arXiv subjects

G. Cvetic

Publications and source records attributed to G. Cvetic.

At least 19 recordsLinked to original sources

arXiv2022

The strong coupling constant: State of the art and the decade ahead

Theoretical predictions for particle production cross sections and decays at colliders rely heavily on perturbative Quantum Chromodynamics (QCD) calculations, expressed as an expansion in powers of the strong coupling constant $\alpha_s$. The current $\mathcal{O}(1\%)$ uncertainty of the QCD coupling evaluated at the reference Z boson mass, $\alpha_s(m_Z) = 0.1179 \pm 0.0009$, is one of the limiting factors to more precisely describe multiple processes at current and future colliders. A reduction of this uncertainty is thus a prerequisite to perform precision tests of the Standard Model as well as searches for new physics. This report provides a comprehensive summary of the state-of-the-art, challenges, and prospects in the experimental and theoretical study of the strong coupling. The current $\alpha_s(m_Z)$ world average is derived from a combination of seven categories of observables: (i) lattice QCD, (ii) hadronic $\tau$ decays, (iii) deep-inelastic scattering and parton distribution functions fits, (iv) electroweak boson decays, hadronic final-states in (v) $e^+e^-$, (vi) e-p, and (vii) p-p collisions, and (viii) quarkonia decays and masses. We review the current status of each of these seven $\alpha_s(m_Z)$ extraction methods, discuss novel $\alpha_s$ determinations, and examine the averaging method used to obtain the world-average value. Each of the methods discussed provides a ``wish list'' of experimental and theoretical developments required in order to achieve the goal of a per-mille precision on $\alpha_s(m_Z)$ within the next decade.

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arXiv2017

Bjorken sum rule in QCD with analytic coupling

We present details of study of the Bjorken polarized sum rule carried out recently in [1] within the range of energies where the data were collected by JLAB collaboration, 0.05 GeV2 < Q2 < 3 GeV2. Three approaches to QCD with analytic (holomorphic) coupling are considered: Analytic Perturbation Theory (APT), Two-delta analytic QCD, and Three-delta lattice-motivated analytic QCD in the three-loop and four-loop MiniMOM schemes. The new frameworks with respective couplings give results which agree well with the experimental data for 0.5 GeV2 < Q2 < 3GeV2 already when only one higher-twist term is taken into account.

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arXiv2010

Forward-backward asymmetry of top quark in unparticle physics

The updated CDF measurement of the forward-backward asymmetry (FBA) in the top quark production p{bar p} -> t{bar t} at Tevatron (with the CMS energy 1.96 TeV) shows a deviation of 2*sigma from the value predicted by the Standard QCD Model. We present calculation of this quantity in the scenario where colored unparticle physics contributes to the s-channel of the process, and obtain the regions in the plane of the unparticle parameters lambda and dU which give the values of the FBA and of the total t{bar t} production cross section compatible with the present measurements.

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arXiv2010

Probing Majorana neutrinos in rare K and D, D_s, B, B_c meson decays

We study lepton number violating decays of charged K, D, D_s, B and B_c mesons of the form M^+\to {M'}^-\ell^+\ell^+, induced by the existence of Majorana neutrinos. These processes provide information complementary to neutrinoless double nuclear beta decays, and are sensitive to neutrino masses and lepton mixing. We explore neutrino mass ranges m_N from below 1 eV to several hundred GeV. We find that in many cases the branching ratios are prohibitively small, however in the intermediate range m_π< m_N < m_{B_c}, in specific channels and for specific neutrino masses, the branching ratios can be at the reach of high luminosity experiments like those at the LHC-b and future Super flavor-factories, and can provide bounds on the lepton mixing parameters.

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arXiv2009

Small-x behavior of the structure function F_2 and its slope partial ln(F_2)/partial ln(1/x) for "frozen" and analytic strong-coupling constants

Using the leading-twist approximation of the Wilson operator product expansion with "frozen" and analytic versions of the strong-coupling constant, we show that the Bessel-inspired behavior of the structure function F_2 and its slope\break partial ln(F_2)/partial ln(1/x) at small values of x, obtained for a flat initial condition in the DGLAP evolution equations, leads to good agreement with experimental data of deep-inelastic scattering at DESY HERA.

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arXiv2007

Method for comparing finite temperature field theory results with lattice data

The values of the presently available truncated perturbative expressions for the pressure of the quark-gluon plasma at finite temperatures and finite chemical potential are trustworthy only at very large energies. When used down to temperatures close to the critical one Tc, they suffer from large uncertainties due to the renormalization scale freedom. In order to reduce these uncertainties, we perform resummations of the pressure by applying Pade-related approximants to the available perturbation series for the short-distance and for the long-distance contributions. In the two contributions, we use two different renormalization scales which reflect different energy regions contributing to the different parts. Application of the obtained expressions at low temperatures is made possible by replacing the usual four-loop barMS beta function for alpha_s by its Borel-Pade resummation, eliminating thus the unphysical Landau singularities of alpha_s. The obtained results are remarkably insensitive to the chosen renormalization scale and can be compared with lattice results -- for the pressure (p), the chemical potential contribution (delta p) to the pressure, and various susceptibilities. A good qualitative agreement with the lattice results is revealed down to temperatures close to Tc.

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arXiv2005

A modification of minimal analytic QCD at low energies

We present an approach which allows for modifications of the (minimal) analytic QCD model of Shirkov, Solovtsov et al. The discontinuity function of the (minimal) analytic QCD coupling parameter is changed at the low time-like momenta by a simple ansatz motivated by the vector meson dominance for the vector channel of the e+e- hadronic decay ratio R_v(s). In this way, the additional low energy parameters introduced in the coupling parameter make it possible to obtain the correct value of the semihadronic tau decay ratio without having to introduce the unusually heavy quark masses of u, d and s quarks, in contrast to the minimal analytic QCD. Some numerical results are presented in the leading skeleton approximation for a simple version of such a model. We hope that a more systematic analysis of such models leads to an approximately universal coupling parameter.

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arXiv2004

Pade-related resummations of the pressure of quark-gluon plasma by approximate inclusion of g**6-terms

We perform various resummations of the hot QCD pressure based on the actual knowledge of the perturbation series which includes the g**6 ln(1/g) and part of the g**6 terms. Resummations are performed separately for the short- and long-distance parts. The g**6 term of the short-distance pressure is estimated on the basis on the known UV cutoff dependence of the long-distance part. The resummations are of the Pade and Borel-Pade type, using in addition the (Pade-)resummed expression for the squared screening mass mE**2 and for the EQCD coupling parameter gE**2. The resummed results depend weakly on the yet unknown g**6 terms and on the the short-range renormalization scale, at all temperatures. The dependence on the long-range renormalization scale is appreciable at low temperatures T < 1 GeV. The resulting dependence of pressure on temperature T is compatible with the results of the lattice calculations at low T.

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arXiv2004

Bottonium mass - evaluation using renormalon cancellation

We present a method of calculating the bottonium mass M[Upsilon(1S)] = [2 mb + E(b barb)]. The binding energy is separated into the soft and ultrasoft components E(b barb)=[E(s)+E(us)] by requiring the reproduction of the correct residue parameter value of the renormalon singularity for the renormalon cancellation in the sum [2 mb + E(s)]. The Borel resummation is then performed separately for (2 mb) and E(s), using the infrared safe MSbar mass [bar mb] as input. E(us) is estimated. Comparing the result with the measured value of M[Upsilon(1S)], the extracted value of the quark mass is [bar mb](mu=[bar mb]) = 4.241 +- 0.068 GeV (for the central value alphas(MZ)=0.1180). This value of [bar mb] is close to the earlier values obtained from the QCD spectral sum rules, but lower than from pQCD evaluations without the renormalon structure for heavy quarkonia.

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arXiv2004

Calculations of binding energies and masses of heavy quarkonia using renormalon cancellation

We use various methods of Borel integration to calculate the binding ground energies and masses of b-bbar and t-tbar quarkonia. The methods take into account the leading infrared renormalon structure of the hard+soft part of the binding energies E(s), and of the corresponding quark pole masses m_q, where the contributions of these singularities in M(s) = 2 m_q + E(s) cancel. Beforehand, we carry out the separation of the binding energy into its hard+soft and ultrasoft parts. The resummation formalisms are applied to expansions of m_q and E(s) in terms of quantities which do not involve renormalon ambiguity, such as MSbar quark mass, and alpha_s. The renormalization scales are different in calculations of m_q, E(s) and E(us). The MSbar mass of b quark is extracted, and the binding energies of t-tbar and the peak (resonance) energies for (t+tbar) production are obtained.

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arXiv2002

QCD effective action with dressing functions - consistency checks in the perturbative regime

In a previous paper, we presented solution to the Slavnov--Taylor identity for the QCD effective action, and argued that the action terms containing (anti)ghost fields are unique. These terms have the same form as those in the classical action, but the gluon and (anti)ghost effective fields are convoluted with gluon and ghost dressing functions G_A and G_c, the latter containing perturbative and nonperturbative effects (but not including the soliton-like vacuum effects). In the present work we show how the perturbative QCD (pQCD) can be incorporated into the framework of this action, and we present explicit one-loop pQCD expressions for G_A and G_c. We then go on to check the consistency of the obtained results by considering an antighost Dyson--Schwinger equation (DSE). By solving the relations that result from the Legendre transformation leading to the effective action, we obtain the effective fields as power expansions of sources. We check explicitly that the aforementioned one-loop functions G_A and G_c fulfil the antighost DSE at the linear source level. We further explicitly check that these one-loop G_A and G_c have the regularization-scale and momentum dependence consistent with the antighost DSE at the quadratic source level. These checks suggest that the the effective action with dressing functions represents a consistent framework for treating QCD, at least at the one-loop level.

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arXiv2002

Resummations of free energy at high temperature

We discuss resummation strategies for free energy in quantum field theories at nonzero temperatures T. We point out that resummations should be performed for the short- and long-distance parts separately in order to avoid spurious interference effects and double-counting. We then discuss and perform Pade resummations of these two parts for QCD at high T. The resummed results are almost invariant under variation of the renormalization and factorization scales. We perform the analysis also in the case of the massless scalar $ϕ^4$ theory.

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arXiv2002

On Lepton Flavor Violation in Tau Decays

We study lepton flavor violation (LFV) in tau decays induced by heavy Majorana neutrinos within two models: (I) the Standard Model with additional right-handed heavy Majorana neutrinos, i.e., a typical seesaw-type model; (II) the Standard Model with left-handed and right-handed neutral singlets, which are inspired by certain scenarios of SO(10)models and heterotic superstring models with E_6 symmetry. We calculate various LFV branching ratios and a T-odd asymmetry. The seesaw Model I predicts very small branching ratios for LFV processes in most of the parameter space, although in a very restricted parameter region it can reach maximal branching ratios Br(tau to mu gamma) approx 10^-9 and Br(tau to 3mu) approx 10^-10. In contrast, Model II may show branching ratios Br(tau to e gamma) approx 10^-8 and Br(tau to 3e) < 10^-9, over a sizable region of the parameter space, large enough to be tested by experiments in the near future.

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arXiv2001

The generalized vector dominance/colour-dipole picture of deep-inelastic scattering at low x

We give a detailed account of the recently formulated generalized vector dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low $x\cong Q^2/W^2$, including photoproduction. The approach, based on $γ^*(q \bar q)$ transitions, $q \bar q$ propagation and diffractive $(q \bar q)p$ scattering via the generic structure of the two-gluon exchange, provides a unique and quantitatively successful theory for the $γ^* p$ total cross section, $σ_{γ^* p} (W^2,Q^2)$, at low $x$. The GVD/CDP is shown to imply the empirical low-$x$ scaling law, $σ_{γ^* p} (W^2,Q^2)=σ_{γ^* p} (η)$ with $η=(Q^2+m_0^2)/Λ^2(W^2)$, that was established by a model-independent analysis of the experimental data.

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arXiv2001

Scale- and scheme-independent extension of Pade approximants; Bjorken polarized sum rule as an example

A renormalization-scale-invariant generalization of the diagonal Padé approximants (dPA), developed previously, is extended so that it becomes renormalization-scheme-invariant as well. We do this explicitly when two terms beyond the leading order (NNLO, $\sim α_s^3$) are known in the truncated perturbation series (TPS). At first, the scheme dependence shows up as a dependence on the first two scheme parameters $c_2$ and $c_3$. Invariance under the change of the leading parameter $c_2$ is achieved via a variant of the principle of minimal sensitivity. The subleading parameter $c_3$ is fixed so that a scale- and scheme-invariant Borel transform of the resummation approximant gives the correct location of the leading infrared renormalon pole. The leading higher-twist contribution, or a part of it, is thus believed to be contained implicitly in the resummation. We applied the approximant to the Bjorken polarized sum rule (BjPSR) at $Q^2_{\rm ph}=5$ and $3 GeV^2$, for the most recent data and the data available until 1997, respectively, and obtained $α_s^{\bar MS}(M_Z^2)=0.119^{+0.003}_{-0.006}$ and $0.113^{+0.004}_{-0.019}$, respectively. Very similar results are obtained with the Grunberg's effective charge method and Stevenson's TPS principle of minimal sensitivity, if we fix $c_3$-parameter in them by the aforementioned procedure. The central values for $α_s^{\bar MS}(M_Z^2)$ increase to 0.120 (0.114) when applying dPA's, and 0.125 (0.118) when applying NNLO TPS.

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arXiv2000

Nonperturbative dispersive sector in strong (quasi-)Abelian fields

In strong (quasi-)Abelian fields, even at the one-loop level of the coupling constant, quantum fluctuations of fermions induce an effective Lagrangian density whose imaginary (absorptive) part is purely nonperturbative and known to be responsible for the fermion-antifermion pair creation. On the other hand, the induced real (dispersive) part has perturbative and nonperturbative contributions. In the one-loop case, we argue how to separate the two contributions from each other for any strength of the field. We show numerically that the nonperturbative contributions are in general comparable with or larger than the induced perturbative ones. We arrive at qualitatively similar conclusions also for the induced energy density. Further, we investigate numerically the quasianalytic continuation of the perturbative results into the nonperturbative sector, by employing (modified) Borel-Pade. It turns out that in the case at hand, we have to integrate over renormalon singularities, but there is no renormalon ambiguity involved.

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arXiv2000

Borel-Pade vs Borel-Weniger method: a QED and a QCD example

Recently, Weniger (delta sequence) method has been proposed by the authors of Ref. [1] (Jentschura et al.) for resummation of truncated perturbation series in quantum field theories. Those authors presented numerical evidence suggesting that this method works better than Pade approximants when we resum a function with singularities in the Borel plane but not on the positive axis. We present here numerical evidence suggesting that in such cases the combined method of Borel-Pade works better than its analog Borel-Weniger, and that it may work better or comparably well in some of the cases when there are singularities on the positive axis in the Borel plane.

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arXiv2000

Achieving renormalization-scale- and scheme-independence in Pade-related resummation in QCD

Previously developed Pade-related method of resummation for QCD observables, which achieves exact renormalization-scale-invariance, is extended so that the scheme-invariance is obtained as well. The dependence on the leading scheme parameter $c_2$ is eliminated by a variant of the method of the principle of minimal sensitivity. The subleading parameter $c_3$ in the approximant is then fixed in such a way that the correct known location of the leading infrared renormalon pole is reproduced. Thus, $beta$-functions which go beyond the last perturbatively calculated order in the observable are used. The $β$-functions in the approximant are quasianalytically continued by Pade approximants. Two aspects of nonperturbative physics are accounted for in the presented resummation: a mechanism of quasianalytic continuation from the weak- into the strong-coupling regime, and the (approximant-specific) contribution of the leading infrared renormalon. The case of the Bjorken polarized sum rule is considered as a specific example of how the method works.

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