Jetlike nature of inclusive hadronic interactions

The dominant inclusive systems formed in the beam and target hemispheres of hadronic reactions are explored for jetlike characteristics in terms of sphericity, thrust, and energy and particle flow. We compare $\text{50}\text{GeV}\text{/c π}{}^{\mathrm{?}}\text{p and}\text{p}\text{p and}\text{100}\text{GeV}\text{/c π}{}^{\mathrm{+}}\text{p and pp}$ interactions and find substantial similarities with neutrino-induced hadronic systems and with hadronic jets in e⁺e^? annihilations. We find no strong evidence for differences in angular size between the forward and backward hadronic systems formed in hadron-hadron reactions, and the single-quark jets and the diquark jets which arise variously in lepton-lepton and lepton-hadron reactions.     

Inclusive properties of the ee annihilation into hadrons

The reaction e⁺e⁻ → hadrons is described on the basis of an hadronic structure for the photon. The structure functions of the inclusive reaction e⁺e⁻ → h + anything describe then the parton constitution of the photon in the scaling limit. Various types of couplings and models are considered (global couplings with vector mesons, fundamental photon-parton-antiparton couplings with formation of jets or fans). Precise shapes for the inclusive spectra and multiplicity distributions are deduced. Detailed comparisons with hadronic collisions A + B → h + anything are proposed in the fragmentation, the phase space boundary and the pionisation regions which could determine the nature of the partons and their interactions.     

Some comments on the sphericity of multiparticle states in e+e− collisions

The deviations from sphericity in e⁺e^? hadron collisions, due to various effects, are calculated analytically. The effect of statistical fluctuations on a previous definition of sphericity is especially strong, and a modified definition is given to take this into account. After this is done, jets of a size typical to hadronic reactions should be clearly visible and cannot be confused with trivial other effects.     

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.     

A QCD model for jet fragmentation including soft gluon interference

A new model for hadronic jet fragmentation in hard processes is presented. It is based on a QCD parton branching mechanism with correct treatment of leading collinear and infra-red singularities (i.e. including soft gluon interference). Hadronization occurs via preconfinement of colour singlet clusters, which decay according to a simple phase-space scheme. Although tightly constrained, the model gives a good account of existing e⁺ e^? annihilation data. It predicts significant differences between quark and gluon jets. Comparisons with datadata on the CERN p?p collider jets suggest that a large fraction of them are gluon jets. The model predicts soft gluon interference effects in hadron distributions which are probably not yet observable for pions but should be clear for kaons and baryons.     

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.     

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 new jet algorithm based on the k-means clustering for the reconstruction of heavy states from jets

A jet algorithm based on the k-means clustering procedure is proposed which can be used for the invariant-mass reconstruction of heavy states decaying to hadronic jets. The proposed algorithm was tested by reconstructing e⁺e^-→tt̄→6 jets and e⁺e^-→W⁺W^-→4 jets processes at using a Monte Carlo simulation. It was shown that the algorithm has a reconstruction efficiency similar to traditional jet-finding algorithms, and leads to 25% and 40% improvement of the top-quark and W mass resolution, respectively, compared to the k_T (Durham) algorithm. In addition, it is expected that the peak positions measured with the new algorithm have smaller systematical uncertainty.     

Gluon identification in planar e+e− events

A method is proposed to identify gluon jets in the planar hadronic events recently found in e⁺e^? annihilation. If a fast hadron is detected at a given angle θ_h ? 30° with respect to the axis opposite to the thrust axis, it is most probably a quark (or antiquark) fragment. An explicit formula then predicts the angle and energy of the unbiased accompanying gluon.     

On the universality of quark jet fragmentation

Universality of inclusive fragmentation density functions in lepton induced processes (ep, νp, e⁺e^?) and purely hadronic processes is discussed from the point of view of the Topological Expansion/Dual Unitarization Scheme. It is shown that planar, single jet dominated processes have universal inclusive distributions and average multiplicities. In multi-jet processes, treated in a simple approximation, 〈x〉 is inversely proportional to the number N of jets and the magnitude of the seagull effect increases as N².     

QCD corrections to jet multiplicity moments

QCD predictions of hadronic multiplicity moments in jets, including corrections of relative order $\text{α}{}_{\mathrm{s}}\text{(Q}{}^2\text{)}$, are presented. They agree with e⁺e^? annihilation data for reasonable values of α_s, but the corrections are so large that terms of yet higher order are unlikely to be negligible.     

On the origin of approximate KNO scaling ine + e − annihilation

KNO scaling of the multiplicity distribution in hadronic final states was originally derived as a consequence of Feynman scaling. We show that in iterative models of hadron production in jets, incorporating Feynman scaling, KNO scaling obtains only in the limit when the width of the multiplicity distribution tends to zero. Within the context of the models currently employed to describee ⁺ e ^? annihilation into hadrons, the apparent KNO scaling observed is an accidental consequence of effects which violate Feynman scaling.     

A new approach to the Sterman-Weinberg jet formula

A new way to define a Sterman-Weinberg jet is described. The fraction of hadronic final states frome ⁺ e ^? annihilation consisting of 2 jets is obtained exactly to order α _s Q ² in QCD perturbation theory using two opposed cones along the thrust axis. Conditions on the parton phase space are discussed in detail. The deviation of the jet angular distribution from 1+cos² θ is also obtained. In contrast to some previously proposed 2 jet definitions we find no restriction of the range of applicability by a relation between the Sterman-Weinberg parameters ? and δ. This definition allows a very easy comparison with experimental data. A rough estimate of the fragmentation effects on the fraction of 2 jet events is also presented.     

Predictions of QCD fragmentation models ine + e − annihilation up to\sqrt s = 200 GeV

Results on event and single particle characteristics are presented for the three most successful perturbative QCD+fragmentation models in the c.m. energy range 12.0≦W≦200 GeV. The models were optimised to describe the properties of the large sample of hadronic events obtained by TASSO atW=35 GeV. The energy evolution of the observables across the range spanned by the PETRA and PEP data, 12.0≦〈W〉≦41.5 GeV, is reasonably well described by all the models. However, for some quantities the predictions of the different models diverge at higher energies; distributions of observables are shown at c.m. energies of 93 and 200 GeV, where data are expected from thee ⁺ e ^? colliders currently under construction. The ability to describe simultaneously, with the same parameter values, hadronic event features both at PETRA/PEP and LEP/SLC energies will provide a severe test of the models. The effects of top quark production on the event properties are illustrated for a top mass of 60 GeV/c². Predictions are given for the rates of multijet events up toW=200 GeV, where ≧4-jet events dominate the cross section and will form a background to events containing heavy states which decay into hadron jets.     

Observation of QCD effects in transverse momenta of e+e− jets

Probability distributions of the total transverse momenta (K⊥) of charged particles produced in hadronic jets in e⁺e^? annihilations have been measured for center of mass energies in the range from 9.2 to 31.6 GeV. A linear increase of the average K_⊥² with Q² is observed. The data are successfully compared with high order QCD predictions (according to a simple q$\text{q}$ picture supplemented by multiple emission of soft gluons). Deviations from this picture at the highest energies and large K_⊥ are then analyzed in terms of hard gluon bremsstrahlung and qualitative agreement is found with first order QCD predictions. Scaling “in the mean” is found to be valid both for jet and single particle transverse momenta.     

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.     

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.     

Search for pair production of heavy objects in 4-jet events at} $sqrt{s}=130-136 {⤪ GeV}$ DELPHI Collaboration

Results are presented of a search for pair production of heavy objects decaying into four hadronic jets, as expected for example from associated or pair production of MSSM Higgs bosons, hA or H⁺H^?, using a data sample of 5.9pb^?1 of e⁺e^? collisions at $sqrt{s}=130-136 {? GeV}$ collected with the DELPHI detector at LEP in November 1995. The data and expectations from standard processes agree after four-jet selections. An analysis based on b-tagging finds no hA candidate with high mass. A study optimized to search for H⁺H^?events with mass in the 40–50 GeV/c² range also finds no candidate. Finally a comparison is made with a recent ALEPH analysis which found an excess of four-jet events with high multiplicity and high mass. Such a signal is not observed in the DELPHI data, although a slight excess in the mass region around 105 GeV/c² is seen.