Nuclear Theory·nucl-th

Thursday·March 28, 2019

8 papers—3 primary·5 cross-listed

01
Classical model for diffusion and thermalization of heavy quarks in a hot medium: memory and out-of-equilibrium effects
Marco Ruggieri🇨🇳 · Marco Frasca🇮🇹 · Santosh Kumar Das🇮🇳
We consider a simple model for the diffusion of heavy quarks in a hot bath, modeling the latter by an ensemble of oscillators distributed accorded to either a thermal distribution or to an out-of-equilibrium distribution with a saturation scale. Within this model it is easy to introduce memory effects by changing the distribution of oscillators: we model these by introducing a gaussian distribution, $d N / d ω$ , which can be deformed continuously from a $δ -$ function giving a Markov dissipation to a broad kernel with memory. Deriving the equation of motion of the heavy quark in the bath we remark how dissipation comes out naturally as an effect of the back-reaction on the bath of oscillators. Moreover, the exact solution of this equation allows for the definition of a thermalization time as the time necessary to remove any memory of the initial condition. We find that the broadening the dissipative kernel while keeping the coupling fixed lowers the thermalization time. We also derive the fluctuation-dissipation theorem for the bath, and use this to estimate the kinematic regime in which momentum diffusion of the heavy quark dominates over drift: we find that diffusion is more important as long as $K_{0} / E$ is small, where $K_{0}$ and $E$ denote the initial energy of the heavy quark and the average energy of the bath respectively.
nucl-thphysics.class-phCPC(2019)·11 citations
02
Scale Dependence of Nucleon-Nucleon Potentials
Omar Benhar
The scale-dependence of the nucleon-nucleon interaction, which in recent years has been extensively analysed within the context of chiral effective field theory, is, in fact, inherent in any potential models constrained by a fit to scattering data. A comparison between a purely phenomenological potential and local interactions derived from chiral effective field theory suggests that--thanks to the ability to describe nucleon-nucleon scattering at higher energies, as well as the deuteron momentum distribution extracted from electro-disintegration data--phenomenological potentials are best suited for the description of nuclear dynamics at the scale relevant to neutron star matter.
nucl-th4 citations
03
Benchmark values for the net proton number fluctuations
Boris Tomasik🇸🇰 · Ivan Melo🇸🇰 · Lukas Laffers🇸🇰 · Marcus Bleicher🇩🇪
We formulate a baseline model for the net-proton number fluctuations in ultrarelativistic heavy-ion collisions. Our model includes total baryon number conservation, fluctuations of the participating nucleon number, limited acceptance, and a possibility that isospin is remembered by the participating wounded nucleons. Most importantly, we formulated the model as consisting of two components: wounded nucleons and the produced nucleon-antinucleon pairs. Those two components have different rapidity distributions. Owing to this feature we obtain also predictions for the rapidity dependence of the cumulants of the net-proton number distribution and their ratios.
nucl-thhep-phnucl-exPoS(2019)·1 citation
04
Dark-matter-nucleus scattering in chiral effective field theory
Martin Hoferichter🇺🇸 · Philipp Klos🇩🇪 · Javier Menéndez🇯🇵 · Achim Schwenk🇩🇪
Chiral effective field theory allows one to calculate the response of few-nucleon systems to external currents, both for currents that can be probed in the Standard Model and ones that only exist in Standard-Model extensions. In combination with state-of-the-art many-body methods, the constraints from chiral symmetry can then be implemented in nuclear structure factors that describe the response of atomic nuclei in direct-detection searches for dark matter. We review the present status of this approach, including the role of coherently enhanced two-body currents, the discrimination of dark matter candidates based on the nuclear response functions, and limits on Higgs-portal dark matter.
↳ hep-phhep-exhep-latnucl-thPoS(2019)·8 citations
05
Electric and magnetic dipole modes in high-resolution inelastic proton scattering at $0^{\circ}$
Peter von Neumann-Cosel (1) · Atsushi Tamii (2) ((1) Institut für Kernphysik, Technische Universität Darmstadt, Germany, (2) Research Center for Nuclear Physics, Osaka University, Japan)
Inelastic proton scattering under extreme forward angles including $0^{\circ}$ and at energies of a few hundred MeV has been established as a new spectroscopic tool for the study of complete dipole strength distributions in nuclei. Such data allow an extraction of the electric dipole polarizability which provides important constraints parameters of the symmetry energy, which determine the neutron skin thickness and the equation of state (EOS) of neutron-rich matter. Also new insight into the much-debated nature of the pygmy dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1 resonance can be studied in heavy nuclei, where only limited experimental information exists so far. Together with much improved results on the isoscalar spin-M1 strength distributions in $N = Z$ nuclei, these data shed new light on the phenomenon of quenching of the nuclear spin response. Using dispersion matching techniques, high energy resolution ( $Δ E / E \leq 1 0^{- 4}$ full width at half maximum, FWHM) can be achieved in the experiments. In spherical-vibrational nuclei considerable fine structure is observed in the energy region of the isovector giant dipole resonance (IVGDR). A quantitative analysis of the fine structure with wavelet methods provides information on the role of different damping mechanisms contributing to the width of the IVGDR. Furthermore, level densities can be extracted from a fluctuation analysis at excitation energies well above neutron threshold, a region hardly accessible by other means. The combination of the gamma strength function (GSF) extracted from the E1 and M1 strength distributions with the independently derived level density permits novel tests of the Brink-Axel hypothesis underlying all calculations of statistical model reaction cross sections in astrophysical applications in the energy region of the PDR.
↳ nucl-exnucl-thEPJA(2019)·52 citations
06
Nucleon Structure Functions from the NJL-Model Chiral Soliton
I. Takyi🇿🇦 · H. Weigel🇿🇦
We present numerical simulations for unpolarized and polarized structure functions in a chiral soliton model. The soliton is constructed self-consistently from quark fields from which the structure functions are extracted. Central to the project is the implementation of regularizing the Dirac sea (or vacuum) contribution to structure functions from first principles. We discuss in detail how sum rules are realized at the level of the quark wave-functions in momentum space. The comparison with experimental data is convincing for the polarized structure functions but exhibits some discrepancies in the unpolarized case. The vacuum contribution to the polarized structure functions is particularly small.
↳ hep-phnucl-thEPJA(2019)·5 citations
07
Isospin breaking decays as a diagnosis of the hadronic molecular structure of the $P_{c} (4457)$
Feng-Kun Guo🇨🇳 · Hao-Jie Jing🇨🇳 · Ulf-G. Meißner🇩🇪 · Shuntaru Sakai🇨🇳
The LHCb Collaboration announced the observation of three narrow structures consistent with hidden-charm pentaquark states. They are candidates of hadronic molecules formed of a pair of a charmed baryon and an anticharmed meson. Among them, the $P_{c} (4457)$ mass is consistent with earlier predictions of a $Σ_{c} D ˉ^{*}$ molecule with $I = 1/2$ . We point out that if such a picture were true, one would have $B (P_{c} (4457) \to J / ψ Δ^{+}) / B (P_{c} (4457) \to J / ψ p)$ at the level ranging from a few percent to about 30%. Such a large isospin breaking decay ratio is two to three orders of magnitude larger than that for normal hadron resonances. It is a unique feature of the $Σ_{c} D ˉ^{*}$ molecular model, and can be checked by LHCb.
↳ hep-phhep-exnucl-thPRD(2019)·149 citations
08
Emergence of a complete heavy-quark spin symmetry multiplet: seven molecular pentaquarks in light of the latest LHCb analysis
Ming-Zhu Liu🇨🇳 · Ya-Wen Pan🇨🇳 · Fang-Zheng Peng🇨🇳 · Mario Sanchez Sanchez🇫🇷 · Li-Sheng Geng🇨🇳 · Atsushi Hosaka🇯🇵 · Manuel Pavon Valderrama🇨🇳
A recent analysis by the LHCb collaboration suggests the existence of three narrow pentaquark-like states --- the $P_{c} (4312)$ , $P_{c} (4440)$ and $P_{c} (4457)$ --- instead of just one in the previous analysis (the $P_{c} (4450)$ ). The closeness of the $P_{c} (4312)$ to the $D ˉ Σ_{c}$ threshold and the $P_{c} (4440)$ / $P_{c} (4457)$ to the $D ˉ^{*} Σ_{c}$ one suggests a molecular interpretation of these resonances. We show that these three pentaquark-like resonances can be naturally accommodated in a contact-range effective field theory description that incorporates heavy-quark spin symmetry. This description leads to the prediction of all the seven possible S-wave heavy antimeson-baryon molecules (that is, there should be four additional molecular pentaquarks in addition to the $P_{c} (4312)$ , $P_{c} (4440)$ and $P_{c} (4457)$ ), providing the first example of a heavy-quark spin symmetry molecular multiplet that is complete. If this is confirmed, it will not only give us an impressive example of the application of heavy-quark symmetries and effective field theories in hadron physics: it will also uncover a clear and powerful ordering principle for the molecular spectrum, reminiscent of the SU(3)-flavor multiplets to which the light hadron spectrum conforms.
↳ hep-phhep-exhep-latnucl-thPRL(2019)·294 citations

Affiliations

first authorsco-authorsvia INSPIRE