arXiv subjects

Gorazd Cvetic

Publications and source records attributed to Gorazd Cvetic.

At least 37 records · Page 2Linked to original sources

arXiv2018

Bjorken polarized sum rule and infrared-safe QCD couplings

Experimental data obtained for the polarized Bjorken sum rule (BSR) $Γ_1^{p-n}(Q^2)$ are fitted by using predictions derived within a truncated operator product expansion (OPE) approach to QCD. Four QCD versions are considered: perturbative QCD (pQCD) in the MSbar scheme, Analytic Perturbation Theory (APT), and 2$δ$ and 3$δ$ analytic QCD versions. In contrast to pQCD, these QCD variants do not have Landau singularities at low positive $Q^2$, which facilitates the fitting procedure significantly. The fitting procedure is applied first to the experimental data of the inelastic part of BSR, and the known elastic contributions are added after the fitting. In general, when 2$δ$ and 3$δ$ QCD coupling is used the fitted curves give the best results, within the $Q^2$-range of the fit as well as in extended $Q^2$-intervals. When the fitting procedure is applied to the total BSR, i.e., to the sum of the experimental data and the elastic contribution, the quality of the results deteriorates significantly.

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arXiv2018

Bjorken sum rule in QCD frameworks with analytic (holomorphic) coupling

We investigate the Bjorken polarized sum rule (BSR) in three approaches to QCD with analytic (holomorphic) coupling: Analytic Perturbation Theory (APT), Two-delta analytic QCD (2$δ$anQCD), and Three-delta lattice-motivated analytic QCD in the three-loop and four-loop MOM scheme (3l3$δ$anQCD, 4l3$δ$anQCD). These couplings do not have unphysical (Landau) singularities, and have finite values when the transferred momentum goes to zero, which allows us to explore the infrared regime. With the exception of APT, these theories at high momenta practically coincide with the underlying perturbative QCD (pQCD) in the same scheme. We apply them in order to verify the Bjorken sum rule within the range of energies available in the data collected by the experimental JLAB collaboration, i.e., $0.05 GeV^2 <Q^2< 3 GeV^2$ and compare the results with those obtained by using the perturbative QCD coupling. The results of the new frameworks with respective couplings (2$δ$ and 3$δ$) are in good agreement with the experimental data for $0.5 GeV^2<Q^2<3 GeV^2$ already when only one higher-twist term is used. In the low-$Q^2$ regime ($Q^2 \lesssim 1 GeV^2$) we use $χ$PT-motivated expression or an expression motivated by the light-front holography (LFH) QCD used earlier in the literature.

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arXiv2017

Nearly perturbative QCD coupling with lattice-motivated zero IR limit

The product of the gluon dressing function and the square of the ghost dressing function in the Landau gauge can be regarded to represent, apart from the inverse power corrections $1/Q^{2 n}$, a nonperturbative generalization $A(Q^2)$ of the perturbative QCD running coupling $a(Q^2)$ ($\equiv α_s(Q^2)/π$). Recent large volume lattice calculations for these dressing functions strongly indicate that such a generalized coupling goes to zero as $A(Q^2) \sim Q^2$ when the squared momenta $Q^2$ go to zero ($Q^2 \ll 1 \ {\rm GeV}^2$). We construct such a QCD coupling $A(Q^2)$ which fulfills also various other physically motivated conditions. At high momenta it becomes the underlying perturbative coupling $a(Q^2)$ to a very high precision. And at intermediately low momenta $Q^2 \sim 1 \ {\rm GeV}^2$ it gives results consistent with the data of the semihadronic $τ$ lepton decays as measured by OPAL and ALEPH. The coupling is constructed in a dispersive way, ensuring as a byproduct the holomorphic behavior of $A(Q^2)$ in the complex $Q^2$-plane which reflects the holomorphic behavior of the spacelike QCD observables. Application of the Borel sum rules to $τ$-decay V + A spectral functions allows us to obtain values for the gluon (dimension-4) condensate and the dimension-6 condensate, which reproduce the measured OPAL and ALEPH data to a significantly better precision than the perturbative MSbar coupling (+OPE) approach. The comparison with the experimental V-channel Adler function, related with the $e^+ e^- \to$ hadrons ratio, at low $Q^2 \sim 1 \ {\rm GeV}^2$, also gives results considerably better than with the usual MSbar pQCD+OPE approach.

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arXiv2017

Nearly perturbative lattice-motivated QCD coupling with zero IR limit

The product of the gluon dressing function and the square of the ghost dressing function in the Landau gauge can be regarded to represent, apart from the inverse power corrections 1/Q^{2n}, a nonperturbative generalization A(Q^2) of the perturbative QCD running coupling a(Q^2)=alpha_s(Q^2)/pi. Recent large volume lattice calculations for these dressing functions indicate that the coupling defined in such a way goes to zero as A(Q^2)~Q^2 when the squared momenta Q^2 go to zero (Q^2<<1 GeV^2). In this work we construct such a QCD coupling A(Q^2) which fulfills also various other physically motivated conditions. At high momenta it becomes the underlying perturbative coupling a(Q^2) to a very high precision. And at intermediate low squared momenta Q^2~1 GeV^2 it gives results consistent with the data of the semihadronic tau lepton decays as measured by OPAL and ALEPH. The coupling is constructed in a dispersive way, resulting as a byproduct in the holomorphic behavior of A(Q^2) in the complex Q^2-plane which reflects the holomorphic behavior of the spacelike QCD observables. Application of the Borel sum rules to tau-decay V+A spectral functions allows us to obtain values for the gluon (dimension-4) condensate and the dimension-6 condensate, which reproduce the measured OPAL and ALEPH data to a significantly better precision than the perturbative MSbar coupling approach.

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arXiv2017

Lattice-motivated holomorphic nearly perturbative QCD

Newer lattice results indicate that, in the Landau gauge at low spacelike momenta, the gluon propagator and the ghost dressing function are finite nonzero. This leads to a definition of the QCD running coupling, in a specific scheme, that goes to zero at low spacelike momenta. We construct a running coupling which fulfills these conditions, and at the same time reproduces to a high precision the perturbative behavior at high momenta. The coupling is constructed in such a way that it reflects qualitatively correctly the holomorphic (analytic) behavior of spacelike observables in the complex plane of the squared momenta, as dictated by the general principles of Quantum Field Theories. Further, we require the coupling to reproduce correctly the nonstrange semihadronic decay rate of tau lepton which is the best measured low-momentum QCD observable with small higher-twist effects. Subsequent application of the Borel sum rules to the V+A spectral functions of tau lepton decays, as measured by OPAL Collaboration, determines the values of the gluon condensate and of the V+A 6-dimensional condensate, and reproduces the data to a significantly higher precision than the usual MSbar running coupling.

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arXiv2017

Rare decays of B mesons via on-shell sterile neutrinos

In view of the projected high number of produced $B$ mesons in Belle II experiment ($\sim 10^{10}$ per year), in addition to the presently ongoing LHC-b, we calculate the rate of decay for the rare decays of $B$ mesons via a sterile on-shell neutrino $N$, which subsequently may decay leptonically or semileptonically within the detector: $B \to (D^{(*)}) \ell_1^{\pm} N$, then $N \to \ell_1^{\pm} \ell_2^{\mp} ν$ or $N \to \ell^{\pm} π^\mp$. Here, $\ell_1 \not= \ell_2$ in order to avoid serious QED background. We account for the possible effects of the strong neutrino lifetime on the observability of the rare decays. If no charmed mesons ($D^{(*)}$) are produced at the first vertex of the sterile neutrino, a strong CKM-suppression becomes effective; this is not true if we consider instead the decays of $B_c$ mesons which can be produced copiously in LHC-b. The production of charmed mesons $D^{(*)}$ at the first vertex offers an attractive possibility because it avoids strong CKM-suppression. Such rare decays of $B$ mesons could be detected at Belle II experiment, with $N$ neutrino either decaying within the detector or manifesting itself as a massive missing momentum.

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arXiv2016

Mass of the bottom quark from Upsilon(1S) at NNNLO: an update

We update our perturbative determination of MSbar bottom quark mass mb(mb), by including the recently obtained four-loop coefficient in the relation between the pole and MSbar mass. First the renormalon subtracted (RS or RS') mass is determined from the known mass of the Upsilon(1S) meson, where we use the renormalon residue Nm obtained from the asymptotic behavior of the coefficient of the 3-loop static singlet potential. MSbar mass is then obtained using the 4-loop renormalon-free relation between the RS (RS') and MSbar mass. We argue that the effects of the charm quark mass are accounted for by effectively using Nf=3 in the mass relations. The extracted value is mb(mb) = 4222(40) MeV, where the uncertainty is dominated by the renormalization scale dependence.

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arXiv2015

Oscillation of heavy sterile neutrino in decay of $B \to μe π$

In a scenario with two almost mass degenerate heavy sterile Majorana neutrinos with a mass $\sim 1$ GeV, we present the semileptonic decay widths of heavy charged pseudoscalars such as $B$ mesons, either lepton-number-violating ($B^{\pm} \to μ^{\pm} e^{\pm} π^{\mp}$), or lepton-number-conserving ($B^{\pm} \to μ^{\pm} e^{\mp} π^{\pm}$), mediated by such on-shell neutrinos. It had been explained in the literature that such decays may be eventually detected, and that they can present even detectable CP violation effects. In this work we point out that, in addition, such decays may present detectable effects of heavy neutrino oscillation, allowing us to extract the oscillation length and thus the heavy neutrino mass difference $ΔM_N$, as well as a CP-violating Majorana phase.

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arXiv2015

anQCD: Fortran programs for couplings at complex momenta in various analytic QCD models

We provide three Fortran programs which evaluate the QCD analytic (holomorphic) couplings $\mathcal{A}_ν(Q^2)$ for complex or real squared momenta $Q^2$. These couplings are holomorphic analogs of the powers $a(Q^2)^ν$ of the underlying perturbative QCD (pQCD) coupling $a(Q^2) \equiv α_s(Q^2)/π$, in three analytic QCD models (anQCD): Fractional Analytic Perturbation Theory (FAPT), Two-delta analytic QCD (2$δ$anQCD), and Massive Perturbation Theory (MPT). The index $ν$ can be noninteger. The provided programs do basically the same job as the Mathematica package anQCD.m in Mathematica published by us previously, Ref.[1], but are now written in Fortran.

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arXiv2015

Probing the Majorana neutrinos and their CP violation in decays of charged scalar mesons $π, K, D, D_s, B, B_c$

Some of the outstanding questions of particle physics today concern the neutrino sector, in particular whether there are more neutrinos than those already known and whether they are Dirac or Majorana particles.There are different ways to explore these issues. In this article we describe neutrino-mediated decays of charged pseudoscalar mesons such as $π^{\pm}$, $K^{\pm}$ and $B^{\pm}$, in scenarios where extra neutrinos are heavy and can be on their mass shell. We discuss semileptonic and leptonic decays of such kinds. We investigate possible ways of using these decays in order to distinguish between the Dirac and Majorana character of neutrinos. Further, we argue that there are significant possibilities of detecting CP violation in such decays when there are at least two almost degenerate Majorana neutrinos involved. This latter type of scenario fits well into the known neutrino minimal standard model ($ν$MSM) which could simultaneously explain the Dark Matter and Baryon Asymmetry of the Universe.

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arXiv2015

Perturbative QCD in acceptable schemes with holomorphic coupling

Perturbative QCD in mass independent schemes leads in general to running coupling $a(Q^2)$ which is nonanalytic (nonholomorphic) in the regime of low spacelike momenta $|Q^2| \lesssim 1 \ {\rm GeV}^2$. Such (Landau) singularities are inconvenient in the following sense: evaluations of spacelike physical quantities ${\cal D}(Q^2)$ with such a running coupling $a(κQ^2)$ ($κ\sim 1$) give us expressions with the same kind of singularities, while the general principles of local quantum field theory require that the mentioned physical quantities have no such singularities. In a previous work, certain classes of perturbative mass independent beta functions were found such that the resulting coupling was holomorphic. However, the resulting perturbation series showed explosive increase of coefficients already at ${\rm N}^4{\rm LO}$ order, as a consequence of the requirement that the theory reproduce the correct value of the $τ$ lepton semihadronic strangeless decay ratio $r_τ$. In this work we successfully extend the construction to specific classes of perturbative beta functions such that the perturbation series do not show explosive increase of coefficients, the perturbative coupling is holomorphic, and the correct value of $r_τ$ is reproduced. In addition, we extract, with Borel sum rule analysis of the V+A channel of the semihadronic strangeless decays of $τ$ lepton, reasonable values of the corresponding D=4 and D=6 condensates.

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arXiv2015

anQCD: a Mathematica package for calculations in general analytic QCD models

We provide a Mathematica package that evaluates the QCD analytic couplings (in the Euclidean domain) $\mathcal{A}_ν(Q^2)$, which are analytic analogs of the powers $a(Q^2)^ν$ of the underlying perturbative QCD (pQCD) coupling $a(Q^2) \equiv α_s(Q^2)/π$, in three analytic QCD models (anQCD): Fractional Analytic Perturbation Theory (FAPT), Two-delta analytic QCD (2$δ$anQCD), and Massive Perturbation Theory (MPT). The analytic (holomorphic) running couplings $\mathcal{A}_ν(Q^2)$, in contrast to the corresponding pQCD expressions $a(Q^2)^ν$, reflect correctly the analytic properties of the spacelike observables ${\cal D}(Q^2)$ in the complex $Q^2$ plane as dictated by the general principles of quantum field theory. They are thus more suited for evaluations of such physical quantities, especially at low momenta $|Q^2| \sim 1 \ {\rm GeV}^2$.

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arXiv2014

Mathematica and Fortran programs for various analytic QCD couplings

We outline here the motivation for the existence of analytic QCD models, i.e., QCD frameworks in which the running coupling $A(Q^2)$ has no Landau singularities. The analytic (holomorphic) coupling $A(Q^2)$ is the analog of the underlying pQCD coupling $a(Q^2) \equiv α_s(Q^2)/π$, and any such $A(Q^2)$ defines an analytic QCD model. We present the general construction procedure for the couplings $A_ν(Q^2)$ which are analytic analogs of the powers $a(Q^2)^ν$. Three analytic QCD models are presented. Applications of our program (in Mathematica) for calculation of $A_ν(Q^2)$ in such models are presented. Programs in both Mathematica and Fortran can be downloaded from the web page: gcvetic.usm.cl.

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arXiv2014

The bottom quark mass from the $Υ(1S)$ system at NNNLO

We obtain an improved determination of the normalization constant of the first infrared renormalon of the pole mass (and the singlet static potential). For $N_f=3$ it reads $N_m=0.563(26)$. Charm quark effects in the bottom quark mass determination are carefully investigated. Finally, we determine the bottom quark mass using the NNNLO perturbative expression for the $Υ(1S)$ mass. We work in the renormalon subtracted scheme, which allows us to control the divergence of the perturbation series due to pole mass renormalon. Our result for the ${\overline {\rm MS}}$ mass reads ${\overline m}_{b}({\overline m}_{b})=4201(43)$ MeV.

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arXiv2014

CP violation in lepton number violating semihadronic decays of $K, D, D_s, B, B_c$

We study the CP violation in lepton number violating meson decays $M^{\pm} \to \ell_1^{\pm} \ell_2^{\pm} M^{' \mp}$, where $M$ and $M^{'}$ are pseudoscalar mesons, $M=K, D, D_s, B, B_c$ and $M^{'}=π, K, D, D_s$, and the charged leptons are $\ell_1, \ell_2 = e, μ$. It turns out that the CP-violating difference $S_{-}(M) \equiv [Γ(M^- \to \ell_1^- \ell_2^- M^{' +})-Γ(M^+ \to \ell_1^+ \ell_2^+ M^{' -})]$ can become appreciable when two intermediate on-shell Majorana neutrinos $N_j$ ($j=1,2$) participate in these decays. Our calculations show that the asymmetry becomes largest when the masses of $N_1$ and $N_2$ are almost degenerate, i.e., when the mass difference $ΔM_N$ becomes comparable with the (small) decay widths $Γ_N$ of these neutrinos: $ΔM_N \not\gg Γ_N$. We show that in such a case, the CP ratio ${\cal A}_{CP}(M) \equiv [Γ(M^- \to \ell_1^- \ell_2^- M^{' +})-Γ(M^+ \to \ell_1^+ \ell_2^+ M^{' -})]/[Γ(M^- \to \ell_1^- \ell_2^- M^{' +})+Γ(M^+ \to \ell_1^+ \ell_2^+ M^{' -})]$ becomes a quantity $\sim 1$. The observation of CP violation in these decays would be consistent with the existence of the well-motivated $ν$MSM model with two almost degenerate heavy neutrinos in the mass range between $M_N \sim 0.1$-$10^1$ GeV.

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arXiv2012

Probing Majorana neutrinos in rare pi+ to e+ e+ mu- nu decays

We study the rare decays of charged π mesons, π+ to e+ e+ μ- anti-ν_μ and π+ to e+ μ- e+ ν_e, induced by a sterile neutrino N with a mass in the range m_μ< m_N < m_π. The first process violates Lepton Number by two units and so occurs only if N is Majorana, while the second process conserves Lepton Number and occurs irrespective of the Majorana or Dirac character of N. We study a way to distinguish the Majorana vs. Dirac character of N in these processes using the muon spectrum. We also find that the branching ratios could be at the reach of high luminosity experiments like Project X at FNAL or any proposed neutrino (or muon) factories worldwide.

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arXiv2010

Reconciling the analytic QCD with the ITEP operator product expansion philosophy

Analytic QCD models are those versions of QCD in which the running coupling parameter a(Q^2) has the same analytic properties as the spacelike physical quantities, i.e., no singularities in the complex Q^2 plane except on the timelike semiaxis. In such models, a(Q^2) usually differs from its perturbative analog by power terms ~(Lambda^2/Q^2)^k for large momenta, introducing thus nonperturbative terms in spacelike physical quantities whose origin is the UV regime. Consequently, it contradicts the ITEP-OPE philosophy which states that such terms can come only from the IR regimes. We investigate whether it is possible to construct analytic QCD models which respect the ITEP-OPE philosophy and, at the same time, reproduce not just the high-energy QCD observables, but also the low-energy ones, among them the well-measured semihadronic tau decay ratio.

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arXiv2010

Modified Contour-Improved Perturbation Theory

The semihadronic tau decay width allows a clean extraction of the strong coupling constant at low energies. We present a modification of the standard "contour improved" method based on a derivative expansion of the Adler function. The approach eliminates ambiguities coming from the existence of different integral expressions for the semihadronic tau decay ratio. Compared to the standard method, renormalization scale dependence is by more than a factor two weaker in modified contour improved perturbation theory. The last term of the expansion is reduced, and renormalization scheme dependence remains approximately equal. The extracted QCD coupling at the tau mass scale is by 2$%$ lower than the "contour improved" value. We find $α_s(M_Z^2)=0.1211\pm 0.0010$.

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