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.     

A search for monojet events produced by virtual Z0 bosons in e+e− annihalation at petra

A search was performed for the associated production of two different Higgs bosons via a virtual Z⁰ in e⁺e^? annihilation (e⁺e^? → h₁⁰h₂⁰) using the JADE detector at PETRA. This was motivated by the interpretation of the monojet events observed at the CERN p$\text{p}$ collider as anomalous Z⁰ decays into two neutral Higgs bosons (h₁⁰ and h₂⁰), where h₁⁰ is stable and escapes detection while h₂⁰ decays into hadrons. Single- or di-jet events with large momentum imbalance are then expected at PETRA energies. No evidence for such events was found in our data; this excludes h₂⁰ masses in the range of 1 to 21 GeV with 95% CL, if the branching fraction for Z⁰ → h₁⁰h₂⁰ is a larger than one half that for $\text{Z}{}^0\text{→ v}{}_{\mathrm{\mu }}\text{v}{}_{\mathrm{\mu }}$. The possibility that the monojets could originate from supersymmetric higgsino production from Z⁰ decay is also examined.     

A determination of quark weak couplings at petra energies

Data on hadron production by e⁺e^? annihilation at c.m. energies between 12 and 36.6 GeV have been collected using the JADE detector. They have been analysed in terms of single-photon and weak neutral-current exchange assuming production of quark-antiquark pairs with only d, u, s, c and b quarks to produce values for the quark weak neutral-current couplings. A further analysis in terms of the Glashow-Salam-Weinberg theory produced the result, sin²θ_{W = 0.22 ± 0.08}. The theory has therefore been tested in a new energy domain and within the context of the neutral weak couplings of the first, second and third generation quarks.     

Observation of a charge asymmetry in e+e− → μ+μ−

The angular distribution and the s dependence of the total cross section for the process e⁺e^? → μ⁺μ^? have been measured using the JADE detector at PETRA. After radiative corrections, a forward-backward asymmetry of ?(11.8±3.8) % was observed at an average centre of mass energy of 33.5 GeV. For comparison, an asymmetry of ?7.8 % is expected on the basis of the standard Glashow-Salam-Weinberg model.     

A measurement of the η radiative width Γη→γγ

The radiative width of the η meson has been measured at PETRA in photon-photon collisions. The resulting value is Γ_η→γγ = 0.53±0.04±0.04 keV.     

A measurement of the electroweak induced charge asymmetry in e+e−→bb̄

The forward-backward charge asymmetry for the process $\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{→}\text{b}\text{b}\text{?}\text{→ μ}{}^{\mathrm{\pm }}\text{+}$ hadrons has been measured using the JADE detector at PETRA. An asymmetry of (?22.8 ± 6.0 ± 2.5)% was observed at an average center of mass energy of 34.6 GeV. For comparison, an asymmetry of ?25.2% is expected on the basis of the standard Glashow-Salam-Weinberg model.     

A measurement of the reaction e+e- →e+e-η′ and the radiative width Γη′→γγ at petra

The reaction e⁺e^- → e⁺e^-η' has been observed in the JADE experiment at PETRA, by detecting the final state π⁺π^-γ, resulting from the decay η' → γ?⁰. The cross section was measured at an average beam energy of 17.15 GeV to be σ(e⁺e^- → e⁺e^-η') = 2.2 ± 0.2 (stat.) ± 0.4(syst.) nb, yielding the radiative width Γ_η'γγ= 5.0 ± 0.5(stat.) ± 0.9 (syst.) keV.     

Test of colour reconnection models using three-jet events in hadronic Z decays

Hadronic Z decays into three jets are used to test QCD models of colour reconnection (CR). A sensitive quantity is the rate of gluon jets with a gap in the particle rapidity distribution and zero jet charge. Gluon jets are identified by either energy-ordering or by tagging two b-jets. The rates predicted by two string-based tunable CR models, one implemented in JETSET (the GAL model), the other in ARIADNE, are too high and disfavoured by the data, whereas the rates from the corresponding non-CR standard versions of these generators are too low. The data can be described by the GAL model assuming a small value for the R₀ parameter in the range 0.01-0.02.     

Comparison of e+e− annihilation with QCD and determination of the strong coupling constant

We have analyzed 1113 events of the reaction e⁺e^? → hadrons at CM energies of 12 and 30 GeV in order to make a detailed comparison with QCD. Perturbative effects can be well separated from effects depending on the quark and gluon fragmentation parameters to yield a reliable measurement of the coupling constant α_S. At 30 GeV, the result is α_S = 0.17 ± 0.02 (statistical) ± 0.03 (systematic). QCD model predictions, using the fragmentation parameters determined along with α_S, agree with both gross properties of the final states and with detailed features of the three-jet states.     

Comparison of three-jet and two-jet cross sections in pp̄ collisions at the CERN SPS pp̄ collider

Results are presented on two-jet and three-jet cross sections, measured in the UA1 experiment at the CERN Super Proton Synchrotron (SPS) pp? Collider, at the highest available subprocess cms energies ($\text{s}\text{?}\text{>150}\text{GeV}$). Precise measurements of the two-jet angular distribution are consistent with previous results but show significant scale-breaking effects. The three-jet Dalitz plot and the three-jet angular distributions show evidence for final- and initial-state bremsstrahlung processes, in agreement with the leading-order QCD predictions. A comparison of the yield of wide-angle three-jet events with the yield of two-jet events at smaller scattering angles gives for the strong interaction coupling constant: $\text{α}{}_{\mathrm{<mtext>s</mtext>}}\text{(}\text{K}{}_{\mathrm{3J}}\text{K}{}_{\mathrm{2J}}\text{)=0.16±0.02±0.03}$ at Q²≈4000 GeV², where the factor $\text{K}{}_{\mathrm{3J}}\text{K}{}_{\mathrm{2J}}$ may plausibly be assumed to be close to unity.     

Hadronic production of gluinonium in three-jet events

The third-order supersymmetric QCD contribution to the high-energy hadronic production of gluino-gluino bound states is calculated. Several experimental cuts are proposed in order to increase the signal-to-background ratio. This ratio is shown to be 2 to 3 times larger in three-jet events than in two-jet events, reaching 5–7%. This gain in efficiency could be crucial in the case of limited statistics and an experimental two-jet mass resolution of a few GeV.     

Axes and scalar measures of two-jet and three-jet events

In a comparison of the jet measures sphericity, spherocity and thrust we find that the spherocity axis exhibits strong instabilities against small changes of particle directions and points in many cases in the direction of a single particle rather than describing the events as a whole. Sphericity does not allow the unique association of particles with jets. Thrust yields such an association and can easily be computed in a non-iterative way. We generalize the concept of thrust for application to triple-jets and present a non-iterative procedure for computing the three axes as well as a scalar quantity measuring the three-jet structure of individual events.     

The topology dependence of charged particle multiplicities in three-jet events

A study of individual jet and whole-event charged particle multiplicities in three-jet events measured in e ⁺ e ^? annihilation at the Z reveals a significant topology dependence. Mean jet multiplicities are inadequately described by jet energies; interjet angles must also be specified. Quantitative tests suggest that it is necessary to use transversemomentum- like scales to describe the data.     

Comparison of radiative energy loss models in a hot QCD medium

The suppression of high pT$p_T$ hadron production in heavy ion collisions is thought to be due to energy loss by gluon radiation off hard partons in a QCD medium. Existing models of QCD radiative energy loss in a color-charged medium give estimates of the coupling strength of the parton to the medium which differ by a factor of 5. We will present a side-by-side comparison of two different formalisms to calculate the energy loss of light quarks and gluons: the multiple soft scattering approximation (ASW-MS) and the opacity expansion formalism (ASW-SH and WHDG-rad). A common time-temperature profile is used to characterize the medium. The results are compared to the single hadron suppression R_AA$R_{AA}$ at RHIC energies.     

Transverse momentum and angular distributions of hadrons in e+e− jets from QCD

Hadron jets in e⁺e^? annihilation will broaden at high energies due to gluon bremsstrahlung. With nonperturbative p_T effects dying out rapidly, the basic features of hadron jets can be calculated in perturbation theory. We examine the p_T distribution of secondarily produced hadrons. This is uniquely connected with the deviation from the 1 + cos²θ dependence of single particle inclusive distributions. We discuss what can be learned about the gluon fragmentation given the p_T and/or angular distributions. A sum rule is derived which establishes a relationship between the average p_T² and α_S.     

Quantum number effects in events with a charged particle of large transverse momentum: (II). Charge correlations in jets

Charge correlations of particles in an event with a large p_T triggering particle have been investigated applying a new method, which has been developed for non-diffractive inelastic hadronic events. The correlation length for charge compensation of the fragments of the hard scattered partons and of the spectators, respectively, are equal to that one in normal inelastic events. Part of the charge of the high p_T trigger particle is compensated by the soft particles of the “away jet”. These results support the idea that the rearrangement of quantum numbers in quark fragmentation shows a universal behaviour.