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Nuclear Theory·nucl-th

Wednesday·September 9, 2020

8 papers—3 primary·5 cross-listed

All8Primary3Cross-lists5
  1. 04

    [Submitted on 7 Sept 2020] (cross-list from cs.LG)

    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.

    Comments:
    29 pages, 4 figures, revised with more focus on neural network perturbation theory, accepted for publication in Phys. Rev. Research
    Subjects:
    Machine Learning (cs.LG); Nuclear Theory (nucl-th); Computational Physics (physics.comp-ph); Machine Learning (stat.ML)
    arXiv:
    2009.03192 [pdf]
    PRResearch(2021)·3 citations
  2. 05

    [Submitted on 7 Sept 2020] (cross-list from hep-ph)

    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 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 . We find that in the hybrid model the distribution of groomed 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 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.

    Comments:
    4 pages; 3 figures; proceedings of Hard Probes 2020
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
    arXiv:
    2009.03316 [pdf]
    PoS(2021)·7 citations
  3. 06

    [Submitted on 8 Sept 2020] (cross-list from hep-ph)

    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.

    Comments:
    Hard Probes 2020 proceedings, 5 pages, 4 figures
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
    arXiv:
    2009.03530 [pdf]
    PoS(2021)·1 citation
  4. 07

    [Submitted on 8 Sept 2020] (cross-list from nucl-ex)

    Effects of centrality fluctuation and deuteron formation on proton number cumulant in Au+Au collisions at = 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 () and correlation functions () of protons up to sixth order in most central ( fm) Au+Au collisions at = 3 GeV from a microscopic transport model (JAM). The results are presented as a function of rapidity acceptance within transverse momentum GeV/. 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.

    Comments:
    9 pages, 8 figures
    Subjects:
    Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
    arXiv:
    2009.03755 [pdf]
    CPC(2021)·13 citations
  5. 08

    [Submitted on 8 Sept 2020] (cross-list from hep-ph)

    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 (corresponding to more than ). 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.

    Comments:
    8 pages, 2 figures, proceedings for LHCP2020
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); High Energy Physics — Lattice (hep-lat); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
    arXiv:
    2009.03877 [pdf]
    PoS(2021)·18 citations

Affiliations

first authorsco-authorsvia INSPIRE