge Segovia🇪🇸 · Cédric Mezrag🇮🇹 · Lei Chang🇨🇳 · Craig D. Roberts🇺🇸
We explain how the emergent phenomenon of dynamical chiral symmetry breaking ensures that Poincaré covariant analyses of the three valence-quark scattering problem in continuum quantum field theory yield a picture of the nucleon as a Borromean bound-state, in which binding arises primarily through the sum of two separate contributions. One involves aspects of the non-Abelian character of Quantum Chromodynamics that are expressed in the strong running coupling and generate tight, dynamical color-antitriplet quark-quark correlations in the scalar-isoscalar and pseudovector-isotriplet channels. This attraction is magnified by quark exchange associated with diquark breakup and reformation, which is required in order to ensure that each valence-quark participates in all diquark correlations to the complete extent allowed by its quantum numbers. Combining these effects, we arrive at a properly antisymmetrised Faddeev wave function for the nucleon and calculate, e.g. the flavor-separated versions of the Dirac and Pauli form factors and the proton's leading-twist parton distribution amplitude. We conclude that available data and planned experiments are capable of validating the proposed picture.
nucl-thhep-exhep-lathep-phFew Body Syst.(2018)·2 citations
08
The MAID Legacy and Future
Lothar Tiator🇩🇪
The MAID project is a collection of theoretical models for pseudoscalar meson photo- and electroproduction from nucleons. It is online available and produces results in real time calculations. In addition to kinematical variables also model parameters, especially for baryon resonances, can be online changed and investigated. Over 20 years MAID has become quite popular and the MAID web pages have been called more than 7.7 million times.
nucl-thhep-phFew Body Syst.(2018)·25 citations
09
$A b ini t i o$ calculations for Gamow-Teller strengths in $s d$ shell
Archana Saxena · Praveen C. Srivastava · Toshio Suzuki
In the present work we perform a systematic shell model study of Gamow-Teller transition strength distributions in $s d$ shell nuclei using $ab ini t i o$ effective interactions. The $ab ini t i o$ effective interactions are based on in-medium similarity renormalization group (IM-SRG) and coupled-cluster effective interaction (CCEI) approaches. The aim of the present work is to test the predictive power of $ab ini t i o$ effective interactions by using the available experimental data of Gamow-Teller strength distributions in $s d$ shell nuclei. We perform calculations for $^{20}$ Ne $\to$ $^{20}$ F, $^{23}$ Na $\to$ $^{23}$ Mg, $^{23}$ Na $\to$ $^{23}$ Ne, $^{24}$ Mg $\to$ $^{24}$ Na, $^{24}$ Mg $\to$ $^{24}$ Al, $^{25}$ Mg $\to$ $^{25}$ Al, $^{26}$ Mg $\to$ $^{26}$ Na, $^{26}$ Mg $\to$ $^{26}$ Al, $^{26}$ Si $\to$ $^{26}$ Al, $^{27}$ Al $\to$ $^{27}$ Si, $^{28}$ Si $\to$ $^{28}$ P, $^{31}$ P $\to$ $^{31}$ Si, and $^{32}$ S $\to$ $^{32}$ P transitions. For comparison we also show the results obtained by using the phenomenological USDB Hamiltonian. The phenomenological USDB results of the Gamow-Teller (GT $_{+}$ /GT $_{-}$ ) strength distributions show reasonable agreements with the experimental data in comparison to the $ab ini t i o$ interactions. We also calculate the electron capture reaction rates for $^{23}$ Na(e $^{-}$ , $ν$ ) $^{23}$ Ne and $^{25}$ Mg(e $^{-}$ , $ν$ ) $^{25}$ Na using $ab ini t i o$ and USDB interactions.
nucl-thnucl-exPRC(2018)·24 citations
10
Transport coefficients in the Polyakov quark meson coupling model: A quasi particle approach
Aman Abhishek🇮🇳 · Sabyasachi Ghosh🇮🇳 · Hiranmaya Mishra🇮🇳
We compute the transport coefficients, namely, the coefficients of shear and bulk viscosities as well as thermal conductivity for hot and dense matter. The calculations are performed within the Polyakov loop extended quark meson model. The estimation of the transport coefficients is made using the Boltzmann kinetic equation within the relaxation time approximation. The energy dependent relaxation time is estimated from meson meson scattering, quark meson scattering and quark quark scattering within the model. In our calculations, the shear viscosity to entropy ratio and the coefficient of thermal conductivity show a minimum at the critical temperature, while the ratio of bulk viscosity to entropy density exhibits a peak at this transition point. The effect of confinement modelled through a Polyakov loop potential plays an important role in the estimation of these dissipative coefficients both below and above the critical temperature.
↳ hep-phnucl-thPoS(2018)·1 citation
11
Surveying exotic pentaquarks with the typical $QQq q q ˉ$ configuration
Qin-Song Zhou🇨🇳 · Kan Chen🇨🇳 · Xiang Liu🇨🇳 · Yan-Rui Liu🇨🇳 · Shi-Lin Zhu🇨🇳
As a hot issue, exploring exotic pentaquarks is full of challenges and opportunities for both theorist and experimentalist. In this work, we focus on a type of pentaquark with the $QQq q q ˉ$ ( $Q = b, c$ ; $q = u, d, s$ ) configuration, where their mass spectrum is estimated systematically. Especially, our result indicates that there may exist some stable or narrow exotic pentaquark states. Obviously, our study may provides valuable information for further experimental search for the $QQq q q ˉ$ pentaquarks. With the running of LHCb and forthcoming Belle II, we have a reason to believe that these predictions present here can be tested.
↳ hep-phhep-latnucl-thPRC(2018)·51 citations
12
On the form factors of $d^{*} (2380)$
Yubing Dong🇨🇳 · Pengnian Shen🇨🇳 · Zongye Zhang🇨🇳
In order to explore the possible physical quantities for judging different structures of the newly observed resonance $d^{*} (2380)$ , we study its electromagnetic form factors. In addition to the electric charge monopole $C 0$ , we calculate its electric quadrupole $E 2$ , magnetic dipole $M 1$ , and six-pole $M 3$ form factors on the base of the realistic coupled $ΔΔ + C C$ channel $d^{*}$ wave function with both the $S$ - and $D$ -partial waves. The results show that the magnetic dipole moment and electric quadrupole deformation of $d^{*}$ are 7.602 and $2.53 \times 1 0^{- 2} fm^{2}$ , respectively. The calculated magnetic dipole moment in the naive constituent quark model is also compared with the result of $D_{12} π$ picture. By comparing with partial results where the $d^{*}$ state is considered with a single $ΔΔ$ and with a $D_{12} π$ structures, we find that in addition to the charge distribution of $d^{*} (2380)$ , the magnetic dipole moment and magnetic radius can be used to discriminate different structures of $d^{*}$ . Moreover, a quite small electric quadrupole deformation indicates that $d^{*}$ is more inclined to an slightly oblate shape due to our compact hexaquark dominated structure of $d^{*} (2380)$ .
↳ hep-phnucl-thPRD(2018)·19 citations
13
Hadronic structure from double parton scattering
Matteo Rinaldi🇪🇸 · Federico Alberto Ceccopieri🇧🇪
In the present paper we consider the so-called effective cross section, a quantity which encodes the experimental knowledge on double parton scattering in hadronic collisions that has been accomulated so far. We show that the effective cross section, under some assumptions close to those adopted in its experimental extractions, can be used to obtain a range of mean transverse distance between an interacting parton pair in double parton scattering. Therefore we have proved that the effective cross section offers a way to access information on the hadronic structure.
↳ hep-phnucl-thPRD(2018)·32 citations
14
Multi Quark Production in p+A collisions: Quantum Interference Effects
Alex Kovner🇺🇸 · Amir H. Rezaeian🇨🇱
We consider forward inclusive production of several quarks in the high energy p-A collisions in the CGC formalism. For three particle production we provide a complete expression in terms of multipole scattering amplitudes on the nucleus and multi particle generalized TMD's of the proton. We then calculate all the terms that are not suppressed by the factor of the area in four particle production, and generalize this result up to terms of order $1/ N_{c}^{2}$ for arbitrary number of produced particles. Our results include the contribution of quantum interference effects both in the final state radiation (HBT) and in the initial projectile wave function (Pauli blocking).
↳ hep-phnucl-thPRD(2018)·25 citations

Affiliations

first authorsco-authorsvia INSPIRE