Showing papers in "European Physical Journal C in 2007"

Interacting holographic phantom

Abstract: In this paper we consider the holographic model of interacting dark energy in non-flat universe. With the choice of c≤0.84, the interacting holographic dark energy can be described by a phantom scalar field. Then we show this phantomic description of the holographic dark energy with c≤0.84 and reconstruct the potential of the phantom scalar field.

Thermal hadronization and Hawking–Unruh radiation in QCD

Abstract: We conjecture that, because of color confinement, the physical vacuum forms an event horizon for quarks and gluons, which can be crossed only by quantum tunneling, i.e., through the QCD counterpart of Hawking radiation at black holes. Since such radiation cannot transmit information to the outside, it must be thermal, of a temperature determined by the chromodynamic force at the confinement surface, and it must maintain color neutrality. We explore the possibility that the resulting process provides a common mechanism for thermal hadron production in high energy interactions, from e+e- annihilation to heavy ion collisions.

Novel constraints on light elementary particles and extra-dimensional physics from the Casimir effect

Abstract: We present supplementary information on the recent indirect measurement of the Casimir pressure between two parallel plates using a micromachined oscillator. The equivalent pressure between the plates is obtained by means of the proximity force approximation after measuring the force gradient between a gold coated sphere and a gold coated plate. The data are compared with a new theoretical approach to the thermal Casimir force based on the use of the Lifshitz formula, combined with a generalized plasma-like dielectric permittivity that takes into account interband transitions of core electrons. The theoretical Casimir pressures calculated using the new approach are compared with those computed in the framework of the previously used impedance approach and also with the Drude model approach. The latter is shown to be excluded by the data at a 99.9% confidence level within the wide separation range from 210 to 620 nm. The level of agreement between the data and theoretical approaches based on the generalized plasma model, or the Leontovich surface impedance, is used to set stronger constraints on the Yukawa forces predicted from the exchange of light elementary particles and/or extra-dimensional physics. The resulting constraints are the strongest in the interaction region from 20 to 86 nm with a largest improvement by a factor of 4.4 at 26 nm.

Electron reconstruction in CMS

Abstract: The reconstruction of the energy and momentum of isolated electrons in CMS combining tracking and electromagnetic calorimetry information is described. The emphasis is put on primary electrons with transverse momentum below 50 GeV/c. The energy deposited in the electromagnetic calorimeter is measured in clusters of clusters (superclusters) which collect bremsstrahlung photons emitted along the electron trajectory in the tracker volume. The electron tracks are built from seeds in the pixel detector found via a cluster-driven pixel hit matching algorithm, followed by a reconstruction of trajectories in the silicon strip tracker with a Gaussian sum filter. Electrons are classified using observables sensitive to the pattern of bremsstrahlung emission and electromagnetic showering in the tracker material. Energy scale corrections depending on the electron class are applied to the supercluster and estimates of associated errors are obtained. The electron energy is deduced from a weighted combination of the corrected supercluster energy and tracker momentum measurements. The electron direction is that of the reconstructed electron track at interaction vertex. The pre-selection of isolated electron candidates for physics analysis is described. Class-dependent observables combining tracking and calorimetry information are discussed for electron identification.

Interacting generalized Chaplygin gas model in non-flat universe

Abstract: We employ the generalized Chaplygin gas of interacting dark energy to obtain the equation of state for the generalized Chaplygin gas energy density in a non-flat universe. By choosing a negative value for B we see that wΛeff<-1, which corresponds to a universe dominated by phantom dark energy.

Non-perturbative QCD effects and the top mass at the Tevatron

Abstract: We present a new, universally applicable toy model of colour reconnections in hadronic final states. The model is based on hadronising strings and has one free parameter. We next present an implementation of this model in the PYTHIA event generator and provide several parameter sets (‘tunes’), constrained by fits to Tevatron minimum-bias data. Finally, we consider the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gives an estimate of order δmtop ∼ ±0.5 GeV from non-perturbative uncertainties, and a further δmtop ∼ ±1 GeV from shower effects.

Noncommutative Induced Gauge Theory

Abstract: We consider an external gauge potential minimally coupled to a renormalisable scalar theory on 4-dimensional Moyal space and compute in position space the one-loop Yang–Mills-type effective theory generated from the integration over the scalar field We find that the gauge-invariant effective action involves, beyond the expected noncommutative version of the pure Yang–Mills action, additional terms that may be interpreted as the gauge theory counterpart of the harmonic oscillator term, which for the noncommutative ϕ4-theory on Moyal space ensures renormalisability The expression of a possible candidate for a renormalisable action for a gauge theory defined on Moyal space is conjectured and discussed

Induced gauge theory on a noncommutative space

Abstract: We consider a scalar φ4 theory on canonically deformed Euclidean space in 4 dimensions with an additional oscillator potential. This model is known to be renormalisable. An exterior gauge field is coupled in a gauge invariant manner to the scalar field. We extract the dynamics for the gauge field from the divergent terms of the 1-loop effective action, using a matrix basis and propose an action for the noncommutative gauge theory, which is a candidate for a renormalisable model.

Dark energy as a massive vector field

Abstract: We propose that the Universe is filled with a massive vector field non-minimally coupled to gravitation. The field equations of the model are consistently derived, and their application to cosmology is considered. The Friedmann equations acquire an extra dark-energy component, which is proportional to the mass of the vector particle. This leads to a late-time accelerated de Sitter type expansion. The free parameters of the model (gravitational coupling constants and initial value of the cosmological vector field) can be estimated by using the PPN solar system constraints. The mass of the cosmological massive vector particle, which may represent the main component of the Universe, is of the order of 10-63 g.

Holographic dark energy in a cyclic universe

Abstract: In this paper we study the cosmological evolution of the holographic dark energy in a cyclic universe, generalizing the model of holographic dark energy proposed by Li. The holographic dark energy with c < 1 can realize a quintom behavior; namely, it evolves from a quintessence-like component to a phantom-like one. The holographic phantom energy density grows rapidly and dominates the late-time expanding phase, helping to realize a cyclic universe scenario in which the high energy regime is modified by the effects of quantum gravity, causing a turn-around (and a bounce) of the universe. The dynamical evolution of holographic dark energy in the regimes of low energy and high energy is governed by two differential equations, respectively. It is of importance to link the two regimes for this scenario. We propose a link condition giving rise to a complete picture of holographic evolution of a cyclic universe.

Generalized global defect solutions

Abstract: We investigate the presence of defect structures in generalized models described by a real scalar field in (1,1) space-time dimensions We work with two distinct generalizations: one in the form of a product of functions of the field and its derivative, and the other as a sum We search for static solutions and study the corresponding linear stability on general grounds We illustrate the results with several examples, where we find stable defect structures of modified profile In particular, we show how the new defect solutions may give rise to evolutions not present in the standard scenario in higher spatial dimensions

Gauge and gravitational anomalies and Hawking radiation of rotating BTZ black holes

Abstract: In this paper we obtain the flux of Hawking radiation from rotating BTZ black holes from the gauge and gravitational anomalies point of view. Then we show that the gauge and gravitational anomaly in BTZ spacetime is cancelled by the total flux of a 2-dimensional blackbody at the Hawking temperature of the spacetime.

Weak interaction effects in top-quark pair production at hadron colliders

Abstract: Top-quark physics plays an important role at hadron colliders such as the Tevatron at Fermilab or the upcoming Large Hadron Collider (LHC) at CERN. Given the planned experimental precision, detailed theoretical predictions are mandatory. In this article we present analytic results for the complete weak corrections to gluon-induced top-quark pair production – neglecting purely photonic corrections, completing our earlier results for the quark-induced reaction. As an application we discuss top-quark pair production at the Tevatron and LHC. In particular we show that, although they are small for inclusive quantities, weak corrections can be sizeable for differential distributions.

Probing anomalous Wtb couplings in top pair decays

Abstract: We investigate several quantities, defined in the decays of top quark pairs, which can be used to explore non-standard Wtb interactions. Two new angular asymmetries are introduced in the leptonic decay of top (anti)quarks. Both are very sensitive to anomalous Wtb couplings, and their measurement allows for a precise determination of the W helicity fractions. We also examine other angular and energy asymmetries, the W helicity fractions and their ratios, as well as spin correlation asymmetries, analysing their dependence on anomalous Wtb couplings and identifing the quantities which are most sensitive to them. It is explicitly shown that spin correlation asymmetries are less sensitive to new interactions in the decay of the top quark; therefore, when combined with the measurement of other observables, they can be used to determine the tt spin correlation even in the presence of anomalous Wtb couplings. We finally discuss some asymmetries which can be used to test CP violation in tt production and complex phases in the effective Wtb vertex.

Is V(tb) =~ 1?

Abstract: The strongest constraint on Vtb presently comes from the 3 x 3 unitarity of the CKM matrix, which fixes Vtb to be very close to one. If the unitarity is relaxed, current information from top production at Tevatron still leaves open the possibility that Vtb is sizably smaller than one. In minimal extensions of the standard model with extra heavy quarks, the unitarity constraints are much weaker and the EW precision parameters entail the strongest bounds on Vtb. We discuss the experimental perspectives of discovering and identifying such new physics models at the Tevatron and the LHC, through a precise measurement of Vtb from the single top cross sections and by the study of processes where the extra heavy quarks are produced. Comment: 19 pages, 8 figures

Covariant realizations of kappa-deformed space

Abstract: We study a Lie algebra type κ-deformed space with an undeformed rotation algebra and commutative vector-like Dirac derivatives in a covariant way. The space deformation depends on an arbitrary vector. Infinitely many covariant realizations in terms of commuting coordinates of undeformed space and their derivatives are constructed. The corresponding coproducts and star products are found and related in a new way. All covariant realizations are physically equivalent. Specially, a few simple realizations are found and discussed. The scalar fields, invariants and the notion of invariant integration is discussed in the natural realization.

Inclusive production of charged pions in p+p collisions at 158 GeV/c beam momentum

Abstract: The production of charged pions in minimum bias p+C interactions is studied using a sample of 377000 inelastic events obtained with the NA49 detector at the CERN SPS at 158 GeV/c beam momentum. The data cover a phase space area ranging from 0 to 1.8 GeV/c in transverse momentum and from -0.1 to 0.5 in Feynman xF. Inclusive invariant cross sections are given on a grid of 270 bins per charge, thus offering for the first time a dense coverage of the projectile hemisphere and of the cross-over region into the target fragmentation zone.

The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector

Abstract: A jet reconstruction algorithm is developed for events with a high particle density in the calorimetric system. The performance of the reconstruction of hard QCD jets with initial parton energies 50–300 GeV is studied in central Pb–Pb collisions with a modified cone jet finder which includes an algorithm for event-by-event background subtraction. The heavy ion background is simulated using the HIJING Monte Carlo generator with \(\text{d} N_{\text{ch}}/\text{d} y\vert_{y=0} = 5000\). Results on the achieved jet reconstruction efficiency, purity, energy and spatial resolution are presented.

Causal viscous hydrodynamics for central heavy-ion collisions

Abstract: We study causal viscous hydrodynamics in the context of central relativistic heavy-ion collisions and provide details of a straightforward numerical algorithm to solve the hydrodynamic equations It is shown that correlation functions of fluctuations provide stringent test cases for any such numerical algorithm Passing these tests, we study the effects of viscosity on the temperature profile in central heavy-ion collisions Also, we find that it is possible to counter-act the effects of viscosity to some extent by re-adjusting the initial conditions However, viscous corrections are strongest for high-mass particles, signaling the breakdown of hydrodynamic descriptions for large η/s

Form factors in the radiative pion decay

Abstract: We perform an analysis of the form factors that rule the structure-dependent amplitude in radiative pion decay. The resonance contributions to π→eνeγ decays are computed through the proper construction of the vector and axial-vector form factors by setting the QCD driven asymptotic properties of the three-point Green functions 〈VVP〉 and 〈VAP〉, and by demanding the smoothening of the form factors at high transfer of momentum. A comparison between theoretical and experimental determination of the form factors is also carried out. We also consider and evaluate the role played by a non-standard tensor form factor. We conclude that, at present and due to the hadronic uncertainties, the search for new physics in this process is not feasible.

DsJ(2860) and DsJ(2715)

Abstract: Recently the Babar Collaboration reported a new cs state, DsJ(2860), and the Belle Collaboration observed DsJ(2715). We investigate the strong decays of the excited cs states using the 3 P 0 model. After comparing the theoretical decay widths and decay patterns with the available experimental data, we are inclined to conclude that: (1) DsJ(2715) is probably the 1-(13 D 1) cs state, although the 1-(23 S 1) assignment is not completely excluded; (2) DsJ(2860) seems unlikely to be the 1-(23 S 1) and 1-(13 D 1) candidate; (3) to consider DsJ(2860) either as a 0+(23 P 0) or as a 3-(13 D 3) cs state is consistent with the experimental data; (4) the experimental search of DsJ(2860) in the channels Dsη, DK*, D*K and Ds *η will be crucial to distinguish the above two possibilities.

Thermodynamics of the PNJL model

Abstract: QCD thermodynamics is investigated by means of the Polyakov-loop-extended Nambu–Jona–Lasinio (PNJL) model, in which quarks couple simultaneously to the chiral condensate and to a background temporal gauge field representing Polyakov loop dynamics. The behaviour of the Polyakov loop as a function of temperature is obtained by minimising the thermodynamic potential of the system. A Taylor series expansion of the pressure is performed. Pressure difference and quark number density are then evaluated up to sixth order in the quark chemical potential and compared to the corresponding lattice data. The validity of the Taylor expansion is discussed within our model through a comparison between the full results and the truncated ones.

Search for direct CP violating charge asymmetries in K ± →π ± π + π - and K ± →π ± π 0 π 0 decays

Abstract: A measurement of the direct CP violating charge asymmetries of the Dalitz plot linear slopes Ag=(g+-g-)/(g++g-) in K±→π±π+π- and K±→π±π0π0 decays by the NA48/2 experiment at CERN SPS is presented. A new technique of asymmetry measurement involving simultaneous K+ and K- beams and a large data sample collected allowed a result of an unprecedented precision. The charge asymmetries were measured to be Ac g=(-1.5±2.2)×10-4 with 3.11×109K±→π±π+π- decays, and An g=(1.8±1.8)×10-4 with 9.13×107K±→π±π0π0 decays. The precision of the results is limited mainly by the size of the data sample.

The pion form factor from lattice QCD with two dynamical flavours

Abstract: We compute the electromagnetic form factor of the pion, using non-perturbatively O(a) improved Wilson fermions. The calculations are done for a wide range of pion masses and lattice spacings. We check for finite size effects by repeating some of the measurements on smaller lattices. The large number of lattice parameters we use allows us to extrapolate to the physical point. For the square of the charge radius we find \(\langle{r^2}\rangle= 0.444(20)\) fm2, in good agreement with experiment.

Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

Abstract: Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q^2<0.01 GeV^2) and deep-inelastic scattering processes (DIS, 4 e X Y, in which the system X, containing at least two jets, is separated from a leading low-mass proton remnant system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5+-0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.

Dispersion representations for hard exclusive processes: beyond the Born approximation

Abstract: Several hard exclusive scattering processes admit a description in terms of generalized parton distributions and perturbative hard-scattering kernels Both the physical amplitude and the hard-scattering kernels fulfill dispersion relations We give a detailed investigation of their consistency at all orders in perturbation theory The results shed light on the information about generalized parton distributions that can be extracted from the real and imaginary parts of exclusive amplitudes They also provide a practical consistency check for models of these distributions in which Lorentz invariance is not exactly satisfied

The topological AC effect on non-commutative phase space

Abstract: The Aharonov–Casher (AC) effect in non-commutative (NC) quantum mechanics is studied. Instead of using the star product method, we use a generalization of Bopp’s shift method. After solving the Dirac equations both on non-commutative space and non-commutative phase space by the new method, we obtain corrections to the AC phase on NC space and NC phase space, respectively.

Two- and three-loop beta function of non-commutative Φ 4 4 theory

Abstract: The simplest non-commutative renormalizable field theory, the φ4 4 model on four dimensional Moyal space with harmonic potential, is asymptotically safe at one loop, as shown by Grosse and Wulkenhaar. We extend this result up to three loops. If this remains true at any loop, it should allow for a full non-perturbative construction of this model.

Supersymmetric benchmarks with non-universal scalar masses or gravitino dark matter

Abstract: We motivate, propose and examine a new set of benchmark supersymmetric scenarios, some of which have non-universal Higgs scalar masses (NUHM) and others have gravitino dark matter (GDM). The scalar masses in these proposed models are either considerably larger or smaller than the narrow range allowed for the same gaugino mass m1/2 in the constrained MSSM (CMSSM) with universal scalar masses m0 and neutralino dark matter. Unlike the CMSSM, the proposed NUHM and GDM models with larger m0 may have large branching ratios for Higgs and/or Z production in the cascade decays of heavier sparticles, whose detection we discuss. The novel phenomenology of the GDM models depends on the nature of the next-to-lightest supersymmetric particle (NLSP), which has a lifetime exceeding 104 s in the proposed benchmark scenarios. In one GDM scenario the NLSP is the lightest neutralino χ, and the supersymmetric collider signatures are similar to those in previous CMSSM benchmarks, but with a distinctive spectrum that would be challenging for the LHC and ILC. In the other GDM scenarios based on minimal supergravity (mSUGRA), the NLSP is the lighter stau slepton \(\tilde\tau_1\), with a lifetime between ∼104 and 3×106 s. Every supersymmetric cascade would end in a \(\tilde\tau_1\), which would have a distinctive time-of-flight signature. Slow-moving \(\tilde\tau_1\)’s might be trapped in a collider detector or outside it, and the preferred detection strategy would depend on the \(\tilde\tau_1\) lifetime. We discuss the extent to which these mSUGRA GDM scenarios could be distinguished from gauge-mediated models.

A generalised manifestly gauge invariant exact renormalisation group for SU(N ) Yang-Mills

Abstract: We take the manifestly gauge invariant exact renormalisation group previously used to compute the one-loop β function in SU(N) Yang–Mills without gauge fixing, and generalise it so that it can be renormalised straightforwardly at any loop order. The diagrammatic computational method is developed to cope with general group theory structures, and new methods are introduced to increase its power, so that much more can be done simply by manipulating diagrams. The new methods allow the standard two-loop β function coefficient for SU(N) Yang–Mills to be computed, for the first time without fixing the gauge or specifying the details of the regularisation scheme.