8,146 research outputs found

### Timelike Compton scattering: exclusive photoproduction of lepton pairs

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

Systems described by an O(n) symmetrical $\phi^4$ Hamiltonian are considered
in a $d$-dimensional film geometry at their bulk critical points. The critical
Casimir forces between the film's boundary planes $\mathfrak{B}_j, j=1,2$, are
investigated as functions of film thickness $L$ for generic symmetry-preserving
boundary conditions $\partial_n\bm{\phi}=\mathring{c}_j\bm{\phi}$. The
$L$-dependent part of the reduced excess free energy per cross-sectional area
takes the scaling form $f_{\text{res}}\approx
D(c_1L^{\Phi/\nu},c_2L^{\Phi/\nu})/L^{d-1}$ when $d<4$, where $c_i$ are scaling
fields associated with the variables $\mathring{c}_i$, and $\Phi$ is a surface
crossover exponent. Explicit two-loop renormalization group results for the
function $D(\mathsf{c}_1,\mathsf{c}_2)$ at $d=4-\epsilon$ dimensions are
presented. These show that (i) the Casimir force can have either sign,
depending on $\mathsf{c}_1$ and $\mathsf{c}_2$, and (ii) for appropriate
choices of the enhancements $\mathring{c}_j$, crossovers from attraction to
repulsion and vice versa occur as $L$ 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

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

Systems with an O(n) symmetrical Hamiltonian are considered in a
$d$-dimensional slab geometry of macroscopic lateral extension and finite
thickness $L$ that undergo a continuous bulk phase transition in the limit
$L\to\infty$. The effective forces induced by thermal fluctuations at and above
the bulk critical temperature $T_{c,\infty}$ (thermodynamic Casimir effect) are
investigated below the upper critical dimension $d^*=4$ 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 $T_{c,\infty}$ make conventional
RG-improved perturbation theory in $4-\epsilon$ 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 $\mathsf{L}\equiv L/\xi_\infty$, where
$\xi_\infty$ is the bulk correlation length. Scaling functions of the
$L$-dependent residual free energy per area are obtained whose $\mathsf{L}\to0$
limits are in conformity with previous results for the Casimir amplitudes
$\Delta_C$ to $O(\epsilon^{3/2})$ and display a more reasonable
small-$\mathsf{L}$ behavior inasmuch as they approach the critical value
$\Delta_C$ monotonically as $\mathsf{L}\to 0$.Comment: 23 pages, 10 figure

### Monte Carlo simulation results for critical Casimir forces

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

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 $d=4-\epsilon$ dimensions. At
$d=4-\epsilon$ we extend, up to the next-to-leading order, the previous
first-order results of the $\sqrt{\epsilon}$ 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

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

Gamma-rays from the decay of $^{26}$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 H$\alpha$ emission. Advantages and disadvantages of using
$^{26}$Al gamma-ray measurements as a tracer of the massive star formation rate
are analyzed. Estimates of the Galactic $^{26}$Al mass derived from COMPTEL
measurements are coupled with a simple, analytical model of the $^{26}$Al
injection rate from massive stars and restrict the Galaxy's recent star
formation rate to \hbox{5 $\pm$ 4 M\sun yr$^{-1}$}. In addition, we show that
the derived $^{26}$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 $^{60}$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

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

Systems described by an O(n) symmetrical $\phi^4$ Hamiltonian are considered
in a $d$-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 $\propto \bm{\phi}^2$ corresponding
to subcritical or critical enhancement of the surface interactions. This
translates into mesoscopic boundary conditions of the generic
symmetry-preserving Robin type $\partial_n\bm{\phi}=\mathring{c}_j\bm{\phi}$.
RG-improved perturbation theory and Abel-Plana techniques are used to compute
the $L$-dependent part $f_{\mathrm{res}}$ of the reduced excess free energy per
film area $A\to\infty$ to two-loop order. When $d<4$, it takes the scaling
form $f_{\mathrm{res}}\approx D(c_1L^{\Phi/\nu},c_2L^{\Phi/\nu})/L^{d-1}$ as
$L\to\infty$, where $c_i$ are scaling fields associated with the
surface-enhancement variables $\mathring{c}_i$, while $\Phi$ is a standard
surface crossover exponent. The scaling function $D(\mathsf{c}_1,\mathsf{c}_2)$
and its analogue $\mathcal{D}(\mathsf{c}_1,\mathsf{c}_2)$ for the Casimir force
are determined via expansion in $\epsilon=4-d$ and extrapolated to $d=3$
dimensions. In the special case $\mathsf{c}_1=\mathsf{c}_2=0$, the expansion
becomes fractional. Consistency with the known fractional expansions of D(0,0)
and $\mathcal{D}(0,0)$ to order $\epsilon^{3/2}$ is achieved by appropriate
reorganisation of RG-improved perturbation theory. For appropriate choices of
$c_1$ and $c_2$, the Casimir forces can have either sign. Furthermore,
crossovers from attraction to repulsion and vice versa may occur as $L$
increases.Comment: Latex source file, 40 pages, 9 figure

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