12,743 research outputs found
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 Ni, Co, and 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
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
~rays should be connected to nucleosynthesis sources: Recently new
light has been shed on this connection, among others though different
measurements of radioactive 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/
Double parton scattering theory overview
The dynamics of double hard scattering in proton-proton collisions is quite
involved compared with the familiar case of single hard scattering. In this
contribution, we review our theoretical understanding of double hard scattering
and of its interplay with other reaction mechanisms.Comment: 25 pages, 7 figures. Prepared for: Multiple Parton Interactions at
the LHC, Eds. P. Bartalini and J. R. Gaunt, World Scientific, Singapor
Double parton scattering in the ultraviolet: addressing the double counting problem
In proton-proton collisions there is a smooth transition between the regime
of double parton scattering, initiated by two pairs of partons at a large
relative distance, and the regime where a single parton splits into a parton
pair in one or both protons. We present a scheme for computing both
contributions in a consistent and practicable way.Comment: 5 pages, 3 figures. To appear in the proeceedings of MPI@LHC 2015,
Trieste, Italy, 23-27 November 201
The Al Gamma-ray Line from Massive-Star Regions
The measurement of gamma rays from the diffuse afterglow of radioactivity
originating in massive-star nucleosynthesis is considered a laboratory for
testing models, when specific stellar groups are investigated, at known
distance and with well-constrained stellar population. Regions which have been
exploited for such studies include Cygnus, Carina, Orion, and
Scorpius-Centaurus. The Orion region hosts the Orion OB1 association and its
subgroups at about 450~pc distance. We report the detection of Al gamma
rays from this region with INTEGRAL/SPI.Comment: Contribution to Symposium "Nuclei in the Cosmos XIV", Niigata, Japan,
Jun 2016; 3 pages, 2 figures; accepted for publication in JPS (Japan Physical
Society) Conference Proceedings http://jpscp.jps.jp
Double hard scattering without double counting
Double parton scattering in proton-proton collisions includes kinematic
regions in which two partons inside a proton originate from the perturbative
splitting of a single parton. This leads to a double counting problem between
single and double hard scattering. We present a solution to this problem, which
allows for the definition of double parton distributions as operator matrix
elements in a proton, and which can be used at higher orders in perturbation
theory. We show how the evaluation of double hard scattering in this framework
can provide a rough estimate for the size of the higher-order contributions to
single hard scattering that are affected by double counting. In a numeric
study, we identify situations in which these higher-order contributions must be
explicitly calculated and included if one wants to attain an accuracy at which
double hard scattering becomes relevant, and other situations where such
contributions may be neglected.Comment: 80 pages, 20 figures. v2: clarifications in section 4, extended
section 8, small changes elsewher
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
Probing the evolving massive star population in Orion with kinematic and radioactive tracers
We assemble a census of the most massive stars in Orion, then use stellar
isochrones to estimate their masses and ages, and use these results to
establish the stellar content of Orion's individual OB associations. From this,
our new population synthesis code is utilized to derive the history of the
emission of UV radiation and kinetic energy of the material ejected by the
massive stars, and also follow the ejection of the long-lived radioactive
isotopes 26Al and 60Fe. In order to estimate the precision of our method, we
compare and contrast three distinct representations of the massive stars. We
compare the expected outputs with observations of 26Al gamma-ray signal and the
extent of the Eridanus cavity. We find an integrated kinetic energy emitted by
the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent
with the energy thought to be required to create the Eridanus superbubble. We
also find good agreement between our model and the observed 26Al signal,
estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion
region. Our population synthesis approach is demonstrated for the Orion region
to reproduce three different kinds of observable outputs from massive stars in
a consistent manner: Kinetic energy as manifested in ISM excavation, ionization
as manifested in free-free emission, and nucleosynthesis ejecta as manifested
in radioactivity gamma-rays. The good match between our model and the
observables does not argue for considerable modifications of mass loss. If
clumping effects turn out to be strong, other processes would need to be
identified to compensate for their impact on massive-star outputs. Our
population synthesis analysis jointly treats kinematic output and the return of
radioactive isotopes, which proves a powerful extension of the methodology that
constrains feedback from massive stars.Comment: Accepted for publication in A&A, 10 page
The Overlap Representation of Skewed Quark and Gluon Distributions
Within the framework of light-cone quantisation we derive the complete and
exact overlap representation of skewed parton distributions for unpolarised and
polarised quarks and gluons. Symmetry properties and phenomenological
applications are discussed.Comment: LaTex, 36 pages. v2: incorrect paper attached originally. v3: erratum
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Solutions of the motion of synchronous satellites with arbitrary eccentricity and inclination
A first order, semianalytical theory for the long term motion of resonant satellites is presented. The theory is valid for all eccentricities and inclinations and for all commensurability ratios. The method allows the inclusion of all the zonal and tesseral harmonics as well as luni solar perturbations and radiation pressure. The method is applied to a synchronous satellite including only the J sub 2 and J sub 22 harmonics. Global, long term solutions for this problem, eccentricity, argument of perigee, and inclination are obtained
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