736 research outputs found
Valence-quark distributions in the pion
We calculate the pion's valence-quark momentum-fraction probability
distribution using a Dyson-Schwinger equation model. Valence-quarks with an
active mass of 0.30 GeV carry 71% of the pion's momentum at a resolving scale
q_0=0.54 GeV = 1/(0.37 fm). The shape of the calculated distribution is
characteristic of a strongly bound system and, evolved from q_0 to q=2 GeV, it
yields first, second and third moments in agreement with lattice and
phenomenological estimates, and valence-quarks carrying 49% of the pion's
momentum. However, pointwise there is a discrepancy between our calculated
distribution and that hitherto inferred from parametrisations of extant
pion-nucleon Drell-Yan data.Comment: 8 pages, 3 figures, REVTEX, aps.sty, epsfig.sty, minor corrections,
version to appear in PR
Geophysical investigations of WWII air-raid shelters in the UK
Just before WW2, the British government prepared for an aerial onslaught that was predicted to raze cities and cause mass casualties. By 1938, the Air Raid Precautions Act officially stated that population protection would be through dispersal, meaning evacuation and small-scale protection, local authority responsibility often devolving to householders. Archaeological records of remaining air-raid shelters are relatively rare and under threat. This paper reports on geophysical surveys on three sites in Stoke-on-Trent and London. Results found three intact Stanton shelters in Stoke-on-Trent, located by GPR, electrical resistivity, magnetometry, gravity and electromagnetic methods. In London, partially demolished shelters and an intact, mass public shelter were both detected by EM and GPR methods, with subsequent intrusive investigations confirming results. Study outcomes shows hitherto-neglected wartime shelters are in varied condition, with geophysical surveys able to detect, characterise and assess them, helping bring WWII British history into the wider scientific community and public domain
A Step Beyond the Bounce: Bubble Dynamics in Quantum Phase Transitions
We study the dynamical evolution of a phase interface or bubble in the
context of a \lambda \phi^4 + g \phi^6 scalar quantum field theory. We use a
self-consistent mean-field approximation derived from a 2PI effective action to
construct an initial value problem for the expectation value of the quantum
field and two-point function. We solve the equations of motion numerically in
(1+1)-dimensions and compare the results to the purely classical evolution. We
find that the quantum fluctuations dress the classical profile, affecting both
the early time expansion of the bubble and the behavior upon collision with a
neighboring interface.Comment: 12 pages, multiple figure
Energy and Flux Measurements of Ultra-High Energy Cosmic Rays Observed During the First ANITA Flight
The first flight of the Antarctic Impulsive Transient Antenna (ANITA)
experiment recorded 16 radio signals that were emitted by cosmic-ray induced
air showers. For 14 of these events, this radiation was reflected from the ice.
The dominant contribution to the radiation from the deflection of positrons and
electrons in the geomagnetic field, which is beamed in the direction of motion
of the air shower. This radiation is reflected from the ice and subsequently
detected by the ANITA experiment at a flight altitude of 36km. In this paper,
we estimate the energy of the 14 individual events and find that the mean
energy of the cosmic-ray sample is 2.9 EeV. By simulating the ANITA flight, we
calculate its exposure for ultra-high energy cosmic rays. We estimate for the
first time the cosmic-ray flux derived only from radio observations. In
addition, we find that the Monte Carlo simulation of the ANITA data set is in
agreement with the total number of observed events and with the properties of
those events.Comment: Added more explanation of the experimental setup and textual
improvement
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Excitation of standing kink oscillations in coronal loops
In this work we review the efforts that have been done to study the
excitation of the standing fast kink body mode in coronal loops. We mainly
focus on the time-dependent problem, which is appropriate to describe flare or
CME induced kink oscillations. The analytical and numerical studies in slab and
cylindrical loop geometries are reviewed. We discuss the results from very
simple one-dimensional models to more realistic (but still simple) loop
configurations. We emphasise how the results of the initial value problem
complement the eigenmode calculations. The possible damping mechanisms of the
kink oscillations are also discussed
Fluctuations, dissipation and the dynamical Casimir effect
Vacuum fluctuations provide a fundamental source of dissipation for systems
coupled to quantum fields by radiation pressure. In the dynamical Casimir
effect, accelerating neutral bodies in free space give rise to the emission of
real photons while experiencing a damping force which plays the role of a
radiation reaction force. Analog models where non-stationary conditions for the
electromagnetic field simulate the presence of moving plates are currently
under experimental investigation. A dissipative force might also appear in the
case of uniform relative motion between two bodies, thus leading to a new kind
of friction mechanism without mechanical contact. In this paper, we review
recent advances on the dynamical Casimir and non-contact friction effects,
highlighting their common physical origin.Comment: 39 pages, 4 figures. Review paper to appear in Lecture Notes in
Physics, Volume on Casimir Physics, edited by Diego Dalvit, Peter Milonni,
David Roberts, and Felipe da Rosa. Minor changes, a reference adde
Measurement of the Deuteron Structure Function F2 in the Resonance Region and Evaluation of Its Moments
Inclusive electron scattering off the deuteron has been measured to extract
the deuteron structure function F2 with the CEBAF Large Acceptance Spectrometer
(CLAS) at the Thomas Jefferson National Accelerator Facility. The measurement
covers the entire resonance region from the quasi-elastic peak up to the
invariant mass of the final-state hadronic system W~2.7 GeV with four-momentum
transfers Q2 from 0.4 to 6 (GeV/c)^2. These data are complementary to previous
measurements of the proton structure function F2 and cover a similar
two-dimensional region of Q2 and Bjorken variable x. Determination of the
deuteron F2 over a large x interval including the quasi-elastic peak as a
function of Q2, together with the other world data, permit a direct evaluation
of the structure function moments for the first time. By fitting the Q2
evolution of these moments with an OPE-based twist expansion we have obtained a
separation of the leading twist and higher twist terms. The observed Q2
behaviour of the higher twist contribution suggests a partial cancellation of
different higher twists entering into the expansion with opposite signs. This
cancellation, found also in the proton moments, is a manifestation of the
"duality" phenomenon in the F2 structure function
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
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