Nuclear Theory·nucl-th

Wednesday·September 9, 2020

8 papers—3 primary·5 cross-listed

01
The Origin of the Giant Dipole Resonance
Richard B. Firestone
The Giant Dipole Resonance (GDR), which is conventionally described as due to collective motion, is instead shown to be the result of a sudden increase in level density at the 2h{\omega} shell closure. The energy of the GDR closely follows the shell model harmonic oscillator energy model where h{\omega} = 39A^-1/3, for heavy nuclei. A better fit covering the entire mass range is given by h{\omega} = 47:55(0:13)(A^-1/3 - A^-2/3). The GDR is shown to be composed of a lower energy peak, E1, corresponding to the population of levels with oblate deformation and a higher energy peak, E2 corresponding to the population of levels with prolate deformation. The peak energy separation is proportional to the \beta_2 deformation and given by E2-E1 = 11:03(0:22)|\beta_2|. The total photonuclear cross section, sigma = sigma1 + sigma2, populating the GDR is proportional to the level density at the GDR and is given by sigma = 0:483(0:006)A^4/3 where sigma1 = sigma2. The widths of the two GDR peaks are consistent with Nilsson model predictions and found to be Gamma1 = 7:41(0:15)A^-1/6 MeV and Gamma2 = 11:13(0:16)A^-1/6 respectively. The Standard Lorentzian model parameters are fitt to high accuracy as a function of mass and deformation and can be applied reliably to all nuclei. It is shown that the energies of pygmy and spin flip resonances correspond to the E = h{\omega} harmonic oscillator energy and that the giant quadrupole (GQR), giant monopole (GMR), and giant octupole (GOR) resonances coincide with the E = 2-4 h{\omega} harmonic oscillator energies where the level density suddenly increases at the shell gaps.
nucl-th3 citations
02
First results from Hybrid Hadronization in small and large systems
M. Kordell II · A. Angerami · S. A. Bass · S. Cao · Y. Chen · J. Coleman · L. Cunqueiro · T. Dai · L. Du · R. Ehlers · H. Elfner · D. Everett and 38 other authors
"Hybrid Hadronization" is a new Monte Carlo package to hadronize systems of partons. It smoothly combines quark recombination applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates with PYTHIA 8 and can be applied to a variety of systems from $e^{+} + e^{-}$ to $A + A$ collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for $A + A$ collisions a thermal background medium is provided, the package allows sampling thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is also part of JETSCAPE since the 2.0 release. In these proceedings we review the physics concepts underlying Hybrid Hadronization and demonstrate how users can use the code with various parton shower Monte Carlos. We present calculations of hadron chemistry and fragmentation functions in small and large systems when Hybrid Hadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular, we discuss observable effects of the recombination of shower partons with thermal partons.
nucl-thhep-phPoS(2021)·1 citation
03
Extended Lipkin-Meshkov-Glick Hamiltonian
R. Romano · X. Roca-Maza · G. Colò · Shihang Shen
The Lipkin-Meshkov-Glick (LMG) model was devised to test the validity of different approximate formalisms to treat many-particle systems. The model was constructed to be exactly solvable and yet non-trivial, in order to capture some of the main features of real physical systems. In the present contribution, we explicitly review the fact that different many-body approximations commonly used in different fields in physics clearly fail to describe the exact LMG solution. With similar assumptions as those adopted for the LMG model, we propose a new Hamiltonian based on a general two-body interaction. The new model (Extended LMG) is not only more general than the original LMG model and, therefore, with a potentially larger spectrum of applicability, but also the physics behind its exact solution can be much better captured by common many-body approximations. At the basis of this improvement lies a new term in the Hamiltonian that depends on the number of constituents and polarizes the system; the associated symmetry breaking is discussed, together with some implications for the study of more realistic systems.
nucl-thcond-mat.mtrl-sciphysics.atom-phquant-phJ.Phys.G(2021)·3 citations
04
A Neural Network Perturbation Theory Based on the Born Series
Bastian Kaspschak · Ulf-G. Meißner
Deep Learning using the eponymous deep neural networks (DNNs) has become an attractive approach towards various data-based problems of theoretical physics in the past decade. There has been a clear trend to deeper architectures containing increasingly more powerful and involved layers. Contrarily, Taylor coefficients of DNNs still appear mainly in the light of interpretability studies, where they are computed at most to first order. However, especially in theoretical physics numerous problems benefit from accessing higher orders, as well. This gap motivates a general formulation of neural network (NN) Taylor expansions. Restricting our analysis to multilayer perceptrons (MLPs) and introducing quantities we refer to as propagators and vertices, both depending on the MLP's weights and biases, we establish a graph-theoretical approach. Similarly to Feynman rules in quantum field theories, we can systematically assign diagrams containing propagators and vertices to the corresponding partial derivative. Examining this approach for S-wave scattering lengths of shallow potentials, we observe NNs to adapt their derivatives mainly to the leading order of the target function's Taylor expansion. To circumvent this problem, we propose an iterative NN perturbation theory. During each iteration we eliminate the leading order, such that the next-to-leading order can be faithfully learned during the subsequent iteration. After performing two iterations, we find that the first- and second-order Born terms are correctly adapted during the respective iterations. Finally, we combine both results to find a proxy that acts as a machine-learned second-order Born approximation.
↳ cs.LGnucl-thphysics.comp-phstat.MLPRResearch(2021)·3 citations
05
Disentangling Jet Modification
Jasmine Brewer🇺🇸 · Quinn Brodsky🇺🇸 · Krishna Rajagopal🇺🇸
Jet modification in heavy-ion collisions is an important probe of the nature and structure of the quark-gluon plasma (QGP) produced in these collisions and also encodes information about how the wakes that jets excite in a droplet of QGP form and relax. However, in experiment, one cannot know what a particular jet in a heavy ion collision would have looked like without quenching, making it difficult to interpret measurements in terms of individual jet modification. The goal of this Monte Carlo study is to gain insight into the modification of jet observables using the hybrid strong/weak coupling model of jet quenching as a test bed. In this Monte Carlo study (but not in experiment) it is possible to watch $the same jet$ as it evolves in vacuum or in QGP. We use this ability to disentangle the effects of modification of individual jets in heavy ion collisions vs. the effects of differing selection bias on the distribution of two observables: fractional energy loss and groomed $Δ R$ . We find that in the hybrid model the distribution of groomed $Δ R$ appears to be unmodified in a sample of jets selected after quenching, as in heavy ion collisions, and confirm that this lack of modification arises because of a selection bias toward jets that lose only a small fraction of their energy. If instead we select jets in a way that avoids this bias, and then follow these selected jets as they are quenched, we show that there is, in fact, a substantial modification of the $Δ R$ of individual jets. We show that this jet modification is principally due to the incorporation of particles coming from the wake that the parton shower excites in the plasma as a component of what an experimentalist reconstructs as a jet. The effects we discuss are substantial in magnitude, suggesting that our qualitative conclusions are more general than the Monte Carlo study in which we obtain them.
↳ hep-phnucl-thPoS(2021)·7 citations
06
Jet charge modification in dense QCD matter
Haitao Li🇺🇸 · Ivan Vitev🇺🇸
In these proceedings we report a recent calculation of the jet charge modification in heavy-ion relative to proton collisions at the LHC. Jets have played an essential role in constraining theories of in-medium parton shower evolution and in determining the properties of the quark-gluon plasma created in ultra-relativistic nuclear reactions. It is important to extend these studies to flavor-tagged jets and explore observables that are sensitive to their partonic origin. The average jet charge, introduced early on in the history of quantum chromodynamics, is a proxy for the electric charge of the quark or gluon that initiates the jet. In the framework of soft-collinear effective theory, we show how to evaluate the jet charge in a dense strongly-interacting matter environments. We identify observables that can isolate the contribution of in-medium branching from isospin effects and present predictions for the transverse momentum dependence of the jet charge distribution in nucleus-nucleus collisions and its modification relative to the proton case.
↳ hep-phnucl-thPoS(2021)·1 citation
07
Effects of centrality fluctuation and deuteron formation on proton number cumulant in Au+Au collisions at $s_{NN}$ = 3 GeV from JAM model
Arghya Chatterjee🇨🇳 · Yu Zhang🇨🇳 · Hui Liu🇨🇳 · Ruiqin Wang🇨🇳 · Shu He🇨🇳 · Xiaofeng Luo🇨🇳
We studied the effects of centrality fluctuation and deuteron formation on the cumulants ( $C_{n}$ ) and correlation functions ( $κ_{n}$ ) of protons up to sixth order in most central ( $b < 3$ fm) Au+Au collisions at $s_{NN}$ = 3 GeV from a microscopic transport model (JAM). The results are presented as a function of rapidity acceptance within transverse momentum $0.4 < p_{T} < 2$ GeV/ $c$ . We compared the results obtained by centrality bin width correction (CBWC) using charged reference particle multiplicity with CBWC done using impact parameter bins. It was found that at low energies the centrality resolution for determining the collision centrality using charged particle multiplicities is not good enough to reduce the initial volume fluctuations effect for higher-order cumulant analysis. New methods need to be developed to classify events with high centrality resolution for heavy-ion collisions at low energies. Finally, we observed that the formation of deuteron will suppress the higher-order cumulants and correlation functions of protons and is found to be similar to the efficiency effect. This work can serve as a noncritical baseline for the QCD critical point search at the high baryon density region.
↳ nucl-exnucl-thCPC(2021)·13 citations
08
Modified lepton couplings and the Cabibbo-angle anomaly
Antonio Coutinho🇨🇭 · Andreas Crivellin🇨🇭 · Claudio Andrea Manzari🇨🇭
Significant discrepancies between the different determinations of the Cabibbo angle have been observed. Here, we point out that this "Cabibbo-angle anomaly" can be explained by lepton flavour universality (LFU) violating New Physics (NP) in the neutrino sector. However, modified neutrino couplings to Standard Model gauge bosons also affect many other observables sensitive to LFU violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent Bayesian global analysis and find that non-zero modifications of electron and muon neutrino couplings are preferred at more than $99.99% C.L.$ (corresponding to more than $4 σ$ ). Our results show that constructive effects in the muon sector are necessary, meaning simple models with right-handed neutrinos are discarded and more sophisticated NP models required.
↳ hep-phhep-exhep-latnucl-ex+1PoS(2021)·18 citations

Affiliations

first authorsco-authorsvia INSPIRE