8,146 research outputs found

    Timelike Compton scattering: exclusive photoproduction of lepton pairs

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    We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, gamma p -> l+ l- p. This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in kinematics where the Compton process should be dominated by quark exchange.Comment: 31 pages, 17 figure

    Crossover from Attractive to Repulsive Casimir Forces and Vice Versa

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    Systems described by an O(n) symmetrical ϕ4\phi^4 Hamiltonian are considered in a dd-dimensional film geometry at their bulk critical points. The critical Casimir forces between the film's boundary planes Bj,j=1,2\mathfrak{B}_j, j=1,2, are investigated as functions of film thickness LL for generic symmetry-preserving boundary conditions nϕ=c˚jϕ\partial_n\bm{\phi}=\mathring{c}_j\bm{\phi}. The LL-dependent part of the reduced excess free energy per cross-sectional area takes the scaling form fresD(c1LΦ/ν,c2LΦ/ν)/Ld1f_{\text{res}}\approx D(c_1L^{\Phi/\nu},c_2L^{\Phi/\nu})/L^{d-1} when d<4d<4, where cic_i are scaling fields associated with the variables c˚i\mathring{c}_i, and Φ\Phi is a surface crossover exponent. Explicit two-loop renormalization group results for the function D(c1,c2)D(\mathsf{c}_1,\mathsf{c}_2) at d=4ϵd=4-\epsilon dimensions are presented. These show that (i) the Casimir force can have either sign, depending on c1\mathsf{c}_1 and c2\mathsf{c}_2, and (ii) for appropriate choices of the enhancements c˚j\mathring{c}_j, crossovers from attraction to repulsion and vice versa occur as LL increases.Comment: 4 RevTeX pages, 2 eps figures; minor misprints corrected and 3 references adde

    Surface critical behavior of driven diffusive systems with open boundaries

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    Using field theoretic renormalization group methods we study the critical behavior of a driven diffusive system near a boundary perpendicular to the driving force. The boundary acts as a particle reservoir which is necessary to maintain the critical particle density in the bulk. The scaling behavior of correlation and response functions is governed by a new exponent eta_1 which is related to the anomalous scaling dimension of the chemical potential of the boundary. The new exponent and a universal amplitude ratio for the density profile are calculated at first order in epsilon = 5-d. Some of our results are checked by computer simulations.Comment: 10 pages ReVTeX, 6 figures include

    Thermodynamic Casimir effects involving interacting field theories with zero modes

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    Systems with an O(n) symmetrical Hamiltonian are considered in a dd-dimensional slab geometry of macroscopic lateral extension and finite thickness LL that undergo a continuous bulk phase transition in the limit LL\to\infty. The effective forces induced by thermal fluctuations at and above the bulk critical temperature Tc,T_{c,\infty} (thermodynamic Casimir effect) are investigated below the upper critical dimension d=4d^*=4 by means of field-theoretic renormalization group methods for the case of periodic and special-special boundary conditions, where the latter correspond to the critical enhancement of the surface interactions on both boundary planes. As shown previously [\textit{Europhys. Lett.} \textbf{75}, 241 (2006)], the zero modes that are present in Landau theory at Tc,T_{c,\infty} make conventional RG-improved perturbation theory in 4ϵ4-\epsilon dimensions ill-defined. The revised expansion introduced there is utilized to compute the scaling functions of the excess free energy and the Casimir force for temperatures T\geqT_{c,\infty} as functions of LL/ξ\mathsf{L}\equiv L/\xi_\infty, where ξ\xi_\infty is the bulk correlation length. Scaling functions of the LL-dependent residual free energy per area are obtained whose L0\mathsf{L}\to0 limits are in conformity with previous results for the Casimir amplitudes ΔC\Delta_C to O(ϵ3/2)O(\epsilon^{3/2}) and display a more reasonable small-L\mathsf{L} behavior inasmuch as they approach the critical value ΔC\Delta_C monotonically as L0\mathsf{L}\to 0.Comment: 23 pages, 10 figure

    Monte Carlo simulation results for critical Casimir forces

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    The confinement of critical fluctuations in soft media induces critical Casimir forces acting on the confining surfaces. The temperature and geometry dependences of such forces are characterized by universal scaling functions. A novel approach is presented to determine them for films via Monte Carlo simulations of lattice models. The method is based on an integration scheme of free energy differences. Our results for the Ising and the XY universality class compare favourably with corresponding experimental results for wetting layers of classical binary liquid mixtures and of 4He, respectively.Comment: 14 pages, 5 figure

    Surface critical behavior of random systems at the ordinary transition

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    We calculate the surface critical exponents of the ordinary transition occuring in semi-infinite, quenched dilute Ising-like systems. This is done by applying the field theoretic approach directly in d=3 dimensions up to the two-loop approximation, as well as in d=4ϵd=4-\epsilon dimensions. At d=4ϵd=4-\epsilon we extend, up to the next-to-leading order, the previous first-order results of the ϵ\sqrt{\epsilon} expansion by Ohno and Okabe [Phys.Rev.B 46, 5917 (1992)]. In both cases the numerical estimates for surface exponents are computed using Pade approximants extrapolating the perturbation theory expansions. The obtained results indicate that the critical behavior of semi-infinite systems with quenched bulk disorder is characterized by the new set of surface critical exponents.Comment: 11 pages, 11 figure

    Diffusion Enhancement in Core-softened fluid confined in nanotubes

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    We study the effect of confinement in the dynamical behavior of a core-softened fluid. The fluid is modeled as a two length scales potential. This potential in the bulk reproduces the anomalous behavior observed in the density and in the diffusion of liquid water. A series of NpTNpT Molecular Dynamics simulations for this two length scales fluid confined in a nanotube were performed. We obtain that the diffusion coefficient increases with the increase of the nanotube radius for wide channels as expected for normal fluids. However, for narrow channels, the confinement shows an enhancement in the diffusion coefficient when the nanotube radius decreases. This behavior, observed for water, is explained in the framework of the two length scales potential.Comment: 17 pages, 8 figures, accept for publication at J. Chem. Phy

    Constraints from 26^{26}Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core Collapse Supernovae Rate

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    Gamma-rays from the decay of 26^{26}Al offer a stringent constraint on the Galaxy's global star formation rate over the past million years, supplementing other methods for quantifying the recent Galactic star formation rate, such as equivalent widths of Hα\alpha emission. Advantages and disadvantages of using 26^{26}Al gamma-ray measurements as a tracer of the massive star formation rate are analyzed. Estimates of the Galactic 26^{26}Al mass derived from COMPTEL measurements are coupled with a simple, analytical model of the 26^{26}Al injection rate from massive stars and restrict the Galaxy's recent star formation rate to \hbox{5 ±\pm 4 M\sun yr1^{-1}}. In addition, we show that the derived 26^{26}Al mass implies a present day \hbox{Type II + Ib} supernovae rate of 3.4 ±\pm 2.8 per century, which seems consistent with other independent estimates of the Galactic core collapse supernova rate. If some independent measure of the massive star initial mass function or star formation rate or \hbox{Type II + Ib} supernovae rate were to become available (perhaps through estimates of the Galactic 60^{60}Fe mass), then a convenient way to restrain, or possibly determine, the other parameters is presented.Comment: 11 pages including 1 figure, ApJ in pres

    Generalized parton distributions in the deuteron

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    We introduce generalized quark and gluon distributions in the deuteron, which can be measured in exclusive processes like deeply virtual Compton scattering and meson electroproduction. We discuss the basic properties of these distributions, and point out how they probe the interplay of nucleon and parton degrees of freedom in the deuteron wave function

    Critical Casimir effect in films for generic non-symmetry-breaking boundary conditions

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    Systems described by an O(n) symmetrical ϕ4\phi^4 Hamiltonian are considered in a dd-dimensional film geometry at their bulk critical points. A detailed renormalization-group (RG) study of the critical Casimir forces induced between the film's boundary planes by thermal fluctuations is presented for the case where the O(n) symmetry remains unbroken by the surfaces. The boundary planes are assumed to cause short-ranged disturbances of the interactions that can be modelled by standard surface contributions ϕ2\propto \bm{\phi}^2 corresponding to subcritical or critical enhancement of the surface interactions. This translates into mesoscopic boundary conditions of the generic symmetry-preserving Robin type nϕ=c˚jϕ\partial_n\bm{\phi}=\mathring{c}_j\bm{\phi}. RG-improved perturbation theory and Abel-Plana techniques are used to compute the LL-dependent part fresf_{\mathrm{res}} of the reduced excess free energy per film area AA\to\infty to two-loop order. When d<4d<4, it takes the scaling form fresD(c1LΦ/ν,c2LΦ/ν)/Ld1f_{\mathrm{res}}\approx D(c_1L^{\Phi/\nu},c_2L^{\Phi/\nu})/L^{d-1} as LL\to\infty, where cic_i are scaling fields associated with the surface-enhancement variables c˚i\mathring{c}_i, while Φ\Phi is a standard surface crossover exponent. The scaling function D(c1,c2)D(\mathsf{c}_1,\mathsf{c}_2) and its analogue D(c1,c2)\mathcal{D}(\mathsf{c}_1,\mathsf{c}_2) for the Casimir force are determined via expansion in ϵ=4d\epsilon=4-d and extrapolated to d=3d=3 dimensions. In the special case c1=c2=0\mathsf{c}_1=\mathsf{c}_2=0, the expansion becomes fractional. Consistency with the known fractional expansions of D(0,0) and D(0,0)\mathcal{D}(0,0) to order ϵ3/2\epsilon^{3/2} is achieved by appropriate reorganisation of RG-improved perturbation theory. For appropriate choices of c1c_1 and c2c_2, the Casimir forces can have either sign. Furthermore, crossovers from attraction to repulsion and vice versa may occur as LL increases.Comment: Latex source file, 40 pages, 9 figure
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