Charged particle multiplicity distributions have been measured with the ALEPH detector in restricted rapidity intervals |Y| ≤0.5, 1.0, 1.5, 2.0 along the thrust axis and also without restriction on rapidity. The distribution for the full range can be parametrized by a log-normal distribution. For smaller windows one finds a more complicated structure, which is understood to arise from perturbative effects. The negative-binomial distribution fails to describe the data both with and without the restriction on rapidity. The JETSET model is found to describe all aspects of the data while the width predicted by HERWIG is in significant disagreement.
Unfolded charged particle multiplicity distribution given the probability to have an hadronic Z0 decay with MULT charged particles.
Unfolded multiplicity distributions for restricted rapidity bin <= 0.5 along the thrust axis.
Unfolded multiplicity distributions for restricted rapidity bin <= 1.0 along the thrust axis.
A study of scaling violations in fragmentation functions performed by the ALEPH collaboration at LEP is presented. Data samples enriched in uds, c, b and gluon jets, respectively, together with measurements of the longitudinal and transverse inclusive cross sections are used to extract the fragmentation function for the gluon and for each flavour. The measurements are compared to data from experiments at energies between 22 GeV and 91 GeV and scaling violations consistent with QCD predictions are observed. From this, a measurement of the strong coupling constant α s ( Mz ) = 0.126 ±0.009 is obtained.
No description provided.
No description provided.
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Elastic $\rho~0$ photoproduction has been measured using the ZEUS detector at HERA. Untagged photoproduction events from $ep$ interactions were used to measure the reaction $\gamma p \rightarrow \rho~0 p$ ($\rho~0 \rightarrow \pi~+ \pi~-$) at photon-proton centre-of-mass energies between 60 and 80GeV and $|t|<0.5$GeV$~2$, where $t$ is the square of the four-momentum transferred at the proton vertex. The differential cross section $d\sigma/dM_{\pi\pi}$, where $M_{\pi\pi}$ is the invariant mass of the two pions, and the integrated cross section, $\sigma_{\gamma p\rightarrow \rho~0 p}$, are presented; the latter was measured to be $14.7\pm 0.4(\mbox{stat.})\pm2.4(\mbox{syst.})\mu\mbox{b}$. The differential cross section $d\sigma/dt$ has an approximately exponential shape; a fit of the type $A~{\prime}_{t} \exp{(-b~{\prime}_{t}|t| + c~{\prime}_{t} t~2)}$ yields a $t$-slope $b~{\prime}_{t}= 9.9\pm1.2(\mbox{stat.})\pm 1.4(\mbox{syst.})\mu\mbox{b}$. The results, when compared to low energy data, show a weak energy dependence of both $\sigma_{\gamma p\rightarrow \rho~0 p}$ and of the $t$-slope. The $\rho~0$ is produced predominantly with transverse polarisation, demonstrating that $s$-channel helicity conservation holds at these energies.
Integrated cross section for exclusive rho0 <pi+ pi-> production where 2Mpi < Mpi pi < Mrho + 5width0.
No description provided.
Applying the Spital and Yennie method to each t bin. No errors given.
The exclusive production of $\rho~0$ mesons in deep inelastic electron-proton scattering has been studied using the ZEUS detector. Cross sections have been measured in the range $7 < Q~2 < 25$ GeV$~2$ for $\gamma~*p$ centre of mass (c.m.) energies from 40 to 130 GeV. The $\gamma~*p \rightarrow \rho~0 p$ cross section exhibits a $Q~{-(4.2 \pm 0.8 ~{+1.4}_{-0.5})}$ dependence and both longitudinally and transversely polarised $\rho~0$'s are observed. The $\gamma~*p \rightarrow \rho~0 p$ cross section rises strongly with increasing c.m. energy, when compared with NMC data at lower energy, which cannot be explained by production through soft pomeron exchange. The data are compared with perturbative QCD calculations where the rise in the cross section reflects the increase in the gluon density at low $x$. the gluon density at low $x$.
No description provided.
Using a sample of about 1.46 million hadronic Z decays collected between 1991 and 1993 with the ALEPH detector at LEP, the energy distribution of the B 0 and B ± mesons produced at the Z resonance is measured by reconstructing semileptonic decays B → ℓ ν ℓ D(X) or B → ℓν ℓ D ∗+ (X) . The charmed mesons are reconstructed through the decay modes D 0 → K − π + , D 0 → K − π + π − π + , D + → K − π + π + and D ∗+ → D 0 π + . The neutrino energy is estimated from the missing energy in the lepton hemisphere. Accounting for B ∗ and B ∗∗ production, the shape of the scaled energy distribution x E (b) for mesons containing a b quark is compared to the predictions of different fragmentation models. The mean value of x E (b) is found to be 〈 x E (b) 〉 = 0.715 ± 0.007(stat) ± 0.013(syst).
SIG/SIG(C=ALL-X-INTERVAL) is fraction of events in bin. Third and fourth systematic errors are due to variation of D** and B** contributions respectively (model dependent, see text).
Data on the inclusive production of the neutral vector mesonsρ 0(770),ω(782), K*0(892), andφ(1020) in hadronic Z decays recorded with the ALEPH detector at LEP are presented and compared to Monte Carlo model predictions. Bose-Einstein effects are found to be important in extracting a reliable value for theρ 0 production rate. An averageρ 0 multiplicity of 1.45±0.21 per event is obtained. Theω is detected via its three pion decay modeω→π + π − π 0 and has a total rate of 1.07±0.14 per event. The multiplicity of the K*0 is 0.83±0.09, whilst that of theφ is 0.122±0.009, both measured using their charged decay modes. The measurements provide information on the relative production rates of vector and pseudoscalar mesons, as well as on the relative probabilities for the production of hadrons containing u, d, and s quarks.
No description provided.
Average multiplicity per hadronic event. Extrapolation to the full X range.
No description provided.
An improved measurement of the average b hadron lifetime is performed using a sample of 1.5 million hadronic Z decays, collected during the 1991–1993 runs of ALEPH, with the silicon vertex detector fully operational. This uses the three-dimensional impact parameter distribution of lepton tracks coming from semileptonic b decays and yields an average b hadron lifetime of 1.533 ± 0.013 ± 0.022 ps.
No description provided.
Jet production in deep inelastic scattering for $120<Q~2<3600$GeV$~2$ has been studied using data from an integrated luminosity of 3.2pb$~{-1}$ collected with the ZEUS detector at HERA. Jets are identified with the JADE algorithm. A cut on the angular distribution of parton emission in the $\gamma~*$-parton centre-of-mass system minimises the experimental and theoretical uncertainties in the determination of the jet rates. The jet rates, when compared to ${\cal O}$($\alpha_{s}$~2$) perturbative QCD calculations, allow a precise determination of $\alpha_{s}(Q)$ in three $Q~2$-intervals. The values are consistent with a running of $\alpha_{s}(Q)$, as expected from QCD. Extrapolating to $Q=M_{Z~0}$ yields $\alpha_{s}(M_{Z~0}) = 0.117\pm0.005(stat)~{+0.004}_{-0.005}(syst_{exp}) {\pm0.007}(syst_{theory})$.
2+1 jet rate as a function of ycut the jet algorithm cut-off value. Statistical errors only.
Measured values of Lambda-QCD in the MS Bar scheme and alpha_s as a function of Q**2. The second systematic uncertainty is related to the theoretical uncertainties .
Strong coupling constant alpha_s extrapolated to the Z0 mass.
Photoproduction events which have two or more jets have been studied in the $W_{\gamma p}$ range 135GeV $< W_{\gamma p} <$ 280GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals ($\Delta\eta$) of up to four units and have transverse energies greater than 6GeV. A gap is defined as the absence between the jets of particles with transverse energy greater than 300MeV. The fraction of events containing a gap is measured as a function of \deta. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of $\Delta\eta \sim 3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet object.
No description provided.
No description provided.
We report on a measurement of the proton structure function $F_2$ in the range $3.5\times10~{-5}\leq x \leq 4\times10~{-3}$ and 1.5 ${\rm GeV~2} \leq Q~2 \leq15$ ${\rm GeV~2}$ at the $ep$ collider HERA operating at a centre-of-mass energy of $\sqrt{s} = 300$ ${\rm GeV}$. The rise of $F_2$ with decreasing $x$ observed in the previous HERA measurements persists in this lower $x$ and $Q~2$ range. The $Q~2$ evolution of $F_2$, even at the lowest $Q~2$ and $x$ measured, is consistent with perturbative QCD.
Data from shifted vertex analysis. Overall normalization error of 3% is notincluded.
Data from shifted vertex analysis. Overall normalization error of 3% is notincluded.
Data from shifted vertex analysis. Overall normalization error of 3% is notincluded.
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