Quantum Chromodynamics Perturbation Theory Research Articles

Fermionic contribution toΛlattice

The ratio $\frac{{\ensuremath{\Lambda}}_{\mathrm{MO}}}{{\ensuremath{\Lambda}}_{\mathrm{latt}}}$ is computed for quantum chromodynamics (QCD) with massless fermions present. The result obtained, using the Wilson form of the lattice action, is $\frac{{\ensuremath{\Lambda}}_{\mathrm{MO}}}{{\ensuremath{\Lambda}}_{\mathrm{latt}}}=117.5$. We show how our methods can readily be extended to other forms of the action. Appendices give details of the technology of QCD perturbation theory on the lattice. Included are derivations of Feynman rules and Ward identities as well as general power-counting arguments and a discussion of the renormalization group.

Asymmetry parameter as a function of transverse momentum in the production of dileptons with polarized beam and target

We calculate in quantum-chromodynamic perturbation theory the asymmetry parameter $A$ in the production of massive dileptons in collisions of polarized hadrons as a function of transverse momentum, using various models of spin-dependent parton distribution functions. We find $A$ to be negative, large, and model-dependent for $p\ensuremath{-}\overline{p}$ collisions, whereas for $p\ensuremath{-}p$ collisions $A$ is not strongly model dependent and varies from -5 to 7% for various values of transverse momentum.

SU 2 quantum chromodynamics perturbation theory in the instanton sector compared to the zeroth sector

In a recent paper, Rouet and the present author performed the renormalization of pure Yang-Mills theory developed around an instanton. We extend these results toSU2 quantum chromodynamics.

Some QCD estimates of the production of heavy quark flavours in hadron-hadron collisions

Quantum chromodynamic perturbation theory is used to investigate the hadronic production of heavy quark flavours. The theory is applied principally to the data on charmed meson production. Particular emphasis is placed on comparing data with this calculation in the specific region of phase space in which the data are taken. The general results of this approach are presented so that comparisons with data from future experiments on charm and heavier flavour production should be possible.

Structure of hadron structure functions

We investigate the comparison of the shapes, as functions of $x$, of nonsinglet structure functions of pions and nucleons, ${F}_{\ensuremath{\pi},N}(x, {Q}^{2})$. We argue that we can predict (to leading twist, all orders of quantum-chromodynamic perturbation theory) the shape of a pion structure function in terms of the observed shape of the nucleon structure function at the same ${Q}^{2}$. The uncertainties due to our ignorance of the details of bound-state dynamics are at the 20% level. Our technology is to exploit in detail the relationship between two approaches to the description of structure functions: the operator-product expansion and the covariant parton model.

Strong violations of the conserved-vector-current hypothesis in pion radiative decay?

We study the question whether large isospin-breaking effects due to the quark mass matrix may be expected in the decay $\ensuremath{\pi}\ensuremath{\rightarrow}e{\ensuremath{\nu}}_{e}\ensuremath{\gamma}$. This has been predicted previously on the basis of quantum-chromodynamic perturbation theory. However, perturbation theory may be rather misleading in this context because it does not take into account the correct ${\ensuremath{\gamma}}_{5}$ structure of the vacuum. We show quite generally that isospin-breaking effects vanish in the chiral limit, independently of $\ensuremath{\xi}=\frac{{m}_{d}}{{m}_{u}}$. An illustration and quantitative estimate is given in the framework of the $\ensuremath{\sigma}$ model.

QCD relation between deep inelastic neutrino reactions and polarized electroproduction

Within the framework of perturbation theory in quantum chromodynamics (QCD), a phenomenologically useful relation is derived between the moments of the structure functions in charged-current neutrino reactions and in polarized electroproduction (eq. (1)). The relation is illustrated by practical QCD results up to the next-to-leading order. The phenomenological consequences of the relation are analyzed with the help of existing data.

Possible existence of quark stars

The problem of existence of a family of quark stars has been investigated using the ground-state equation of state for the quark gas in second-order perturbation theory in quantum chromodynamics.

Signals ofW±andZ0production in hadronic collisions as predicted by quantum-chromodynamics perturbation theory

We point out that the observation of high-${p}_{T}$ charged leptons ${\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}$ in high-energy $\mathrm{pp}$, $\overline{p}p$ collisions can provide a clean way for detecting ${W}^{\ifmmode\pm\else\textpm\fi{}}$ and ${Z}^{0}$, including the determination of their masses. The calculations are done within the Weinberg-Salam model using the generalization of the Drell-Yan formula for ${W}^{\ifmmode\pm\else\textpm\fi{}}$,${Z}^{0}$ production to first order in quark-gluon couplings in asymptotically free quantum chromodynamics. We present the expressions and curves for various differential cross sections in the reactions $(pp,\overline{p}p)\ensuremath{\rightarrow}{W}^{+}+X$, ${W}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu}$; $(pp,\overline{p}p)\ensuremath{\rightarrow}{W}^{+}+\mathrm{jet}+X$, ${W}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu}$; and $(pp,\overline{p}p)\ensuremath{\rightarrow}({Z}^{0},\ensuremath{\gamma})+X$, $({Z}^{0},\ensuremath{\gamma})\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ for c.m. energy range $\sqrt{s}=100\ensuremath{-}800$ GeV. Different forms for scale violation of the structure functions are incorporated and compared with the case when they are absent. Implications of the results for the detection of intermediate vector bosons ${W}^{\ifmmode\pm\else\textpm\fi{}}$ and ${Z}^{0}$ are discussed.

Tensor analysis of hadronic jets in quantum chromodynamics

We define tensors in momentum space for the events produced in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation. They are calculable in quantum-chromodynamics (QCD) perturbation theory and therefore can be used as a test of QCD. We have studied their distributions and average values which give characteristics of the global structure of hadronic jets in QCD.

Jet invariant mass in quantum chromodynamics

We give a heuristic argument that a new class of observables related to the invariant mass of jets in e +e − annihilation is infrared finite to all orders of perturbation theory in Quantum Chromodynamics. We calculate the lowest order QCD predictions for the mass distribution as well as for the double differential cross section to produce back to back jets of invariant mass M 1 and M 2. The resulting cross sections are quite different from that expected in simple hadronic fireball models and should provide experimentally accessible tests of QCD.

High-pThadronic-jet production

The production of high-${p}_{T}$ hadronic jets is discussed using quantum-chromodynamic perturbation theory. Predictions are given for both single-jet and two-jet cross sections. Effects due to the transverse motion of the interacting partons are discussed at length. Various parton-model tests are discussed and performed using existing data.

Transverse-momentum distributions in lepton-hadron scattering from quantum chromodynamics

We use quantum-chromodynamic perturbation theory to calculate $\frac{d\ensuremath{\sigma}}{d{x}_{H}\mathrm{dyd}{\ensuremath{\Pi}}_{T}}$ in electromagnetic lepton-hadron scattering, where ${\ensuremath{\Pi}}_{T}$ is the sum over all produced hadrons of the absolute value of the component of three-momentum perpendicular to the virtual-photon direction.

Quantum-chromodynamic predictions for high-pTbaryon production

Predictions for high-${p}_{T}$ hadron production based on lowest-order quantum-chromodynamic (QCD) perturbation theory are reexamined in the light of new precise data relevant to the determination of the parton fragmentation functions. In particular, predictions for both $p$ and $\overline{p}$ production are given. It is found that for meson production in $\mathrm{pp}$ interactions the lowest-order QCD perturbation-theory predictions become dominant in the region $\sqrt{s}>50$ GeV and ${p}_{T}>5$ GeV/c. On the contrary, for baryon production the lowest-order predictions do not dominate until significantly larger ${p}_{T}$ values are reached.

Quark Structure Functions of Mesons and the Drell-Yan Process

For massive-lepton pair production in meson-induced reactions, we use quantum chromodynamics perturbation theory to predict that the decay angular distribution in the pair rest frame will change from predominantly $1+{cos}^{2}\ensuremath{\theta}$ to ${sin}^{2}\ensuremath{\theta}$ as the longitudinal-momentum fraction of the pair ${x}_{\mathrm{F}}\ensuremath{\rightarrow}+1$. The two angular distributions are associated respectively with ${(1\ensuremath{-}x)}^{2}$ and ${Q}^{\ensuremath{-}2}{(1\ensuremath{-}x)}^{0}$ components of the valence-quark structure function of the meson.

Quantum-chromodynamic predictions for large-pThadron production with transversely polarized beam and target

We apply a hard-scattering model based on quantum-chromodynamic perturbation theory to the production of hadrons at large transverse momentum by transversely polarized beam and target protons. We find that, at large ${x}_{T}=\frac{2{p}_{T}}{\sqrt{s}}$, the spin-spin asymmetry ${A}_{\mathrm{NN}}$ for the case of transversely polarized protons is significantly smaller than the corresponding asymmetry ${A}_{\mathrm{LL}}$ for longitudinally polarized protons. This situation is due to the relatively smaller size of both the quark-quark scattering asymmetry and the spin-dependent distribution functions for quarks in the transversely polarized proton.

Observables for the Analysis of Event Shapes ine+e−Annihilation and Other Processes

We present a set of rotationally invariant observables which characterizes the of events, and is calculable in quantum-chromodynamics perturbation theory for final states consisting of quarks and gluons ($G$). We include the effects of fragmentation to hadrons in comparing the shapes of events from the processes ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}$, ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}G$, and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{heavy}\mathrm{resonance}\ensuremath{\rightarrow}\mathrm{GGG}$, and from heavy-quark and lepton production. We indicate how our analysis may be extended to deep-elastic lepton-hadron interactions and hadron-hadron collisions involving large transverse momenta.

Quantum-Chromodynamic Spin-Spin Asymmetries in Large-pTHadron Production with Polarized Beam and Target

We calculate in quantum-chromodynamic perturbation theory the spin-spin asymmetrics for the fundamental 2 ..-->.. 2 scattering processes involving quarks and gluons. Using simple, reasonable assumptions concerning the distribution functions for definite-helicity quarks and gluons in a spinning proton we find that the idea that large-p/sub T/ events arise from the hard scattering of quarks and gluons leads to a measurable asymmetry for inclusive hadron production at large P/sub T/ from a polarized beam and target.

Simple Quantum-Chromodynamics Prediction of Jet Structure ine+e−Annihilation

We propose a simple alternative to the sphericity as a measure of jet structure in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation. Our variable has the property that it can be reliably calculated in perturbation theory in quantum chromodynamics (QCD) for large ${Q}^{2}$. It is not sensitive to the details of quark and gluon decay into color-singlet hadrons. We discuss the nonperturbative effects which are important at moderate ${Q}^{2}$.