Are monojets due to unusual Z0 decays?

We examine and compare implications of several recent suggestions that events of the form $\text{p}\text{p}\text{→}\text{jet}\text{+(}\text{missing transverse momentum}\text{)}$ could be due to unusual Z⁰ decays. Two possibilities are noted: (a) $\text{Z}{}^0\text{→}\text{N}\text{N}$, where N (or $\text{N}$) is a neutral heavy lepton decaying to three neutrinos with a 10% branching ratio, and (b) Z⁰→h₁h₂, where h₁ and h₂ are two distinct spinless particles, and only the decay of h₁ is seen in the detector. Early tests of both possibilities are noted.     

Bose–Einstein study of position–momentum correlations of charged pions in hadronic Z0 decays

A study of Bose–Einstein correlations in pairs of identically charged pions produced in e⁺e^- annihilations at the Z⁰ peak has been performed for the first time assuming a non-static emitting source. The results are based on the high statistics data obtained with the OPAL detector at LEP. The correlation functions have been analyzed in intervals of the average pair transverse momentum and of the pair rapidity, in order to study possible correlations between the pion production points and their momenta (position–momentum correlations). The Yano–Koonin and the Bertsch–Pratt parameterizations have been fitted to the measured correlation functions to estimate the geometrical parameters of the source as well as the velocity of the source elements with respect to the overall centre-of-mass frame. The source rapidity is found to scale approximately with the pair rapidity, and both the longitudinal and transverse source dimensions are found to decrease for increasing average pair transverse momenta.     

The production of neutral kaons in Z0 decays and their Bose-Einstein correlations

Updated measurements of the B⁰ and B⁺ meson lifetimes are presented. From a data sample of 1.72 million hadronic Z⁰ decays recorded during the period 1991 to 1993, a sample of approximately 1000 semileptonic B meson decays containing a D⁰, D⁺ or D^*+ has been isolated. From the distribution of decay times in the different samples the lifetimes of the B⁰ and B⁺ mesons are determined to be 1.53±0.12±0.08 ps and 1.52±0.14±0.09 ps, respectively, where the first error is statistical and the second systematic. The ratio of the B⁺ to B⁰ lifetimes is measured to be 0.99±0.14 _–0.04 ^+0.05 , confirming expectations that the lifetimes are similar.     

Charged particle multiplicity distributions inZ0 hadronic decays

This paper presents an analysis of the multiplicity distributions of charged particles produced inZ ⁰ hadronic decays in the DELPHI detector. It is based on a sample of 25364 events. The average multiplicity is n _ch>=20.71±0.04(stat)±0.77(syst) and the dispersionD=6.28±0.03(stat)±0.43(syst). The data are compared with the results at lower energies and with the predictions of phenomenological models. The Lund parton shower model describes the data reasonably well. The multiplicity distributions show approximate KNO-scaling. They also show positive forward-backward correlations that are strongest in the central region of rapidity and for particles of opposite charge.     

Measuring higher-order chromodynamics on the Z0

We point out the importance of measuring the transverse momentum distribution of the Z⁰. The experiment directly probes the higher-order perturbative structure of quantum chromodynamics and is a phenomenological laboratory for deciphering the lepton signature of W^±.     

A study of charged particle multiplicities in hadronic decays of theZ0

We present an analysis of multiplicity distributions of charged particles produced inZ ⁰ hadronic decays. The results are based on the analysis of 82941 events collected within 100 MeV of theZ ⁰ peak energy with the OPAL detector at LEP. The charged particle multiplicity distribution, corrected for initial-state radiation and for detector acceptance and resolution, was found to have a mean n _ch=21.40±0.02(stat.)±0.43(syst.) and a dispersionD=6.49±0.02(stat.)±0.20(syst.). The shape is well described by the Lognormal and Gamma distributions. A negative binomial parameterisation was found to describe the shape of the multiplicity distribution less well. A comparison with results obtained at lower energies confirms the validity of KNO(-G) scaling up to LEP energies. A separate analysis of events with low sphericity, typically associated with two-jet final states, shows the presence of features expected for models based on a stochastic production mechanism for particles. In all cases, the features observed in the data are well described by the Lund parton shower model JETSET.deceased 25th March 1991     

Charged particle multiplicity distributions for fixed number of jets inZ0 hadronic decays

The multiplicity distributions of charged particles in full phase space and in restricted rapidity intervals for events with a fixed number of jets measured by the DELPHI detector are presented. The data are well reproduced by the Lund Parton Shower model and can also be well described by fitted negative binomial distributions. The properties of these distributions in terms of the clan model are discussed. In symmetric 3-jet events the candidate gluon jet is found not to be significantly different in average multiplicity than the mean of the other two jets, thus supporting previous results of the HRS and OPAL experiments. Similar results hold for events generated according to the LUND PS and to the HERWIG models, when the jets are defined by the JADE jet finding algorithm. The method seems to be insensitive for measuring the color charge ratio between gluons and quarks.     

Charged particle multiplicity distributions in restricted rapidity intervals inZ0 hadronic decays

The multiplicity distributions of charged particles in restricted rapidity intervals inZ ⁰ hadronic decays measured by the DELPHI detector are presented. The data reveal a shoulder structure, best visible for intervals of intermediate size, i.e. for rapidity limits around ±1.5. The whole set of distributions including the shoulder structure is reproduced by the Lund Parton Shower model. The structure is found to be due to important contributions from 3-and 4-jet events with a hard gluon jet. A different model, based on the concept of independently produced groups of particles, clans, fluctuating both in number per event and particle content per clan, has also been used to analyse the present data. The results show that for each interval of rapidity the average number of clans per event is approximately the same as at lower energies.This paper is dedicated to the memory of Léon Van Hove     

A calculation of Z0 → 2 gluinos

We report the results of a one-loop calculation of $\text{Z}{}^0\text{→}\text{gg}$, where $\text{g}$ is a gluino, the proposed supersymmetric partner of a gluon. Depending on the masses of the scalar quarks and of the top quark, the branching ratio for the decay is in the 10^?5 to 10^?4 range for gluino mass below about 40 GeV. The signature for gluinos should allow detection in this range.     

Tests of composite models with Z0 decay modes

If Z⁰ and W^± gauge bosons are composite objects, several multiphoton and multigluon Z⁰ decay modes can become well observable and can be used to test the nature of the subconstituents.     

Search for sleptons and squarks at the Z0 peak in e-e+ colliders

We investigate single scalar lepton and scalar quark production processes $\text{e}{}^{\mathrm{-}}\text{e}{}^{\mathrm{+}}\text{→ Z}{}^0\text{→}\text{?}{}^{\mathrm{\pm }}\text{γ}\text{?}{}^{\mathrm{\pm }}\text{,}\text{q}\text{γ}\text{q}\text{and}\text{q}\text{g}\text{q}$ at the Z⁰ peak. We find that a detectable number of these scalars should be produced at the SLC and LEP-I colliders even if their masses substantially exceed the beam energy E = m_Z/2?45 GeV.     

The needle in the large pT haystack: Higgs versus quark or gluon jets together with Z0 in hadronic reactions

The possibility of observing a Higgs particle via high p_T bremsstrahlung from a weak Z₀ in p$\text{p}$ and pp collisions is examined. It appears that, although very weak, some signal might be observed in the future p$\text{p}$ collision experiments.     

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.     

Intermittency and correlations in hadronic Z^0 decays

A multidimensional study of local multiplicity fluctuations and multiparticle correlations of hadrons produced in Z decays is performed. The study is based on the data sample of more than events recorded with the OPAL detector at LEP. The fluctuations and correlations are analysed in terms of the normalized scaled factorial moments and cumulants up to the fifth order. The moments are observed to have intermittency-like behaviour, which is found to be more pronounced with increasing dimension. The large data sample allows for the first time a study of the factorial cumulants in annihilation. The analysis of the cumulants shows the existence of genuine multiparticle correlations with a strong intermittency rise up to higher orders. These correlations are found to be stronger in higher dimensions. The decomposition of the factorial moments into lower-order correlations shows that the dynamical fluctuations have important contributions from genuine many-particle correlations. The Monte Carlo models Jetset 7.4 and Herwig 5.9 are found to reproduce the trend of the measured moments and cumulants but they underestimate the magnitudes. The results are found to be consistent with QCD jet formation dynamics, although additional contributions from other mechanisms cannot be excluded. Received: 8 February 1999 / Published online: 8 September 1999     

Inclusive neutral vector meson production in hadronicZ0 decays

Results are reported of a study of neutral vector meson production in multihadronicZ ⁰ decays in the OPAL experiment at LEP. Pions and kaons have been identified by specific ionisation energy loss andK ^± andK ⁺ K ^– mass spectra have been fitted, in bins of the scaled momentum variablex _p , to combinations of resonance signals and non-resonant backgrounds. Rates are given forK ^*(892)° and ø(1020), and production cross sections are compared to the predictions of Monte Carlo models. Overall multiplicities have been determined as 0.76±0.07±0.06K ^*(892)° and 0.086±0.015±0.010 ø(1020) per hadronicZ ⁰ decay (the quoted errors are respectively statistical and systematic). Momentum dependent distortions of the mass spectra, possibly associated indirectly with Bose-Einstein effects, have prevented reliable measurement of the (770)° cross section in this study.     

Search for low mass Higgs bosons produced inZ0 decays

A search for light Higgs bosons was performed using the data sample collected in 1990 by the DELPHI detector at LEP, at centre of mass energies between 88.2 and 94.2 GeV. Using the process e⁺e^–H ⁰+Z ^0*,Z ^0*ff, it is possible to exclude the existence of the standard model Higgs particle with a mass between 0 and 210 MeV/c² at the 99% confidence level. Extending this analysis to the minimal supersymmetric standard model restricts the lightest neutral Higgs boson to masses above 28 GeV/c² irrespective of the value of the mixing angle.     

Strange baryon production in hadronic Z0 decays

The production of the $J^{P}={1?er 2}^{+}$ octet baryons Λ and Ξ^?, the $J^{P}={3?er 2}^{+}$ decuplet baryons Σ(1385)^±Ξ(1530)⁰, and Ω^?, and the $J^{P}={3?er 2}^{-}$ orbitally excited state Λ(1520) has been measured in a sample of approximately 3.65 million hadronic Z⁰ decays. The integrated rates and the differential cross-sections as a function of x _E, the scaled energy, are determined. The differential cross-sections of the Λ and Ξ^? baryons are found to be softer than those predicted by both the JETSET and HERWIG Monte Carlo generators. The measured baryon yields are found to disagree with the simple diquark picture where only one tuning parameter for spin 1 diquarks is allowed. The yields are further compared with a thermodynamic model of hadron production which includes the production of orbitally excited mesons and baryons. The momentum spectra of Λ, Ξ^?, Σ(1385)^±Ξ(1530)⁰, and Λ(1520) are also compared to the predictions of an analytical QCD formula.     

Polarization effects for W± and Z0 production in pp collisions

Polarization effects for the leptonic decays of W^± and Z⁰ are calculated and discussed in $\text{p}\text{p}$ collisions with longitudinally polarized proton beams. Numerical examples are given for the CERN $\text{p}\text{p}$ collider and Fermilab Tevatron.