Showing papers in "European Physical Journal C in 2005"

CP Violation and the CKM Matrix : Assessing the Impact of the Asymmetric B Factories

Abstract: We update the profile of the CKM matrix. The apex (rhobar,etabar) of the Unitarity Triangle is given by means of a global fit. We propose to include therein sin2alpha from the CP-violating asymmetries in B0->rho+rho-, using isospin to discriminate the penguin contribution. The constraint from epsilon'/epsilon is briefly discussed. We study the impact from the measurement of the rare decay K+->pi+nunu-bar, and from a future observation of KL->pi0nunubar. The B system is investigated in detail, beginning with 2beta+gamma and gamma from B0->D(*)+-pi-+ and B+->D(*)0K+. A significant part of this paper is dedicated to B decays into pipi, Kpi, rhopi and rhorho. Various phenomenological and theoretical approaches are studied. Within QCD Factorization we find a remarkable agreement of the pipi and Kpi data with the other UT constraints. A fit of QCD FA to all pipi and Kpi data leads to precise predictions of the related observables. We analyze separately the B->Kpi decays, and in particular the impact of electroweak penguins in response to recent phenomenological discussions. We do not observe any unambiguous sign of New Physics, whereas there is some evidence for potentially large rescattering effects. Finally we use a model-independent description of a large class of New Physics effects in both BBbar mixing and B decays, namely in the b->d and b->s gluonic penguin amplitudes, to perform a new numerical analysis. Significant non-standard corrections cannot be excluded yet, however standard solutions are favored in most cases.

Final results from phase II of the Mainz neutrino mass searchin tritium ${\beta}$ decay

Abstract: This paper reports on the improved Mainz experiment on tritium $\beta$ spectroscopy which yields a 10 times higher signal to background ratio than before. The main experimental effects and systematic uncertainties have been investigated in side experiments, and possible error sources have been eliminated. Extensive data taking took place in the years 1997 to 2001. A residual analysis of the data sets yields for the square of the electron antineutrino mass the final result of $m^2( u_e) = (-0.6 \pm 2.2_{\mathrm{{stat}}} \pm 2.1_{\mathrm{{syst}}})$ eV2/c4. We derive an upper limit of $m( u_e)\leq 2.3$ eV/c2 at 95% confidence level for the mass itself.

Physics potential and experimental challenges of the LHC luminosity upgrade

Abstract: We discuss the physics potential and the experimental challenges of an upgraded LHC running at an instantaneous luminosity of 1035 cm-2s-1. The detector R&D needed to operate ATLAS and CMS in a very high radiation environment and the expected detector performance are discussed. A few examples of the increased physics potential are given, ranging from precise measurements within the Standard Model (in particular in the Higgs sector) to the discovery reach for several New Physics processes.

Transverse-momentum-ordered showers and interleaved multiple interactions

Abstract: We propose a sophisticated framework for high-energy hadronic collisions, wherein different QCD physics processes are interleaved in a common sequence of falling transverse-momentum values. Thereby phase-space competition is introduced between multiple parton-parton interactions and initial-state radiation. As a first step we develop new transverse-momentum-ordered showers for initial- and final-state radiation, which should be of use also beyond the scope of the current article. These showers are then applied in the context of multiple interactions, and a few tests of the new model are presented. The article concludes with an outlook on further aspects, such as the possibility of a shower branching giving partons participating in two different interactions.

Parton distributions incorporating QED contributions

Abstract: We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${{\mathcal{O}}}(\alpha_{\rm QED})$ corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p eq d^n$ , which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2\theta_W$ anomaly. A second consequence is the appearance of photon parton distributions $\gamma(x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to e\gamma X$ ; our predictions are in agreement with the present data.

Leading-particle suppression in high energy nucleus-nucleus collisions

Abstract: Parton energy loss effects in heavy-ion collisions are studied with the Monte Carlo program PQM (Parton Quenching Model) constructed using the BDMPS quenching weights and a realistic collision geometry. The merit of the approach is that it contains only one free parameter that is tuned to the high-pt nuclear modification factor measured in central Au-Au collisions at √ sNN = 200 GeV. Once tuned, the model is consistently applied to all the high-pt observables at 200 GeV: the centrality evolution of the nuclear modification factor, the suppression of the away-side jet-like correlations, and the azimuthal anisotropies for these observables. Predictions for the leading-particle suppression at nucleon-nucleon centre-of-mass energies of 62.4 and 5500 GeV are presented. The limits of the eikonal approximation in the BDMPS approach, when applied to finite-energy partons, are discussed.

Exclusive radiative and electroweak $b\to d$ and $b\to s$ penguin decays at NLO

Abstract: We provide standard model expectations for the rare radiative decays $B\to K^*\gamma$ , $B\to \rho\gamma$ and $B\to\omega\gamma$ , and the electroweak penguin decays $B\to K^*\ell^ + \ell^-$ and $B\to\rho \ell^ + \ell^-$ at the next-to-leading order (NLO), extending our previous results to $b\to d$ transitions. We consider branching fractions, isospin asymmetries and direct CP asymmetries. For the electroweak penguin decays, the lepton-invariant mass spectrum and forward-backward asymmetry is also included. Radiative and electroweak penguin transitions in $b\to d$ are mainly interesting in the search for new flavor-changing neutral current interactions, but in addition the $B\to \rho\gamma$ decays provide constraints on the CKM parameters $(\bar\rho,\bar\eta)$ . The potential impact of these constraints is discussed.

Generalized parton distributions from nucleon form factor data

Abstract: We present a simple empirical parameterization of the x- and t-dependence of generalized parton distributions at zero skewness, using forward parton distributions as input. A fit to experimental data for the Dirac, Pauli and axial form factors of the nucleon allows us to discuss quantitatively the interplay between longitudinal and transverse partonic degrees of freedom in the nucleon (“nucleon tomography”). In particular we obtain the transverse distribution of valence quarks at given momentum fraction x. We calculate various moments of the distributions, including the form factors that appear in the handbag approximation to wide-angle Compton scattering. This allows us to estimate the minimal momentum transfer required for reliable predictions in that approach to be around $\vert t\vert \simeq 3 {\rm GeV}^2$ . We also evaluate the valence contributions to the energy-momentum form factors entering Ji’s sum rule.

Photon events with missing energy in e(+)e(-) collisions at root s=130 to 209 GeV

Abstract: The production of single- and multi-photon events has been studied in the reaction e+e- -> gamma (gamma) + invisible particles. The data collected with the DELPHI detector during the years 1999 and 2000 at centre-of-mass energies between 191 GeV and 209 GeV was combined with earlier data to search for phenomena beyond the Standard Model. The measured number of light neutrino families was consistent with three and the absence of an excess of events beyond that predicted by the Standard Model processes was used to set limits on new physics. Both model-independent searches and searches for new processes predicted by supersymmetric and extra-dimensional models have been made. Limits on new non-standard model interactions between neutrinos and electrons were also determined.

Study of the neutron quantum states in the gravity field

Abstract: We have studied neutron quantum states in the potential well formed by the earth's gravitational field and a horizontal mirror. The estimated characteristic sizes of the neutron wave functions in the two lowest quantum states correspond to expectations with an experimental accuracy. A position-sensitive neutron detector with an extra-high spatial resolution of ∼ 2 µm was developed and tested for this particular experiment, to be used to measure the spatial density distribution in a standing neutron wave above a mirror for a set of some of the lowest quantum states. The present experiment can be used to set an upper limit for an additional short-range fundamental force. We studied methodological uncertainties as well as the feasibility of improving further the accuracy of this experiment.

On the analysis of lepton scattering on longitudinally or transversely polarized protons

Abstract: We discuss polarized lepton-proton scattering with special emphasis on the difference between target polarization defined relative to the lepton beam or to the virtual photon direction. In particular, this difference influences azimuthal distributions in the final state. We provide a general framework of analysis and apply it to the specific cases of semi-inclusive deep inelastic scattering, of exclusive meson production, and of deeply virtual Compton scattering.

High-precision predictions for the MSSM Higgs sector at ${{\mathcal{O}}}(\alpha_b\alpha_s)$

Abstract: We evaluate $\mathcal{O}{\alpha_b \alpha_s}$ corrections in the Higgs boson sector of the $\mathcal{C}P$ -conserving MSSM, generalising the known result in the literature to arbitrary values of $\tan\beta$ . A detailed analysis of the renormalisation in the bottom/scalar bottom sector is performed. Concerning the lightest MSSM Higgs boson mass, we find relatively small corrections for positive μ, while for $\mu < 0$ the genuine two-loop $\mathcal{O}{\alpha_b \alpha_s}$ corrections can amount up to 3 GeV. Different renormalisation schemes are applied and numerically compared. It is demonstrated that some care has to be taken in choosing an appropriate renormalisation prescription in order to avoid artificially large corrections. The residual dependence on the renormalisation scale is investigated, and the remaining theoretical uncertainties from unknown higher-order corrections in this sector are discussed for different regions of the MSSM parameter space.

Charmonium at finite temperature in quenched lattice QCD

Abstract: We study charmonium correlators in pseudoscalar and vector channels at finite temperature using lattice QCD simulation in the quenched approximation. Anisotropic lattices are used in order to have sufficient numbers of degrees of freedom in the Euclidean temporal direction. We focus on the low energy structure of the spectral function, corresponding to the ground state in the hadron phase, by applying the smearing technique to enhance the contribution to the correlator from this region. We employ two analysis procedures: the maximum entropy method (MEM) for the extraction of the spectral function without assuming a specific form, to estimate the shape of the spectral function, and the standard χ2 fit analysis using typical forms in accordance with the result of MEM, for a more quantitative evaluation. To verify the applicability of the procedures, we first analyze the smeared correlators as well as the point correlators at zero temperature. We find that by shortening the t-interval used for the analysis (a situation inevitable at T>0) the reliability of MEM for point correlators is lost, while it subsists for smeared correlators. Then the smeared correlators at T≃ 0.9 T c and 1.1 T c are analyzed. At T≃ 0.9 T c , the spectral function exhibits a strong peak, well approximated by a delta function corresponding to the ground state with almost the same mass as at T=0. At T≃ 1.1 T c , we find that the strong peak structure still persists at almost the same place as below T c , but with a finite width of a few hundred MeV. This result indicates that the correlators possess a nontrivial structure even in the deconfined phase. PACS: 12.38.Gc Lattice QCD calculations – 12.38.Mh Quark-gluon plasma

Vector-meson electroproduction at small Bjorken-x and generalized parton distributions

Abstract: We analyze electroproduction of light vector mesons ( $V = \rho, \phi$ ) at small Bjorken-x in an approach that includes the gluonic generalized parton distributions and a partonic subprocess, $\gamma g \to (q\bar{q}) g$ , $q\bar{q}\to V$ . The subprocess is calculated to lowest order of perturbative QCD taking into account the transverse momenta of the quark and antiquark as well as Sudakov suppressions. Our approach allows us to investigate the transition amplitudes for all kinds of polarized virtual photons and polarized vector mesons. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal and transverse cross sections at large photon virtualities as well as the spin-density matrix elements for the vector mesons. Our results are in fair agreement with the findings of recent experiments performed at HERA.

Measurement of event shape distributions and moments in e+ e- --> hadrons at 91-GeV - 209-GeV and a determination of alpha(s)

Abstract: We have studied hadronic events from e(+)e(-) annihilation data at centre-of-mass energies from 91 to 209 GeV. We present distributions of event shape observables and their moments at each energy and compare with QCD Monte Carlo models. From the event shape distributions we extract the strong coupling alpha(s) and test its evolution with energy scale. The results are consistent with the running of alpha(s) expected from QCD. Combining all data, the value of alpha(s)(M-Z) is determined to bealpha(s)(M-Z) = 0.1191 +/- 0.0005 (stat.) +/- 0.0010 (expt.) +/- 0.0011 (hadr.) +/- 0.0044 (theo.).The energy evolution of the moments is also used to determine a value of alpha(s) with slightly larger errors: alpha(s)(M-Z) = 0.1223 +/- 0.0005 (stat.) +/- 0.0014 (expt.) +/- 0.0016 (hadr.)(-0.0036)(+0.0054) (theo.).

Non-minimal coupling to a Lorentz-violating background and topological implications

Abstract: The non-minimal coupling of fermions to a background responsible for the breaking of Lorentz symmetry is introduced in Dirac’s equation; the non-relativistic regime is contemplated, and the Pauli equation is used to show how an Aharonov-Casher phase may appear as a natural consequence of the Lorentz violation, once the particle is placed in a region where there is an electric field. Different ways of implementing the Lorentz breaking are presented and, in each case, we show how to relate the Aharonov-Casher phase to the particular components of the background vector or tensor that realizes the violation of Lorentz symmetry.

Collinear subtractions in hadroproduction of heavy quarks

Abstract: We present a detailed discussion of the collinear subtraction terms needed to establish a massive variable-flavor-number scheme for the one-particle inclusive production of heavy quarks in hadronic collisions. The subtraction terms are computed by convoluting appropriate partonic cross sections with perturbative parton distribution and fragmentation functions relying on the method of mass factorization. We find (with one minor exception) complete agreement with the subtraction terms obtained in a previous publication by comparing the zero-mass limit of a fixed-order calculation with the genuine massless results in the ${\overline{\mathrm{MS}}}$ scheme. This presentation will be useful for extending the massive variable-flavor-number scheme to other processes.

Renormalization-group improved calculation of the $B\to X_s\gamma$ branching ratio

Abstract: Using results on soft-collinear factorization for inclusive B-meson decay distributions, a systematic study of the partial $B\to X_s\gamma$ decay rate with a cut $E_\gamma\ge E_0$ on photon energy is performed. For values of E 0 below about 1.9 GeV, the rate can be calculated without reference to shape functions using a multi-scale operator product expansion (MSOPE). The transition from the shape-function region to the MSOPE region is studied analytically. The resulting prediction for the $B\to X_s\gamma$ branching ratio depends on three large scales: m b , $\sqrt{m_b\Delta}$ , and $\Delta = m_b-2E_0$ . Logarithms associated with these scales are resummed at next-to-next-to-leading logarithmic order. While power corrections in $\Lambda_{\mathrm {QCD}}/\Delta$ turn out to be small, the sensitivity to the scale $\Delta\approx 1.1$ GeV (for $E_0\approx 1.8$ GeV) introduces significant perturbative uncertainties, which so far have been ignored. The new theoretical prediction for the $B\to X_s\gamma$ branching ratio with $E_\gamma\ge 1.8$ GeV is $\mbox{Br}(B\to X_s\gamma) = \smash{(3.38_{ -0.42}^{ + 0.31}\pm 0.31)}\times 10^{-4}$ , where the first error is an estimate of perturbative uncertainties and the second one reflects uncertainties in input parameters. With this cut $\smash{(89_{ -7}^{ + 6} \pm 1)}\%$ of all events are contained. When this fraction is combined with the previously best prediction for the total decay rate, one obtains $\mbox{Br}(B\to X_s\gamma) = (3.30_{ -0.35}^{ + 0.31}\pm 0.17)\times 10^{-4}$ , with a somewhat less conservative estimate of parametric uncertainties. The implications of larger theory uncertainties for new physics searches are briefly explored with the example of the type-II two-Higgs-doublet model, for which the lower bound on the charged-Higgs mass is reduced compared with previous estimates to approximately 200 GeV at 95% confidence level.

An NLO QCD analysis of inclusive cross-section and jet-production data from the ZEUS experiment

Abstract: The ZEUS inclusive differential cross-section data from HERA, for charged and neutral current processes taken with e(+) and e(-) beams, together with differential cross-section data on inclusive jet production in e+ p scattering and dijet production in gamma p scattering, have been used in a new NLO QCD analysis to extract the parton distribution functions of the proton. The input of jet-production data constrains the gluon and allows an accurate extraction of alpha(s)( M-Z) at NLO;alpha(s)( M-Z) = 0.1183 +/- 0.0028( exp.) +/- 0.0008( model).An additional uncertainty from the choice of scales is estimated as +/- 0.005. This is the first extraction of as( M-Z) from HERA data alone.

A flavor symmetry for quasi-degenerate neutrinos: \(L_\mu - L_\tau\)

Abstract: We consider the flavor symmetry $L_\mu - L_\tau$ for the neutrino mass matrix. The most general neutrino mass matrix conserving $L_\mu - L_\tau$ predicts quasi-degenerate neutrino masses with one maximal and two zero mixing angles. The presence of $L_\mu - L_\tau$ can also be motivated by the near-bimaximal form of the neutrino mixing matrix. Furthermore, it is a special case of $\mu \tau$ symmetric mass matrices. Breaking the flavor symmetry by adding a small flavor-blind term to the neutrino mass matrix and/or by applying radiative corrections is shown to reproduce the observed neutrino oscillation phenomenology. Both the normal and inverted mass ordering can be accommodated within this scheme. Moderate cancellation for neutrinoless double beta decay is expected. The observables |U e3|2 and $\vert 1/2 - \sin^2\theta_{23}\vert$ are proportional to the inverse of the fourth power of the common neutrino mass scale. We comment on whether the atmospheric neutrino mixing is expected to lie above or below $\pi/4$ . We finally present a model based on the see-saw mechanism which generates a light neutrino mass matrix with an (approximate) $L_\mu - L_\tau$ flavor symmetry. This is a minimal model with just one standard Higgs doublet and three heavy right-handed neutrinos. It needs only small values for the soft $L_\mu - L_\tau$ breaking terms to reproduce the phenomenological viable mass textures analyzed.

e + e − -pair production in Pb-Au collisions at 158 GeV per nucleon

Abstract: We present the combined results on electron-pair production in 158GeV/n Pb-Au ( √ s=17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data analysis. The enhancement over the reference of neutral meson decays amounts to a factor of 2.31±0.19 (stat.) ± 0.55(syst.) ±0.69 (decays) for semi-central collisions (28% σ/σgeo) when yields are in- tegrated over m >200MeV/c 2 in invariant mass. The measured yield, its stronger-than-linear scaling with Nch, and the dominance of low pair pt strongly suggest an interpretation as thermal radiation from pion annihilation in the hadronic fireball. The shape of the excess centring at m ≈500MeV/c 2 , however, cannot be described without strong medium modifications of the ρ meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.

Measurement of F-2(c(c)over-bar) and F-2(b(b)over-bar) at high Q(2) using the H1 vertex detector at HERA

Abstract: Measurements are presented of inclusive charm and beauty cross sections in e$^+$p collisions at HERA for values of photon virtuality Q$^2$ > 150 GeV$^2$ and of inelasticity 0.1 < y < 0.7. The charm and beauty fractions are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 vertex detector. The data are divided into four regions in Q$^2$ and Bjorken x, and values for the structure functions $F_2^{c\overline{c}}$ and $F_2^{b\overline{b}}$ are obtained. The results are found to be compatible with the predictions of perturbative quantum chromodynamics.

Exploring Little Higgs models with ATLAS at the LHC

Abstract: We discuss possible searches for the new particles predicted by Little Higgs Models at the LHC. By using a simulation of the ATLAS detector, we demonstrate how the predicted quark, gauge bosons and additional Higgs bosons can be found and estimate the mass range over which their properties can be constrained.

Spin densities in the transverse plane and generalized transversity distributions

Abstract: We show how generalized quark distributions in the nucleon describe the density of polarized quarks in the impact parameter plane, both for longitudinal and transverse polarization of the quark and the nucleon. This density representation entails positivity bounds including chiral-odd distributions, which tighten the known bounds in the chiral-even sector. Using the quark equations of motion, we derive relations between the moments of chiral-odd generalized parton distributions of twist two and twist three. We exhibit the analogy between polarized quark distributions in impact parameter space and transverse momentum dependent distribution functions.

Operator product expansion and quark condensate from lattice QCD in coordinate space

Abstract: We present a lattice QCD determination of the chiral quark condensate based on a new method. We extract the quark condensate from the operator product expansion of the quark propagator at short euclidean distances, where it represents the leading contribution in the chiral limit. From this study we obtain $ {\left\langle {\bar{q}q} \right\rangle }^{{\overline{{{\text{MS}}}} }} {\left( {2\;{\text{GeV}}} \right)} = - {\left( {265 \pm 5 \pm 22\;{\text{MeV}}} \right)}^{3} $ , in good agreement with determinations of this quantity based on different approaches. The simulation is performed by using the ${\cal O}(a)$ -improved Wilson action at $\beta = 6.45$ on a volume 323 × 70 in the quenched approximation.

Concerning the Landau pole in 3-3-1 models

Abstract: Some 3-3-1 models predict the existence of a non-perturbative regime at the TeV scale We study in these models and their supersymmetric extensions, the energy at which the non-perturbative limit and a Landau-like pole arise An order of magnitude for the mass of the extra neutral vector boson, $Z^\prime$ , present in these models is also obtained

Infrared regularization for spin-1 fields

Abstract: We extend the method of infrared regularization to spin-1 fields. As applications, we discuss the chiral extrapolation of the rho meson mass from lattice QCD data and the pion-rho sigma term.

The standard model on non-commutative space-time: electroweak currents and the Higgs sector

Abstract: We review the electroweak charged and neutral currents in the non-commutative standard model (NCSM) and compute the Higgs and Yukawa parts of the NCSM action. With the aim to make the NCSM accessible to phenomenological considerations, all relevant expressions are given in terms of physical fields, and Feynman rules are provided.

Summary of the CMS potential for the Higgs boson discovery

Abstract: This work summarizes the studies for the Higgs boson searches in CMS at the LHC collider. The main discovery channels are presented and the potential is given for the discovery of the SM Higgs boson and the Higgs bosons of the MSSM. The phenomenology, detector, trigger and reconstruction issues are briefly discussed.

Modeling the pion and kaon form factors in the timelike region

Abstract: New, accurate measurements of the pion and kaon electromagnetic form factors are expected in the near future from experiments at electron-positron colliders, using the radiative return method. We construct a model for the timelike pion electromagnetic form factor that is valid also at momentum transfers far above the $\rho$ resonance. The ansatz is based on vector dominance and includes a pattern of radial excitations expected from dual resonance models. The form factor is fitted to the existing data in the timelike region, continued to the spacelike region and compared with the measurements there and with the QCD predictions. Furthermore, the model is extended to the kaon electromagnetic form factor. Using isospin and SU(3)-flavor symmetry relations we extract the isospin-one contribution and predict the kaon weak form factor accessible in semileptonic $\tau$ decays.