Ref HAL: hal-02302945_v1
Ref Arxiv: 1908.08363
Ref INSPIRE: 1750592
DOI: 10.1103/PhysRevD.100.114006
WoS: 000499979100004
Ref. & Cit.: NASA ADS
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2 Citations
Résumé:

We apply the renormalization group optimized perturbation theory (RGOPT) to evaluate the QCD (matter) pressure at the two-loop level considering three flavors of massless quarks in a dense and cold medium. Already at leading order (αs0), which builds on the simple one loop (RG resummed) term, our technique provides a nontrivial nonperturbative approximation which is completely renormalization group invariant. At the next-to-leading order the comparison between the RGOPT and the perturbative QCD predictions shows that the former method provides results which are in better agreement with the state-of-the-art higher order perturbative results, which include a contribution of order αs3ln2αs. At the same time one also observes that the RGOPT predictions are less sensitive to variations of the arbitrary MS¯ renormalization scale than those obtained with perturbative QCD. These results indicate that the RGOPT provides an efficient resummation scheme which may be considered as an alternative to lattice simulations at high baryonic densities.

Ref HAL: hal-01677723_v1
Ref Arxiv: 1709.03457
DOI: 10.1103/PhysRevD.96.116009
WoS: WOS:000417758200005
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3 Citations
Résumé:

A recently developed variational resummation technique, incorporating renormalization group properties consistently, has been shown to solve the scale dependence problem that plagues the evaluation of thermodynamical quantities, e.g., within the framework of approximations such as in the hard-thermal-loop resummed perturbation theory. This method is used in the present work to evaluate thermodynamical quantities within the two-dimensional nonlinear sigma model, which, apart from providing a technically simpler testing ground, shares some common features with Yang-Mills theories, like asymptotic freedom, trace anomaly and the nonperturbative generation of a mass gap. The present application confirms that nonperturbative results can be readily generated solely by considering the lowest-order (quasi-particle) contribution to the thermodynamic effective potential, when this quantity is required to be renormalization group invariant. We also show that when the next-to-leading correction from the method is accounted for, the results indicate convergence, apart from optimally preserving, within the approximations here considered, the sought-after scale invariance.

Commentaires: 24 pages, 10 figures. v2: some corrections in a few figures, more explanations on the difference with standard optimized perturbation or hard thermal loop resummation. One reference added. To appear in Phys. Rev. D. Réf Journal: Phys. Rev. D 96, 116009 (2017)

Ref HAL: hal-01669701_v1
Ref INSPIRE: 1643380
DOI: 10.5506/APhysPolBSupp.10.1099
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Résumé:

We illustrate how our recently developed renormalization group optimized perturbation (RGOPT) efficiently resums perturbative expansions in thermal field theories. The residual renormalization scale dependence of optimized thermodynamical quantities is drastically improved as compared to either standard perturbative expansions, or related methods such as the screened perturbation or (resummed) hard-thermal-loop perturbation. Our approach is illustrated briefly for the nonlinear sigma model, as a toy model for thermal QCD. Finally, preliminary applications of RGOPT to hard thermal loop resummation for the QCD pressure are sketched.

Ref HAL: hal-01624698_v1
DOI: 10.1088/1742-6596/880/1/012049
WoS: WOS:000412424700049
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Résumé:

The recently proposed renormalization group improved optimized perturbation theory is employed to evaluate the pressure of the two dimensional non linear sigma model at finite temperatures. We explicitly show how this powerful resummation method can turn the lowest (one loop) perturbative contribution to the pressure, which is not RG invariant, into a non perturbative quantity exhibiting scale invariance.

Ref HAL: hal-01939695_v1
Ref Arxiv: 1512.05194
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This document provides a writeup of all contributions to the workshop on "High precision measurements of $\alpha_s$: From LHC to FCC-ee" held at CERN, Oct. 12--13, 2015. The workshop explored in depth the latest developments on the determination of the QCD coupling $\alpha_s$ from 15 methods where high precision measurements are (or will be) available. Those include low-energy observables: (i) lattice QCD, (ii) pion decay factor, (iii) quarkonia and (iv) $\tau$ decays, (v) soft parton-to-hadron fragmentation functions, as well as high-energy observables: (vi) global fits of parton distribution functions, (vii) hard parton-to-hadron fragmentation functions, (viii) jets in $e^\pm$p DIS and $\gamma$-p photoproduction, (ix) photon structure function in $\gamma$-$\gamma$, (x) event shapes and (xi) jet cross sections in $e^+e^-$ collisions, (xii) W boson and (xiii) Z boson decays, and (xiv) jets and (xv) top-quark cross sections in proton-(anti)proton collisions. The current status of the theoretical and experimental uncertainties associated to each extraction method, the improvements expected from LHC data in the coming years, and future perspectives achievable in $e^+e^-$ collisions at the Future Circular Collider (FCC-ee) with $\cal{O}$(1--100 ab$^{-1}$) integrated luminosities yielding 10$^{12}$ Z bosons and jets, and 10$^{8}$ W bosons and $\tau$ leptons, are thoroughly reviewed. The current uncertainty of the (preliminary) 2015 strong coupling world-average value, $\alpha_s(m_Z)$ = 0.1177 $\pm$ 0.0013, is about 1\%. Some participants believed this may be reduced by a factor of three in the near future by including novel high-precision observables, although this opinion was not universally shared. At the FCC-ee facility, a factor of ten reduction in the $\alpha_s$ uncertainty should be possible, mostly thanks to the huge Z and W data samples available.

Commentaires: 135 pages, 56 figures. CERN-PH-TH-2015-299, CoEPP-MN-15-13. This document is dedicated to the memory of Guido Altarelli

Ref HAL: hal-01405689_v1
DOI: 10.1016/j.nuclphysbps.2015.01.014
WoS: WOS:000381173300014
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Résumé:

A new version of the optimized perturbation (OPT), implementing renormalization group properties consistently, allows to calculate the nonperturbative ratio of the pion decay constant and the basic QCD scale in the MSbar scheme. From the experimental Fπ input value it gives a new determination of $\Lambda_{MSbar}$ for nf=2 and nf=3, and thus of the QCD coupling constant $\alpha_S$ at various scales once combined with a standard perturbative evolution. The empirical convergence properties of the RGOPT modified series is exhibited up to the third order. We examine the various sources of theoretical uncertainties and obtain $\alpha_S(mZ)= 0.1174 ^{+.001}_{-.0005} \pm .001\pm .0005_{evol}$, where the first errors are estimates of the intrinsic theoretical uncertainties of our method, and the second errors come from present uncertainties in Fπ/F0, where F0 is Fπ in the exact chiral SU(3) limit.

Ref HAL: hal-01264155_v1
Ref Arxiv: 1512.03406
DOI: 10.1016/j.nuclphysbps.2016.02.021
WoS: WOS:000375626800021
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Résumé:

Our renormalization group consistent variant of optimized perturbation, RGOPT, is used to calculate the nonperturbative QCD spectral density of the Dirac operator and the related chiral quark condensate $\langle \bar q q \rangle$, for $n_f=2$ and $n_f=3$ massless quarks. Sequences of approximations at two-, three-, and four-loop orders are very stable and give $\langle \bar q q \rangle^{1/3}_{n_f=2}(2\, {\rm GeV}) = -(0.833-0.845) \bar\Lambda_2 $, and $ \langle \bar q q \rangle^{1/3}_{n_f=3}(2\, {\rm GeV}) = -(0.814-0.838) \bar\Lambda_3 $ where the range is our estimated theoretical error and $\bar\Lambda_{n_f}$ the basic QCD scale in the $\rm \bar{MS}$-scheme. We compare those results with other recent determinations (from lattice calculations and spectral sum rules).

Commentaires: 5 pages, talk given at 18th International Conference in Quantum Chromodynamics (QCD 15) Montpellier, published in the proceedings