Showing papers in "European Physical Journal A in 2012"

Physics opportunities with the 12 GeV upgrade at Jefferson Lab

Abstract: We summarize the scientific opportunities for utilization of the upgraded 12GeV Continuous Electron Beam Accelerator Facility (CEBAF) and associated experimental equipment at Jefferson Lab. This is based primarily on the 52 proposals recommended for approval by the Jefferson Lab Program Advisory Committee to date. The upgraded facility will enable a new experimental program with substantial discovery potential to address important topics in nuclear, hadronic, and electroweak physics.

Properties of baryon resonances from a multichannel partial wave analysis

Abstract: Properties of nucleon and Δ resonances are derived from a multichannel partial wave analysis of pion and photo-induced reactions off protons. This paper summarizes the latest results on masses, widths, and decay properties of nucleon and Δ resonances.

Relativistic thermodynamics: Transverse momentum distributions in high-energy physics

Abstract: Transverse momentum distributions measured by the STAR and PHENIX Collaborations at the Relativistic Heavy Ion Collider and by the ALICE, ATLAS and CMS Collaborations at the Large Hadron Collider can be considered in the framework of relativistic thermodynamics using the Tsallis distribution. Theoretical issues are clarified concerning the thermodynamic consistency in the case of relativistic high-energy quantum distributions. An improved form is proposed for describing the transverse momentum distribution and fits are presented together with estimates of the parameter q and the temperature T .

Nuclear quantum many-body dynamics: from collective vibrations to heavy-ion collisions

Abstract: A summary of recent researches on nuclear dynamics with realistic microscopic quantum approaches is presented. The Balian-Veneroni variational principle is used to derive the time-dependent Hartree-Fock (TDHF) equation describing the dynamics at the mean-field level, as well as an extension including small-amplitude quantum fluctuations which is equivalent to the time-dependent random-phase approximation (TDRPA). Such formalisms as well as their practical implementation in the nuclear physics framework with modern three-dimensional codes are discussed. Recent applications to nuclear dynamics, from collective vibrations to heavy-ion collisions are presented. Particular attention is devoted to the interplay between collective motions and internal degrees of freedom. For instance, the harmonic nature of collective vibrations is questioned. Nuclei are also known to exhibit superfluidity due to pairing residual interaction. Extensions of the theoretical approach to study such pairing vibrations are now available. Large amplitude collective motions are investigated in the framework of heavy-ion collisions leading, for instance, to the formation of a compound system. How fusion is affected by the internal structure of the collision partners, such as their deformation, is discussed. Other mechanisms in competition with fusion, and responsible for the formation of fragments which differ from the entrance channel (transfer reactions, deep-inelastic collisions, and quasi-fission) are investigated. Finally, studies of actinide collisions forming, during very short times of few zeptoseconds, the heaviest nuclear systems available on Earth, are presented.

Three particles in a finite volume

Abstract: Within the non-relativistic potential scattering theory, we derive a generalized version of the Luscher formula, which includes three-particle inelastic channels. Faddeev equations in a finite volume are discussed in detail. It is proved that, even in the presence of the three-particle intermediate states, the discrete spectrum in a finite box is determined by the infinite-volume elements of the scattering S -matrix up to corrections, exponentially suppressed at large volumes.

Bottomonia in the quark-gluon plasma and their production at RHIC and LHC

Abstract: We study the production of bottomonium states in heavy-ion reactions at collider energies available at RHIC and LHC. We employ an earlier constructed rate equation approach which accounts for both suppression and regeneration mechanisms in the quark-gluon plasma (QGP) and hadronization phases of the evolving thermal medium. Our previous predictions utilizing two limiting cases of strong and weak bottomonium binding in the QGP are updated by i) checking the compatibility of the pertinent spectral functions with lattice-QCD results for euclidean correlators, ii) adapting the initial conditions of the rate equation by updating bottom-related input cross-sections and the charged-particle multiplicity of the fireball, and iii) converting our calculations into observables as recently measured by the STAR and CMS experiments. Our main findings are a preference for strong ϒ binding as well as a significant regeneration component at the LHC.

Analysis of the Λb → Λℓ+ℓ− transition in the SM4 using form factors from full QCD

Abstract: Using the responsible form factors calculated via full QCD, we analyze the Λ b → Λl+l− transition in the standard model containing fourth-generation quarks (SM4). We discuss the effects of the presence of the t’ fourth-family quark on related observables like branching ratio, forward-backward asymmetry, baryon polarization as well as double-lepton polarization asymmetries. We also compare our results with those obtained in the SM as well as with predictions of the SM4 but using form factors calculated within heavy-quark effective theory. The obtained results on the branching ratio indicate that the l transition is more probable in full QCD comparing to the heavy-quark effective theory. It is also shown that the results on all considered observables in the SM4 deviate considerably from the SM predictions when m t′ ≥ 400 GeV.

The size of the proton: Closing in on the radius puzzle

Abstract: We analyze the recent electron-proton scattering data from Mainz using a dispersive framework that respects the constraints from analyticity and unitarity on the nucleon structure. We also perform a continued fraction analysis of these data. We find a small electric proton charge radius, rEp = 0.84−0.01+0.01 fm, consistent with the recent determination from muonic hydrogen measurements and earlier dispersive analyses. We also extract the proton magnetic radius, rMp = 0.86−0.03+0.02 fm, consistent with earlier determinations based on dispersion relations.

Particlization in hybrid models

Abstract: In hybrid models, which combine hydrodynamical and transport approaches to describe different stages of heavy-ion collisions, conversion of fluid to individual particles, particlization, is a non-trivial technical problem. We describe in detail how to find the particlization hypersurface in a 3+1 dimensional model, and how to sample the particle distributions evaluated using the Cooper-Frye procedure to create an ensemble of particles as an initial state for the transport stage. We also discuss the role and magnitude of the negative contributions in the Cooper-Frye procedure.

Study of $$qqqc\bar c$$ five quark system with three kinds of quark-quark hyperfine interaction

Abstract: The low-lying energy spectra of five quark systems uudc (c) over bar (I = 1/2, S = 0) and udsc (c) over bar (I = 0, S = -1) are investigated with three kinds of schematic interaction: the chromomagnetic interaction, the flavor-spin-dependent interaction and the instanton-induced interaction. In all the three models, the lowest five-quark state (uudc (c) over bar or udsc (c) over bar) has an orbital angular momentum L = 0 and the spin-parity J(P) = 1/2(-); the mass of the lowest udsc (c) over bar state is heavier than the lowest uudc (c) over bar state.

JYFLTRAP: a Penning trap for precision mass spectroscopy and isobaric purification

Abstract: In this article a comprehensive description and performance of the double Penning-trap setup JYFLTRAP will be detailed. The setup is designed for atomic mass measurements of both radioactive and stable ions and additionally serves as a very high-resolution mass separator. The setup is coupled to the IGISOL facility at the accelerator laboratory of the University of Jyvaskyla. The trap has been online since 2003 and it was shut down in the summer of 2010 for relocation to the upgraded IGISOL facility. Numerous atomic mass and decay energy measurements have been performed using the time-of-flight ion-cyclotron resonance technique. The trap has also been used in several decay spectroscopy experiments as a high-resolution mass filter.

Proton polarisability contribution to the Lamb shift in muonic hydrogen at fourth order in chiral perturbation theory

Abstract: We calculate the amplitude T1 for forward doubly virtual Compton scattering in heavy-baryon chiral perturbation theory, to fourth order in the chiral expansion and with the leading contribution of the $ \gamma$ N $ \Delta$ form factor. This provides a model-independent expression for the amplitude in the low-momentum region, which is the dominant one for its contribution to the Lamb shift. It allows us to significantly reduce the theoretical uncertainty in the proton polarisability contributions to the Lamb shift in muonic hydrogen. We also stress the importance of consistency between the definitions of the Born and structure parts of the amplitude. Our result leaves no room for any effect large enough to explain the discrepancy between proton charge radii as determined from muonic and normal hydrogen.

Hybrid stars in a strong magnetic field

Abstract: We study the effects of high magnetic fields on the particle population and equation of state of hybrid stars using an extended hadronic and quark SU(3) non-linear realization of the sigma model. In this model the degrees of freedom change naturally from hadrons to quarks as the density and/or temperature increases. The effects of high magnetic fields and anomalous magnetic moment are visible in the macroscopic properties of the star, such as mass, adiabatic index, moment of inertia, and cooling curves. Moreover, at the same time that the magnetic fields become high enough to modify those properties, they make the star anisotropic.

Scalar mesons moving in a finite volume and the role of partial wave mixing

Abstract: Phase shifts and resonance parameters can be obtained from finite-volume lattice spectra for interacting pairs of particles, moving with non-zero total momentum. We present a simple derivation of the method that is subsequently applied to obtain the ππ and πK phase shifts in the sectors with total isospin I = 0 and I = 1/2 , respectively. Considering different total momenta, one obtains extra data points for a given volume that allow for a very efficient extraction of the resonance parameters in the infinite-volume limit. Corrections due to the mixing of partial waves are provided. We expect that our results will help to optimize the strategies in lattice simulations, which aim at an accurate determination of the scattering and resonance properties.

Strangeness nuclear physics: a critical review on selected topics

Abstract: Selected topics in strangeness nuclear physics are critically reviewed. This includes production, structure and weak decay of Λ-Hypernuclei, the $$\bar K$$ nuclear interaction and the possible existence of $$\bar K$$ bound states in nuclei. Perspectives for future studies on these issues are also outlined.

Derivation of fluid dynamics from kinetic theory with the 14-moment approximation

Abstract: We review the traditional derivation of the fluid-dynamical equations from kinetic theory according to Israel and Stewart. We show that their procedure to close the fluid-dynamical equations of motion is not unique. Their approach contains two approximations, the first being the so-called 14-moment approximation to truncate the single-particle distribution function. The second consists in the choice of equations of motion for the dissipative currents. Israel and Stewart used the second moment of the Boltzmann equation, but this is not the only possible choice. In fact, there are infinitely many moments of the Boltzmann equation which can serve as equations of motion for the dissipative currents. All resulting equations of motion have the same form, but the transport coefficients are different in each case.

The low-pressure focal plane detector of the MAGNEX spectrometer

Abstract: The focal plane detector of the MAGNEX large acceptance magnetic spectrometer is presented. It is based on a large low-pressure gas-filled tracker followed by a wall of silicon detectors to stop the particles. It has been designed for the stringent requirements from the ion optics and the foreseen use with heavy ions. The performances of the device are demonstrated using 4MeV/u oxygen beams. The results of the tests are described and discussed in view of the application of the particle identification and trajectory reconstruction techniques to recover the kinematic properties of the reaction products. They demonstrate the suitability of such detector in a wide range of experiments where low-energy thresholds, large-accepted solid angle, precise momentum measurement and good particle identification are needed.

Have superheavy elements been produced in nature

Abstract: We discuss the possibility whether superheavy elements can be produced in Nature by the astrophysical rapid neutron capture process. To this end we have performed fully dynamical network r-process calculations assuming an environment with neutron-to-seed ratio large enough to produce superheavy nuclei. Our calculations include two sets of nuclear masses and fission barriers and include all possible fission channels and the associated fission yield distributions. Our calculations produce superheavy nuclei with \(\ensuremath A\approx 300\) that however decay on time scales of days.

Consequences of temperature fluctuations in observables measured in high-energy collisions

Abstract: We review the consequences of intrinsic, nonstatistical temperature fluctuations as seen in observables measured in high-energy collisions. We do this from the point of view of nonextensive statistics and Tsallis distributions. Particular attention is paid to multiplicity fluctuations as a first consequence of temperature fluctuations, to the equivalence of temperature and volume fluctuations, to the generalized thermodynamic fluctuations relations allowing us to compare fluctuations observed in different parts of the phase space, and to the problem of the relation between Tsallis entropy and Tsallis distributions. We also discuss the possible influence of conservation laws on these distributions and provide some examples of how one can get them without considering temperature fluctuations.

Three particles in a finite volume: The breakdown of spherical symmetry

Abstract: Lattice simulations of light nuclei necessarily take place in finite volumes, thus affecting their infrared properties. These effects can be addressed in a model-independent manner using Effective Field Theories. We study the model case of three identical bosons (mass m with resonant two-body interactions in a cubic box with periodic boundary conditions, which can also be generalized to the three-nucleon system in a straightforward manner. Our results allow for the removal of finite-volume effects from lattice results as well as the determination of infinite-volume scattering parameters from the volume dependence of the spectrum. We study the volume dependence of several states below the break-up threshold, spanning one order of magnitude in the binding energy in the infinite volume, for box side lengths L between the two-body scattering length a and L = 0.25a. For example, a state with a three-body energy of −3/(ma 2) in the infinite volume has been shifted to −10/(ma 2) at L = a. Special emphasis is put on the consequences of the breakdown of spherical symmetry and several ways to perturbatively treat the ensuing partial-wave admixtures. We find their contributions to be on the sub-percent level compared to the strong volume dependence of the S-wave component. For shallow bound states, we find a transition to boson-diboson scattering behavior when decreasing the size of the finite volume.

Nucleon axial and pseudoscalar form factors from the covariant Faddeev equation

Abstract: We compute the axial and pseudoscalar form factors of the nucleon in the Dyson-Schwinger approach. To this end, we solve a covariant three-body Faddeev equation for the nucleon wave function and determine the matrix elements of the axialvector and pseudoscalar isotriplet currents. Our only input is a well-established and phenomenologically successful ansatz for the non-perturbative quark-gluon interaction. As a consequence of the axial Ward-Takahashi identity that is respected at the quark level, the Goldberger-Treiman relation is reproduced for all current-quark masses. We discuss the timelike pole structure of the quark-antiquark vertices that enters the nucleon matrix elements and determines the momentum dependence of the form factors. Our result for the axial charge underestimates the experimental value by 20 -25% which might be a signal of missing pion-cloud contributions. The axial and pseudoscalar form factors agree with phenomenological and lattice data in the momentum range above Q 2 ~ 1...2 GeV2.

T-REX. A new setup for transfer experiments at REX-ISOLDE

Abstract: Transfer reactions in inverse kinematics using the radioactive ion beams provided by the REX-ISOLDE facility at CERN are very good tools to gain insight into the single-particle properties of exotic nuclei. The new silicon particle detector setup T-REX, optimized for these transfer reactions, is presented in this article. T-REX consists of position sensitive ΔE -E telescopes in a compact configuration that covers up to 66% of the solid angle. The setup allows the identification of the light reaction products and the measurement of their angular distribution for a large range of polar angles. Simultaneous detection of the elastically scattered target-like nuclei enables the determination of the optical model parameters used in the analysis of the experiments.

Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model

Abstract: We present dilepton spectra from p+p, d+p and p+Nb reactions at SIS energies, which were simulated with the GiBUU transport model in a resonance model approach. These spectra are compared to the data published by the HADES and DLS collaborations. It is shown that the $\rho$ spectral function includes non-trivial effects already in elementary reactions, due to production via baryon resonances, which can yield large contributions to the dilepton spectrum. Dilepton spectra from nuclear reactions in the energy range of the HADES experiment are thus found to be sensitive also to properties of nucleon resonances in the nuclear medium.

Electromagnetic transitions in an effective chiral Lagrangian with the $ \eta^{{\prime}}_{}$ and light vector mesons

Abstract: We consider the chiral Lagrangian with a nonet of Goldstone bosons and a nonet of light vector mesons The mixing between the pseudoscalar mesons eta and eta' is taken into account A novel countin

Effects of pseudoscalar-baryon channels in the dynamically generated vector-baryon resonances

Abstract: We study the interaction of vector mesons with the octet of stable baryons in the framework of the local hidden gauge formalism using a coupled-channels unitary approach, including also the pseudoscalar-baryon channels which couple to the same quantum numbers. We examine the scattering amplitudes and their poles, which can be associated to the known J P = 1/2−, 3/2− baryon resonances, and determine the role of the pseudoscalar-baryon channels, changing the width and eventually the mass of the resonances generated with only the basis of vector-baryon states.

Inclusive dielectron spectra in p+p collisions at 3.5 GeV kinetic beam energy

Abstract: We present the inclusive invariant-mass, transverse momentum and rapidity distributions of dielectrons (e + e pairs) in p+p interactions at 3.5 GeV beam kinetic energy. In the vector-meson mass region, a distinct peak corresponding to direct ! decays is reconstructed with 2% mass resolution. The data is compared to predictions from three model calculations. Due to the large acceptance of the HADES apparatus for e + e invariant masses above 0.2 GeV/c 2 and for transverse pair momenta pt < 1 GeV/c, acceptance corrections are to a large extent model independent. This allows us to extract from dielectron data for the rst time at this energy the inclusive production cross sections for light vector mesons. Inclusive production cross sections for o and mesons are also reported. The obtained results will serve as an important reference for the study of vector meson production in proton-nucleus and heavy-ion collisions. Furthermore, using this data, an improved value for the upper bound of the branching ratio for direct decays into the electron-positron channel is obtained.

The collinear cluster tri-partition (CCT) of 252 Cf (sf): New aspects from neutron gated data

Abstract: Results of two different experiments for the study of fission of 252Cf (sf) events in coincidence with neutrons are reported. Two time-of-flight-energy (TOF-E detectors systems have been used. The fission fragment masses were obtained in a double arm coincidence set-up, where the missing mass in the binary decay is used to characterise ternary fission as a collinear cluster tri-partition (CCT). The 3He filled neutron counters have been arranged so as to detect principally neutrons emitted from an isotropic source in the laboratory frame. The fission events connected to the larger experimental neutron multiplicities show a wide range in the missing-mass spectrum, down to $ \alpha$ -particles, carbon and oxygen isotopes. These are linked with magic nuclei in the binary mass-mass correlations of the fission fragments. These neutron gated data are virtually free from background events from scattered binary fission fragments. The ungated spectra are compared to those of the previous data from our previous article (Eur. Phys. J. A. 45, 29 (2010)), the observed structures agree well with the manifestations of the collinear cluster tri-partition of 252Cf (sf) observed earlier. Several new families of the CCT modes are observed.

Exact solutions of relativistic perfect fluid hydrodynamics for a QCD equation of state

Abstract: We generalize a previously known class of exact analytic solutions of relativistic perfect fluid hydrodynamics for the first time to arbitrary temperature-dependent Equation of State. We investigate special cases of this class of solutions, in particular, we present hydrodynamical solutions with an Equation of State determined from lattice QCD calculations. We discuss the phenomenological relevance of these solutions as well.

Light cone QCD sum rules study of the semileptonic heavy ΞQ and Ξ′Q transitions to Ξ and Σ baryons

Abstract: The semileptonic decays of heavy spin-1/2, Ξ b(c) and Ξ′ b(c) baryons to the light spin-1/2, Ξ and Σ baryons are investigated in the framework of light cone QCD sum rules. In particular, using the most general form of the interpolating currents for the heavy baryons as well as the distribution amplitudes of the Ξ and Σ baryons, we calculate all form factors entering the matrix elements of the corresponding effective Hamiltonians in full QCD. Having calculated the responsible form factors, we evaluate the decay rates and branching fractions of the related transitions.

Shear viscosity of a hot pion gas

Abstract: The shear viscosity of an interacting pion gas is studied using the Kubo formalism as a microscopic description of thermal systems close to global equilibrium. We implement the skeleton expansion in order to approximate the retarded correlator of the viscous part of the energy-momentum tensor. After exploring this in gφ 4 theory we show how the skeleton expansion can be consistently applied to pions in chiral perturbation theory. The shear viscosity η is determined by the spectral width, or equivalently, the mean free path of pions in the heat bath. We derive a new analytical result for the mean free path which is well conditioned for numerical evaluation and discuss the temperature and pion-mass dependence of the mean free path and the shear viscosity. The ratio η/s of the interacting pion gas exceeds the lower bound 1/4π from AdS/CFT correspondence.