Showing papers in "European Physical Journal A in 2013"

Performance of the neutron time-of-flight facility n_TOF at CERN

Abstract: The neutron time-of-flight facility n_TOF features a white neutron source produced by spallation through 20GeV/c protons impinging on a lead target. The facility, aiming primarily at the measurement of neutron-induced reaction cross sections, was operating at CERN between 2001 and 2004, and then underwent a major upgrade in 2008. This paper presents in detail all the characteristics of the new neutron beam in the currently available configurations, which correspond to two different collimation systems and two choices of neutron moderator. The characteristics discussed include the intensity and energy dependence of the neutron flux, the spatial profile of the beam, the in-beam background components and the energy resolution/broadening. The discussion of these features is based on dedicated measurements and Monte Carlo simulations, and includes estimations of the systematic uncertainties of the mentioned quantities.

Controlling complex Langevin dynamics at finite density

Abstract: At nonzero chemical potential the numerical sign problem in lattice field theory limits the use of standard algorithms based on importance sampling. Complex Langevin dynamics provides a possible solution, but it has to be applied with care. In this review, we first summarise our current understanding of the approach, combining analytical and numerical insight. In the second part we study SL(C, ℂ) gauge cooling, which was introduced recently as a tool to control complex Langevin dynamics in nonabelian gauge theories. We present new results in Polyakov chain models and in QCD with heavy quarks and compare various adaptive cooling implementations.

Coupled-channel dynamics in the reactions πN → πN, ηN, KΛ, KΣ

Abstract: Elastic πN scattering and the world data of the family of reactions π − p → ηn, K 0 Λ, K 0 Σ 0, K + Σ −, and π + p → K + Σ + are described simultaneously in an analytic, unitary, coupled-channel approach. SU(3) flavor symmetry is used to relate both the t - and the u - channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. Partial-wave amplitudes are determined and the resonance content is extracted from the analytic continuation, including resonance positions and branching ratios, and possible sources of uncertainties are discussed. The results provide the final-state interactions for the ongoing analysis of photo- and electroproduction data.

The Miniball spectrometer

Abstract: The Miniball germanium detector array has been operational at the REX (Radioactive ion beam EXperiment) post accelerator at the Isotope Separator On-Line facility ISOLDE at CERN since 2001. During the last decade, a series of successful Coulomb excitation and transfer reaction studies have been performed with this array, utilizing the unique and high-quality radioactive ion beams which are available at ISOLDE. In this article, an overview is given of the technical details of the full Miniball setup, including a description of the \(\gamma\)-ray and particle detectors, beam monitoring devices and methods to deal with beam contamination. The specific timing properties of the REX-ISOLDE facility are highlighted to indicate the sensitivity that can be achieved with the full Miniball setup. The article is finalized with a summary of some physics highlights at REX-ISOLDE and the utilization of the Miniball germanium detectors at other facilities.

Compton scattering from the proton in an effective field theory with explicit Delta degrees of freedom

Abstract: We analyse the proton Compton-scattering differential cross section for photon energies up to 325 MeV using Chiral Effective Field Theory (χEFT) and extract new values for the electric and magnetic polarisabilities of the proton. Our approach builds in the key physics in two different regimes: photon energies ω ≲ m π (“low energy”), and the higher energies where the Δ(1232) resonance plays a key role. The Compton amplitude is complete at N4LO, $$\mathcal{O}\left( {e^2 \delta ^4 } \right)$$ , in the low-energy region, and at NLO, $$\mathcal{O}\left( {e^2 \delta ^0 } \right)$$ , in the resonance region. Throughout, the Delta-pole graphs are dressed with π N loops and γNΔ vertex corrections. A statistically consistent database of proton Compton experiments is used to constrain the free parameters in our amplitude: the M1 γNΔ transition strength b 1 (which is fixed in the resonance region) and the polarisabilities α E1 and β M1 (which are fixed from data below 170 MeV). In order to obtain a reasonable fit, we find it necessary to add the spin polarisability γ M1M1 as a free parameter, even though it is, strictly speaking, predicted in χEFT at the order to which we work. We show that the fit is consistent with the Baldin sum rule, and then use that sum rule to constrain α E1 + β M1. In this way we obtain α E1 = [10.65 ± 0.35(stat) ± 0.2(Baldin) ± 0.3(theory)] × 10−4 fm3 and β M1 = [3.15 ∓ 0.35(state) ± 0.2(Baldin) ∓ 0.3()theory] × 10−4 fm3, with χ2 = 113.2 for 135 degrees of freedom. A detailed rationale for the theoretical uncertainties assigned to this result is provided.

High-accuracy determination of the neutron flux at n_TOF

Abstract: The neutron flux of the n_TOF facility at CERN was measured, after installation of the new spallation target, with four different systems based on three neutron-converting reactions, which represent accepted cross sections standards in different energy regions. A careful comparison and combination of the different measurements allowed us to reach an unprecedented accuracy on the energy dependence of the neutron flux in the very wide range (thermal to 1 GeV) that characterizes the n_TOF neutron beam. This is a pre-requisite for the high accuracy of cross section measurements at n_TOF. An unexpected anomaly in the neutron-induced fission cross section of 235U is observed in the energy region between 10 and 30keV, hinting at a possible overestimation of this important cross section, well above currently assigned uncertainties.

Technical design report for the PANDA (AntiProton Annihilations at Darmstadt) Straw Tube Tracker

Abstract: This document describes the technical layout and the expected performance of the Straw Tube Tracker (STT), the main tracking detector of the PANDA target spectrometer. The STT encloses a Micro-Vertex-Detector (MVD) for the inner tracking and is followed in beam direction by a set of GEM stations. The tasks of the STT are the measurement of the particle momentum from the reconstructed trajectory and the measurement of the specific energy loss for a particle identification. Dedicated simulations with full analysis studies of certain proton-antiproton reactions, identified as being benchmark tests for the whole PANDA scientific program, have been performed to test the STT layout and performance. The results are presented, and the time lines to construct the STT are described.

Quark-gluon plasma connected to finite heat bath

Abstract: We derive entropy formulas for finite reservoir systems, Sq, from universal thermostat independence and obtain the functional form of the corresponding generalized entropy-probability relation. Our result interprets thermodynamically the subsystem temperature, T1, and the index q in terms of the temperature, T , entropy, S , and heat capacity, C of the reservoir as \( T_1=T \exp(-S/C)\) and \( q=1-1/C\) . In the infinite C limit, irrespective of the value of S , the Boltzmann-Gibbs approach is fully recovered. We apply this framework for the experimental determination of the original temperature of a finite thermostat, T , from the analysis of hadron spectra produced in high-energy collisions, by analyzing frequently considered simple models of the quark-gluon plasma.

Systematic analysis of pT -distributions in p + p collisions

Abstract: A systematic analysis of transverse momentum distribution of hadrons produced in ultra-relativistic p + p collisions is presented. We investigate the effective temperature and the entropic index from the non-extensive thermodynamic theory of strong interaction. We conclude that the existence of a limiting effective temperature and of a limiting entropic index is in accordance with experimental data.

The electric dipole moment of the deuteron from the QCD θ-term

Abstract: The two-nucleon contributions to the electric dipole moment (EDM) of the deuteron, induced by the QCD θ-term, are calculated in the framework of effective field theory up-to-and-including next-to- next-to-leading order. In particular we find for the difference of the deuteron EDM and the sum of proton and neutron EDM induced by the QCD θ-term a value of ( 5.4 ±3.9) ¯ ×10 4 efm. The by far dominant uncertainty comes from the CP- and isospin-violating πNN coupling constant.

Simple solution to the Strangeness Horn description puzzle

Abstract: We propose to use the thermal model formulation that employs the multi-component hard-core radii to describe the hadron yield ratios from the low AGS to the highest RHIC energies. It is demonstrated that the variation of the hard-core radii of pions and kaons enable us to drastically improve the fit quality of the measured mid-rapidity data and for the first time to completely describe the Strangeness Horn behavior as the function of the energy of collision without spoiling the fit quality of other ratios. The best global fit is found for an almost vanishing hard-core radius of pions of 0.1fm and for the hard-core radius of kaons being equal to 0.38fm, whereas the hard-core radius of all other mesons is found to be 0.4fm and that one of baryons is equal to 0.2fm. It is argued that the multi-component hadron resonance gas model opens us a principal possibility to determine the second virial coefficients of hadron-hadron interaction.

New experimental limits on the α decays of lead isotopes

Abstract: For the first time ancient Roman lead was used to grow a crystal of PbWO4 , and this crystal has subsequently been used as a cryogenic particle detector. The new device provides independent readout of heat and scintillation light and is able to discriminate between $ \beta/\gamma$ interactions and alpha interactions down to few keV. Stringent limits on the $ \alpha$ decays of the lead isotopes are presented. In particular a limit of $ T_{1/2}>1.4\cdot 10^{20}$ y at a 90% C.L. was evaluated for the $ \alpha$ decay of 204Pb to 200Hg .

Dependence of the triple-alpha process on the fundamental constants of nature

Abstract: We present an ab initio calculation of the quark mass dependence of the ground state energies of 4He , 8Be and 12C , and of the energy of the Hoyle state in 12C . These investigations are performed within the framework of lattice chiral Effective Field Theory. We address the sensitivity of the production rate of carbon and oxygen in red giant stars to the fundamental constants of nature by considering the impact of variations in the light quark masses and the electromagnetic fine-structure constant on the reaction rate of the triple-alpha process. As carbon and oxygen are essential to life as we know it, we also discuss the implications of our findings for an anthropic view of the Universe. We find strong evidence that the physics of the triple-alpha process is driven by alpha clustering, and that shifts in the fundamental parameters at the ≃ 2–3% level are unlikely to be detrimental to the development of life. Tolerance against much larger changes cannot be ruled out at present, given the relatively limited knowledge of the quark mass dependence of the two-nucleon S -wave scattering parameters. Lattice QCD is expected to provide refined estimates of the scattering parameters in the future.

Phenomenology of axial-vector and pseudovector mesons: decays and mixing in the kaonic sector

Abstract: We study the decays of the lightest axial-vector and pseudovector mesons, interpreted as quark-antiquark states, into a vector and a pseudoscalar meson. We show that the quarkonium assignment delivers a good description of the decays and allows also to make further testable predictions. In the kaonic sector, the physical resonances K 1(1270) and K 1(1400) emerge as mixed objects of an axial-vector state K 1,A and a pseudovector state K 1,B . We determine the mixing angle as |θ K | = (33.6 ± 4.3)°, a value compatible with previous studies but with a smaller uncertainty. This result may be helpful for testing models beyond the Standard Model of particle physics in which decays into the kaonic resonances K 1(1270) and K 1(1400) are investigated.

Chiral three-nucleon forces and bound excited states in neutron-rich oxygen isotopes

Abstract: We study the spectra of neutron-rich oxygen isotopes based on chiral two- and three-nucleon interactions. First, we benchmark our many-body approach by comparing ground-state energies to coupled-cluster results for the same two-nucleon interaction, with overall good agreement. We then calculate bound excited states in 21, 22, 23O , focusing on the role of three-nucleon forces, in the standard sd shell and an extended \( sdf_{7/2}p_{3/2}\) valence space. Chiral three-nucleon forces provide important one- and two-body contributions between valence neutrons. We find that both these contributions and an extended valence space are necessary to reproduce key signatures of novel shell evolution, such as the N = 14 magic number and the low-lying states in 21O and 23O , which are too compressed with two-nucleon interactions only. For the extended space calculations, this presents first work based on nuclear forces without adjustments. Future work is needed and open questions are discussed.

Fermion Bag Approach to Fermion Sign Problems

Abstract: The fermion bag approach is a new method to tackle fermion sign problems in lattice field theories. Using this approach it is possible to solve a class of sign problems that seem unsolvable by traditional methods. The new solutions emerge when partition functions are written in terms of fermion bags and bosonic worldlines. In these new variables it is possible to identify hidden pairing mechanisms which lead to the solutions. The new solutions allow us for the first time to use Monte Carlo methods to solve a variety of interesting lattice field theories, thus creating new opportunities for understanding strongly correlated fermion systems.

New search for correlated e+e- pairs in the alpha decay of 241Am

Abstract: A new search for production of correlated $e^{+}e^{-}$ pairs in the $\alpha$ decay of 241Am has been carried out deep underground at the Gran Sasso National Laboratory of the INFN by using pairs of NaI(Tl) detectors of the DAMA/LIBRA set-up The experimental data show an excess of double coincidences of events with energy around 511keV in faced pairs of detectors, which are not explained by known side reactions This measured excess gives a relative activity $\lambda=(470\pm 063)\times 10^{-9}$ for the Internal Pair Production (IPP) with respect to the alpha decay of 241Am; this value is of the same order of magnitude as previous determinations In a conservative approach the upper limit $\lambda < 55\times 10^{-9}$ (90% CL) can be derived It is worth noting that this is the first result on IPP obtained in an underground experiment, and that the $ \lambda$ value obtained in the present work is independent of the live-time estimate

Coherent correlated states and low-energy nuclear reactions in non stationary systems

Abstract: In this paper the universal mechanism of optimization of low-energy nuclear reactions on the basis of coherent correlated states of interacting particles is discussed. The formation of these states is the result of the special nonstationary low-energy action to any one of these interacting particles. We have considered the peculiarities and investigated the efficiency of the creation of a correlated state under monotonous or periodic action on the particle that is situated in the parabolic potential. This method is shown to lead to the rapid formation of a strongly correlated particle state that provides an almost complete clearing of the potential barrier even for a narrow range of oscillator frequency variations. The successful low-energy fusion experiment based on the use of correlated states of interacting particles at laser irradiation is discussed.

Phase transitions and gluodynamics in 2-colour matter at high density

Abstract: We investigate 2-colour QCD with 2 flavours of Wilson fermion at nonzero temperature T and quark chemical potential μ, with a pion mass of 700 MeV (m π /m ρ = 0.8). From temperature scans at fixed μ we find that the critical temperature for the superfluid to normal transition depends only very weakly on μ above the onset chemical potential, while the deconfinement crossover temperature is clearly decreasing with μ. We find indications of a region of superfluid but deconfined matter at high μ and intermediate T. The static quark potential determined from the Wilson loop is found to exhibit a “string tension” that increases at large μ in the “deconfined” region. The electric (longitudinal) gluon propagator in Landau gauge becomes strongly screened with increasing temperature and chemical potential. The magnetic (transverse) gluon shows little sensitivity to temperature, and exhibits a mild enhancement at intermediate μ before becoming suppressed at large μ.

Analysis of the vector and axialvector Bc mesons with QCD sum rules

Abstract: In this article, we study the vector and axialvector Bc mesons with QCD sum rules, and make reasonable predictions for the masses and decay constants, then calculate the leptonic decay widths. The present predictions for the masses and decay constants can be confronted with the experimental data in the future. We can also take the masses and decay constants as basic input parameters and study other phenomenological quantities with the three-point vacuum correlation functions via QCD sum rules.

Study of multi-nucleon transfer reactions in 58, 64 Ni + 207 Pb collisions at the velocity filter SHIP

Abstract: We investigated multi-nucleon transfer reactions in collisions of 58Ni + 207Pb and 64Ni + 207Pb at Coulomb barrier energies. The new aspect is that we used a velocity filter (SHIP at GSI) for the separation of the heavy target-like transfer products from background events. The isotopic identification was performed via the $ \alpha$ decay properties of the reaction products. The goal of the experiment was to study the characteristics of multi-nucleon transfer reactions in the region of heavy nuclei and the applicability of existing separation and detection techniques, which are usually used for identification of heavy fusion-evaporation residues, to heavy transfer products. This was motivated by recent theoretical results from macroscopic-microscopic models which suggest deep inelastic transfer reactions in heavy systems as a means to produce new neutron-rich isotopes in the region of N = 126 and in the region of superheavy nuclei. In this paper we present the isotopic yields, the excitation functions and the excitation energies of the heavy transfer products with Z > 82 as well as the influence of shell effects on the reaction products. The influence of the different neutron numbers of the projectiles is also discussed.

Adiabatic projection method for scattering and reactions on the lattice

Abstract: We demonstrate and test the adiabatic projection method, a general new framework for calculating scattering and reactions on the lattice. The method is based upon calculating a low-energy effective theory for clusters which becomes exact in the limit of large Euclidean projection time. As a detailed example we calculate the adiabatic two-body Hamiltonian for elastic fermion-dimer scattering in lattice effective field theory. Our calculation corresponds to neutron-deuteron scattering in the spin-quartet channel at leading order in pionless effective field theory. We show that the spectrum of the adiabatic Hamiltonian reproduces the spectrum of the original Hamiltonian below the inelastic threshold to arbitrary accuracy. We also show that the calculated s -wave phase shift reproduces the known exact result in the continuum and infinite-volume limits. When extended to more than one scattering channel, the adiabatic projection method can be used to calculate inelastic reactions on the lattice in future work.

GARFIELD + RCo digital upgrade: A modern set-up for mass and charge identification of heavy-ion reaction products

Abstract: An upgraded GARFIELD + Ring Counter (RCo) apparatus is presented with improved performances as far as electronics and detectors are concerned. On the one hand fast sampling digital read out has been extended to all detectors, allowing for an important simplification of the signal processing chain together with an enriched extracted information. On the other hand a relevant improvement has been made in the forward part of the set-up (RCo): an increased granularity of the CsI(Tl) crystals and a higher homogeneity in the silicon detector resistivity. The renewed performances of the GARFIELD + RCo array make it suitable for nuclear reaction measurements both with stable and with Radioactive Ion Beams (RIB), like the ones planned for the SPES facility, where the physics of isospin can be studied.

Nucleon-nucleon interactions via Lattice QCD: Methodology HAL QCD approach to extract hadronic interactions in lattice QCD

Abstract: We review the potential method in lattice QCD, which has recently been proposed to extract nucleon-nucleon interactions via numerical simulations. We focus on the methodology of this approach by emphasizing the strategy of the potential method, the theoretical foundation behind it, and special numerical techniques. We compare the potential method with the standard finite volume method in lattice QCD, in order to make pros and cons of the approach clear. We also present several numerical results for nucleon-nucleon potentials.

Fusion of 60 Ni + 100 Mo near and below the Coulomb barrier

Abstract: The fusion excitation function of 60 Ni + 100 Mo has been measured from above the Coulomb barrier down to a cross section around 2 μb, looking for coupling and hindrance effects in this soft medium-mass system with positive Q-values for several neutron transfer channels. A comparison is made with previous results for 64 Ni + 100 Mo where no Q > 0 transfer channels exist and the hindrance effect is quite clear. The two excitation functions are very similar, as well as the corresponding logarithmic derivatives showing analogous saturations below the barrier. It appears that transfer couplings to Q > 0 channels seem to play a marginal role near and below the barrier for 60 Ni + 100 Mo , even if measurements of cross sections lower than 1 μb would be needed also for this system. Coupled-channels calculations confirm these observations and indicate that multi-phonon excitations dominate the fusion dynamics in the whole measured energy range.

Neutron-induced background by an α-beam incident on a deuterium gas target and its implications for the study of the 2H(α, γ)6Li reaction at LUNA

Abstract: The production of the stable isotope 6Li in standard Big Bang nucleosynthesis has recently attracted much interest. Recent observations in metal-poor stars suggest that a cosmological 6Li plateau may exist. If true, this plateau would come in addition to the well-known Spite plateau of 7Li abundances and would point to a predominantly primordial origin of 6Li , contrary to the results of standard Big Bang nucleosynthesis calculations. Therefore, the nuclear physics underlying Big Bang 6Li production must be revisited. The main production channel for 6Li in the Big Bang is the 2H(α,γ)6Li reaction. The present work reports on neutron-induced effects in a high-purity germanium detector that were encountered in a new study of this reaction. In the experiment, an α-beam from the underground accelerator LUNA in Gran Sasso, Italy, and a windowless deuterium gas target are used. A low neutron flux is induced by energetic deuterons from elastic scattering and, subsequently, the 2H(d,n)3He reaction. Due to the ultra-low laboratory neutron background at LUNA, the effect of this weak flux of 2-3MeV neutrons on well-shielded high-purity germanium detectors has been studied in detail. Data have been taken at 280 and 400keV α-beam energy and for comparison also using an americium-beryllium neutron source.

Dyson-Schwinger approach to color superconductivity at finite temperature and density

Abstract: We investigate the phases of dense QCD matter at finite temperature with Dyson-Schwinger equations for the quark propagator for N f = 2 + 1 flavors. For the gluon propagator we take a fit to quenched lattice data and add quark-loop effects perturbatively in a hard-thermal-loop-hard-dense-loop approximation. We consider 2SC and CFL-like pairing with chiral up and down quarks and massive strange quarks and present results for the condensates and the phase diagram. We find a dominant CFL phase at chemical potentials larger than 500-600MeV. At lower values of the chemical potential we find a 2SC phase, which also exists in a small band at higher temperatures for larger chemical potentials. With values of 20–30 MeV, the critical temperatures to the normal phase turn out to be quite small.

Helicity amplitudes for photoexcitation of nucleon resonances off neutrons

Abstract: The helicity amplitudes $A^{1/2}_{n}$ and $A^{3/2}_{n}$ for the photoexcitation of nucleon resonances off neutrons are determined in a multichannel partial wave analysis.

Hidden beauty baryon states in the local hidden gauge approach with heavy quark spin symmetry

Abstract: Using a coupled channel unitary approach, combining the heavy quark spin symmetry and the dynamics of the local hidden gauge, we investigate the meson-baryon interaction with hidden beauty and obtain several new states of N ∗ around 11 GeV. We consider the basis of states ηbN, �N, Bb, Bb, B ∗ �b, B ∗ �b, B ∗ � ∗ and find four basic bound states which correspond to Bb, B� ∗ , B ∗ �b and B ∗ � ∗ , decaying mostly into ηbN andN and with a binding energy about 50−130 MeV with respect to the thresholds of the corresponding channel. All of them have isospin I = 1/2, and there are no bound states or resonances in I = 3/2. The Bb state appears in J = 1/2, the B� ∗ in J = 3/2, the B ∗ �b appears nearly degenerate in J = 1/2, 3/2 and the B ∗ � ∗ appears nearly degenerate in J = 1/2, 3/2, 5/2. These states have a width from 6 ∼ 45 MeV except for the one in J = 5/2 which has zero width since it cannot decay into any of the states of the basis chosen.

Neutron total cross section measurements of gold and tantalum at the nELBE photoneutron source

Abstract: Neutron total cross sections of 197Au and natTa have been measured at the nELBE photoneutron source in the energy range 0.1–10MeV with a statistical uncertainty of up to 2% and a total systematic uncertainty of 1%. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5ps) with a short flight path of 7m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.