29 research outputs found
Light-Quark Flavour Splitting of Heavy-Light Constituent Diquark Masses and Doubly-Strange Diquarks from QCD Sum-Rules
QCD Laplace sum-rules are used to examine the constituent mass spectrum of
heavy-light [Qq] diquarks with and
. As in previous sum-rule studies, the negative parity
[Qq] diquark mass predictions do not stabilize, so the
sum-rule analysis focuses on positive parity [Qq] diquarks. Doubly-strange
[ss] diquarks are also examined, but the resulting sum rules do not
stabilize. Hence there is no sum-rule evidence for [ss] diquark
states, aiding the interpretation of sum-rule analyses of fully-strange
tetraquark states. The SU(3) flavour splitting effects for [Qq] diquarks are
obtained by calculating QCD correlation functions of
diquark composite operators up to next-to-leading order in perturbation theory,
leading-order in the strange quark mass, and in the chiral limit for
non-strange (u,d) quarks with an isospin-symmetric vacuum . Apart from the strange quark mass parameter , the strange
quark condensate parameter has an important impact
on SU(3) flavour splittings. A Laplace sum-rule analysis methodology is
developed for the mass difference between the strange and
non-strange heavy-light diquarks to reduce the theoretical uncertainties from
all other QCD input parameters. The mass splitting is found to decrease with
increasing , providing an upper bound on where the
mass hierarchy reverses. In the typical QCD sum-rule range
, and , with a slight tendency for larger splittings for
the channels. These constituent mass splitting results are discussed
in comparison with values used in constituent diquark models for tetraquark and
pentaquark hadronic states.Comment: 30 pages, 19 figures, 7 tables. v2 contains extended discussio
Observational learning of two visual discriminations by pigeons: a within-subjects design.
Pigeon's observational learning of successive visual discrimination was studied using within-subject comparisons of data from three experimental conditions. Two pairs of discriminative stimuli were used; each bird was exposed to two of the three experimental conditions, with different pairs of stimuli used in a given bird's two conditions. In one condition, observers were exposed to visual discriminative stimuli only. In a second condition, subjects were exposed to a randomly alternating sequence of two stimuli where the one that would subsequently be used as S+ was paired with the operation of the grain magazine. In a third experimental condition, subjects were exposed to the performance of a conspecific in the operant discrimination procedure. After exposures to conspecific performances, there was facilitation of discriminative learning, relative to that which followed exposures to stimulus and reinforcement sequences or exposures to stimulus sequences alone. Exposure to stimulus and food-delivery sequences enhanced performance relative to exposure to stimulus sequences alone. The differential effects of these three types of exposure were not attributable to order effects or to task difficulty; rather, they clearly were due to the type of exposure