2,350 research outputs found
A -ray determination of the Universe's star-formation history
The light emitted by all galaxies over the history of the Universe produces
the extragalactic background light (EBL) at ultraviolet, optical, and infrared
wavelengths. The EBL is a source of opacity for rays via photon-photon
interactions, leaving an imprint in the spectra of distant -ray
sources. We measure this attenuation using {739} active galaxies and one
gamma-ray burst detected by the {\it Fermi} Large Area Telescope. This allows
us to reconstruct the evolution of the EBL and determine the star-formation
history of the Universe over 90\% of cosmic time. Our star-formation history is
consistent with independent measurements from galaxy surveys, peaking at
redshift . Upper limits of the EBL at the epoch of re-ionization
suggest a turnover in the abundance of faint galaxies at .Comment: Published on Science. This is the authors' version of the manuscrip
On the vacancy decay in endohedral atoms
It is demonstrated that the fullerene shell affects dramatically the
radiative and Auger vacancy decay of an endohedral atom A@C60. The
collectivized electrons of the C60 shell add new possibilities for radiative
and non-radiative decays similar to that in ordinary atoms where initial and
final state vacancies almost always belong to different subshells.
It is shown that the smallness of the atomic shell radii as compared to that
of the fullerenes shell provides an opportunity to derive the simple formulas
for the probabilities of the electron transitions. It is shown that the
radiative and Auger (or Koster-Kronig) widths of a vacancy decay due to
electron transition in the atom A in A@C60 acquire an additional factors that
can be expressed via the polarizability of the C60 at transition energy.
It is demonstrated that due to opening of the non-radiative decay channel for
vacancies in subvalent subshells the decay probability increases by five -- six
orders of magnitude.Comment: 14 pages, 4 figure
Novel designs for Penning ion traps
We present a number of alternative designs for Penning ion traps suitable for
quantum information processing (QIP) applications with atomic ions. The first
trap design is a simple array of long straight wires which allows easy optical
access. A prototype of this trap has been built to trap Ca+ and a simple
electronic detection scheme has been employed to demonstrate the operation of
the trap. Another trap design consists of a conducting plate with a hole in it
situated above a continuous conducting plane. The final trap design is based on
an array of pad electrodes. Although this trap design lacks the open geometry
of the traps described above, the pad design may prove useful in a hybrid
scheme in which information processing and qubit storage take place in
different types of trap. The behaviour of the pad traps is simulated
numerically and techniques for moving ions rapidly between traps are discussed.
Future experiments with these various designs are discussed. All of the designs
lend themselves to the construction of multiple trap arrays, as required for
scalable ion trap QIP.Comment: 11 pages, 10 figure
Heavy-light quark pseudoscalar and vector mesons at finite temperature
The temperature dependence of the mass, leptonic decay constant, and width of
heavy-light quark peseudoscalar and vector mesons is obtained in the framework
of thermal Hilbert moment QCD sum rules. The leptonic decay constants of both
pseudoscalar and vector mesons decrease with increasing , and vanish at a
critical temperature , while the mesons develop a width which increases
dramatically and diverges at , where is the temperature for
chiral-symmetry restoration. These results indicate the disappearance of
hadrons from the spectral function, which then becomes a smooth function of the
energy. This is interpreted as a signal for deconfinement at . In
contrast, the masses show little dependence on the temperature, except very
close to , where the pseudoscalar meson mass increases slightly by 10-20
%, and the vector meson mass decreases by some 20-30
An upper limit on hypertriton production in collisions of Ar(1.76 A GeV) + KCl
A high-statistic data sample of Ar(1.76 AGeV)+KCl events recorded with HADES is used to search for a hypertriton signal. An upper production limit per centrality-triggered event of x on the level is derived. Comparing this value with the number of successfully reconstructed hyperons allows to determine an upper limit on the ratio , which is confronted with statistical and coalescence-type model calculations.Peer Reviewe
The dp-elastic cross section measurement at the deuteron kinetic energy of 2.5-GeV
New results on the differential cross section in deuteron-proton elastic scattering are obtained at the deuteron kinetic energy of 2.5 GeV with the HADES spectrometer. The angular range of 69deg. - 125deg. in the center of mass system is covered. The obtained results are compared with the relativistic multiple scattering model calculation using the CD-Bonn deuteron wave function. The data at fixed scattering angles in the c.m. are in qualitative agreement with the constituent counting rules prediction.Peer Reviewe
Lambda hyperon production and polarization in collisions of p(3.5 GeV)+Nb
Results on hyperon production are reported for collisions of p(3.5 GeV) + Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. The transverse mass distributions in rapidity bins are well described by Boltzmann shapes with a maximum inverse slope parameter of about MeV at a rapidity of , i.e. slightly below the center-of-mass rapidity for nucleon-nucleon collisions, . The rapidity density decreases monotonically with increasing rapidity within a rapidity window ranging from 0.3 to 1.3. The phase-space distribution is compared with results of other experiments and with predictions of two transport approaches which are available publicly. None of the present versions of the employed models is able to fully reproduce the experimental distributions, i.e. in absolute yield and in shape. Presumably, this finding results from an insufficient modelling in the transport models of the elementary processes being relevant for production, rescattering and absorption. The present high-statistics data allow for a genuine two-dimensional investigation as a function of phase space of the self-analyzing polarization in the weak decay . Finite negative values of the polarization in the order of are observed over the entire phase space studied. The absolute value of the polarization increases almost linearly with increasing transverse momentum for MeV/c and increases with decreasing rapidity for .Peer Reviewe
Searching a Dark Photon with HADES
We present a search for the e+e- decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL.Peer Reviewe
Inclusive pion and η production in p+Nb collisions at 3.5 GeV beam energy
Data on inclusive pion and η production measured with the dielectron spectrometer HADES in the reaction p+93Nb at a kinetic beam energy of 3.5 GeV are presented. Our results, obtained with the photon-conversion method, supplement the rather sparse information on neutral-meson production in proton-nucleus reactions existing for this bombarding energy regime. The reconstructed e+e−e+e− transverse-momentum and rapidity distributions are confronted with transport-model calculations, which account fairly well for both π0 and η production.Peer Reviewe
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