Search

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb−1 taken at centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from ∝s ≈ Mz to place limits on B(H0 → γγ within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scalar resonance decaying into di-photons. Upper limits on B(H0 →4 γγ x σ(e+e− → H°Z°) of 290 – 830 fb are obtained over 40 < MH < 160 GeV. Higgs scalars which couple only to gauge bosons with Standard Model strength are ruled out up to a mass of 76.5 GeV at the 95% confidence level. [ABSTRACT FROM AUTHOR]

We have searched for unstable neutral and charged heavy leptons, N and L±, and for excited states of neutral and charged leptons, ν* e*, μ* and τ*, in e+e− collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, ϕ/λ, are determined for masses up to the kinematic limit. [ABSTRACT FROM AUTHOR]

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with χ L > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on t is measured in the interval 0.073 < | t| < 0.4 GeV2 and is found to be described by an exponential shape with the slope parameter b = 7.2 ± 1.1(stat.)(syst.) GeV−2. The diffractive structure function FD (4) is presented as 0.9 a function of χ H ≃ 1 − χ L and β, the momentum fraction of the struck quark with respect to χ H, and averaged over the t interval 0.073 < |t′ < 0.4 GeV2 and the photon virtuality range 5 < Q2 < 20 GeV2. In the kinematic range 4 × 104 < χ p < 0.03 and 0.015 < β < 0.5, the χ p dependence of F D(4) is fitted with a form (1/χ p)α , yielding a − 1.00 ± 0.09 (stat.)(syst.). Upon integration over L, the structure function F2 D(3) is determined in a kinematic range extending to higher χ p and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory. [ABSTRACT FROM AUTHOR]

The double-differential inclusive di-jet cross section in photoproduction processes is measured with the H1 detector at HERA. The cross section is determined as a function of the average transverse jet energy E for ranges of the fractional energy ξ of the parton from the photon side. An effective leading order parton distribution in the photon is determined at large parton fractional energies for scale between 80 < p < 1250 GeV2. The measurement is compatible with the logarithmic scale dependence that is predicted by perturbative QCD. [ABSTRACT FROM AUTHOR]

Measurements of open charm hadro-production from CERN and Fermilab experiments are reviewed, with particular emphasis on the absolute cross sections and on their A and √ s dependences. Differential pT and xF cross sections calculated with the Pythia event generator are found to be in reasonable agreement with recent data. The calculations are scaled to nucleus-nucleus collisions and the expected lepton pair yield is deduced. The charm contribution to the low mass dilepton continuum observed by the CERES experiment is found to be negligible. In particular, it is shown that the observed low mass dilepton excess in S-Au collisions cannot be explained by charm enhancement. [ABSTRACT FROM AUTHOR]

A polarized proton beam extracted from SATURNE II and the Saclay polarized proton target were used to determine the spin correlation parameter Aoosk and the rescattering observables Kos″ so; Dos″ok, Nos″sn, and Nonsk at 1.80 and 2.10 GeV. The beam polarization was oriented perpendicular to the beam direction in the horizontal scattering plane and the target polarization was directed either along the vertical axis or longitudinally. Left-right and up-down asymmetries in the second scattering were measured. A check for the beam optimization with the beam and target polarizations oriented vertically provided other observables, of which results for Donon and Konno at 1.80, 1.85, 2.04, and 2.10 GeV are listed here. The new data at 2.10 GeV suggest a smooth energy dependence of spin triplet scattering amplitudes at fixed angles in the vicinity of this energy. [ABSTRACT FROM AUTHOR]

We study ππ correlations in the exclusive reaction pp → 2π+2π− at rest measured by the CPLEAR experiment. Avoiding the introduction of an arbitrary reference sample, we analyse differential distributions for equal charge pion pairs removing the phase-space factor event by event. A peak at small relative momenta is most pronounced for large total momentum of the pair. The physical implications of bosonic symmetrization for the properties of the pion source, in particular its radius, are briefly discussed. The two extremes considered are the chaotic Hanbury-Brown-Twiss mechanism and the coherent Skyrmion model. [ABSTRACT FROM AUTHOR]

We analyze the contribution of the SUSY particles to the coupling of the lightest Higgs boson to two photons in supersymmetric theories. We discuss to what extent these contributions can be large enough to allow for a discrimination between the lightest SUSY and the standard Higgs particles in the decoupling limit where all other Higgs bosons are very heavy and no supersymmetric particle has been discovered at future colliders. We find that only chargino and top squark loops can generate a sizeable difference between the standard and the SUSY Higgs-photon couplings. For masses above 250 GeV, the effect of chargino loops on the two-photon width is however smaller than ≈ 10% in the entire SUSY parameter space. Top squarks heavier than 250 GeV can induce deviations larger than 10% only if their couplings to the Higgs boson are large. Since top squark contributions can be sizeable, we derive the two-loop QCD correction to squark loops and show that they are well under control. [ABSTRACT FROM AUTHOR]

We analyze the coupling of CP-even and CP-odd Higgs bosons to a photon and a Z boson in extensions, of the Standard Model. In particular, we study in detail the effect of charged Higgs bosons in two-Higgs doublet models;. and the contribution of SUSY particle loops in the minimal supersymmetric extension of the Standard Model: The Higgs-γZ coupling can be measured in the decay Z → γ+Higgs at e+ e− colliders running on the Z resonance, or in the reverse process Higgs → Zγ with the Higgs boson produced at LHC. We show that a measurement of this coupling with a precision at the percent level, which could be the case at future e+ e− colliders, would allow to distinguish between the lightest SUSY and standard Higgs bosons in large areas of the parameter space. [ABSTRACT FROM AUTHOR]

We discuss how to measure anomalous WWZ- and WWγ-couplings with minimal statistical error using integrated observables, without having to assume that the anomalous couplings are small. We propose a parametrisation of these couplings which is well suited for the extraction of both single and many parameters, and which leads to a very simple form of the integrated cross section, from which additional information on the couplings can be obtained. [ABSTRACT FROM AUTHOR]

Isospin violating signals in the τ− → (3π)−ντ decay mode are discussed. For the τ− → π−π−π+ντ decay mode, isospin violation arises from the vector current contribution in the τ− → ωπ−ντ decay with the subsequent isospin violating ω decay into π+π−. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current, i. e. through a measurement of the structure functions WF, W G, W H and WI. In the case of the τ− → π0π0π−ντ decay mode, a vector current contribution is generated by ηπ0 mixing in the decay chain τ− → ηϱ−ντ → π0π0π−ντ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics. [ABSTRACT FROM AUTHOR]

In recent studies of inclusive B decays, it has been suggested that either B mesons decay much more copiously to final states with no open charm than currently assumed, or B(D0 → K−π+) has to be reduced significantly. This note takes the experimental B(D0 → K−π+) at its face value and estimates B(b → no open charm) using complementary methods: one accounts for the c quark in b → c transitions, the other accounts for the c quark in b → ccs transitions. Through cancellation of errors, the average gives our best estimate of B(b → no open charm), and the difference measures the consistency. The results of the methods are consistent with each other, strongly suggesting a much enhanced B(b → no open charm). This observation indicates that non-perturbative QCD effects are probably causing a sizable fraction of the b → ccs transitions to be seen as charmless b → s processes, contrary to smaller traditional expectations. This mechanism has generally been overlooked and may explain the existing experimental data within the framework of the standard model. We then briefly discuss implications on baryon production governed by b → ccs processes, rare hadronic B decays and CP violation studies. [ABSTRACT FROM AUTHOR]

We employ OPE techniques within HQET to calculate the inclusive semileptonic decays of polarized Λb baryons. Lepton mass effects are included which enables us to also discuss rates into polarized τ-leptons. We present explicit results for the longitudinal polarization of the τ in the Λb rest frame as well as in the (τ−, vτ) c.m. frame. In both the Λb rest frame and in the (τ−, vτ) c.m. frame we make use of novel calculational techniques which considerably simplify the calculations. The transverse polarization components of the τ are calculated in the (τ−, vτ) c.m. frame. We delineate how to measure the full set of 14 polarized and unpolarized structure functions of the decay process by angular correlation measurements. A set of observables are identified that allow one to isolate the contributions of the two O(1/ mt 2) nonperturbative matrix elements Kb and εb. [ABSTRACT FROM AUTHOR]

Heavy to light baryon weak form factors are investigated in a lightcone constituent quark model. In a SU(4) symmetry broken scheme, both charged and neutral weak current-induced form factors are calculated at the q2 = 0 point including the leading relativistic effects in the spin composition of baryons. The corresponding semileptonic decays are described by assuming dipole dependence of form factors on q2. [ABSTRACT FROM AUTHOR]

We calculate the inclusive decays B → X sΨ and B → X s ν c using factorization assumption. To investigate the bound state effect of the decaying B meson in these inclusive decays we take into account the motion of the b quark using a Gaussian momentum distribution model. The resulting correction to free quark decay approximation is around 6% at most. Utilizing a potential model evaluation of the ratio of the decay constants fνc 2/ fΨ 2, it is shown that the ratio R = Γ(B → Xsνc)/Γ(B → XsΨ) can be used a possible test of factorization assumption. [ABSTRACT FROM AUTHOR]

All the possible schemes of neutrino mixing with four massive neutrinos inspired by the existing experimental indications in favor of neutrino mixing are considered in a model independent way. Assuming that in short-baseline experiments only one mass-squared difference is relevant, it is shown that the scheme with a neutrino mass hierarchy is not compatible with the experimental results. Only two schemes with two pairs of neutrinos with close masses separated by a mass difference of the order of 1 eV are in agreement with the results of all experiments. One of these schemes leads to possibly observable effects in3H and (β,β)oν experiments. [ABSTRACT FROM AUTHOR]

The resummation of the α ln2 ( z) non-singlet contributions is performed for initial state QED corrections. As examples, the effect of the resummation on neutral-current deep-inelastic scattering and the e+ e− → μ+μ− scattering cross section near the Zo-peak is investigated. [ABSTRACT FROM AUTHOR]

We present a next-to-leading order calculation for the virtual photoproduction of one and two jets in ep collisions. Soft and collinear singularities are extracted using the phase space slicing method. The collinear photon initial state singularity depends logarithmically on the mass of the virtual photon and is absorbed into the virtual photon structure function. An $$\overline {MS} $$ factorization scheme is defined similarly to the real photon case. Numerical results are presented for HERA conditions using the Snowmass jet definition for inclusive single jet and dijet cross sections. We study the dependence of these cross sections on the transverse energies and rapidities of the jets. Finally, we compare the ratio of the experimentally defined resolved and direct cross sections with recent ZEUS data as a function of the photon virtuality P2. [ABSTRACT FROM AUTHOR]

We study the high energy behaviour of elastic scattering amplitudes within the leading logarithm approximation. In particular, we cast the amplitude in a form which allows us to study the internal dynamics of the BFKL Pomeron for general momentum transfer. We demonstrate that the momentum transfer acts as an effective infrared cut-off which ensures that the dominant contribution arises from short distance physics. [ABSTRACT FROM AUTHOR]

Starting from fixed-order perturbation theory (FOPT) we derive expressions for the heavy-flavour components of the deep-inelastic structure functions ( Fi,H ( x, Q2, mH 2), i = 2, L; H = c, b, t) in the variable-flavour number scheme (VFNS). These expressions are valid in all orders of perturbation theory. This derivation establishes a relation between the parton densities parametrized at nf and nf + 1 light flavours. One of the results is that the heavy quark parton density does not vanish when the factorization scale becomes equal to mH contrary to what is assumed in the literature. Further we observe that in charm electroproduction the exact and asymptotic expressions for the heavy-quark coefficient functions yield identical results for F2,c(ξ, Q2, mc 2) when x < 0.01 and Q2 > 20 (GeV/ c)2. From this observation an analysis of the size of the higher order corrections we conclude that in this region the VFNS description and of F2,c is better than the one given by FOPT. On the other hand in the charm threshold region i.e. x > 0.01 and Q2 < 20 (GeV/ c)2 it turns out that the reverse is true. [ABSTRACT FROM AUTHOR]

Single pseudoscalar and vector mesons hard semi-inclusive photoproduction γh → MX via higher twist mechanism is calculated using the QCD running coupling constant method. It is proved that in the context of this method a higher twist contribution to the photoproduction cross section cannot be normalized in terms of the meson electromagnetic form factor. The structure of infrared renormalon singularities of the higher twist subprocess cross section and the resummed expression (the Borel sum) for it are found. Comparisons are made with earlier results, as well as with leading twist cross section. Phenomenological effects of studied contributions for π, K, ϱ-meson photoproduction are discussed. [ABSTRACT FROM AUTHOR]

We study various classical solutions of the baby-Skyrmion model in (2+1) dimensions. We point out the existence of higher energy states, interpret them as resonances of Skyrmions and anti-Skyrmions and study their decays. Most of the discussion involves a highly exited Skyrmion-like state with winding number one which decays into an ordinary Skyrmion and a Skyrmion-anti-Skyrmion pair. We also study wave-like solutions of the model and show that some of such solutions can be constructed from the solutions of the sine-Gordon equation. We also show that the baby-Skyrmion model has non-topological stationary solutions. We study their interactions with Skyrmions. [ABSTRACT FROM AUTHOR]

It is shown that an (anti-) self-dual homogeneous vacuum gluon field appears in a natural way within the problem of calculation of the QCD partition function in the form of Euclidean functional integral with periodic boundary conditions. There is no violation of cluster property within this formulation, nor are parity, color and rotational symmetries broken explicitly. The massless limit of the product of the quark masses and condensates, $$m_f \left\langle {\bar \psi _f \psi _f } \right\rangle $$ , is calculated to all loop orders. This quantity does not vanish and is proportional to the gluon condensate appearing due to the nonzero strength of the vacuum gluon field. We conclude that the gluon condensate can be considered as an order parameter both for confinement and chiral symmetry breaking. [ABSTRACT FROM AUTHOR]

The void probability in the QCD-inspired FRITIOF model is studied for both unrestricted and restricted phase space. Comparison to phenomenological clan model is presented. The observed enhancement of void probability can be accommodated with the clan model when the randomly produced clans decay with the geometric distribution instead of the logarithmic one. A simple parameterization is introduced to reproduce the shift of weighting factor with the size of restricted phase space. The predictions to the multiplicity distributions of pp collision at $$\sqrt s = 10$$ TeV is presented. [ABSTRACT FROM AUTHOR]

We propose a thermodynamically consistent equation of state (EOS) for a hot and dense handron gas (HG) incorporating the hard-core finite-size correction for baryons which result in a Van-der Waals excluded volume effect. We develop a complete description within a thermal hadron gas model to determine the production of strange particles in relativistic nucleus-nucleus collisions and calculate the strange particle ratios as function of baryon chemical potential pB and temperature T. The results are compared with other existing thermodynamically consistent EOS. Furthermore we extract the thermal freeze-out parameters for different strange particles using the latest data obtained by the CERN experiments. [ABSTRACT FROM AUTHOR]

The implication of recent electroweak precision data for left-right symmetric models is examined. We establish a lower bound on the charged and neutral right-handed gauge bosons independent of the right-handed neutrino mass and of any restrictions or implied symmetries on the right KM matrix. [ABSTRACT FROM AUTHOR]

Currently popular search strategies for supersymmetric particles may be significantly affected due to relatively light sneutrinos which decay dominantly into invisible channels. In certain cases the second lightest neutralino may also decay invisibly leading to two extra carriers of missing energy (in addition to the lightest supersymmetric particle (LSP) ) — the virtual LSPs (VLSPs). A tree lavel calculation shows that if the sneutrino mass happens to be in the small but experimentally allowed range ( m≈ν ≈ 45–55 GeV), these particles together with neutralino pairs may contribute significantly to the missing energy in the process e+e− → γ+ E at LEP-2 energies as an enhancement over the Standard Model or the conventional MSSM predictions. It is further shown that a much larger region of the parameter space can be scanned at a high luminosity e+e− collider at 500 GeV like the proposed NLC machine. Moreover, at both LEP-2 and NLC this process may play a complementary role to direct chargino searches, which may fail due to a near mass degeneracy of the chargino and the sneutrino. Formulae for the cross sections taking into account full mixings of the charginos and the neutralinos are derived. The signal remains observable even in the context of more restricted models based on N=1 SUGRA with common scalar and gaugino masses. A preliminary study of the QED radiative corrections due to soft multiple photon emission as well as hard collinear bremsstrahlung indicates that these corrections play a crucial role in estimating the background. [ABSTRACT FROM AUTHOR]

This paper describes the measurement of the W boson mass, $M_W$, and decay width, $\Gamma_{\rm W}$, from the direct reconstruction of the invariant mass of its decay products in W pair events collected at a mean centre-of-mass energy of $\sqrt s=172.12$ GeV with the OPAL detector at LEP. Measurements of the W pair production cross-section, the W decay branching fractions and properties of the W decay final states are also described. A total of 120 candidate ${\rm W^+W^-}$ events has been selected for an integrated luminosity of 10.36 pb $^{-1}$. The ${\rm W^+W^-}$ production cross-section is measured to be $\sigma_{\rm WW}=12.3\pm1.3{\rm (stat.)}\pm0.3{\rm (syst.)}$ pb, consistent with the Standard Model expectation. The ${\rm W^+W^-\to q\bar q \ell \bar \nu_\ell}$ and ${\rm W^+W^-} \to{\rm q \overline q q \overline q}$ final states are used to obtain a direct measurement of $\Gamma_{\rm W} =1.30^{+0.70}_{-0.55}{\rm (stat.)}\pm0.18{\rm (syst.)}$ GeV. Assuming the Standard Model relation between $M_{\rm W}$ and $\Gamma_{\rm W}$, the W boson mass is measured to be $M_{\rm W}=80.32\pm0.30{\rm (stat.)}\pm0.09{\rm (syst.)}$ GeV. The event properties of the fully-hadronic decays of ${\rm W^+W^-}$ events are compared to those of the semi-leptonic decays. At the current level of precision there is no evidence for effects of colour reconnection in the observables studied. Combining data recorded by OPAL at $\sqrt s\sim161$–172 GeV, the W boson branching fraction to hadrons is determined to be $69.8^{+3.0}_{-3.2}{\rm (stat.)}\pm0.7{\rm (syst.)}$%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the ${\rm W^+W^-}$ production cross-sections measured at $\sqrt s\sim161$ and $\sqrt s\sim172$ GeV is $M_{\rm W} = 80.35 \pm 0.24{\rm (stat.)}\pm0.07{\rm (syst.)}$ GeV. [ABSTRACT FROM AUTHOR]

This paper describes a search for the Standard Model Higgs boson using data from ${\rm e^+e^-}$ collisions collected at center-of-mass energies of 161, 170 and 172 GeV by the OPAL detector at LEP. The data collected at these energies correspond to integrated luminosities of 10.0, 1.0 and 9.4 pb $^{-1}$, respectively. The search is sensitive to the main final states from the process in which the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electron, muon or tau leptons. One candidate event is observed, in agreement with the Standard Model background expectation. In combination with previous OPAL searches at center-of-mass energies close to the Z $^0$ resonance and the revised previous OPAL searches at 161 GeV , we derive a lower limit of 69.4 GeV for the mass of the Standard Model Higgs boson at the 95% confidence level. [ABSTRACT FROM AUTHOR]

The production rates of ${\rm D}^{*\pm}$ mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary ${\rm c\bar c}$ pairs in hadronic ${\rm Z}^0$ decays have been measured at LEP using almost 4.4 million hadronic ${\rm Z}^0$ decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed ${\rm D}^{*\pm}$ mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into ${\rm D}^{*\pm}$ mesons have been found to be \[ {\rm f(b \to D^{*+}}X) = 0.173 \pm 0.016 \pm 0.012\quad {\rm and }\quad {\rm f (c \to D^{*+}}X) = 0.222 \pm 0.014 \pm 0.014\; . \] The fraction of ${\rm c\bar c}$ events in hadronic ${\rm Z}^0$ decays, $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}=\Gamma({\rm Z}^0\to{\rm c\bar c}) / \Gamma({\rm Z}^0\to\rm hadrons)$, is determined to be \[ \Gamma_{\rm c\bar c}/\Gamma_{\rm had} = 0.180\pm 0.011 \pm 0.012 \pm 0.006 \ . \] In all cases the first error is statistical, and the second one systematic. The last error quoted for $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}$ is due to external branching ratios. [ABSTRACT FROM AUTHOR]

Data recorded by the JADE experiment at the PETRA ${\rm e^+e^-}$ collider were used to measure the event shape observables thrust, heavy jet mass, wide and total jet broadening and the differential 2-jet rate in the Durham scheme. For the latter three observables, no experimental results have previously been presented at these energies. The distributions were compared with resummed QCD calulations ( $\mathcal{O}(\alpha_s^2)$+NLLA), and the strong coupling constant $\alpha_s(Q)$ was determined at different energy scales $Q=\sqrt{s}$. The results, \[ \alpha_s(22 \mathrm{GeV}) = 0.161 ^{+0.016}_{-0.011} , \ \alpha_s(35 \mathrm{GeV}) = 0.143 ^{+0.011}_{-0.007} , \ \alpha_s(44 \mathrm{GeV}) = 0.137 ^{+0.010}_{-0.007} \; , \] are in agreement with previous combined results of PETRA albeit with smaller uncertainties. Together with corresponding data from LEP, the energy dependence of $\alpha_s$ is significantly tested and is found to be in good agreement with the QCD expectation. Similarly, mean values of the observables were compared to analytic QCD predictions where hadronisation effects are absorbed in calculable power corrections. [ABSTRACT FROM AUTHOR]

Gluon jets are identified in ${\rm e^+e^-}$ hadronic annihilation events by tagging two quark jets in the same hemisphere of an event. The gluon jet is defined inclusively as all the particles in the opposite hemisphere. Gluon jets defined in this manner have a close correspondence to gluon jets as they are defined for analytic calculations, and are almost independent of a jet finding algorithm. The charged particle multiplicity distribution of the gluon jets is presented, and is analyzed for its mean, dispersion, skew, and curtosis values, and for its factorial and cumulant moments. The results are compared to the analogous results found for a sample of light quark (uds) jets, also defined inclusively. We observe differences between the mean, skew and curtosis values of gluon and quark jets, but not between their dispersions. The cumulant moment results are compared to the predictions of QCD analytic calculations. A calculation which includes next-to-next-to-leading order corrections and energy conservation is observed to provide a much improved description of the data compared to a next-to-leading order calculation without energy conservation. There is agreement between the data and calculations for the ratios of the cumulant moments between gluon and quark jets. [ABSTRACT FROM AUTHOR]

A thrust analysis of Large-Rapidity-Gap events in deep-inelastic $ep$ collisions is presented, using data taken with the H1 detector at HERA in 1994. The average thrust of the final states $X$, which emerge from the dissociation of virtual photons in the range $10 < Q^2 < 100 {\rm GeV}^2$, grows with hadronic mass $M_X$ and implies a dominant 2-jet topology. Thrust is found to decrease with growing $P_t$, the thrust jet momentum transverse to the photon-proton collision axis. Distributions of $P_t^2$ are consistent with being independent of $M_X$. They show a strong alignment of the thrust axis with the photon-proton collision axis, and have a large high $-P_t$ tail. The correlation of thrust with $M_X$ is similar to that in $e^+e^-$annihilation at $\sqrt s_{ee}=M_X$, but with lower values of thrust in the $ep$ data. The data cannot be described by interpreting the dissociated system $X$ as a $q\bar q$ state but inclusion of a substantial fraction of $q\bar q g$ parton configurations leads naturally to the observed properties. The soft colour exchange interaction model does not describe the data. [ABSTRACT FROM AUTHOR]

We study the sensitivity of an asymmetric $B$-factory to indirect $CPT$ violating effects. We find that both dilepton signals and nonleptonic asymmetries can be used to bound the space of parameters describing possible $CPT$ violation in the mixing matrix of the neutral $B$ system. [ABSTRACT FROM AUTHOR]

We show how radiative QCD corrections calculated in terms of quarks can be incorporated at the hadron level in inclusive semileptonic $B$-meson decays. The bound state effects are described by a momentum distribution function of the $b$ quark. The summation over the final states and the averaging over the momentum distribution of the decaying quark render the radiative corrections finite. With this coherent formalism we investigate the shape of the electron spectra for $b\to u$ and $b\to c$ decays as a function of the parameters of the theory. The resultant $b\to c$ electron energy spectrum is in agreement with the experimental data. [ABSTRACT FROM AUTHOR]

We present a full account of the two-loop electroweak, two-photon mediated short-distance dispersive $K_L \rightarrow\mu^+\mu^-$ decay amplitude. QCD corrections change the sign of this amplitude and reduce it by an order of magnitude. Thus, the QCD-corrected two-loop amplitude represents only a small fraction (with the central value of 5%) of the one-loop weak short-distance contribution, and has the same sign. In combination with a recent measurement, the standard-model prediction of the short-distance amplitude, completed in this paper, provides a constraint on the otherwise uncertain long-distance dispersive amplitude. [ABSTRACT FROM AUTHOR]

We investigate the power-suppressed corrections to the mean values of various quantities that characterise the shapes of final states in deep inelastic lepton scattering. Our method is based on an analysis of one-loop Feynman graphs containing a massive gluon, which is equivalent to the evaluation of leading infrared renormalon contributions. As in $e^+e^-$ annihilation, we find that the leading corrections are proportional to $1/Q$. We give quantitative estimates based on the hypothesis of a universal low-energy effective coupling. [ABSTRACT FROM AUTHOR]

We consider the production of charm by real and virtual photons. Special attention is paid to diffractive charm production, which provides information on the gluonic content of the Pomeron. Our calculations are based on the gluon distributions of the CKMT-model, which is shown to lead to agreement with the data on open charm production in deep inelastic scattering. We compare predictions for diffractive charm production of different models for the distribution of gluons in the Pomeron. Experiments at HERA should be able to discriminate between them. Predictions for beauty production in diffractive and non-diffractive interactions of photons are also given. [ABSTRACT FROM AUTHOR]

We perform a calculation of the full momentum dependence of the gluon and ghost propagators in pure SU(3) Yang-Mills theory by integrating Wilson's exact renormalization group equations with respect to an infrared cutoff $k$. The heavy quark potential in the quenched approximation can be expressed in terms of these propagators. Our results strongly indicate a $1/p^4$-behaviour of the heavy quark potential for $p^2 \to 0$. We show in general, that effective actions which satisfy Schwinger-Dyson equations, correspond to (quasi-) fixed points of Wilson's exact renormalization group equations. [ABSTRACT FROM AUTHOR]

Two–gluon coupling of gluonic mesons is estimated in a spectator model to be considerably larger than that of $Q\bar{Q}$. As an example of a gluonic meson that may be produced in $J/\psi$ radiative decay, we analyse $\eta(1760)$. We deduce from experimental data that its branching ratio to two gluons is bounded, $0.4\pm 0.3 \stackrel{<}{\scriptstyle \sim} BR(\eta(1760)\rightarrow gg)\stackrel{<}{\scriptstyle \sim} 0.9\pm 0.3$, consistent with a gluonic meson, but not compelling it. We propose further tests, and estimate the two–photon coupling of $\eta(1760)$ to be $1.4\pm 0.7$ keV. [ABSTRACT FROM AUTHOR]

We study the current-algebra approach to the Gerasimov-Drell-Hearn sum rule, paying particular attention to the infinite-momentum limit. Employing the ${\cal O}(\alpha^2)$ Weinberg-Salam model of weak interactions as a testing ground, we find that the legitimacy of the infinite-momentum limit is intimately connected with the validity of the naive equal-times algebra of electric charge densities. Our results considerably reduce the reliability of a recently proposed modification of the Gerasimov-Drell-Hearn sum rule, originating from an anomalous charge-density algebra. [ABSTRACT FROM AUTHOR]

The Yano-Koonin-Podgoretskii (YKP) parametrisation of Hanbury Brown-Twiss (HBT) two-particle correlation functions opens new strategies for extracting the emission duration and testing the longitudinal expansion in heavy-ion collisions. Based on the recently derived model-independent expressions, we present a detailed parameter study of the YKP parameters for a finite, hydrodynamically expanding source model of heavy-ion collisions. For the class of models studied here, we show that the three YKP radius parameters have an interpretation as longitudinal extension, transverse extension and emission duration of the source in the YKP frame. This frame is specified by the fourth fit parameter, the Yano-Koonin velocity which describes to a good approximation the velocity of the fluid element with highest emissivity and allows to test for the longitudinal expansion of the source. Deviations from this interpretation of the YKP parameters are discussed quantitatively. [ABSTRACT FROM AUTHOR]

We study 3-point functions at finite temperature in the closed time path formalism. We give a general decomposition of the eight component tensor in terms of seven vertex functions. We derive a spectral representation for these seven functions in terms of two independent real spectral functions. We derive relationships between the seven functions and obtain a representation of the vertex tensor that greatly simplifies calculations in real time. [ABSTRACT FROM AUTHOR]

The complete matrix elements for $e^+e^-\rightarrow b\bar bW^\pm H^\mp$ and $e^+e^-\rightarrow b\bar bH^+H^-$ are computed at tree-level within the Minimal Supersymmetric Standard Model. Rates of interest to phenomenological analyses at the Next Linear Collider are given. In particular, we analyse: (i) $t\bar t$ pair production followed by the top decays into $b\bar bW^\pm H^\mp$ and $b\bar b H^+H^-$ final states; (ii) $H^+H^-$ signals in which $H^\pm\to h W^{\pm} \to b\bar b W^{\pm}$ and $H^\pm\to tb\to b\bar b W^\pm$. Top and Higgs finite width effects are included as well as all those due to the irreducible backgrounds. [ABSTRACT FROM AUTHOR]

Many of the ingredients are explored which are needed to develop a supersymmetric $SO(10) \times U(1)_F$ grand unified model based on the Yukawa structure of a model previously constructed in collaboration with S. Nandi to explain the quark and lepton masses and mixings in a particular neutrino scenario. The $U(1)_F$ family symmetry can be made anomaly-free with the introduction of one conjugate pair of $SO(10)$-singlet neutrinos with the same $U(1)_F$ charge. Due to a plethora of conjugate pairs of supermultiplets, the model develops a Landau singularity within a factor of 1.5 above the GUT scale. With the imposition of a $Z_2$ discrete symmetry and under certain conditions, all higgsino triplets can be made superheavy while just one pair of higgsino doublets remains light and results in mass matrix textures previously obtained from the bottom-up approach. Diametrically opposite splitting of the first and third family scalar quark and lepton masses away from the second family ones results from the nonuniversal D-term contributions. [ABSTRACT FROM AUTHOR]

The effective action of the linear meson model generates the mesonic $n$–point functions with all quantum effects included. Based on chiral symmetry and a systematic quark mass expansion we derive relations between meson masses and decay constants. The model “predicts” values for $f_\eta$ and $f_{\eta^\prime}$ which are compatible with observation. This involves a large momentum dependent $\eta$– $\eta^\prime$ mixing angle which is different for the on–shell decays of the $\eta$ and the $\eta^\prime$. We also present relations for the masses of the $0^{++}$ octet. The parameters of the linear meson model are computed and related to cubic and quartic couplings among pseudoscalar and scalar mesons. We also discuss extensions for vector and axialvector fields. In a good approximation the exchange of these fields is responsible for the important nonminimal kinetic terms and the $\eta$– $\eta^\prime$ mixing encountered in the linear meson model. [ABSTRACT FROM AUTHOR]

We study the non-compact version of the U(1) gauge-Higgs model in three dimensions for $m_H = 30$ GeV. We found that, using this formulation, rather modest lattices approach quite well the infinite volume behaviour.The phase transition is first order, as expected for this Higgs mass. The latent heat (in units of $T_{cr}^4$) is compatible with the predictions of the two-loop effective potential; it is an order of magnitude less than the corresponding SU(2) value. The transition temperature and $\langle \varphi^* \varphi\rangle $ in units of the critical temperature are also compatible with the perturbative results. [ABSTRACT FROM AUTHOR]

Based on a generalized gauge theory on $M^4\times Z_2\times Z_3$, we reconstruct the realistic SU(5) grand unified model by a suitable assignment of fermion fields. The action of group elements $Z_2$ on fermion fields is the charge conjugation, and the action of $Z_3$ elements represent generation translation. We find that a linear term of curvature has to be introduced to accommodate the spontaneous symmetry breaking and gauge hierarchy of the SU(5) model. A new mass relation is obtained in our reconstructed model. [ABSTRACT FROM AUTHOR]

We present an $N=2$ multiplet including a three–index antisymmetric tensor gauge potential, and describe it as a solution to the Bianchi identities for the associated fieldstrength superform, subject to some covariant constraints, in extended central charge superspace. We find that this solution is given in terms of an $8+8$ tensor multiplet subject to an additional constraint. We give the transformation laws for the multiplet as well as invariant superfield and component field lagrangians, and mention possible couplings to other multiplets. We also allude to the relevance of the 3–form geometry for generic invariant supergravity actions. [ABSTRACT FROM AUTHOR]

The $N=2$ super-Yang-Mills gauge theory is reconstructed in a non-commutative differential geometry (NCG). Our NCG with one-form bases $dx^\mu$ on the Minkowski space $M_4$ and $\chi$ on the discrete space $Z_2$ is a generalization of the ordinary differential geometry on the continuous manifold. Thus, the generalized gauge field is written as ${\cal A}(x,y)=A_\mu(x,y)dx^\mu+\Phi(x,y)\chi$ where $y$ is the argument in $Z_2$. $\Phi(x,y)$ corresponds to the scalar and pseudo-scalar bosons in the $N=2$ super Yang-Mills gauge theory whereas it corresponds to the Higgs field in the ordinary spontaneously broken gauge theory. Using the generalized field strength constructed from ${\cal A}(x,y)$ we can obtain the bosonic Lagrangian of the $N=2$ super Yang-Mills gauge theory in the same way as Chamseddine first introduced the supersymmetric Lagrangian of the $N=2$ and $N=4$ super Yang-Mills gauge theories within the framework of Connes's NCG. The fermionic sector is introduced so as to satisfy the invariance of the total Lagrangian with respect to supersymmetry. [ABSTRACT FROM AUTHOR]