Quantum Chromodynamics Perturbation Theory Research Articles

Constraining Palatini gravity with GR-independent equations of state for neutron stars

We demonstrate how to construct GR-independent equations of state for a neutron star from the information available in the literature. We emphasize the importance of using theory-based principles instead of relying solely on astrophysical observables and General Relativity. We propose a set of equations of state based on first microscopic principles, including chiral perturbation theory and perturbation theory in quantum chromodynamics. Interpolation methods are employed with assumptions on the thermodynamic stability and causality in the intermediate region. These equations of state are then used to constrain quadratic Palatini f(ℛ) gravity, indicating that its free parameter can at most lie within the range around -6.47 ≲ β ≲ 1.99 km2. Additionally, we briefly discuss the problem of phase transitions and twin stars.

NNLO QCD predictions of the asymmetry probe of the Z γ pair-production process

The paper presents for the first time a novel idea of exploiting asymmetry between differential cross sections of the off-shell Z γ pair-production in proton-proton (pp) collisions for the final states of a charged-lepton pair plus a photon pp → Z γ → l + l − γ (leptonic decay) and of a neutrino pair plus a photon (invisible decay). Asymmetry between the leptonic and invisible decays of the Z γ process is investigated by using fixed-order predictions through inclusion of next-to-next-to-leading (NNLO) radiative corrections in quantum chromodynamics (QCD) perturbation theory. NNLO QCD predictions are presented at various pp-collision energies as functions of several key observables including transverse momenta and azimuthal-angle separation, regarding the Z γ decay products. The predicted distributions for the Z γ asymmetry are provided based on realistic fiducial phase-space requirements in line with the related hadron-collider measurements. The predicted distributions are assessed at various pp-collision energies and in different phase-space regions such as with higher lepton-pair invariant mass or higher neutrino-pair transverse momentum requirements. The Z γ asymmetry is shown to be significantly sensitive in different regions of phase space including high- and high- regions. The Z γ asymmetry can therefore be translated into an important quantity for probing deviation from the Standard Model (SM) predictions. In this regard, the asymmetry probe is proposed as a sensitive indicator for indirect searches for physics beyond the SM encompassing high-mass resonances and dark-matter sector.

Measurement of the [formula omitted] charge asymmetry in events with highly Lorentz-boosted top quarks in pp collisions at [formula omitted] TeV

The measurement of the charge asymmetry in top quark pair events with highly Lorentz-boosted top quarks decaying to a single lepton and jets is presented. The analysis is performed using proton-proton collisions at s=13TeV with the CMS detector at the LHC and corresponding to an integrated luminosity of 138 fb−1. The selection is optimized for top quarks produced with large Lorentz boosts, resulting in nonisolated leptons and overlapping jets. The top quark charge asymmetry is measured for events with a tt¯ invariant mass larger than 750 GeV and corrected for detector and acceptance effects using a binned maximum likelihood fit. The measured top quark charge asymmetry of (0.42−0.69+0.64)% is in good agreement with the standard model prediction at next-to-next-to-leading order in quantum chromodynamic perturbation theory with next-to-leading-order electroweak corrections. The result is also presented for two invariant mass ranges, 750–900 and >900GeV.

Evidence for the charge asymmetry in pp → toverline{t} production at sqrt{s} = 13 TeV with the ATLAS detector

Inclusive and differential measurements of the top–antitop ( toverline{t} ) charge asymmetry {A}_{textrm{C}}^{toverline{t}} and the leptonic asymmetry {A}_{textrm{C}}^{ell overline{ell}} are presented in proton–proton collisions at sqrt{s} = 13 TeV recorded by the ATLAS experiment at the CERN Large Hadron Collider. The measurement uses the complete Run 2 dataset, corresponding to an integrated luminosity of 139 fb−1, combines data in the single-lepton and dilepton channels, and employs reconstruction techniques adapted to both the resolved and boosted topologies. A Bayesian unfolding procedure is performed to correct for detector resolution and acceptance effects. The combined inclusive toverline{t} charge asymmetry is measured to be {A}_{textrm{C}}^{toverline{t}} = 0.0068 ± 0.0015, which differs from zero by 4.7 standard deviations. Differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the toverline{t} system. Both the inclusive and differential measurements are found to be compatible with the Standard Model predictions, at next-to-next-to-leading order in quantum chromodynamics perturbation theory with next-to-leading-order electroweak corrections. The measurements are interpreted in the framework of the Standard Model effective field theory, placing competitive bounds on several Wilson coefficients.

Chiral perturbation theory: reflections on effective theories of the standard model

The pseudoscalar particles pions, kaons and the $\eta$-particle are considerably lighter than the other hadrons such as protons or neutrons. Their lightness was understood as a consequence of approximate chiral symmetry breaking. This led to current algebra, a way to express the relations imposed by the symmetry breaking. It was realized by Weinberg that because of their low mass, it is possible to formulate a purely pionic (effective) field theory at experimental energies, which carries all information on the (non-perturbative) dynamics, symmetries, and their spontaneous breaking of quantum chromodynamics (QCD) and allows for systematic calculations of observables. In this review, we trace these developments and present recent activities in this field. We make the connection to other effective theories, more generally introduced by Wilson, as approximate field theories at low energies. Indeed, principles and paradigms introduced first for pions have become ubiquitous in particle physics and the standard model. Lastly, we turn to the latest development where the present (fundamental) standard model itself is considered as an effective field theory of a - yet to be formulated - even more fundamental theory. We also discuss important techniques that were developed in order to turn chiral perturbation theory into a predictive framework and briefly review some connections between lattice QCD and chiral perturbation theory (ChPT).

Dense and Hot QCD at Strong Coupling

We present a novel framework for the equation of state of dense and hot quantum chromodynamics (QCD), which focuses on the region of the phase diagram relevant for neutron star mergers and core-collapse supernovae. The model combines predictions from the gauge/gravity duality with input from lattice field theory, QCD perturbation theory, chiral effective theory, and statistical modeling. It is therefore, by construction, in good agreement with theoretical constraints both at low and high densities and temperatures. The main ingredients of our setup are the nonperturbative V-QCD model based on the gauge/gravity duality, a van der Waals model for nucleon liquid, and the DD2 version of the Hempel-Schaffner-Bielich statistical model of nuclear matter. By consistently combining these models, we also obtain a description for the nuclear to quark matter phase transition and its critical end point. The parameter dependence of the model is represented by three (soft, intermediate, and stiff) variants of the equation of state, all of which agree with observational constraints from neutron stars and their mergers. We discuss resulting constraints for the equation of state, predictions for neutron stars, and the location of the critical point.3 MoreReceived 18 January 2022Revised 8 June 2022Accepted 29 August 2022DOI:https://doi.org/10.1103/PhysRevX.12.041012Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEquations of state of nuclear matterGauge-gravity dualitiesNuclear matter in neutron starsQCD phase transitionsQuantum chromodynamicsParticles & FieldsGravitation, Cosmology & AstrophysicsNuclear Physics

Variational perturbation theory and nonperturbative calculations in QCD

A nonperturbative approach based on the variational perturbation theory in quantum chromodynamics is developed. The variational series is different from the conventional perturbative expansion and can be used to go beyond the weak-coupling regime. The approach suggested takes into account the summation of threshold singularities and the involvement of nonperturbative light quark masses. Phenomenological applications of this approach to describe physical quantities connected with the hadronic τ-decay data: the Rτ ratio, the light-quark Adler function, and the smeared RΔ function are presented. The description of examined quantities includes an infrared region and, therefore, they cannot be directly calculated within the standard perturbation theory. It is shown that in spite of this fact the approach suggested gives a rather good result for these quantities down to the lowest energy scale.

Factorization Theorems for Exclusive Heavy-Quarkonium Production

We outline the proofs of the factorization theorems for exclusive two-body charmonium production in B-meson decay and e;{+}e;{-} annihilation to all orders in perturbation theory in quantum chromodynamics. We find that factorized expressions hold up to corrections of order m_{c}/m_{b} in B-meson decay and corrections of order m_{c};{2}/s in e;{+}e;{-} annihilation, where m_{c} is the charm-quark mass, m_{b} is the bottom-quark mass, and sqrt[s] is the e;{+}e;{-} center-of-momentum energy.

Using π2(1670)→b1(1235)π to constrain hadronic models

We show that current analyses of experimental data indicate that the strong decay mode π2→b1π is anomalously small. Non-relativistic quark models with spin-1 quark pair creation, such as 3P0, 3S1 and 3D1 models, as well as instanton and lowest order one-boson (in this case π) emission models, can accommodate the analyses of experimental data, because of a quark-spin selection rule. Models and effects that violate this selection rule, such as higher order one-boson emission models, as well as mixing with other Fock states, may be constrained by the small π2→b1π decay. This can provide a viability check on newly proposed decay mechanisms. We show that for mesons made up of a heavy quark and anti-quark, the selection rule is exact to all orders of quantum chromodynamics (QCD) perturbation theory.

Transverse momentum and total cross section of e(+)e(-) pairs in the Z-boson region from p&pmacr; collisions at sqrt

The transverse momentum and total cross section of e(+)e(-) pairs in the Z-boson region of 66<M(ee)<116 GeV/c(2) from p&pmacr; collisions at sqrt[s] = 1.8 TeV are measured using 110 pb(-1) of collisions taken by the Collider Detector at Fermilab during 1992-1995. The total cross section is measured to be 248+/-11 pb. The differential transverse momentum cross section is compared with calculations that match quantum chromodynamics perturbation theory at high transverse momentum with the gluon resummation formalism at low transverse momentum.

Isolated prompt photon production in hadronic final states of e+e- annihilation.

We provide complete analytic expressions for the isolated prompt photon production cross section in $e^+e^-$ annihilation reactions through one-loop order in quantum chromodynamics (QCD) perturbation theory. Functional dependences on the isolation cone size $\delta$ and isolation energy parameter $\epsilon$ are derived. The energy dependence as well as the full angular dependence of the cross section on $\theta_\gamma$ are displayed, where $\theta_\gamma$ specifies the direction of the photon with respect to the $e^+e^-$ collision axis. We point out that conventional perturbative QCD factorization breaks down for isolated photon production in $e^+e^-$ annihilation reactions in a specific region of phase space. We discuss the implications of this breakdown for the extraction of fragmentation functions from $e^+e^-$ annihilation data and for computations of prompt photon production in hadron-hadron reactions.

Inclusive prompt photon production in hadronic final states of e+e- annihilation.

We provide complete analytic expressions for the inclusive prompt photon production cross section in hadronic final states of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ annihilation reactions through one-loop order in quantum chromodynamics perturbation theory. Computed explicitly are direct photon production through first order in the electromagnetic strength ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{em}}$ and the quark-to-photon and gluon-to-photon fragmentation contributions through first order in the strong coupling ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$. The full angular dependence of the cross sections is displayed, separated into transverse (1+${\mathrm{cos}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$) and longitudinal (${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$) components, where ${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$ specifies the direction of the photon with respect to the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collision axis. We discuss extraction of fragmentation functions from ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ data. \textcopyright{} 1996 The American Physical Society.

Re+e− at low energies in variational perturbation theory

We calculate R e + e − at low energies using method of variational perturbation theory in quantum chromodynamics. We apply the “smearing” method to compare the obtained theoretical prediction for R e + e − with experimental data down to lowest energies. The problem of the “freezing” of the QCD coupling constant is discussed.

NONPERTURBATIVE β-FUNCTION IN QUANTUM CHROMODYNAMICS

We propose a method by which it is possible to go beyond the scope of quantum chromodynamics perturbation theory. By using a new small parameter we formulate a systematic nonperturbative expansion and derive a renormalization β-function in quantum chromodynamics.

The increase in the total cross sections for hadronic interactions with increasing energy

The reasons for an increase in the cross sections and the range of interaction of hadrons with increasing energy are discussed. A picture of fast hadron collisions that enables one to explain on the basis of quantum chromodynamics perturbation theory the main qualitative features of present-day experimental data is examined.

Physics of $\Upsilon$ resonances: ten years later

This paper reviews the present status of the physics of bottomonium: a bound system consisting of a heavy b quark and the antiquark . The basic experimental data on the levels of bottomonium are presented. Theoretical methods for describing the properties of these levels are discussed. Questions pertaining to the spectroscopy of bottomonium, including the fine and hyperfine splittings, radiative transitions between levels, and annihilation decays of the b system, are discussed. Effects which are not describable by quantum-chromodynamics perturbation theory are taken into account. Transitions between bottomonium levels involving the emission of light muons are discussed. The possibilities of a search for hypothetical new particles and effects in the decays of resonances are also discussed.

Heavy quarks and perturbation theory in quantum chromodynamics

Heavy quarks and perturbation theory in quantum chromodynamics

Тяжелые кварки и теория возмущений квантовой хромодинамики

Тяжелые кварки и теория возмущений квантовой хромодинамики

Introduction to QCD thermodynamics and the quark-gluon plasma

These lectures review recent theoretical work suggesting that hadronic matter may dissolve into a weakly interacting quark-gluon plasma phase at energy densities only one order of magnitude above the ground state energy density of nuclei. Basic techniques of field theory used for calculating thermodynamic properties of Quantum Chromodynamics (QCD) are introduced. Functional methods are applied to develop QCD perturbation theory at finite temperatures and chemical potentials. The relevance of asymptotic freedom at high T, μ is motivated. We then confront the main skeleton in the QCD closet, namely, the nonperturbative color magnetic sector. Techniques of lattice gauge theories to get beyond the limitations of perturbation theory are then discussed. Recent numerical results are critically assessed.

Singularities in quantum-chromodynamic perturbation theory forx∼1

The QCD perturbation series has been observed to converge poorly near phase-space boundaries; for example, as $x\ensuremath{\rightarrow}1$ in structure functions. Methods have been proposed for improving the convergence by including kinematical corrections. We compare these methods, and apply them to the calculation of the photon structure function.