289 research outputs found
Anomaly-induced Quadrupole Moment of the Neutron in Magnetic Field
The neutrons cannot possess a quadrupole moment in the vacuum. Nevertheless,
we show that in the presence of an external magnetic field the neutrons acquire
a new type of quadrupole moment involving the
components of spin and magnetic field. This "chiral magnetic" quadrupole moment
arises from the interplay of the chiral anomaly and the magnetic field; we
estimate its value for the neutron in the static limit, and find . The detection of the quadrupole moment of the
neutron would provide a novel test of the role of the chiral anomaly in
low-energy QCD and can be possible in the presence of both magnetic and
inhomogeneous electric fields. The quadrupole moment of the neutron may affect
e.g. the properties of neutron stars and magnetars.Comment: 2 pages; extended versio
Potassium channels in cell cycle and cell proliferation
Normal cell-cycle progression is a crucial task for every multicellular organism, as it determines body size and shape, tissue renewal and senescence, and is also crucial for reproduction. On the other hand, dysregulation of the cell-cycle progression leading to uncontrolled cell proliferation is the hallmark of cancer. Therefore, it is not surprising that it is a tightly regulated process, with multifaceted and very complex control mechanisms. It is now well established that one of those mechanisms relies on ion channels, and in many cases specifically on potassium channels. Here, we summarize the possible mechanisms underlying the importance of potassium channels in cell-cycle control and briefly review some of the identified channels that illustrate the multiple ways in which this group of proteins can influence cell proliferation and modulate cell-cycle progression
The Gemini NICI Planet-Finding Campaign: The Offset Ring of HR 4796 A
We present J, H, CH_4 short (1.578 micron), CH_4 long (1.652 micron) and
K_s-band images of the dust ring around the 10 Myr old star HR 4796 A obtained
using the Near Infrared Coronagraphic Imager (NICI) on the Gemini-South 8.1
meter Telescope. Our images clearly show for the first time the position of the
star relative to its circumstellar ring thanks to NICI's translucent focal
plane occulting mask. We employ a Bayesian Markov Chain Monte Carlo method to
constrain the offset vector between the two. The resulting probability
distribution shows that the ring center is offset from the star by 16.7+/-1.3
milliarcseconds along a position angle of 26+/-3 degrees, along the PA of the
ring, 26.47+/-0.04 degrees. We find that the size of this offset is not large
enough to explain the brightness asymmetry of the ring. The ring is measured to
have mostly red reflectivity across the JHK_s filters, which seems to indicate
micron-sized grains. Just like Neptune's 3:2 and 2:1 mean-motion resonances
delineate the inner and outer edges of the classical Kuiper Belt, we find that
the radial extent of the HR 4796 A and Fomalhaut rings could correspond to the
3:2 and 2:1 mean-motion resonances of hypothetical planets at 54.7 AU and 97.7
AU in the two systems, respectively. A planet orbiting HR 4796 A at 54.7 AU
would have to be less massive than 1.6 Mjup so as not to widen the ring too
much by stirring.Comment: Accepted to A&A for publication on April 23, 2014 (15 pages, 9
figures, 4 tables
Dilepton and Photon Emission Rates from a Hadronic Gas III
We extend our early analyses of the dilepton and photon emission rates from a
hadronic gas to account for strange mesons using a density expansion. The
emission rates are reduced to vacuum correlation functions using three-flavor
chiral reduction formulas, and the latters are assessed in terms of empirical
data. Using a fire-ball, we compare our results to the low and intermediate
mass dilepton data available from CERN. Our results suggest that a baryon free
hadronic gas does not account for the excess of low mass dielectrons observed
at CERES but do well in accounting for the intermediate dimuons at HELIOS. The
same observations apply to the recent low and high dielectron rates from
CERES.Comment: 12 pages LaTeX, 11 eps figure
Topology of the gauge-invariant gauge field in two-color QCD
We investigate solutions to a nonlinear integral equation which has a central
role in implementing the non-Abelian Gauss's Law and in constructing
gauge-invariant quark and gluon fields. Here we concern ourselves with
solutions to this same equation that are not operator-valued, but are functions
of spatial variables and carry spatial and SU(2) indices. We obtain an
expression for the gauge-invariant gauge field in two-color QCD, define an
index that we will refer to as the ``winding number'' that characterizes it,
and show that this winding number is invariant to a small gauge transformation
of the gauge field on which our construction of the gauge-invariant gauge field
is based. We discuss the role of this gauge field in determining the winding
number of the gauge-invariant gauge field. We also show that when the winding
number of the gauge field is an integer , the gauge-invariant
gauge field manifests winding numbers that are not integers, and are
half-integers only when .Comment: 26 pages including 6 encapsulated postscript figures. Numerical
errors have been correcte
NICI: combining coronagraphy, ADI, and SDI
The Near-Infrared Coronagraphic Imager (NICI) is a high-contrast AO imager at
the Gemini South telescope. The camera includes a coronagraphic mask and dual
channel imaging for Spectral Differential Imaging (SDI). The instrument can
also be used in a fixed Cassegrain Rotator mode for Angular Differential
Imaging (ADI). While coronagraphy, SDI, and ADI have been applied before in
direct imaging searches for exoplanets. NICI represents the first time that
these 3 techniques can be combined. We present preliminary NICI commissioning
data using these techniques and show that combining SDI and ADI results in
significant gains.Comment: Proc. SPIE, Vol. 7014, 70141Z (2008
The Spitzer c2d Survey of Nearby Dense Cores: III: Low Mass Star Formation in a Small Group, L1251B
We present a comprehensive study of a low-mass star-forming region,L1251B, at
wavelengths from the near-infrared to the millimeter. L1251B, where only one
protostar, IRAS 22376+7455, was known previously, is confirmed to be a small
group of protostars based on observations with the Spitzer Space Telescope. The
most luminous source of L1251B is located 5" north of the IRAS position. A
near-infrared bipolar nebula, which is not associated with the brightest object
and is located at the southeast corner of L1251B, has been detected in the IRAC
bands. OVRO and SMA interferometric observations indicate that the brightest
source and the bipolar nebula source in the IRAC bands are deeply embedded disk
sources.Submillimeter continuum observations with single-dish telescopes and
the SMA interferometric observations suggest two possible prestellar objects
with very high column densities. Outside of the small group, many young stellar
object candidates have been detected over a larger region of 12' x 12'.
Extended emission to the east of L1251B has been detected at 850 micron; this
"east core" may be a site for future star formation since no point source has
been detected with IRAC or MIPS. This region is therefore a possible example of
low-mass cluster formation, where a small group of pre- and protostellar
objects (L1251B) is currently forming, alongside a large starless core (the
east core).Comment: 35 pages, 15 figures, accepted for publication in ApJ, for the full
resolution paper, visit
"http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap27.pub.pdf
Prompt Quark Production by exploding Sphalerons
Following recent works on production and subsequent explosive decay of QCD
sphaleron-like clusters, we discuss the mechanism of quark pair production in
this process. We first show how the gauge field explosive solution of Luscher
and Schechter can be achieved by non-central conformal mapping from the
O(4)-symmetric solution. Our main result is a new solution to the Dirac
equation in real time in this configuration, obtained by the same inversion of
the fermion O(4) zero mode. It explicitly shows how the quark acceleration
occurs, starting from the spherically O(3) symmetric zero energy chiral quark
state to the final spectrum of non-zero energies.
The sphaleron-like clusters with any Chern-Simons number always produce quarks, and the antisphaleron-like clusters the
chirality opposite.
The result are relevant for hadron-hadron and nucleus-nucleus collisions at
large , wherein such clusters can be produced
Holographic Pomeron and the Schwinger Mechanism
We revisit the problem of dipole-dipole scattering via exchanges of soft
Pomerons in the context of holographic QCD. We show that a single closed string
exchange contribution to the eikonalized dipole-dipole scattering amplitude
yields a Regge behavior of the elastic amplitude; the corresponding slope and
intercept are different from previous results obtained by a variational
analysis of semi-classical surfaces. We provide a physical interpretation of
the semi-classical worldsheets driving the Regge behavior for (-t)>0 in terms
of worldsheet instantons. The latter describe the Schwinger mechanism for
string pair creation by an electric field, where the longitudinal electric
field E_L=\sigma_T tanh(\chi/2) at the origin of this non-perturbative
mechanism is induced by the relative rapidity {\chi} of the scattering dipoles.
Our analysis naturally explains the diffusion in the impact parameter space
encoded in the Pomeron exchange; in our picture, it is due to the Unruh
temperature of accelerated strings under the electric field. We also argue for
the existence of a "micro-fireball" in the middle of the transverse space due
to the soft Pomeron exchange, which may be at the origin of the thermal
character of multiparticle production in ep/pp collisions. After summing over
uncorrelated multi-Pomeron exchanges, we find that the total dipole-dipole
cross section obeys the Froissart unitarity bound.Comment: 17 pages, 4 figures, version 2: minor typos corrected, references
adde
- …