4,402 research outputs found
Superstatistical energy distributions of an ion in an ultracold buffer gas
An ion in a radiofrequency ion trap interacting with a buffer gas of
ultracold neutral atoms is a driven dynamical system which has been found to
develop a non-thermal energy distribution with a power law tail. The exact
analytical form of this distribution is unknown, but has often been represented
empirically by q-exponential (Tsallis) functions. Based on the concepts of
superstatistics, we introduce a framework for the statistical mechanics of an
ion trapped in an RF field subject to collisions with a buffer gas. We derive
analytic ion secular energy distributions from first principles both neglecting
and including the effects of the thermal energy of the buffer gas. For a buffer
gas with a finite temperature, we prove that Tsallis statistics emerges from
the combination of a constant heating term and multiplicative energy
fluctuations. We show that the resulting distributions essentially depend on
experimentally controllable parameters paving the way for an accurate control
of the statistical properties of ion-atom hybrid systems.Comment: Main text 6 pages, 4 figures. Total length including supplemental
material 11 pages, 6 figure
The energy distribution of an ion in a radiofrequency trap interacting with a nonuniform neutral buffer gas
An ion in a radiofrequency (rf) trap sympathetically cooled by a
simultaneously trapped neutral buffer gas exhibits deviations from thermal
statistics caused by collision-induced coupling of the rf field to the ion
motion. For a uniform density distribution of the buffer gas, the energy
distribution of the ion can be described by Tsallis statistics. Moreover,
runaway heating of the ion occurs if the buffer gas particles are sufficiently
heavy relative to the ion. In typical experiments, however, ultracold buffer
gases are confined in traps resulting in localised, non-uniform density
distributions. Using a superstatistical approach, we develop an analytical
model for an ion interacting with a localised buffer gas. We demonstrate
theoretically that limiting collisions to the centre of the ion trap enables
cooling at far greater mass ratios than achievable using a uniform buffer gas,
but that an upper limit to the usable mass ratio exists even in this case.
Furthermore, we analytically derive the energy distribution for an ion
interacting with a buffer gas held in a harmonic potential. The analytical
distribution obtained is found to be in excellent agreement with the results of
numerical simulations.Comment: 27 pages, 6 figure
Superstatistical velocity distributions of cold trapped ions in molecular dynamics simulations
We present a realistic molecular-dynamics treatment of laser-cooled ions in
radiofrequency ion traps which avoids previously made simplifications such as
modeling laser cooling as a friction force and combining individual heating
mechanisms into a single effective heating force. Based on this implementation,
we show that infrequent energetic collisions of single ions with background gas
molecules lead to pronounced heating of the entire ion ensemble and a
time-varying secular ensemble temperature which manifests itself in a
superstatistical time-averaged velocity distribution of the ions. The effect of
this finding on the experimental determination of ion temperatures and rate
constants for cold chemical reactions is discussed.Comment: 13 pages, 12 figures, accepted for publication in Phys. Rev.
A queueing model of pilot decision making in a multi-task flight management situation
Allocation of decision making responsibility between pilot and computer is considered and a flight management task, designed for the study of pilot-computer interaction, is discussed. A queueing theory model of pilot decision making in this multi-task, control and monitoring situation is presented. An experimental investigation of pilot decision making and the resulting model parameters are discussed
A model of human event detection in multiple process monitoring situations
It is proposed that human decision making in many multi-task situations might be modeled in terms of the manner in which the human detects events related to his tasks and the manner in which he allocates his attention among his tasks once he feels events have occurred. A model of human event detection performance in such a situation is presented. An assumption of the model is that, in attempting to detect events, the human generates the probability that events have occurred. Discriminant analysis is used to model the human's generation of these probabilities. An experimental study of human event detection performance in a multiple process monitoring situation is described and the application of the event detection model to this situation is addressed. The experimental study employed a situation in which subjects simulataneously monitored several dynamic processes for the occurrence of events and made yes/no decisions on the presence of events in each process. Input to the event detection model of the information displayed to the experimental subjects allows comparison of the model's performance with the performance of the subjects
Auxin regulates SCFTIR1-dependent degradation of AUX/IAA proteins
The plant hormone auxin is central in many aspects of plant development. Previous studies have implicated the ubiquitin-ligase SCFTIR1 and the AUX/IAA proteins in auxin response. Dominant mutations in several AUX/IAA genes confer pleiotropic auxin-related phenotypes, whereas recessive mutations affecting the function of SCFTIR1 decrease auxin response. Here we show that SCFTIR1 is required for AUX/IAA degradation. We demonstrate that SCFTIR1 interacts with AXR2/IAA7 and AXR3/IAA17, and that domain II of these proteins is necessary and sufficient for this interaction. Further, auxin stimulates binding of SCFTIR1 to the AUX/IAA proteins, and their degradation. Because domain II is conserved in nearly all AUX/IAA proteins in Arabidopsis, we propose that auxin promotes the degradation of this large family of transcriptional regulators, leading to diverse downstream effects
Financial Aid Packages and College Enrollment Decisions: An Econometric Case Study
We study the effects of a change in financial aid policy introduced by a Northeastern university in 1998. Prior to that time, the university's financial aid packages for low-income students consisted of grants, loans, and campus jobs. After the change, the entire loan portion of the package for low-income students was replaced with grants. We find the program increased the likelihood of matriculation by low-income students by about 3 percentage points, although the effect is not statistically significant. The effect among low-income minority students was between 8 and 10 percentage points and statistically significant at the 10 percent level.
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