Showing papers in "European Physical Journal C in 2004"

A New measurement of J/psi suppression in Pb-Pb collisions at 158-GeV per nucleon

Abstract: We present a new measurement of $J/\psi$ production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the ($J/\psi$)/Drell-Yan cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the ($J/\psi$)/Drell-Yan cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions.

A new analysis of pi K scattering from Roy and Steiner type equations

Abstract: With the aim of generating new constraints on the OZI suppressed couplings of chiral perturbation theory a set of six equations of the Roy and Steiner type for the S- and P-waves of the \(\pi K\) scattering amplitudes is derived. The range of validity and the multiplicity of the solutions are discussed. Precise numerical solutions are obtained in the range \(E\lesssim 1\) GeV which make use as input, for the first time, of the most accurate experimental data available at \(E\gtrsim 1\) GeV for both \(\pi K\to\pi K\) and \(\pi\pi\to K\overline{K}\) amplitudes. Our main result is the determination of a narrow allowed region for the two S-wave scattering lengths. Present experimental data below 1 GeV are found to be in generally poor agreement with our results. A set of threshold expansion parameters, as well as sub-threshold parameters are computed. For the latter, a matching with the SU(3) chiral expansion at NLO is performed.

Studies of QCD at e(+)e(-) centre-of-mass energies between 91 and 209 GeV

Abstract: The hadronic final states observed with the ALEPH detector at LEP in e+e- annihilation are analysed using 730 pb-1 of data collected between 91 and 209 GeV in the framework of QCD. In particular event-shape variables and inclusive charged particle spectra are measured. The energy evolution of quantities derived from these measurements is compared to analytic QCD predictions. The mean charged particle multiplicity, the charged particle momentum spectrum and its peak position are compared to predictions of the modified-leading-logarithmic approximation. The strong coupling constant alpha_s is determined from a fit of the QCD prediction to distributions of six event-shape variables at eight centre-of-mass energies. A study of non-perturbative power law corrections is presented

Symmetry realization of texture zeros

Abstract: We show that it is possible to enforce texture zeros in arbitrary entries of the fermion mass matrices by means of Abelian symmetries; in this way, many popular mass-matrix textures find a symmetry justification. We propose two alternative methods which allow one to place zeros in any number of elements of the mass matrices that one wants. They are applicable simultaneously in the quark and lepton sectors. They are also applicable in grand unified theories. The number of scalar fields required by our methods may be large; still, in many interesting cases this number can be reduced considerably. The larger the desired number of texture zeros is, the simpler are the models which reproduce the texture.

QCD/String holographic mapping and glueball mass spectrum

Abstract: Recently Polchinski and Strassler reproduced the high energy QCD scaling at fixed angles from a gauge string duality inspired by the AdS/CFT correspondence. In their approach a confining gauge theory is taken as approximately dual to an AdS space with an IR cut-off. Considering such an approximation (AdS slice) we found a one to one holographic mapping between bulk and boundary scalar fields. Associating the bulk fields with dilatons and the boundary fields with glueballs of the confining gauge theory we also found the same high energy QCD scaling. Here, using this holographic mapping, we give a simple estimate for the mass ratios of the glueballs assuming the AdS slice approximation to be valid at low energies. We also compare these results to those coming from supergravity and lattice QCD.

Uncertainties of predictions from parton distributions II: theoretical errors

Abstract: We study the uncertainties in parton distributions, determined in global fits to deep inelastic and related hard scattering data, due to so-called theoretical errors. Amongst these, we include potential errors due to the change of perturbative order (NLO $\to$ NNLO), $\ln(1/x)$ and $\ln(1-x)$ effects, absorptive corrections and higher-twist contributions. We investigate these uncertainties both by including explicit corrections to our standard global analysis and by examining the sensitivity to changes of the x,Q 2,W 2 cuts on the data that are fitted. In this way we expose those kinematic regions where the conventional DGLAP description is inadequate. As a consequence we obtain a set of NLO, and of NNLO, conservative partons where the data are fully consistent with DGLAP evolution, but over a restricted kinematic domain. We also examine the potential effects of such issues as the choice of input parametrisation, heavy target corrections, assumptions about the strange quark sea and isospin violation. Hence we are able to compare the theoretical errors with those uncertainties due to errors on the experimental measurements, which we studied previously. We use W and Higgs boson production at the Tevatron and the LHC as explicit examples of the uncertainties arising from parton distributions. For many observables the theoretical error is dominant, but for the cross section for W production at the Tevatron both the theoretical and experimental uncertainties are small, and hence the NNLO prediction may serve as a valuable luminosity monitor.

Updated post-WMAP benchmarks for supersymmetry

Abstract: We update a previously-proposed set of supersymmetric benchmark scenarios, taking into account the precise constraints on the cold dark matter density obtained by combining WMAP and other cosmological data, as well as the LEP and $b \rightarrow s \gamma$ constraints. We assume that R parity is conserved and work within the constrained MSSM (CMSSM) with universal soft supersymmetry-breaking scalar and gaugino masses m 0 and m 1/2. In most cases, the relic density calculated for the previous benchmarks may be brought within the WMAP range by reducing slightly m 0, but in two cases more substantial changes in m 0 and m 1/2 are made. Since the WMAP constraint reduces the effective dimensionality of the CMSSM parameter space, one may study phenomenology along “WMAP lines” in the (m 1/2, m 0) plane that have acceptable amounts of dark matter. We discuss the production, decays and detectability of sparticles along these lines, at the LHC and at linear e + e - colliders in the sub- and multi-TeV ranges, stressing the complementarity of hadron and lepton colliders, and with particular emphasis on the neutralino sector. Finally, we preview the accuracy with which one might be able to predict the density of supersymmetric cold dark matter using collider measurements.

Spectrum and decays of kaonic hydrogen

Abstract: By using the non-relativistic effective Lagrangian approach to bound states, a complete expression for the isospin-breaking corrections to the energy levels and the decay widths of kaonic hydrogen is obtained up-to-and-including \(O(\alpha,m_d-m_u)\) in QCD. It is demonstrated that, although the leading-order corrections at \(O(\alpha^{1/2},(m_d-m_u)^{1/2})\) emerging due to the unitarity cusp are huge, they can be expressed solely in terms of the KN S-wave scattering lengths. Consequently, at leading order, it is possible to derive parameter-free modified Deser-type relations, which can be used to extract the scattering lengths from the hadronic atom data.

Prospects for the search for a standard model Higgs boson in ATLAS using vector boson fusion

Abstract: The potential for the discovery of a Standard Model Higgs boson in the mass range m H < 2 m Z in the vector boson fusion mode has been studied for the ATLAS experiment at the LHC. The characteristic signatures of additional jets in the forward regions of the detector and of low jet activity in the central region allow for an efficient background rejection. Analyses for the H → W W (*) and H → τ τ decay modes have been performed using a realistic simulation of the expected detector performance. The results obtained demonstrate the large discovery potential in the H → W W (*) decay channel and the sensitivity to Higgs boson decays into τ-pairs in the low-mass region around 120 GeV/c2.

A modified cluster-hadronisation model

Abstract: A new phenomenological cluster-hadronisation model is presented. Its specific features are the incorporation of soft colour reconnection, a more general treatment of diquarks including their spin and giving rise to clusters with baryonic quantum numbers, and a dynamic separation of the regimes of clusters and hadrons according to their masses and flavours. The distinction between the two regions automatically leads to different cluster decay and transformation modes. Additionally, these aspects require an extension of individual cluster-decay channels that were available in previous versions of such models.

The pion mass in finite volume

Abstract: We determine the relative pion mass shift $M_\pi(L)/M_\pi-1$ due to the finite spatial extent L of the box by means of two-flavor chiral perturbation theory and the one-particle Luscher formula. We use as input the expression for the infinite volume $\pi \pi$ forward scattering amplitude up to next-to-next-to-leading order and can therefore control the convergence of the chiral series. A comparison to the full leading order chiral expression for the pion mass in finite volume allows us to check the size of subleading terms in the large-L expansion.

The CMS trigger system

Abstract: The CMS trigger system must reduce an input data rate from the LHC bunch-crossing frequency of 40 MHz to a rate which will be written to permanent storage. A detailed study has recently been made of the performance of this system. This paper presents key elements of the results obtained and gives details of a draft “trigger table” for the Level-1 Trigger and the High-Level Trigger selection at a “start-up” luminosity of 2× 1033 cm – 2s – 1. High efficiencies for most physics objects are attainable with a selection that remains inclusive and avoids detailed topological or other requirements on the event.

Prospective analysis of spin- and CP-sensitive variables in H rightarrow ZZ rightarrow l + 1 l - 1 l + 2 l - 2 at the LHC

Abstract: A possibility to prove spin and CP-eigenvalue of a Standard Model (SM) Higgs boson is presented. We exploit angular correlations in the subsequent decay H \(\rightarrow\) ZZ \(\rightarrow\) 4l (muons or electrons) for Higgs masses above 200 GeV. We compare the angular distributions of the leptons originating from the SM Higgs with those resulting from decays of hypothetical particles with differing quantum numbers. We restrict our analysis to the use of the Atlas-detector which is one of two multi-purpose detectors at the upcoming 14 TeV proton-proton-collider (LHC) at CERN. By applying a fast simulation of the Atlas detector it can be shown that these correlations will be measured sufficiently well that consistency with the spin-CP hypothesis 0 + of the Standard Model can be verified and the 0- and \(1^\pm\) can be ruled out with an integrated luminosity of 100 fb-1.

Study of tau-pair production in photon-photon collisions at LEP and limits on the anomalous electromagnetic moments of the tau lepton

Abstract: Tau-pair production in the process e(+)e(-) --> e(+)e(-) tau(+)tau(-) was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb(-1). The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.

Electroweak model and constraints on new physics

Abstract: 10.

Interactions of heavy stable hadronising particles

Abstract: In this article, we study the interactions of stable, hadronising new states, arising in certain extensions of the standard model. A simple model, originally intended for stable gluino hadrons, is developed to describe the nuclear interactions of hadrons containing any new colour triplet or octet stable parton. Hadron mass spectra, nuclear scattering cross sections and interaction processes are discussed. Furthermore, an implementation of the interactions of heavy hadrons in GEANT 3 is presented, signatures are studied, and a few remarks about possible detection with the ATLAS experiment are given.

Non-commutativity of space-time and the hydrogen atom spectrum

Abstract: There has been disagreement in the literature on whether the hydrogen atom spectrum receives any tree-level correction due to non-commutativity. Here we shall clarify this issue and show that indeed a general argument on the structure of the proton as a non-elementary particle leads to the appearance of such corrections. As a showcase, we evaluate the corrections in a simple non-relativistic quark model with a result in full agreement with the previous one we had obtained by considering the electron moving in the external electric field of proton. Thus the previously obtained bound on the non-commutativity parameter, $\theta < (10^4 {\mathrm {GeV}})^{-2}$ , using the Lamb shift data, remains valid.

Reduction method for dimensionally regulatedone-loop N-point Feynman integrals

Abstract: We present a systematic method for reducing an arbitrary one-loop N-point massless Feynman integral with generic 4-dimensional momenta to a set comprised of eight fundamental scalar integrals: six box integrals in D = 6, a triangle integral in D = 4, and a general two-point integral in D space-time dimensions All the divergences present in the original integral are contained in the general two-point integral and associated coefficients The problem of vanishing of the kinematic determinants has been solved in an elegant and transparent manner Being derived with no restrictions regarding the external momenta, the method is completely general and applicable for arbitrary kinematics In particular, it applies to the integrals in which the set of external momenta contains subsets comprised of two or more collinear momenta, which are unavoidable when calculating one-loop contributions to the hard-scattering amplitude for exclusive hadronic processes at large-momentum transfer in PQCD The iterative structure makes it easy to implement the formalism in an algebraic computer program

The $\beta$ -function in duality-covariant non-commutative $\phi^4$ -theory

Abstract: We compute the one-loop $\beta$ -functions describing the renormalisation of the coupling constant $\lambda$ and the frequency parameter $\Omega$ for the real four-dimensional duality-covariant non-commutative $\phi^4$ -model, which is renormalisable to all orders. The contribution from the one-loop four-point function is reduced by the one-loop wavefunction renormalisation, but the $\beta_\lambda$ -function remains non-negative. Both $\beta_\lambda$ and $\beta_\Omega$ vanish at the one-loop level for the duality-invariant model characterised by $\Omega = 1$ . Moreover, $\beta_\Omega$ also vanishes in the limit $\Omega\to 0$ , which defines the standard non-commutative $\phi^4$ -quantum field theory. Thus, the limit $\Omega\to 0$ exists at least at the one-loop level.

Measurements of atmospheric muon neutrino oscillations, global analysis of the data collected with MACRO detector

Abstract: The final analysis of atmospheric neutrino events collected with the MACRO detector is presented. Three different classes of events, generated by neutrinos in different energy ranges, are studied l ...

Gauge theories on the κ-Minkowski spacetime

Abstract: This study of gauge field theories on $\kappa$ -deformed Minkowski spacetime extends previous work on field theories on this example of a non-commutative spacetime. We construct deformed gauge theories for arbitrary compact Lie groups using the concept of enveloping algebra-valued gauge transformations and the Seiberg-Witten formalism. Derivative-valued gauge fields lead to field strength tensors as the sum of curvature- and torsion-like terms. We construct the Lagrangians explicitly to first order in the deformation parameter. This is the first example of a gauge theory that possesses a deformed Lorentz covariance.

Searches for Neutral Higgs Bosons in Extended Models

Abstract: Searches for neutral Higgs bosons produced at LEP in association with Z bosons, in pairs and in the Yukawa process are presented in this paper. Higgs boson decays into b quarks, tau leptons, or other Higgs bosons are considered, giving rise to four-b, four-b+jets, six-b and four-tau final states, as well as mixed modes with b quarks and tau leptons. The whole mass domain kinematically accessible at LEP in these topologies is searched. The analysed data set covers both the LEP1 and LEP2 energy ranges and exploits most of the luminosity recorded by the DELPHI experiment. No convincing evidence for a signal is found, and results are presented in the form of mass-dependent upper bounds on coupling factors (in units of model-independent reference cross-sections) for all processes, allowing interpretation of the data in a large class of models.

Complete description of polarization effects in emission of a photon by an electron in the field of a strong laser wave

Abstract: We consider the emission of a photon by an electron in the field of a strong laser wave. Polarization effects in this process are important for a number of physical problems. The probability of this process for circularly or linearly polarized laser photons and for arbitrary polarization of all other particles is calculated. We obtain the complete set of functions which describe such a probability in a compact invariant form. Besides this, we discuss in some detail the polarization effects in the kinematics relevant to the problem of $e\to \gamma$ conversion at $\gamma \gamma$ and $\gamma e$ colliders.

Bose-Einstein correlations for Levy stable source distributions

Abstract: The peak of the two-particle Bose-Einstein correlation functions has a very interesting structure. It is often believed to have a multivariate Gaussian form. We show here that for the class of stable distributions, characterized by the index of stability $0 < \alpha \le 2$ , the peak has a stretched exponential shape. The Gaussian form corresponds then to the special case of $\alpha = 2$ . We give examples for the Bose-Einstein correlation functions for univariate as well as multivariate stable distributions, and we check the model against two-particle correlation data.

Charmonium production and nuclear absorption in p-A interactions at 450 GeV

Abstract: The production of J/ψ and ψ � charmonium states in proton-nucleus interactions has been investigated by the NA50 experiment, at the CERN SPS. High statistics data sets were collected with collisions induced by 450 GeV protons incident on Be, Al, Cu, Ag and W targets. The J/ψ and ψ � production cross-sections have been determined for each p-A system and their dependences on the nucleus size have been studied, leading to the so-called normal nuclear absorption. Comparing the two patterns we see that the nuclear absorption is stronger for the ψ � than for the J/ψ. Given the high statistics of the data samples, the xF (or rapidity) differential cross-sections of the J/ψ and ψ � states have also been studied, for each of the target nuclei.

Exclusive photoproduction of a heavy vector meson in QCD

Abstract: The process of exclusive heavy vector meson photoproduction, $\gamma p \to V p$ , is studied in the framework of QCD factorization. The mass of the produced meson, $V = \Upsilon$ or $J/\Psi$ , provides a hard scale for the process. We demonstrate that, in the heavy quark limit and at the one-loop order in perturbation theory, the amplitude factorizes in a convolution of a perturbatively calculable hard-scattering amplitude with the generalized parton densities and the non-relativistic QCD matrix element $\langle O_1\rangle_{V}$ . We evaluate the hard-scattering amplitude at one-loop order and compare the data with theoretical predictions using an available model for generalized parton distributions.

Isospin breaking in K ---> pi pi decays

Abstract: We perform a complete analysis of isospin breaking in $K \to 2 \pi$ amplitudes in chiral perturbation theory, including both strong isospin violation ( $m_u eq m_d$ ) and electromagnetic corrections to next-to-leading order in the low-energy expansion. The unknown chiral couplings are estimated at leading order in the 1/N c expansion. We study the impact of isospin breaking on CP conserving amplitudes and rescattering phases. In particular, we extract the effective couplings g 8 and g 27 from a fit to $K \rightarrow \pi\pi$ branching ratios, finding small deviations from the isospin-limit case. The ratio ${\rm Re} A_0 /{\rm Re} A_2$ measuring the $ \Delta I = 1/2$ enhancement is found to decrease from $22.2 \pm 0.1$ in the isospin limit to $20.3 \pm 0.5$ in the presence of isospin breaking. We also analyze the effect of isospin violation on the CP violation parameter $\epsilon^\prime$ , finding a destructive interference between three different sources of isospin violation. Within the uncertainties of large-N c estimates for the low-energy constants, the isospin violating correction for $\epsilon^\prime$ is below 15%.

Moments of semileptonic B decay distributionsin the 1/ m b expansion

Abstract: We report the OPE-based predictions for a number of lepton energy and hadronic mass moments in the inclusive semileptonic $B \to X_c \ell u$ decays with a lower cut on the charged lepton energy. We rely on the direct OPE approach where no expansion in the charm mass is employed and the theoretical input is a limited set of underlying OPE parameters including m b and m c . A Wilsonian treatment with a “hard” cutoff is applied using running low-scale masses $m_Q(\mu)$ and a kinetic expectation value $\mu_\pi^2(\mu)$ . This leaves for perturbative corrections only genuinely short-distance effects and makes them numerically small. Predictions are also given for the modified hadronic moments of the kinematic variable ${\cal N}_X^2$ which is a combination of M X 2 and E X . The measurement of such moments would allow a more reliable extraction to be made of higher-order non-perturbative heavy quark parameters from experiment.

The photon-neutrino interaction induced by non-commutativity and astrophysical bounds

Abstract: In this work we propose a possible mechanism of left- and right-handed neutrino couplings to photons, which arises quite naturally in non-commutative field theory. We estimate the predicted additional energy-loss in stars induced by space-time non-commutativity. The usual requirement that any new energy-loss mechanism in globular stellar clusters should not excessively exceed the standard neutrino losses implies a scale of non-commutative gauge theory above the scale of weak interactions.

Search for charged Higgs bosons at LEP in general two Higgs doublet models

Abstract: A search for pair-produced charged Higgs bosons was performed in the data col- lected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 209 GeV. Five different final states, τ + ντ τ − ¯τ ,cscs, csτ − ¯ ντ ,W ∗ AW ∗ Aa nd W ∗ Aτ − ¯τ were considered, accounting for the major expected decays in type I and type II Two Higgs Doublet Models. No significant excess of data compared to the expected Standard Model processes was observed. The existence of a charged Higgs boson with mass lower than 76.7 GeV/c 2 (type I) or 74.4 GeV/c 2 (type II) is ex- cluded at the 95% confidence level, for a wide range of the model parameters. Model independent cross-section limits have also been calculated.