QCD predictions for four-jet final states in e+e− annihilation

We have calculated the four-jet production processes $\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→q}\text{q}\text{gg}$ and $\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→q}\text{q}\text{q}\text{q}$ to lowest-order QCD perturbation theory. We find that (q$\text{q}$q$\text{q}$) production is small compared to the dominant process $\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→q}\text{q}\text{gg}$ which can in part be traced to the fact that the latter process is more singular as the 2- and 3-jet phase-space limits are approached. We present differential 4-jet acoplanarity distributions and compare them with non-perturbative acoplanarity distributions at maximum PETRA and PEP energies. Leading log cross-section formulae are derived for various cut-off procedures and are compared to the results of our numerical integrations. We also present results on associated heavy quark production in e⁺e^? annihilation.     

Hadronic jets with a large Q⊥ lepton pair trigger

We propose to observe hadron jets in correlation with large Q⊥ transverse momentum lepton pairs. This would allow to test the theoretical idea that the large transverse momentum of the pair is mainly produced through a scattering subprocess $\text{a}\text{+}\text{b}\text{→}\text{c}\text{+γ}{}^1$. The quantum number content of these jets is a specific signature of the subprocess, especially of those involved in perturbative QCD.     

Vorticity in nuclear fluid dynamics

We consider a model for gloun jet fragmentation based on QCD in which the fast hadrons in the jet are produced by the sequential reaction gluon $\text{→}\text{q}\text{q}\text{?}\text{q}\text{→}$ hadrons. The resulting jet shows an oblate transverse momentum structure, with a major axis preferential oriented normally to the direction of linear polarization of the gluon. We discuss jet-jet oblateness angular correlations in decays of heavy QQ? pseudoscalar and vector systems.     

Gluons in quarkonium decay

We discuss what can be learned from the ³S₁ quarkonium decay $\text{Q}\text{Q}\text{→ 3}\text{gluons}$, $\text{Q}\text{Q}\text{→ γ + 2}\text{gluons}$. The former is a way to find gluon jets and test QCD. The latter also allows us to measure gluon + gluon → hadrons, and look for pure gluonic resonances (glueballs).     

Heavy-quark production by hadrons: Gluon jet fragmentation versus fusion

We show that at $\text{p}\text{p}$ collider energies heavy quarks are dominantly produced by the fragmentation of gluon jets into $\text{Q}\text{Q}$ with Q = c,b. This is because the underlying gg → gg jet cross section exceeds the fusion process $\text{gg → Q}\text{Q}$ by over two orders of magnitude. We compute the perturbative contribution to the gluon fragmentation from 2 → 3 processes such as gg $\text{→ gQ}\text{Q}$. We point out the observable consequences for single-lepton production and same-opposite charge dileptons. In particular, the observed production of dileptons with large p_T and low effective mass is explained.     

A theory of the J/gY→η(η′)γ decays

It is argued that the $\text{J}\text{gY}\text{→η(η′)γ}$ decay rates are directly calculable within quantum chromodynamics. The calculation of $\text{Γ(}\text{J}\text{Ψ}\text{→ηγ)}$ is most straightforward and involves no new hypotheses as compared to, say, the $\text{J}\text{Ψ}\text{→}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}$ decay in detail earlier. To the contrary, evaluation of the $\text{J}\text{Ψ}\text{→η′γ}$ width puts on trial dynamical results concerning the η′ coupling to gluons. The latter depends crucially on non-perturbative power terms in the QCD sum rules. The calculated numbers for $\text{Γ(}\text{J}\text{Ψ}\text{→ηγ)}$ and $\text{Γ(}\text{J}\text{Ψ}\text{→η′γ)}$ agree well with the data.     

The effect of 1Q2 and αs corrections on tests of QCD

We discuss in detail the use of the structure function F₃(x, Q²) of deep-inelastic neutrino scattering for testing quantum chromodynamics. QCD is entirely consistent with all data. However, we show that higher-twist (order $\text{1}\text{Q}{}^2$) contributions, which are commonly neglected, can have a dramatic impact on interpretation of this result. At present the data are not accurate enough to determine the magnitudes of these $\text{1}\text{Q}{}^2$ contributions within the context of QCD. Furthermore, the possible presence of higher-twist terms makes it impossible to unambiguously detect the logarithmic Q² dependence and anomalous dimensions which distinguish QCD from hypothetical alternative theories. As a result, more precise data with higher Q² are needed to provide definitive tests of QCD. The corrections of second-order in α_s introduce fewer complications for testing QCD, and provide a useful context for understanding critical ambiguities in the definitions of α_s and Λ.     

Photon production by gluon jets

The fragmentation function for photons from a gluon jet is uniquely calculable in QCD. A phenomenological investigation of its properties indicates that the best place to observe it is in the continuum photon spectrum in the two-gluon decays of heavy Q$\text{Q}$ P-wave or pseudo-scalar bound states. The fragmentation function may be observable in an experiment with sufficient efficiency in rejecting photons from π⁰ decay.     

Target mass effects in polarized electroproduction

We examine the target mass effects in spin-dependent deep inelastic electroproduction within the framework of Wilson's operator product expansion. Simple analytic expressions for the Nachtmann moments of polarized structure functions g₁ and g₂ are obtained and their Q² evolutions are considered in QCD. The Bjorken and Burkhardt-Cottingham sum rules including target mass effects are derived and the QCD $\text{g}{}^2$ corrections are discussed.     

A comment on higher twist contributions to σLσT

We consider the contribution to $\text{σ}{}_{\mathrm{L}}\text{σ}{}_{\mathrm{t}}$ in deep inelastic lepton-hadron scattering from a certain class of 4-quark operators in the framework of QCD. We find for this case by explicit calculation that $\text{σ}{}_{\mathrm{L}}\text{σ}{}_{\mathrm{T}}$ is of order g²(Q²), with g²(Q²) the running coupling strength of QCD, in agreement with a general result of Politzer.     

On the QCD perturbative predictions for the signal ofW± production in hadronic collisions

Evidence for the intermediate bosons in high energy pp and p?p collisions as signal in the cross section of large p_T charged lepton pp, $\text{p}\text{p}\text{→}\text{W}{}^{\mathrm{\pm }}\text{+}\text{X}$, W^±→?^±ν_? is considered for CM energy range $\text{s}\text{=100?800}\text{GeV}$. Besides the usual Drell-Yan term which gives a very sharp peak in the lepton p_T distribution, the first order QCD contribution is included thus producing the intermediate boson with transverse momentum and smearing the lepton distribution. Scaling violation of the structure functions is incorporated and compared with the case when it is absent.     

Transverse momentum in Drell-Yan processes

We study the transverse momentum distribution of muon pairs from Drell-Yan processes in QCD. In particular the dependence of 〈k_⊥²〉 on Q² is considered. QCD predicts an approximately linear rise of 〈k_⊥²〉 with S or Q² only at fixed τ = Q²/S. The slope as a function of τ is quantitatively studied for P$\text{P}$ and P-nucleus scattering. The most recent data showing a rather flat 〈k_⊥²〉 in Q² at fixed S are found to be consistent with QCD.     

An appraisal of Lepton signatures for the top quark in collider events with consideration of gluon radiation effects

Lepton signatures for top particles in $\text{p}\text{p}$ events at the CERN collider are discussed in detail. QCD radiation effects are duly included. It is shown that because of the QCD effects, the contaminating background is considerably more difficult to handle, especially in view of the present statistical limitations.     

Super-cosmology

By using the jet calculus technique, the size of the colour singlet clusters of emitted partons of mass Q₀ is estimated. For $\text{α(Q}{}^2{}_0\text{)}\text{π}\text{?1}$, the average mass of these clusters turns out to be a few GeV, thus suggesting phenomenological applications of preconfinement in pertubbative QCD. This result is very sensitive to the correct treatment of infrared singularities.     

Transverse momentum distribution of partons in QCD

The renormalization group approach is applied to the study of transverse momentum distribution of partons in QCD. We generalize the method of Altarelli and Parisi to obtain a formula for the mean squared average transverse momentum as a function of Q² and x, the virtual photon mass squared and the longitudinal momentum fraction of the parton. For large Q², it decreases with x in addition to the expected increase with Q². This x-dependence reduces substantially the transverse momentum of massive μ-pairs produced by hadronic collisions from its linear dependence on $\text{(}\text{Q}{}^2\text{ln}\text{Q}{}^2\text{)}\text{1}\text{2}$.     

Event shapes in deep inelastic lepton-hadron scattering

The structure of hadronic final states in deep inelastic scattering expected from QCD is analyzed in terms of the shape parameters H_l and C_l. We find that the effects of the fragmentation of quarks and gluons into hadrons are typically governed by $\text{√s = [Q}{}^2\text{(1/x?1)]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. For √s?30 GeV, the distributions of events in H₂ and C₂ should allow a test of the perturbative QCD prediction of three-jet events.     

Perturbative QCD corrections in Drell-Yan processes

The influence of lowest-order QCD corrections on the Drell-Yan cross section $\text{Q}{}^4\text{(}\text{dσ}\text{dQ}{}^2\text{)(τ, Q}{}^2\text{)}$ is determined and compared with the asymptotic freedom (AF) corrections. The perturbative calculation exhibits the AF-characteristics of a (strongly) rising Q²-dependence for √τ?0.1 (qg-scattering) and falling for √τ?0.2 (qq?-annihilation). Qualitative agreement between the two calculation methods in the entire √τ-range is obtained with α_s = 0.3.