16 research outputs found
Energy Dependence of the Delta Resonance: Chiral Dynamics in Action
There is an important connection between the low energy theorems of QCD and
the energy dependence of the Delta resonance in pi-N scattering, as well as the
closely related gamma^{*} N -> pi N reaction. The resonance shape is due not
only to the strong pi-N interaction in the p wave but the small interaction in
the s wave; the latter is due to spontaneous chiral symmetry breaking in QCD
(i.e. the Nambu-Goldstone nature of the pion). A brief overview of experimental
tests of chiral perturbation theory and chiral based models is presentedComment: 11 pages, 6 figures, Festschrift for S.N. yan
Electroproduction of nucleon resonances
The unitary isobar model MAID has been extended and used for a partial wave
analysis of pion photo- and electroproduction in the resonance region W < 2
GeV. Older data from the world data base and more recent experimental results
from Mainz, Bates, Bonn and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed
and the Q^2 dependence of the helicity amplitudes have been extracted for a
series of four star resonances. We compare single-Q^2 analyses with a
superglobal fit in a new parametrization of Maid2003 together with predictions
of the hypercentral constituent quark model. As a result we find that the
helicity amplitudes and transition form factors of constituent quark models
should be compared with the analysis of bare resonances, where the pion cloud
contributions have been subtracted.Comment: 6 pages Latex including 5 figures, Invited talk at ICTP 4th
International Conference on Perspectives in Hadronic Physics, Trieste, Italy,
12-16 May 200
Chiral dynamics in systems with strangeness
In this talk a brief review of several problems involving systems with strangeness is made. In the first place one shows how the Λ(1405), Λ(1670) and Σ(1620) states, for S=−1, and the Ξ(1620) for S=−2 are generated dynamically in the context of unitarized chiral perturbation theory. The results for the K¯N interaction are then used to evaluate the K−d scattering length. Results obtained for the kaon selfenergy in a nuclear medium within this approach, with application to K− atoms, are also mentioned. Finally a few words are said about recent developments in the weak decay of Λ hypernuclei and the puzzle of the Γn/Γp ratio
Cold Accretion Disks and Lineless Quasars
The optical-UV continuum of quasars is broadly consistent with the emission
from a geometrically thin optically thick accretion disk (AD). The AD produces
the ionizing continuum which powers the broad and narrow emission lines. The
maximum AD effective temperature is given by Teff=fmax(Mdot/M^2)^1/4, where M
is the black hole mass, Mdot the accretion rate, and fmax is set by the black
hole spin a_*. For a low enough value of Mdot/M^2 the AD may become too cold to
produce ionizing photons. Such an object will form a lineless quasar. This
occurs for a local blackbody (BB) AD with a luminosity Lopt=10^46 erg/s for
M>3.6E9 Msun, when a_*=0, and for M>1.4E10 Msun, when a_*=0.998. Using the AD
based Mdot, derived from M and Lopt, and the reverberation based M, derived
from Lopt and the Hbeta FWHM, v, gives Teff \propto Lopt^-0.13v^-1.45. Thus,
Teff is mostly set by v. Quasars with a local BB AD become lineless for v>
8,000 km/s, when a_*=0, and for v> 16,000 km/s, when a_*=0.998. Higher values
of v are required if the AD is hotter than a local BB. The AD becoming
non-ionizing may explain why line emitting quasars with v>10,000 km/s are rare.
Weak low ionization lines may still be present if the X-ray continuum is
luminous enough, and such objects may form a population of weak emission line
quasars (WLQ). If correct, such WLQ should show a steeply falling SED at
lambda<1000A. Such an SED was observed by Hryniewicz et al. in SDSS
J094533.99+100950.1, a WLQ observed down to 570A, which is well modeled by a
rather cold AD SED. UV spectroscopy of z~1-2 quasars is required to eliminate
potential intervening Lyman limit absorption by the intergalactic medium (IGM),
and to explore if the SEDs of lineless quasars and some additional WLQ are also
well fit by a cold AD SED.Comment: Accepted for publication in MNRA