63,148 research outputs found
The eta transition form factor from space- and time-like experimental data
The transition form factor is analysed for the first time in both
space- and time-like regions at low and intermediate energies in a
model-independent approach through the use of rational approximants. The
experimental data provided by the A2
Collaboration in the very low energy region of the dilelectron invariant mass
distribution allows for the extraction of the most precise up-to-date slope and
curvature parameters of the form factors as well as their values at zero and
infinity. The impact of these new results on the mixing parameters of the
- system, together with the role played by renormalisation
dependent effects, and on the determination of the couplings from
and radiative decays are also discussed.Comment: 16 pages, 17 figures; v2: additional comments, references added;
mathces published version in EPJ
The UV-upturn in brightest cluster galaxies
This paper is part of a series devoted to the investigation of a large sample
of brightest cluster galaxies (BCGs), their properties and the relationships
between these and the properties of the host clusters. In this paper, we
compare the stellar population properties derived from high signal-to-noise,
optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements
for 36 nearby BCGs to understand the diversity in the most rapidly evolving
feature in old stellar systems, the UV-upturn. We investigate: (1) the possible
differences between the UV-upturn of BCGs and those of a control sample of
ordinary ellipticals in the same mass range, as well as possible correlations
between the UV-upturn and other general properties of the galaxies; (2)
possible correlations between the UV-upturn and the properties of the host
clusters; (3) recently proposed scenarios where helium-sedimentation in the
cluster centre can produce an enhanced UV-upturn. We find systematic
differences between the UV-colours of BCGs and ordinary ellipticals, but we do
not find correlations between these colours and the properties of the host
clusters. Furthermore, the observations do not support the predictions made by
the helium-sedimentation model as an enhancer of the UV-upturn.Comment: 12 pages, accepted for publication in MNRA
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