"The European Physical Journal C"

al., P. Abreu et

doi: 10.1007/s100529800885pmid: N/A

Abstract. The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the Delphi detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation \(C_A/C_F\). The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution \(y\), with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is \(2.77\pm0.11\pm0.10\). Due to non-perturbative effects, the data are below the expectation at small \(y\). The transition from the perturbative to the non-perturbative domain appears at smaller \(y\) for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets.

al., K. Ackerstaff et

doi: 10.1007/s100529800886pmid: N/A

Abstract. Inclusive production of the \({\rm f_0}(980)\), \({\rm f}_2(1270)\) and \(\phi(1020)\) resonances has been studied in a sample of 4.3 million hadronic \({\rm Z}^0\) decays from the OPAL experiment at LEP. A coupled channel analysis has been used for the \({\rm f}_0\) in simultaneous fits to the resonances in inclusive \(\pi^+\pi^-\) and \({\rm K^+K^-}\) mass spectra. Fragmentation functions are reported for the three states. Total inclusive rates are measured to be \(0.141 \pm 0.007 \pm 0.011\) \({\rm f}_0\), \(0.155\pm0.011\pm0.018\) \({\rm f}_2\) and \(0.091\pm0.002\pm0.003\) \(\phi\) mesons per hadronic \({\rm Z}^0\) decay. The production properties of the \({\rm f}_0\), including those in three-jet events, are compared with those of the \({\rm f}_2\) and \(\phi\), and with the Lund string model of hadron production. All measurements are consistent with the hypothesis that the \({\rm f_0}(980)\) is a conventional \({\rm q\bar q}\) scalar meson.

al., R. Barate et

doi: 10.1007/s100529800879pmid: N/A

Abstract. From a sample of about 160k \(\mbox{Z}\!\!\to\!\!\tau^+\tau^-\) candidates collected with the ALEPH detector at LEP between 1991 and 1995, \(\tau\) lepton decays involving \(K^0_S\!\to\!\pi^+\pi^-\) are studied. The \(K^0_SK^0_L\) associated production in \(\tau\) decays is also investigated. The branching ratios are measured for the inclusive decay \(B(\tau^-\!\!\to\!\!K^0_SX^-\nu_\tau)=(9.70\!\p m\!0.58\!\pm\!0.62)\times10^{-3}\), where \(X^-\) can be anything, and for the exclusive decays \[ \begin{array}{rcl} B(\tau^-\!\!\to\!\!\overline{K}^0\pi^-\nu_\tau)&=& (8.55\!\pm\!1.17\! \pm\!0.66)\times10^{-3} ,\\ B(\tau^-\!\!\to\!\!\overline{K}^0\pi^-\pi^0\nu_\tau)&=& (2.94\!\pm\! 0.73\!\pm\!0.37)\times10^{-3},\\ B(\tau^-\!\!\to\!\!\overline{K}^0K^-\nu_\tau)&=& (1.58\!\pm\!0.42\!\pm\! 0.17)\times10^{-3},\\ B(\tau^-\!\!\to\!\!\overline{K}^0K^-\pi^0\nu_\tau)&=& (1.52\!\p m\!0.76\! \pm\!0.21)\times10^{-3}. \end{array} \] The decay \(\tau^-\!\!\to\!\!K^0_SK^0_L\pi^-\nu_\tau\) is studied for the first time, giving a branching ratio \[ \begin{array}{rcl} B(\tau^-\!\!\to\!\!K^0_SK^0_L\pi^-\nu_\tau)&=& (1.01\!\pm\!0.23\!\pm\!0.13)\times10^{-3}. \end{array} \] The channels \(\tau^- \!\!\!\to\!\!\! K^0_SK^0_S\pi^-\nu_\tau \), \(\tau^- \!\!\!\to\!\!\! K^0_SK^0_S\pi^-\pi^0\nu_\tau \), \(\tau^-\!\!\!\to\!\!\! K^0_SK^0_L\pi^-\pi^0\nu_\tau \), \(\tau^- \!\!\!\to\!\!\! \overline{K}^0\pi^-\pi^0\pi^0\nu_\tau\), \(\tau^- \!\!\!\to\!\!\! K^0K^-\pi^0\pi^0\nu_\tau\) and \(\tau^- \!\!\!\to\!\!\! K^0h^+h^-h^-\nu_\tau\) are also investigated. In addition, mass spectra in the \(K^0_Sh^-\) and \(K^0_Sh^-\pi^0\) final states are analysed to provide information on the intermediate states produced in the decays.

al., K. Ackerstaff et

doi: 10.1007/PL00021655pmid: N/A

Abstract. Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb\(^{-1}\) at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from \(\mathrm{W}^+\mathrm{W}^-\) production with each W decaying l eptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

Ishikawa, T.;Kurihara, Y.;Skrzypek, M.;Wąs, Z.

doi: 10.1007/s100529800742pmid: N/A

Abstract. We discuss theoretical predictions for \(W\)-pair production and decay at LEP2 and higher energies in a form suitable for comparison with raw data. We present a practical framework for calculating uncertainties of predictions given by the KORALW and grc4f Monte Carlo programs. As an example we use observables in the \(s\bar s c \bar c\) decay channel: the total four-quark (four-jet) cross section and two-quark/jet invariant-mass distribution and cross section, in the case when the other two may escape detection. Effects of QED bremsstrahlung, effective couplings, running \(W\) and \(Z\) widths, Coulomb interaction and the complete tree level set of diagrams are discussed. We also revisit the question of technical precision of the new version 1.21 of the KORALW Monte Carlo code as well as of version 1.2(26) of the grc4f one. Finally we find predictions of the two programs to have an overall physical uncertainty of 2%. As a side result we show, on the example of an \(s\bar s\) invariant mass distribution, the strong interplay of spin correlations and detector cut-offs in the case of four-fermion final states.

Braun, Mikhail;Vacca, Gian Paolo

doi: 10.1007/s100529800753pmid: N/A

Abstract. We describe the calculation of the evolution of the gluon density in \(x\) from an initial value \(x=x_{0}=0.01\) to smaller values, up to \(x=10^{-8}\) in the hard pomeron formalism with a running coupling introduced on the basis of the bootstrap equation. The obtained gluon density is used to calculate the singlet part of the proton structure function. Comparison with experiment and the results following from the fixed coupling evolutio n is made.

Engel, R.;Ivanov, D.Y.;Kirschner, R.;Szymanowski, L.

doi: 10.1007/s100529800752pmid: N/A

Abstract. We calculate the cross section of diffractive charge-parity \(C=+1\) neutral meson production in virtual photon proton collision at high energies. Due to the opposite \(C\)-parities of photon and meson \(M^+\) (\(M^+ = \eta_C, \pi^0, a_2\)) this process probes the \(t\)-channel \(C=-1\) odderon exchange which is described here as noninteracting three–gluon exchange. Estimates for the cross section of inelastic diffractive process \(\gamma^* p\to \eta_C X_p\) are presented. The total cross section of diffractive \(\eta_C\) meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons (\(\pi^0, a_2\)) in \(\gamma^* p\) collisions are of the same order if the photon virtuality \(Q^2\) is \( m^2_C\).

Meggiolaro, Enrico

doi: 10.1007/s100529800749pmid: N/A

Abstract. It is known that the high-energy quark-quark scattering amplitude can be described by the expectation value of two lightlike Wilson lines, running along the classical trajectories of the two colliding particles. Generalizing the results of a previous paper, we give here the general proof that the expectation value of two infinite Wilson lines, forming a certain hyperbolic angle in Minkowski space-time, and the expectation value of two infinite Euclidean Wilson lines, forming a certain angle in Euclidean four-space, are connected by an analytic continuation in the angular variables. This result could be used to evaluate the high-energy scattering amplitude directly on the lattice.

Motyka, L.;Zalewski, K.

doi: 10.1007/s100529800743pmid: N/A

Abstract. A nonrelativistic Hamiltonian with plausible spin dependent corrections is proposed for the quarkonia below their respective strong decay thresholds. With only six free parameters this model reproduces the nine known masses of the bottomonia within about 1 MeV, the six known masses of the charmonia within a few MeV and the five known leptonic decay widths of the \({}^3S_1\) states within about 20%. The model is then used to predict the masses of the remaining 43 qu arkonia (some of them for the first time) and of the leptonic decay widths of the two \({}^1S_0(\overline{b}c)\) states. Comparison with some other models is made.

Vasconcellos, C.A.Z.;Coelho, H.T.;Pilotto, F.G.;Bodmann, B.E.J.;Dillig, M.;Razeira, M.

doi: 10.1007/s100529800762pmid: N/A

Abstract. In the present work we propose a new bag model for hadrons, called the modified fuzzy bag model (MFBM). The distinguishing feature of this model is the suppression of the pion field, as it enters the bag, by means of a scalar potential for the pions, while still preserving chiral symmetry. The mechanism of pion suppression in the MFBM is similar to the mechanism of quark suppression in the fuzzy bag model (FBM). The standard chiral transformation for the pion field suffers a natural alteration in the MFBM, and as a result the model is chiral invariant. We present also a discussion of the FBM and study, in the quark sector, the implications of the soft surface of the bag on the expectation value of the mass operator. In the pion-quark sector, we study the effects of the suppression of the pion field on the form factor for the pion-nucleon interaction, on the pion-nucleon coupling constant \(g_{ \pi{\scriptscriptstyle{N}}{\scriptscriptstyle{N}} }\) and on the nucleon axial charge \(g_A\). Calculations of the pion-nucleon form factor exhibit, in particular, an improvement over previous results. The pionic axial current induces, in the MFBM, a nonvanishing and orientation dependent contribution to axial charge. An analysis of the asymptotic behaviour of the axial charge shows that the role of the surface is to increase the difference of the contributions associated to different orientations.