244 research outputs found
Recoil Polarization Measurements for Neutral Pion Electroproduction at Q^2=1 (GeV/c)^2 Near the Delta Resonance
We measured angular distributions of differential cross section, beam
analyzing power, and recoil polarization for neutral pion electroproduction at
Q^2 = 1.0 (GeV/c)^2 in 10 bins of W across the Delta resonance. A total of 16
independent response functions were extracted, of which 12 were observed for
the first time. Comparisons with recent model calculations show that response
functions governed by real parts of interference products are determined
relatively well near 1.232 GeV, but variations among models is large for
response functions governed by imaginary parts and for both increases rapidly
with W. We performed a nearly model-independent multipole analysis that adjusts
complex multipoles with high partial waves constrained by baseline models.
Parabolic fits to the W dependence of the multipole analysis around the Delta
mass gives values for SMR = (-6.61 +/- 0.18)% and EMR = (-2.87 +/- 0.19)% that
are distinctly larger than those from Legendre analysis of the same data.
Similarly, the multipole analysis gives Re(S0+/M1+) = (+7.1 +/- 0.8)% at
W=1.232 GeV, consistent with recent models, while the traditional Legendre
analysis gives the opposite sign because its truncation errors are quite
severe. Finally, using a unitary isobar model (UIM), we find that excitation of
the Roper resonance is dominantly longitudinal with S1/2 = (0.05 +/- 0.01)
GeV^(-1/2) at Q^2=1. The ReS0+ and ReE0+ multipoles favor pseudovector coupling
over pseudoscalar coupling or a recently proposed mixed-coupling scheme, but
the UIM does not reproduce the imaginary parts of 0+ multipoles well.Comment: 60 pages, 54 figure
The Quasielastic 3He(e,e'p)d Reaction at Q^2 = 1.5 GeV^2 for Recoil Momenta up to 1 GeV/c
We have studied the quasielastic 3He(e,e'p)d reaction in perpendicular
coplanar kinematics, with the energy and momentum transferred by the electron
fixed at 840 MeV and 1502 MeV/c, respectively. The 3He(e,e'p)d cross section
was measured for missing momenta up to 1000 MeV/c, while the A_TL asymmetry was
extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150
MeV/c, the measured cross section is described well by calculations that use a
variational ground-state wave function of the 3He nucleus derived from a
potential that includes three-body forces. For missing momenta from 150 to 750
MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c,
the experimental cross section is more than an order of magnitude larger than
predicted by available theories. The A_TL asymmetry displays characteristic
features of broken factorization, and is described reasonably well by available
models.Comment: 5 pages, 3 figures, submitted to Physical Review Letters, v3: changed
conten
The effect of travel restrictions on the spread of a moderately contagious disease
BACKGROUND: Much research in epidemiology has been focused on evaluating conventional methods of control strategies in the event of an epidemic or pandemic. Travel restrictions are often suggested as an efficient way to reduce the spread of a contagious disease that threatens public health, but few papers have studied in depth the effects of travel restrictions. In this study, we investigated what effect different levels of travel restrictions might have on the speed and geographical spread of an outbreak of a disease similar to severe acute respiratory syndrome (SARS). METHODS: We used a stochastic simulation model incorporating survey data of travel patterns between municipalities in Sweden collected over 3 years. We tested scenarios of travel restrictions in which travel over distances >50 km and 20 km would be banned, taking into account different levels of compliance. RESULTS: We found that a ban on journeys >50 km would drastically reduce the speed and geographical spread of outbreaks, even when compliance is < 100%. The result was found to be robust for different rates of intermunicipality transmission intensities. CONCLUSION: This study supports travel restrictions as an effective way to mitigate the effect of a future disease outbreak
Phase transitions in contagion processes mediated by recurrent mobility patterns
Human mobility and activity patterns mediate contagion on many levels,
including the spatial spread of infectious diseases, diffusion of rumors, and
emergence of consensus. These patterns however are often dominated by specific
locations and recurrent flows and poorly modeled by the random diffusive
dynamics generally used to study them. Here we develop a theoretical framework
to analyze contagion within a network of locations where individuals recall
their geographic origins. We find a phase transition between a regime in which
the contagion affects a large fraction of the system and one in which only a
small fraction is affected. This transition cannot be uncovered by continuous
deterministic models due to the stochastic features of the contagion process
and defines an invasion threshold that depends on mobility parameters,
providing guidance for controlling contagion spread by constraining mobility
processes. We recover the threshold behavior by analyzing diffusion processes
mediated by real human commuting data.Comment: 20 pages of Main Text including 4 figures, 7 pages of Supplementary
Information; Nature Physics (2011
Simulation of an SEIR infectious disease model on the dynamic contact network of conference attendees
The spread of infectious diseases crucially depends on the pattern of
contacts among individuals. Knowledge of these patterns is thus essential to
inform models and computational efforts. Few empirical studies are however
available that provide estimates of the number and duration of contacts among
social groups. Moreover, their space and time resolution are limited, so that
data is not explicit at the person-to-person level, and the dynamical aspect of
the contacts is disregarded. Here, we want to assess the role of data-driven
dynamic contact patterns among individuals, and in particular of their temporal
aspects, in shaping the spread of a simulated epidemic in the population.
We consider high resolution data of face-to-face interactions between the
attendees of a conference, obtained from the deployment of an infrastructure
based on Radio Frequency Identification (RFID) devices that assess mutual
face-to-face proximity. The spread of epidemics along these interactions is
simulated through an SEIR model, using both the dynamical network of contacts
defined by the collected data, and two aggregated versions of such network, in
order to assess the role of the data temporal aspects.
We show that, on the timescales considered, an aggregated network taking into
account the daily duration of contacts is a good approximation to the full
resolution network, whereas a homogeneous representation which retains only the
topology of the contact network fails in reproducing the size of the epidemic.
These results have important implications in understanding the level of
detail needed to correctly inform computational models for the study and
management of real epidemics
Measurement of GEp/GMp in ep -> ep to Q2 = 5.6 GeV2
The ratio of the electric and magnetic form factors of the proton, GEp/GMp,
was measured at the Thomas Jefferson National Accelerator Facility (JLab) using
the recoil polarization technique. The ratio of the form factors is directly
proportional to the ratio of the transverse to longitudinal components of the
polarization of the recoil proton in the elastic
reaction. The new data presented in this article span the range 3.5 < Q2 < 5.6
GeV2 and are well described by a linear Q2 fit. Also, the ratio QF2p/F1p
reaches a constant value above Q2=2 GeV2.Comment: 6 pages, 4 figures Added two names to the main author lis
Modeling influenza epidemics and pandemics: insights into the future of swine flu (H1N1)
Here we present a review of the literature of influenza modeling studies, and discuss how these models can provide insights into the future of the currently circulating novel strain of influenza A (H1N1), formerly known as swine flu. We discuss how the feasibility of controlling an epidemic critically depends on the value of the Basic Reproduction Number (R0). The R0 for novel influenza A (H1N1) has recently been estimated to be between 1.4 and 1.6. This value is below values of R0 estimated for the 1918–1919 pandemic strain (mean R0~2: range 1.4 to 2.8) and is comparable to R0 values estimated for seasonal strains of influenza (mean R0 1.3: range 0.9 to 2.1). By reviewing results from previous modeling studies we conclude it is theoretically possible that a pandemic of H1N1 could be contained. However it may not be feasible, even in resource-rich countries, to achieve the necessary levels of vaccination and treatment for control. As a recent modeling study has shown, a global cooperative strategy will be essential in order to control a pandemic. This strategy will require resource-rich countries to share their vaccines and antivirals with resource-constrained and resource-poor countries. We conclude our review by discussing the necessity of developing new biologically complex models. We suggest that these models should simultaneously track the transmission dynamics of multiple strains of influenza in bird, pig and human populations. Such models could be critical for identifying effective new interventions, and informing pandemic preparedness planning. Finally, we show that by modeling cross-species transmission it may be possible to predict the emergence of pandemic strains of influenza
Analysis of the archetypal functional equation in the non-critical case
We study the archetypal functional equation of the form (), where is a probability measure on ; equivalently, , where is expectation with respect to the distribution of random coefficients . Existence of non-trivial (i.e. non-constant) bounded continuous solutions is governed by the value ; namely, under mild technical conditions no such solutions exist whenever (and ) then there is a non-trivial solution constructed as the distribution function of a certain random series representing a self-similar measure associated with . Further results are obtained in the supercritical case , including existence, uniqueness and a maximum principle. The case with is drastically different from that with ; in particular, we prove that a bounded solution possessing limits at must be constant. The proofs employ martingale techniques applied to the martingale , where is an associated Markov chain with jumps of the form
Recoil Polarization for Delta Excitation in Pion Electroproduction
We measured angular distributions of recoil-polarization response functions
for neutral pion electroproduction for W=1.23 GeV at Q^2=1.0 (GeV/c)^2,
obtaining 14 separated response functions plus 2 Rosenbluth combinations; of
these, 12 have been observed for the first time. Dynamical models do not
describe quantities governed by imaginary parts of interference products well,
indicating the need for adjusting magnitudes and phases for nonresonant
amplitudes. We performed a nearly model-independent multipole analysis and
obtained values for Re(S1+/M1+)=-(6.84+/-0.15)% and Re(E1+/M1+)=-(2.91+/-0.19)%
that are distinctly different from those from the traditional Legendre analysis
based upon M1+ dominance and sp truncation.Comment: 5 pages, 2 figures, for PR
Precision Measurement of the Neutron Spin Asymmetry and Spin-Flavor Decomposition in the Valence Quark Region
We have measured the neutron spin asymmetry with high precision at
three kinematics in the deep inelastic region at , 0.47 and 0.60, and
, 3.5 and 4.8 (GeV/c), respectively. Our results unambiguously
show, for the first time, that crosses zero around and becomes
significantly positive at . Combined with the world proton data,
polarized quark distributions were extracted. Our results, in general, agree
with relativistic constituent quark models and with perturbative quantum
chromodynamics (pQCD) analyses based on the earlier data. However they deviate
from pQCD predictions based on hadron helicity conservation.Comment: 5 pages, 2 figures, this is the final version appeared in Phys. Rev.
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