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  • 1
    Online Resource
    Online Resource
    Stichting SciPost ; 2023
    In:  SciPost Physics Vol. 15, No. 4 ( 2023-10-10)
    In: SciPost Physics, Stichting SciPost, Vol. 15, No. 4 ( 2023-10-10)
    Abstract: We calculate the spectral function of light quark flavours in 2+1 flavour vacuum QCD in the isospin-symmetric approximation. We employ spectral Dyson-Schwinger equations and compute the non-perturbative quark propagator directly in real-time, using recent spectral reconstruction results from Gaussian process regression of gluon propagator data in 2+1 flavour lattice QCD. Our results feature a pole-like peak structure at time-like momenta larger than the propagator’s gapping scale as well as a negative scattering continuum, which we exploit assuming an analytic pole-tail split during the iterative solution. The computation is augmented with a general discussion of the impact of the quark-gluon vertex and the gluon propagator on the analytic structure of the quark propagator. In particular, we investigate under which conditions the quark propagator shows unphysical complex poles. Our results offer a wide range of applications, encompassing the ab-initio calculation of transport as well as resonance properties in QCD.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 2886659-9
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  • 2
    Online Resource
    Online Resource
    Stichting SciPost ; 2022
    In:  SciPost Physics Vol. 13, No. 2 ( 2022-08-31)
    In: SciPost Physics, Stichting SciPost, Vol. 13, No. 2 ( 2022-08-31)
    Abstract: Lattice simulations along with studies in continuum QCD indicate that non-perturbative quantum fluctuations lead to an infrared regularisation of the gluon propagator in covariant gauges in the form of an effective mass-like behaviour. In the present work we propose an analytic understanding of this phenomenon in terms of gluon condensation through a dynamical version of the Higgs mechanism, leading to the emergence of color condensates. Within the functional renormalisation group approach we compute the effective potential of covariantly constant field strengths, whose non-trivial minimum is related to the color condensates. In the physical case of an $SU(3)$ gauge group this is an octet condensate. The value of the gluon mass obtained through this procedure compares very well to lattice results and the mass gap arising from alternative dynamical scenarios.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2022
    detail.hit.zdb_id: 2886659-9
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  • 3
    In: SciPost Physics Core, Stichting SciPost, Vol. 6, No. 3 ( 2023-09-04)
    Abstract: We derive renormalised finite functional flow equations for quantum field theories in real and imaginary time that incorporate scale transformations of the renormalisation conditions, hence implementing a flowing renormalisation. The flows are manifestly finite in general non-perturbative truncation schemes also for regularisation schemes that do not implement an infrared suppression of the loops in the flow. Specifically, this formulation includes finite functional flows for the effective action with a spectral Callan-Symanzik cutoff, and therefore gives access to Lorentz invariant spectral flows. The functional setup is fully non-perturbative and allows for the spectral treatment of general theories. In particular, this includes theories that do not admit a perturbative renormalisation such as asymptotically safe theories. Finally, the application of the Lorentz invariant spectral functional renormalisation group is briefly discussed for theories ranging from real scalar and Yukawa theories to gauge theories and quantum gravity.
    Type of Medium: Online Resource
    ISSN: 2666-9366
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 3071450-3
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  • 4
    Online Resource
    Online Resource
    Stichting SciPost ; 2023
    In:  SciPost Physics Vol. 14, No. 4 ( 2023-04-13)
    In: SciPost Physics, Stichting SciPost, Vol. 14, No. 4 ( 2023-04-13)
    Abstract: We investigate dynamical chiral symmetry breaking and the emergence of mesonic bound states from the infrared dynamics of four-quark scatterings. Both phenomena originate from the resonant scalar-pseudoscalar channel of the four-quark scatterings, and we compute the functional renormalisation group (fRG) flows of the Fierz-complete four-quark interaction of up and down quarks with its t t channel momentum dependence. This is done in the isospin symmetric case, also including the flow of the quark two-point function. This system can be understood as the fRG analogues of the complete Bethe-Salpeter equations and quark gap equation. The pole mass of the pion is determined from both direct calculations of the four-quark flows in the Minkowski regime of momenta and the analytic continuation based on results in the Euclidean regime, which are consistent with each other.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 2886659-9
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  • 5
    Online Resource
    Online Resource
    Stichting SciPost ; 2023
    In:  SciPost Physics Vol. 15, No. 3 ( 2023-09-20)
    In: SciPost Physics, Stichting SciPost, Vol. 15, No. 3 ( 2023-09-20)
    Abstract: We present a comprehensive non-perturbative study of the phase structure of the asymptotically safe Standard Model. The physics scales included range from the asymptotically safe trans-Planckian regime in the ultraviolet, the intermediate high-energy regime with electroweak symmetry breaking to strongly correlated QCD in the infrared. All flows are computed with a self-consistent functional renormalisation group approach, using a vertex expansion in the fluctuation fields. In particular, this approach takes care of all physical threshold effects and the respective decoupling of ultraviolet degrees of freedom. Standard Model and gravity couplings and masses are fixed by their experimental low energy values. Importantly, we accommodate for the difference between the top pole mass and its Euclidean analogue. Both, the correct mass determination and the threshold effects have a significant impact on the qualitative properties, and in particular on the stability properties of the specific ultraviolet-infrared trajectory with experimental Standard Model physics in the infrared. We show that in the present rather advanced approximation the matter part of the asymptotically safe Standard Model has the same number of relevant parameters as the Standard Model, and is asymptotically free. This result is based on the novel UV fixed point found in the present work: the fixed point Higgs potential is flat but has two relevant directions. These results and their analysis are accompanied by a thorough discussion of the systematic error of the present truncation, also important for systematic improvements.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 2886659-9
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  • 6
    Online Resource
    Online Resource
    Stichting SciPost ; 2018
    In:  SciPost Physics Vol. 5, No. 6 ( 2018-12-21)
    In: SciPost Physics, Stichting SciPost, Vol. 5, No. 6 ( 2018-12-21)
    Abstract: We compute correlation functions of three-dimensional Landau-gauge Yang-Mills theory with the Functional Renormalisation Group. Starting from the classical action as only input, we calculate the non-perturbative ghost and gluon propagators as well as the momentum-dependent ghost-gluon, three-gluon, and four-gluon vertices in a comprehensive truncation scheme. Compared to the physical case of four spacetime dimensions, we need more sophisticated truncations due to significant contributions from non-classical tensor structures. In particular, we apply a special technique to compute the tadpole diagrams of the propagator equations, which captures also all perturbative two-loop effects, and compare our correlators with lattice and Dyson-Schwinger results.
    Type of Medium: Online Resource
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2018
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  • 7
    Online Resource
    Online Resource
    Stichting SciPost ; 2018
    In:  SciPost Physics Vol. 5, No. 5 ( 2018-11-05)
    In: SciPost Physics, Stichting SciPost, Vol. 5, No. 5 ( 2018-11-05)
    Abstract: One of the main challenges in simulations on Lefschetz thimbles is the computation of the relative weights of contributing thimbles. In this paper we propose a solution to that problem by means of computing those weights using a reweighting procedure. Besides we present recipes for finding parametrizations of thimbles and anti-thimbles for a given theory. Moreover, we study some approaches to combine the Lefschetz thimble method with the Complex Langevin evolution. Our numerical investigations are carried out by using toy models among which we consider a one-site z^4 z 4 model as well as a U(1) U ( 1 ) one-link model.
    Type of Medium: Online Resource
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2018
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  • 8
    Online Resource
    Online Resource
    Stichting SciPost ; 2019
    In:  SciPost Physics Vol. 6, No. 5 ( 2019-05-09)
    In: SciPost Physics, Stichting SciPost, Vol. 6, No. 5 ( 2019-05-09)
    Abstract: Low-energy effective theories have been used very successfully to study the low-energy limit of QCD, providing us with results for a plethora of phenomena, ranging from bound-state formation to phase transitions in QCD. These theories are consistent quantum field theories by themselves and can be embedded in QCD, but typically have a physical ultraviolet cutoff that restricts their range of validity. Here, we provide a discussion of the concept of renormalization group consistency, aiming at an analysis of cutoff effects and regularization-scheme dependences in general studies of low-energy effective theories. For illustration, our findings are applied to low-energy effective models of QCD in different approximations including the mean-field approximation. More specifically, we consider hot and dense as well as finite systems and demonstrate that violations of renormalization group consistency affect significantly the predictive power of the corresponding model calculations.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2019
    detail.hit.zdb_id: 2886659-9
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  • 9
    In: SciPost Physics, Stichting SciPost, Vol. 12, No. 1 ( 2022-01-26)
    Abstract: The approximation of quantum states with artificial neural networks has gained a lot of attention during the last years. Meanwhile, analog neuromorphic chips, inspired by structural and dynamical properties of the biological brain, show a high energy efficiency in running artificial neural-network architectures for the profit of generative applications. This encourages employing such hardware systems as platforms for simulations of quantum systems. Here we report on the realization of a prototype using the latest spike-based BrainScaleS hardware allowing us to represent few-qubit maximally entangled quantum states with high fidelities. Bell correlations of pure and mixed two-qubit states are well captured by the analog hardware, demonstrating an important building block for simulating quantum systems with spiking neuromorphic chips.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2022
    detail.hit.zdb_id: 2886659-9
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  • 10
    Online Resource
    Online Resource
    Stichting SciPost ; 2018
    In:  SciPost Physics Vol. 5, No. 4 ( 2018-10-04)
    In: SciPost Physics, Stichting SciPost, Vol. 5, No. 4 ( 2018-10-04)
    Abstract: We investigate the asymptotic safety scenario for a scalar-gravity system. This system contains two avatars of the dynamical Newton coupling, a gravitational self-coupling and a scalar-graviton coupling. We uncover an effective universality for the dynamical Newton coupling on the quantum level: its momentum-dependent avatars are in remarkable quantitative agreement in the scaling regime of the UV fixed point. For the background Newton coupling, this effective universality is not present, but qualitative agreement remains.
    Type of Medium: Online Resource
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2018
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