35,444 research outputs found

### Exclusive two-photon annihilation at large energy or large virtuality

I review recent progress in the theory of gamma gamma annihilation into meson
or baryon pairs at large energy, and of the process gamma* gamma* -> pi0 at
large photon virtuality.Comment: 6 pages, 4 figures. Presented at PHOTON 2003, Frascati, Italy, 7-11
April 200

### News from Cosmic Gamma-ray Line Observations

The measurement of gamma rays at MeV energies from cosmic radioactivities is
one of the key tools for nuclear astrophysics, in its study of nuclear
reactions and how they shape objects such as massive stars and supernova
explosions. Additionally, the unique gamma-ray signature from the annihilation
of positrons falls into this same astronomical window, and positrons are often
produced from radioactive beta decays. Nuclear gamma-ray telescopes face
instrumental challenges from penetrating gamma rays and cosmic-ray induced
backgrounds. But the astrophysical benefits of such efforts are underlined by
the discoveries of nuclear gamma~rays from the brightest of the expected
sources. In recent years, both thermonuclear and core-collapse supernova
radioactivity gamma~rays have been measured in spectral detail, and complement
conventional supernova observations with measurements of origins in deep
supernova interiors, from the decay of $^{56}$Ni, $^{56}$Co, and $^{44}$Ti. The
diffuse afterglow in gamma rays of radioactivity from massive-star
nucleosynthesis is analysed on the large (galactic) scale, with findings
important for recycling of matter between successive stellar generations: From
$^{26}$Al gamma-ray line spectroscopy, interstellar cavities and superbubbles
have been recognised in their importance for ejecta transport and recycling.
Diffuse galactic emissions from radioactivity and positron-annihilation
$\gamma$~rays should be connected to nucleosynthesis sources: Recently new
light has been shed on this connection, among others though different
measurements of radioactive $^{60}$Fe, and through spectroscopy of positron
annihilation gamma~rays from a flaring microquasar and from different parts of
our Galaxy.Comment: Invited talk at the international symposium "Nuclei in the Cosmos
XIV", June 2016, at Niigata, Japan. Six pages, two figures. Accepted for
publication in JPS (Japan Physical Society) Conference Proceedings
(http://jpscp.jps.jp/

### Critical Casimir amplitudes for $n$-component $\phi^4$ models with O(n)-symmetry breaking quadratic boundary terms

Euclidean $n$-component $\phi^4$ theories whose Hamiltonians are O(n)
symmetric except for quadratic symmetry breaking boundary terms are studied in
films of thickness $L$. The boundary terms imply the Robin boundary conditions
$\partial_n\phi_\alpha =\mathring{c}^{(j)}_\alpha \phi_\alpha$ at the boundary
planes $\mathfrak{B}_{j=1,2}$ at $z=0$ and $z=L$. Particular attention is paid
to the cases in which $m_j$ of the $n$ variables $\mathring{c}^{(j)}_\alpha$
take the special value $\mathring{c}_{m_j\text{-sp}}$ corresponding to critical
enhancement while the remaining ones are subcritically enhanced. Under these
conditions, the semi-infinite system bounded by $\mathfrak{B}_j$ has a
multicritical point, called $m_j$-special, at which an $O(m_j)$ symmetric
critical surface phase coexists with the O(n) symmetric bulk phase, provided
$d$ is sufficiently large. The $L$-dependent part of the reduced free energy
per area behaves as $\Delta_C/L^{d-1}$ as $L\to\infty$ at the bulk critical
point. The Casimir amplitudes $\Delta_C$ are determined for small
$\epsilon=4-d$ in the general case where $m_{c,c}$ components $\phi_\alpha$ are
critically enhanced at both boundary planes, $m_{c,D} + m_{D,c}$ components are
enhanced at one plane but satisfy asymptotic Dirichlet boundary conditions at
the respective other, and the remaining $m_{D,D}$ components satisfy asymptotic
Dirichlet boundary conditions at both $\mathfrak{B}_j$. Whenever $m_{c,c}>0$,
these expansions involve integer and fractional powers $\epsilon^{k/2}$ with
$k\ge 3$ (mod logarithms). Results to $O(\epsilon^{3/2})$ for general values of
$m_{c,c}$, $m_{c,D}+m_{D,c}$, and $m_{D,D}$ are used to estimate the $\Delta_C$
of 3D Heisenberg systems with surface spin anisotropies when $(m_{c,c},
m_{c,D}+ m_{D,c}) = (1,0)$, $(0,1)$, and $(1,1)$.Comment: Latex source file with 5 eps files; version with minor amendments and
corrected typo

### Energy Momentum Tensor in Conformal Field Theories Near a Boundary

The requirements of conformal invariance for the two point function of the
energy momentum tensor in the neighbourhood of a plane boundary are
investigated, restricting the conformal group to those transformations leaving
the boundary invariant. It is shown that the general solution may contain an
arbitrary function of a single conformally invariant variable $v$, except in
dimension 2. The functional dependence on $v$ is determined for free scalar and
fermion fields in arbitrary dimension $d$ and also to leading order in the
\vep expansion about $d=4$ for the non Gaussian fixed point in $\phi^4$
theory. The two point correlation function of the energy momentum tensor and a
scalar field is also shown to have a unique expression in terms of $v$ and the
overall coefficient is determined by the operator product expansion. The energy
momentum tensor on a general curved manifold is further discussed by
considering variations of the metric. In the presence of a boundary this
procedure naturally defines extra boundary operators. By considering
diffeomorphisms these are related to components of the energy momentum tensor
on the boundary. The implications of Weyl invariance in this framework are also
derived.Comment: 22 pages, TeX with epsf.tex, DAMTP/93-1. (original uuencoded file was
corrupted enroute - resubmitted version has uuencoded figures pasted to the
ended of the Plain TeX file

### Dynamic surface scaling behavior of isotropic Heisenberg ferromagnets

The effects of free surfaces on the dynamic critical behavior of isotropic
Heisenberg ferromagnets are studied via phenomenological scaling theory,
field-theoretic renormalization group tools, and high-precision computer
simulations. An appropriate semi-infinite extension of the stochastic model J
is constructed, the boundary terms of the associated dynamic field theory are
identified, its renormalization in d <= 6 dimensions is clarified, and the
boundary conditions it satisfies are given. Scaling laws are derived which
relate the critical indices of the dynamic and static infrared singularities of
surface quantities to familiar static bulk and surface exponents. Accurate
computer-simulation data are presented for the dynamic surface structure
factor; these are in conformity with the predicted scaling behavior and could
be checked by appropriate scattering experiments.Comment: 9 pages, 2 figure

### Hard exclusive reactions and hadron structure

The generalized Bjorken regime of exclusive reactions opens new ways to
explore the hadron structure. We shortly review the present status of this
domain where generalized parton distributions, generalized distribution
amplitudes and transition distribution amplitudes describe various aspects of
confinement physics.Comment: 7 pages LATEX, 5 figures, invited talk at the Vth Conference on
Hadronic Physics held at ICTP, Trieste, Italy, May 200

### Exclusive production of pentaquarks in the scaling regime

We investigate two exclusive reactions with a Theta+ pentaquark in the final
state: electroproduction of a K meson on the nucleon, and K+ scattering on a
neutron target producing a lepton pair. These reactions offer unique
opportunities to investigate the structure of pentaquark baryons at parton
level. We discuss the generalized parton distributions for the N --> Theta+
transition and give the leading order amplitude for these processes in the
Bjorken regime.Comment: 5 pages in LATEX, 1 .eps figure, talk at the Baryons04 Conference,
October 25-29, 2004, Palaiseau, Franc

### Boundary critical behavior at m-axial Lifshitz points for a boundary plane parallel to the modulation axes

The critical behavior of semi-infinite $d$-dimensional systems with
$n$-component order parameter $\bm{\phi}$ and short-range interactions is
investigated at an $m$-axial bulk Lifshitz point whose wave-vector instability
is isotropic in an $m$-dimensional subspace of $\mathbb{R}^d$. The associated
$m$ modulation axes are presumed to be parallel to the surface, where $0\le
m\le d-1$. An appropriate semi-infinite $|\bm{\phi}|^4$ model representing the
corresponding universality classes of surface critical behavior is introduced.
It is shown that the usual O(n) symmetric boundary term $\propto \bm{\phi}^2$
of the Hamiltonian must be supplemented by one of the form $\mathring{\lambda}
\sum_{\alpha=1}^m(\partial\bm{\phi}/\partial x_\alpha)^2$ involving a
dimensionless (renormalized) coupling constant $\lambda$. The implied boundary
conditions are given, and the general form of the field-theoretic
renormalization of the model below the upper critical dimension
$d^*(m)=4+{m}/{2}$ is clarified. Fixed points describing the ordinary, special,
and extraordinary transitions are identified and shown to be located at a
nontrivial value $\lambda^*$ if $\epsilon\equiv d^*(m)-d>0$. The surface
critical exponents of the ordinary transition are determined to second order in
$\epsilon$. Extrapolations of these $\epsilon$ expansions yield values of these
exponents for $d=3$ in good agreement with recent Monte Carlo results for the
case of a uniaxial ($m=1$) Lifshitz point. The scaling dimension of the surface
energy density is shown to be given exactly by $d+m (\theta-1)$, where
$\theta=\nu_{l4}/\nu_{l2}$ is the anisotropy exponent.Comment: revtex4, 31 pages with eps-files for figures, uses texdraw to
generate some graphs; to appear in PRB; v2: some references and additional
remarks added, labeling in figure 1 and some typos correcte

### Generalized parton distributions: recent results

I review progress on selected issues connected with generalized parton
distributions. Topics range from the description of hard exclusive reactions to
the spatial distribution of quarks in the nucleon and the contribution of their
orbital angular momentum to the nucleon spin.Comment: 9 pages, 5 figures. To appear in the proceedings of the Particles and
Nuclei International Conference (PANIC 05), Santa Fe, NM, USA, 24-28 Oct 200

### Gamma-ray line measurements from supernova explosions

Gamma ray lines are expected to be emitted as part of the afterglow of
supernova explosions, because radioactive decay of freshly synthesised nuclei
occurs. Significant radioactive gamma ray line emission is expected from 56Ni
and 44Ti decay on time scales of the initial explosion (56Ni, tau~days) and the
young supernova remnant (44Ti,tau~90 years). Less specific, and rather
informative for the supernova population as a whole, are lessons from longer
lived isotopes such as 26Al and 60Fe. From isotopes of elements heavier than
iron group elements, any interesting gamma-ray line emission is too faint to be
observable. Measurements with space-based gamma-ray telescopes have obtained
interesting gamma ray line emissions from two core collapse events, Cas A and
SN1987A, and one thermonuclear event, SN2014J. We discuss INTEGRAL data from
all above isotopes, including all line and continuum signatures from these two
objects, and the surveys for more supernovae, that have been performed by gamma
ray spectrometry. Our objective here is to illustrate what can be learned from
gamma-ray line emission properties about the explosions and their astrophysics.Comment: 7 pages, 4 figures. IAU Symposium 331 "SN1987A 30 years after", La
Reunion, Feb. 2017. Accepted for publication in IAU Conf Pro

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