Selection of jets in multihadron final states produced bye + e −-annihilation

Fore ⁺ e ^?-annihilations a procedure is described to select events of a given multiplicity of jets. The procedure is demonstrated by applying it to a Monte-Carlo sample ofe ⁺ e ^? multihadron final states. Selection of three jet events is shown to be somewhat better than more conventional techniques.     

QCD effects and b-tagging at LEP I

We analyze the impact of using b-tagged samples in studying non-Abelian effects due to QCD in e ⁺ e ^? → 4jet events at ${sqrt s=M_{Z^0}}$, using angular variable analyses and comparisons with e ⁺ e ^? → 3jetγ events. We find that QCD effects are largely enhanced in b-quark samples with respect to ‘unflavoured’ ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that the b-quark mass influences the angular distributions significantly and should not be neglected.     

Can the Aleph four-jet excess be explained in a supersymmetric model with R-parity violation?

We investigate the possibility that the excess of four-jet events in e ⁺ e ^? collisions at LEP-1.5 which has been reported by the ALEPH Collaboration could be due to the production of charginos or neutralinos, followed by their decay into quark jets through baryon number-violating (λ″) couplings. An estimate at the parton level shows, however, that these events cannot be due to neutralinos because of the low cross-section, and is unlikely to be due to the production of chargino pairs because of the largely different event shapes.     

QCD effects andb-tagging at LEP I

We analyze the impact of usingb-tagged samples in studying non-Abelian effects due to QCD ine ⁺ e ^? → 4jet events at √s=M _z ⁰, using angular variable analyses and comparisons withe ⁺ e ^? → 3jetγ events. We find that QCD effects are largely enhanced inb-quark samples with respect to ‘unflavoured’ ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that theb-quark mass influences the angular distributions significantly and should not be neglected.     

Charged particle multiplicity distributions ine + e − annihilation at 29 GeV: a comparison with hadronic data

The charged particle multiplicity distributions for two-jet events ine ⁺ e ^? annihilation at 29 GeV have been measured using the High Resolution Spectrometer at PEP. A Poisson distribution describes the data for both the complete event and for the single jets. In addition, no correlation is observed between the multiplicities in the two jets of an event. For fixed values of the prong number of the complete event, the multiplicity sharing between the two jets is in good agreement with a binomial distribution. The rapidity gap distribution is exponential with a slope equal to the mean rapidity density. These observations, which are consistent with a picture of independent emission of single particles, are contrasted to the results from soft hadronic collisions and conclusions are drawn about the nature of clusters.     

Three-jet distributions ine + e − annihilation: Electromagnetic and weak production

Hard gluon bremsstrahlung off quarks ine ⁺ e ^? annihilation is expected to produce three-jet events. The angular distributions of the jets are calculated for electromagnetic and weak production in the high energy region (Z resonance decay included). Numerical estimates are presented for the vectoraxialvector interference terms within the Weinberg-Salam theory. We discuss (i) angular correlations between jets, (ii) semiinclusive pion distributions and (iii) we comment on angular asymmetries of leptons originating from heavy quark decays.     

Analysis of the fragmentation properties of quark and gluon jets at the CERN SPS pp̄ collider

A sample of two-jet events from the UA1 experiment at the CERN pp̄ Collider has been used to study the fragmentation of high-energy quark and gluon jets into charged hadrons. Compared with lower-energy jets observed in e⁺e⁻ and pp collisions, the fragmentation function measured in the present experiment is softer (i.e. peaked to smaller values of z) and the mean internal transverse momentum is larger, mainly because of the effects of the QCD scaling violations. Using our knowledge of the quark and gluon structure functions in the proton, together with the QCD matrix elements, a statistical separation of quark and gluon jets is achieved within the present experiment. The fragmentation function for the gluon jets is found to be softer, and the angular spread of the fragmentation products larger, than is the case for quark jets.     

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).     

Search for large-rapidity-gap events in e+e− annihilation

We investigate the cross section for the production of a low-mass colour-singlet cluster ine ⁺ e ^? annihilation with a large rapidity gap between the colour-singlet cluster and the other jets. It is argued that such events are the cross-channel analogue of large-rapidity-gap events in deep-inelastic scattering, and therefore could in principle be used to investigate the analytic continuation of the BFKL pomeron to the positive-t kinematic regime, where one would expect the trajectory to pass through glueball states. The cross section can be calculated in perturbative QCD, so that the infrared scale arising from non-perturbative effects, which prevents an exponential fall-off with rapidity gap in the case of deep-inelastic scattering, is absent ine ⁺ e ^? annihilation. Correspondingly, the cross section for such events decreases rapidly with increasing rapidity gap.     

A new quantity for finding four-jet events in electron-positron annihilation into hadrons

We introduce a new jet measure, tripodityD ₃, which offers good prospects to identify four-jet events ine ⁺ e ^? annihilation into hadrons. TheoreticalD ₃ distributions are presented for quantum chromodynamics, for an abelian model and for a phase space distribution of four jets.     

Tests of quantum chromodynamics and a direct measurement of the strong coupling constant αs at √s=30 GeV

We report the measurement of the reaction e⁺ + e^? → hadronic jets at a center-of-mass energy √s=30 GeV using the MARK-J detector at PETRA. By measuring the energy and angular distribution of both neutrals and charged particles we were able to isolate unambiguously the three-jet events in a kinematic region where the backgrounds from q$\text{q}$ and phase space contributions and other processes are small. Various comparisons of the data with quantum chromodynamics were made. The relative yield of three-jet events and the shape distribution of the events enable us to determine α_s = 0.23 ± 0.02 (statistical error) with a systematic error of ± 0.04.     

Electron-positron annihilation into three jets in the integer-charge quark model

We discuss e⁺e^? → 3 jets in the integer-charge qaurk model. In the lowest non-trivial order, ignoring coloured Higgs scalars, we find that three-gluon jets would be observable at present PETRA energies if the gluon mass m_g ? 2 GeV. As an experimental test, the angular distribution is presented.     

Four-jet events in e+e− annihilation: Testing the three-gluon vertex

The distribution of the azimuthal angle between the planes formed by the two highest energy jets and the two lowest energy jets in four-jet events of e⁺e⁻ annihilation, depends characteristically on the triple-gluon vertex.     

Large transverse momentum jets in high-energy proton-proton collisions

We present a study of pp collisions producing large transverse momentum π⁰'s. The data collected at c.m. energies √s=52.4 and 62.7 GeV, give evidence for a two-jet structure. The fragmentation of the jets is studied and correlations between the two jets are investigated. Results are compared with e⁺e^? and νp data.     

Experimental investigation of the energy dependence of the strong coupling strength

The energy dependence of the relative production rate of three-jet events is studied in hadronic e⁺e⁻ annihilation events at center of mass energies between 22 and 46.7 GeV. Three-jet events are defined by a jet finding algorithm which is closely related to the definition of resolvable jets used in O(α_s²) perturbative QCD calculations, where the relative production rate of three-jet events is roughly proportional to the size of the strong coupling strength. The production rates of three-jet events in the data decrease significantly with increasing centre of mass energy. The experimental rates, which are independent of fragmentation model calculations, can be directly compared to theoretically calculated jet production rates and are in good agreement with the QCD expectations of a running coupling strength. The hypothesis of an energy independent coupling constant can be excluded with a significance of four standard derivations.     

A cluster algorithm for jet studies

A procedure is described which determines the number of jets in hadronic final states by means of a cluster algorithm. In addition it yields a measurement of the energy and the direction of each jet. The properties of this method are studied using Monte Carlo simulations of different types ofe ⁺ e ^?-annihilation final states. It is shown that in the case of 3-jet events direct comparison with the underlying parton structure can be made. Possible further applications of this method are discussed.     

Observation of planar three-jet events in e+e− annihilation and evidence for gluon bremsstrahlung

Topological distributions of charged and neutral hadrons from the reaction e⁺e^? → multihadrons are studied at √s of about 30 GeV. An excess of planar events is observed at a rate which cannot be explained by statistical fluctuations in the standard two-jet process. The planar events, mostly consisting of a slim jet on one side and a broader jet on the other, are shown actually to possess three-jet structure by demonstrating that the broader jet itself consists of two collinear jets in its own rest system. Detailed agreement between data and predictions is obtained if the process $\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→q}\text{q}\text{?}\text{g}$ is taken into account. This strongly suggests gluon bremsstrahlung as the origin of the planar three-jet events. By comparison of the data with the qq?g-model we obtain a value for the strong coupling constant of α_S(q² = 0.17 ± 0.04.     

Gluon jet-like structure in the upsilon decay

The properties of hadronic jets in e⁺ e^– annihilation in quantum chromodynamics have been studied. Motivated by the observation of the hadronic jets (arising from quark jets), the partial cross sections from the two-gluon jet-like events are calculated perturbatively. Estimates for the jet angular radius and its energy dependence are discussed.The author would like to thank Dr. Mubarak Ahmad for his guidance and encouragement.     

A test for the gluon selfcoupling in the reactionse + e −→4 jets andZ 0→4 jets

We consider the reactionse ⁺ e ^?→γ^*→4 jets andZ ^o→4 jets with the 4 jets coming in two pairs of essentially back to back jets of high and low energy. We calculate the angular distribution of the low energy jet axis with respect to the high energy jet axis in QCD, in an abelian gluon model “QED” and a phase space model (PS). Using simple helicity arguments we show that our angular distribution is very sensitive to the triple gluon coupling in QCD. This is then confirmed by a complete calculation. Our correlation offers, therefore, a direct test for QCD as a non-abelian gauge theory.     

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.