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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
    In: SciPost Physics, Stichting SciPost, Vol. 14, No. 3 ( 2023-03-16)
    Abstract: The fluctuations of the atom number between a Bose-Einstein condensate and the surrounding thermal gas have been the subject of a long standing theoretical debate. This discussion is centered around the appropriate thermodynamic ensemble to be used for theoretical predictions and the effect of interactions on the observed fluctuations. Here we introduce the so-called Fock state sampling method to solve this classic problem of current experimental interest for weakly interacting gases. A suppression of the predicted peak fluctuations is observed when using a microcanonical with respect to a canonical ensemble. Moreover, interactions lead to a shift of the temperature of peak fluctuations for harmonically trapped gases. The absolute size of the fluctuations furthermore depends on the total number of atoms and the aspect ratio of the trapping potential. Due to the interplay of these effect, there is no universal suppression or enhancement of fluctuations.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 2886659-9
    Location Call Number Limitation Availability
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  • 8
    Online Resource
    Online Resource
    Stichting SciPost ; 2023
    In:  SciPost Physics Vol. 15, No. 2 ( 2023-08-31)
    In: SciPost Physics, Stichting SciPost, Vol. 15, No. 2 ( 2023-08-31)
    Abstract: In the present work we set up a general functional renormalisation group framework for the computation of complex effective actions. For explicit computations we consider both flows of the Wilsonian effective action and the one-particle irreducible (1PI) effective action. The latter is based on an appropriate definition of a Legendre transform for complex actions, and we show its validity by comparison to exact results in zero dimensions, as well as a comparison to results for the Wilsonian effective action. In the present implementations of the general approaches, the flow of the Wilsonian effective action has a wider range of applicability and we obtain results for the effective potential of complex fields in \phi^4 ϕ 4 -theories from zero up to four dimensions. These results are also compared with results from the 1PI effective action within its range of applicability. The complex effective action also allows us to determine the location of the Lee-Yang zeros for general parameter values. We also discuss the extension of the present results to general theories including QCD.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2023
    detail.hit.zdb_id: 2886659-9
    Location Call Number Limitation Availability
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  • 9
    Online Resource
    Online Resource
    Stichting SciPost ; 2018
    In:  SciPost Physics Vol. 5, No. 6 ( 2018-12-20)
    In: SciPost Physics, Stichting SciPost, Vol. 5, No. 6 ( 2018-12-20)
    Abstract: We reconstruct the gluon spectral function in Landau gauge QCD from numerical data for the gluon propagator. The reconstruction relies on two novel ingredients: Firstly we derive analytically the low frequency asymptotics of the spectral function. Secondly we construct a functional basis from a careful consideration of the analytic properties of the gluon propagator in Landau gauge. This allows us to reliably capture the non-perturbative regime of the gluon spectrum. We also compare different reconstruction methods and discuss the respective systematic errors.
    Type of Medium: Online Resource
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2018
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  • 10
    Online Resource
    Online Resource
    Stichting SciPost ; 2022
    In:  SciPost Physics Vol. 12, No. 1 ( 2022-01-04)
    In: SciPost Physics, Stichting SciPost, Vol. 12, No. 1 ( 2022-01-04)
    Abstract: We reconstruct the Lorentzian graviton propagator in asymptotically safe quantum gravity from Euclidean data. The reconstruction is applied to both the dynamical fluctuation graviton and the background graviton propagator. We prove that the spectral function of the latter necessarily has negative parts similar to, and for the same reasons, as the gluon spectral function. In turn, the spectral function of the dynamical graviton is positive. We argue that the latter enters cross sections and other observables in asymptotically safe quantum gravity. Hence, its positivity may hint at the unitarity of asymptotically safe quantum gravity.
    Type of Medium: Online Resource
    ISSN: 2542-4653
    Language: Unknown
    Publisher: Stichting SciPost
    Publication Date: 2022
    detail.hit.zdb_id: 2886659-9
    Location Call Number Limitation Availability
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