Angular intermittency in QCD jets

Using two methods, via fluctuations and correlations, an analytical formula is derived for the factorial multiplicity moments in a QCD jet at the Double Leading Logarithm accuracy. The resulting self-similar dependence on the solid-angle cell size is characteristic of an intermittency behaviour in angular variables. The intermittency indices depend on the diffusion angle through the running of α_S. Physical features of jet fluctuations such as collimation at large angles and saturation at small angles are well described in the perturbative framework. A parameter-free prediction of angular intermittency is proposed for Z⁰ decays into hadrons, assuming hadronparton duality.

     

Probing the transverse momentum structure of QCD jets

Large logarithms are resummed to give QCD predictions for the differing transverse momentum structure of quark and gluon jets.     

Quantum correlations in QCD jets

The spin of the gluon gives rise to quantum correlations between azimuthal angles in QCD jet evolution, which are usually ignored or integrated out. Here they are computed and their effects are shown to be small.     

Baryon production in QCD jets

Recent data from several sources—Υ decays, high energy deep inelastic scattering,e ⁺ e ^? annihilation at PETRA—reveal substantial baryon production in QCD jets. We discuss the likely mechanisms within the context of QCD, and formulate experimental tests to distinguish amongst them.     

Measuring the charges of QCD jets

We investigate the effects of perturbative branching upon the accuracy with which one can determine the charge of the underlying QCD quantum from the charge structure of a given hadronic jet. We show explicitly how at asymptoticQ ² we lose all such charge information. We investigate these effects at current PETRA energies using the Monte Carlo program of Fox and Wolfram; and find that a reasonably accurate charge determination is still possible at these energies. We suggest the variation of the jet charge structure with multiplicity, at a given energy, as a sensitive probe of the onset of perturbative branching inside jets.     

Angular distribution of energy in jets

The transverse momentum distribution of heavy C-even resonances produced in hadron collisions probes, according to QCD, the spatial spread of color charge in gluons. An all-orders calculation of the effective doubly logarithmic gluon form factor is supported by recent data on χ (3.5) production. We predict a very wide p_t distribution of C-even b$\text{b}$ resonances associated with ?.     

Angular distribution of lepton-pairs and QCD

The recently measured angular distribution of one of the leptons produced in π⁻N → μ⁺μ^-+… is compared with theoretical calculations done in the framework of QCD.     

Heavy particle content in QCD jets

We calculate the multiplicity of heavy quark pairs in gluon jets in lowest order QCD, along with the non-perturbative correction related to the gluon condensate 〈(α/π)F²〉. The value of the non-perturbative correction is

where M is the mass of the heavy quark, N is the number of colors, and C_F and C_A are the values of the Casimir operators in the fundamental and adjoint representation respectively. α(M) is the running coupling constant at the scale of the heavy quark, and 〈F²〉 is the gluon condensate, usually determined to be 〈α/πF²〉 ∼ 0.012 GeV⁴. The non-perturbative correction is extremely small.     

Generalised factorial moments and QCD jets

Zeitschrift für Physik C Particles and Fields - In this paper we present a natural and comprehensive generalisation of the standard factorial moments (F q ) analysis of a multiplicity...     

On the transverse spread of QCD jets

Recent QCD results on two-particle longitudinal spectra inside quark and gluon jets are extended to the case of a fixed relative transverse momentum q_T with Λ ≈ 0.5 GeV ? 2|q_T| ? √Q². Broad q_T distributions, especially for gluon jets, are obtained which smooth out automatically the perturbative result and whose integrated versions scale in η_Max ≡ $\text{log}\text{(}\text{2q}{}_{\mathrm{<mtext>T</mtext>}}{}^{\mathrm{<mtext>Max</mtext>}}\text{Λ}\text{)/}\text{log}\text{(}\text{Q}\text{Λ}\text{)}$.     

Collimation of energy in medium-modified QCD jets

The collimation of energy inside medium-modified jets is investigated in the leading logarithmic approximation of QCD. The Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) evolution equations are slightly modified by introducing splitting functions enhanced in the infrared sector. As compared to elementary collisions in the vacuum, the angular distribution of the jet energy is found to broaden in QCD media.     

Angular energy flow in perturbative QCD

We discuss a general way in which QCD can be checked in lepton-induced reactions — even with low statistics data. This is the “angular energy flow”, or the average energy fraction outside a cone of opening angle 2σ about the principal jet axis. in the final state of deep inelastic reactions. We illustrate this method by a perturbative calculation of the angular energy flow in $e^{+}{e}^{?}\to q\overline{q}g\to$ jets. This perturbative approach can be extended to other deep inelastic reactions wit h gluon jets. At high enough energy, it should test QCD beyond lowest order in α_s.     

Collimation of average multiplicity in QCD jets

The collimation of average multiplicity inside quark and gluon jets is investigated in perturbative QCD in the modified leading logarithmic approximation (MLLA). The role of higher order corrections accounting for energy conservation and the running of the coupling constant leads to smaller multiplicity collimation as compared to leading logarithmic approximation (LLA) results. The collimation of jets produced in heavy-ion collisions has also been explored by using medium-modified splitting functions enhanced in the infrared sector. As compared to elementary collisions, the angular distribution of the jet multiplicity is found to broaden in QCD media at all energy scales.     

On the multiplicity of hadrons in QCD jets

A three-loop calculation is presented for the jet multiplicity of produced slightly-off-shell gluons for a pure Yang-Mills theory. Planar and non-planar graphs are found to be equally important in an axial gauge. If the three-loop calculation is indicative of what happens at higher orders $\text{n}\text{(Q}{}^2\text{) ∝}\text{exp}\text{{[(}\text{2C}{}_{\mathrm{<mtext>A</mtext>}}\text{πb}\text{) 1}\text{n}\text{Q}{}^2\text{]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{}}$ where C_A = 3 and $\text{b =}\text{(33 ? 2n}{}_{\mathrm{<mtext>f</mtext>}}\text{)}\text{12π}$ in QCD.     

Angular energy flow in leptoproduction and QCD

The perturbative QCD prediction of the angular energy flow in deep inelastic leptoproduction is discussed. A numerical estimate of its magnitude is given in the Breit frame and in the c.m. frame of the virtual photon and proton.     

QCD predictions for jets in e+e− annihilation: Angular correlations and asymmetries

The topological features of hadron jets in e⁺e^? annihilation may reliably be calculated in QCD perturbation theory. To second order the final state hadrons fall into quark-antiquark-gluon initiated jets. The latter induce angular asymmetries which are extensively studied.     

Angular distribution of lepton pairs in perturbative QCD

We compute to first order in perturbative QCD the decay angular distribution of the lepton pairs produced in high energy hadronic collisions. The results, integrated over the transverse momentum of the pair, are found in reasonable agreement with experimental data.     

Coherent effects in the perturbative QCD parton jets

The influence of coherence in a soft parton bremsstrahlung on hadron spectra in jets is discussed. We present a simple functional method for calculation of different inclusive parton distributions in the “plateau” region. Inclusive parton cross sections, spectra, multiplicities, energy and angular correlations, etc. are derived in the Double Logarithmic approximation.     

Factorial correlators: angular scaling within QCD jets

Factorial correlators measure the amount of dynamical correlation in the multiplicity between two separated phase-space windows. We present the analytical derivation of factorial correlators for a QCD jet described at the double logarithmic (DL) accuracy. We obtain a new angular scaling property for properly normalized correlators between two solid-angle cells or two rings around the jet axis. Normalized QCD factorial correlators scale with the angular distance and are independent of the window size. Scaling violations are expected beyond the DL approximation, in particular from the subjet structure. Experimental tests are feasible, and thus would be welcome. Received: 12 January 2001 / Published online: 7 September 2001