674 research outputs found
Non-exponential one-body loss in a Bose-Einstein condensate
We have studied the decay of a Bose-Einstein condensate of metastable helium
atoms in an optical dipole trap. In the regime where two- and three-body losses
can be neglected we show that the Bose-Einstein condensate and the thermal
cloud show fundamentally different decay characteristics. The total number of
atoms decays exponentially with time constant tau; however, the thermal cloud
decays exponentially with time constant (4/3)tau and the condensate decays much
faster, and non-exponentially. We show that this behaviour, which should be
present for all BECs in thermal equilibrium with a considerable thermal
fraction, is due to a transfer of atoms from the condensate to the thermal
cloud during its decay.Comment: The intuitive explanation of the atomic transfer effect has been
correcte
Universal three-body parameter in ultracold 4He*
We have analyzed our recently-measured three-body loss rate coefficient for a Bose-Einstein condensate of spin-polarized metastable triplet 4He atoms in terms of Efimov physics. The large value of the scattering length for these atoms, which provides access to the Efimov regime, arises from a nearby potential resonance. We find the loss coefficient to be consistent with the three-body parameter (3BP) found in alkali-metal experiments, where Feshbach resonances are used to tune the interaction. This provides new evidence for a universal 3BP, the first outside the group of alkali-metal elements. In addition, we give examples of other atomic systems without Feshbach resonances but with a large scattering length that would be interesting to analyze once precise measurements of three-body loss are available
Magnetic-field dependent trap loss of ultracold metastable helium
We have experimentally studied the magnetic-field dependence of the decay of
a Bose-Einstein condensate of metastable 4He atoms confined in an optical
dipole trap, for atoms in the m=+1 and m=-1 magnetic substates, and up to 450
G. Our measurements confirm long-standing calculations of the two-body loss
rate coefficient that show an increase above 50 G. We demonstrate that for m=-1
atoms, decay is due to three-body recombination only, with a three-body loss
rate coefficient of 6.5(0.4)(0.6)10^(-27)cm^6s^(-1), which is interesting in
the context of universal few-body theory. We have also searched for a
recently-predicted d-wave Feshbach resonance, but did not observe it.Comment: 7 pages, 6 figure
Reexamination of helium fine structure
In order to explain discrepancies between theoretical predictions and
experimental data for the helium fine structure, we check and recalculate all
theoretical contributions up to orders m\alpha^7 and m^2/M\alpha^6. The
previous result for the m\alpha^7 correction is improved by a much more
accurate calculation of relativistic corrections to the Bethe logarithm. The
theoretical values of the 2^3P_0-2^3P_1 and 2^3P_1-2^3P_2 fine structure
intervals in helium are, correspondingly, \nu_{01} = 29616946.2(1.6) kHz and
\nu_{12} = 2291177.3(1.6) kHz, with the uncertanties being due to higher-order
effects. For the small interval \nu_{12}, the theoretical value agrees with the
experimental data, whereas for the large interval \nu_{01}, a discrepancy of
about 3 standard deviations is present.Comment: 19 page
Projectile Coherence Effects in Simple Atomic Systems
Recent studies of projectile coherence effects in ion-atom collisions are presented. For intermediate-energy proton collisions an extensive literature provides strong support for the importance of such effects. In this regime coherence effects are now used as a tool to study the few-body dynamics very sensitively. In contrast, for high-energy ion impact the literature is much sparser and here an important role of coherence effects cannot be regarded as being established. In this context, a recent claim that in COLTRIMS experiments the coherence properties are determined only by the target beam is rebutted
Permanent school closures and crime: evidence from Scotland
In this article we study the effects of permanent school closures on crime. We leverage the closure of over 300 schools in Scotland between the school years 2006/07 and 2018/19, and employ a staggered
difference-in-differences design on a matched sample. We find that neighbourhoods affected by school closures experience a reduction in crime of about 9% of a standard deviation, relative to areas where schools remained open. This effect is mainly driven by a reduction in violent and property crimes. We provide evidence on several mechanisms explaining the negative crime effect, such as changes in neighbourhood composition and reductions in school-level segregation
Dynamics of Sleep-Wake Transitions During Sleep
We study the dynamics of the awakening during the night for healthy subjects
and find that the wake and the sleep periods exhibit completely different
behavior: the durations of wake periods are characterized by a scale-free
power-law distribution, while the durations of sleep periods have an
exponential distribution with a characteristic time scale. We find that the
characteristic time scale of sleep periods changes throughout the night. In
contrast, there is no measurable variation in the power-law behavior for the
durations of wake periods. We develop a stochastic model which agrees with the
data and suggests that the difference in the dynamics of sleep and wake states
arises from the constraints on the number of microstates in the sleep-wake
system.Comment: Final form with some small corrections. To be published in
Europhysics Letters, vol. 57, issue no. 5, 1 March 2002, pp. 625-63
Peculiar properties of the cluster-cluster interaction induced by the Pauli exclusion principle
Role of the Pauli principle in the formation of both the discrete spectrum
and multi-channel states of the binary nuclear systems composed of clusters is
studied in the Algebraic Version of the resonating-group method. Solutions of
the Hill-Wheeler equations in the discrete representation of a complete basis
of the Pauli-allowed states are discussed for 4He+n, 3H+3H, and 4He+4He binary
systems. An exact treatment of the antisymmetrization effects are shown to
result in either an effective repulsion of the clusters, or their effective
attraction. It also yields a change in the intensity of the centrifugal
potential. Both factors significantly affect the scattering phase behavior.
Special attention is paid to the multi-channel cluster structure 6He+6He as
well as to the difficulties arising in the case when the two clustering
configurations, 6He+6He and 4He+8He, are taken into account simultaneously. In
the latter case the Pauli principle, even in the absence of a potential energy
of the cluster-cluster interaction, leads to the inelastic processes and
secures an existence of both the bound state and resonance in the 12Be compound
nucleus.Comment: 17 pages, 14 figures, 1 table; submitted to Phys.Rev.C Keywords:
light neutron-rich nuclei, cluster model
Comparative overwiew of brain perfusion imaging techniques
Background and Purpose - Numerous imaging techniques have been developed and applied to evaluate brain hemodynamics. Among these are positron emission tomography, single photon emission computed tomography, Xenon-enhanced computed tomography, dynamic perfusion computed tomography, MRI dynamic susceptibility contrast, arterial spin labeling, and Doppler ultrasound. These techniques give similar information about brain hemodynamics in the form of parameters such as cerebral blood flow or cerebral blood volume. All of them are used to characterize the same types of pathological conditions. However, each technique has its own advantages and drawbacks.
Summary of Review - This article addresses the main imaging techniques dedicated to brain hemodynamics. It represents a comparative overview established by consensus among specialists of the various techniques.
Conclusions - For clinicians, this article should offer a clearer picture of the pros and cons of currently available brain perfusion imaging techniques and assist them in choosing the proper method for every specific clinical setting
Comparative overview of brain perfusion imaging techniques Epub
Background and Purpose - Numerous imaging techniques have been developed and applied to evaluate brain hemodynamics. Among these are positron emission tomography, single photon emission computed tomography, Xenon-enhanced computed tomography, dynamic perfusion computed tomography, MRI dynamic susceptibility contrast, arterial spin labeling, and Doppler ultrasound. These techniques give similar information about brain hemodynamics in the form of parameters such as cerebral blood flow or cerebral blood volume. All of them are used to characterize the same types of pathological conditions. However, each technique has its own advantages and drawbacks.
Summary of Review - This article addresses the main imaging techniques dedicated to brain hemodynamics. It represents a comparative overview established by consensus among specialists of the various techniques.
Conclusions - For clinicians, this article should offer a clearer picture of the pros and cons of currently available brain perfusion imaging techniques and assist them in choosing the proper method for every specific clinical setting
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