Quark and gluon jets in the breit frame of lepton-nucleon scattering

Gluon bremsstrahlung,q→Gq, and quark pair production from gluons,laggy, in deep inelastic reactions is investigated in the Breit frame (moving alongQ in the laboratory). These QCD effects diminish the overall forward momentum. There are also events with asingle largep _⊥ forward jet. One spectacular class of events is predicted in which no forward going hadrons emerge, in the Breit frame. These effects are not mimicked by nonperturbative (limitedp _⊥) parton jets at large but attainableQ ².     

Properties of gluon jets

We compute the expected properties of gluon jets in a model based on the KUV jet calculus and recombination. Emphasis is placed on a) the production of baryons, and b) the question of whether hadrons produced by the decays of Zweig rule stable quarkonia (e.g. the upsilon) ine ⁺ e ^? have markedly different energy spectra from those produced by the adjacent quark-antiquark continuum.     

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.     

Spotting glueballs in collinear gluon jets

We suggest looking for pure gluon hadronic states (glueballs) in ?→ 3g → low spherically final state (collinear gluon jets). In these events one gluon has the maximum energy, $\text{M?}\text{2}$, favouring fragmentation into a glueball. The signature for a true C = G + glueball is its prominence at the ? resonance in e⁺e^? →? → (glueball → four charged pions) + … and its absence in q$\text{q}$ jets off (we do not expect significant quark fragmentation into glueballs).     

A comparison of b and uds quark jets to gluon jets

Symmetric three-jet events are selected from hadronic Z⁰ decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For thek _⊥ jet finder withy _cut=0.02, we find as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.     

A comparison of b and uds quark jets to gluon jets

Symmetric three-jet events are selected from hadronic Z⁰ decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For the k _⊥ jet finder with y _cut=0.02, we find $${«ngle n^{? ch.}»ngle {? gluon}?er «ngle n^{? ch.}»ngle {? b} {? quark}}=1.089pm 0.024 ({? stat.})pm0.024 ({? syst.})$$ $${«ngle n^{? ch.}»ngle {? gluon}?er «ngle n^{? ch.}»ngle {? uds} {? quark}}=1.390pm 0.038 ({? stat.})pm0.032 ({? syst.})$$ as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.     

Identified charged particles in quark and gluon jets

A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphi detector at Lep during 1994–1995 was used for an improved measurement of inclusive distributions of and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, , of the -distributions for kaons in gluon and quark jets are observed to be different. Received: 24 January 2000 / Revised version: 15 May 2000 / Published online: 8 September 2000     

Effective photon theory and photon-photon scattering

It is shown that the assumptions made by Eimerl in deriving the unusually high photon-photon scattering cross section from the effective photon theory of Panarella are unfounded.     

Search for stimulated photon-photon scattering in vacuum

We have searched for stimulated photon scattering in vacuum at a center of mass photon energy of 0.8 eV. The QED contribution to this process is equivalent to four wave mixing in vacuum. No evidence for scattering was observed. The corresponding upper limit of the cross-section is . Received 29 September 1999     

The production of large transverse momentum jets in plarized photon-photon collisions

A comprehensive survey is presented of the polarization structure of the leading order QCD processes for the production of jets at largep _t in photon-photon collisions which are the result of colliding positrons. Results are given for asymmetries. It is shown that by separating events with various jet topologies it is possible to investigate different aspects of polarization and QCD. For instance the four anf three jet process asymmetries are found to be sensitive to the relative magnitudes of the perturbative anf non-perturbative components of the photon structure function. A discussion is given of the usefulness of polarization in separating the two gluon jet subprocess. It is concluded that such experiments could be very useful in understanding QCD and the strong interactions.     

Nonlinear phase shift from photon-photon scattering in vacuum

We show that QED nonlinear effects imply a phase correction to the linear evolution of electromagnetic waves in vacuum. We provide explicit solutions of the modified Maxwell equations for the propagation of a superposition of two plane waves and calculate analytically and numerically the corresponding phase shift. This provides a new framework for the search of all-optical signatures of photon-photon scattering in vacuum. In particular, we propose an experiment for measuring the phase shift in projected high-power laser facilities.     

A three-dimensional model for quark and gluon jets

Based on the assumption that a color force-field has a stringlike character (like a stretchedout bag_ with no excited degrees of freedom transverse to the field direction (which is strongly supported by the observed polarization of inclusively produced Λ-particles) we derive the probability to produce heavy flavor quark-antiquark pairs and pairs with transverse momentum in the field. We show how to incorporate the results into a soft hadronisation scheme for particle distributions in quark and gluon jets. We point to some non-trivial effects from the finite (longitudinal) size of the force-field which result on the one hand in important correlations between the longitudinal scaling variable and the transverse momentum and on the other hand leads to correstions to the simple iterative cascade scheme.     

Investigation of the splitting of quark and gluon jets

The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the Delphi detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation . The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution , with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is . Due to non-perturbative effects, the data are below the expectation at small . The transition from the perturbative to the non-perturbative domain appears at smaller for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets. Received: 18 February 1998 / Published online: 24 April 1998     

Color dipole BFKL-Regge factorization and high-energy photon-photon scattering

Based on the color dipole representation, we investigate consequences for the γ*γ*, γ *γ scattering of the finding by Fadin, Kuraev, and Lipatov that the incorporation of asymptotic freedom into the BFKL equation makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL-Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to the γ *γ*, γ*γ scattteirng and the deep inelastic scattering on protons. Numerical predictions based on our early work on the color dipole BFKL phenomenology of the deep inelastic scattering on protons gives a good agreement with the recent experimental data from OPAL and L3 experiments at LEP200. We discuss the role of nonperturbative dynamics and predict a pronounced effect of the Regge-factorization breaking due to large unfactorizable nonperturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL-Regge expansion for the γ*γ*, γ*γ scattering including the issue of the decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance.