10,230 research outputs found
The static potential: lattice versus perturbation theory in a renormalon-based approach
We compare, for the static potential and at short distances, perturbation
theory with the results of lattice simulations. We show that a
renormalon-dominance picture explains why in the literature sometimes
agreement, and another disagreement, is found between lattice simulations and
perturbation theory depending on the different implementations of the latter.
We also show that, within a renormalon-based scheme, perturbation theory agrees
with lattice simulations.Comment: 18 pages, 11 figures, lattice data of Necco and Sommer introduced,
references added, some lengthier explanations given, physical results
unchange
Breakdown of the operator product expansion in the 't Hooft model
We consider deep inelastic scattering in the 't Hooft model. Being solvable,
this model allows us to directly compute the moments associated with the cross
section at next-to-leading order in the 1/Q^2 expansion. We perform the same
computation using the operator product expansion. We find that all the terms
match in both computations except for one in the hadronic side, which is
proportional to a non-local operator. The basics of the result suggest that a
similar phenomenon may occur in four dimensions in the large N_c limit.Comment: 4 page
A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
ABRIDGED: Context: HI and CO large scale surveys of the Milky Way trace the
diffuse atomic clouds and the dense shielded regions of molecular hydrogen
clouds. However, until recently, we have not had spectrally resolved C+ surveys
to characterize the photon dominated interstellar medium, including, the H2 gas
without C, the CO-dark H2, in a large sample of clouds. Aims: To use a sparse
Galactic plane survey of the 1.9 THz [C II] spectral line from the Herschel
Open Time Key Programme, Galactic Observations of Terahertz C+ (GOT C+), to
characterize the H2 gas without CO in a statistically significant sample of
clouds. Methods: We identify individual clouds in the inner Galaxy by fitting
[CII] and CO isotopologue spectra along each line of sight. We combine these
with HI spectra, along with excitation models and cloud models of C+, to
determine the column densities and fractional mass of CO-dark H2 clouds.
Results: We identify 1804 narrow velocity [CII] interstellar cloud components
in different categories. About 840 are diffuse molecular clouds with no CO, 510
are transition clouds containing [CII] and 12CO, but no 13CO, and the remainder
are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are
concentrated between Galactic radii 3.5 to 7.5 kpc and the column density of
the CO-dark H2 layer varies significantly from cloud-to-cloud with an average
9X10^(20) cm-2. These clouds contain a significant fraction of CO-dark H2 mass,
varying from ~75% for diffuse molecular clouds to ~20% for dense molecular
clouds. Conclusions: We find a significant fraction of the warm molecular ISM
gas is invisible in HI and CO, but is detected in [CII]. The fraction of
CO-dark H2 is greatest in the diffuse clouds and decreases with increasing
total column density, and is lowest in the massive clouds.Comment: 21 pages, 19 figures, accepted for publication in A&A (2014
Kinematics of dense gas in the L1495 filament
We study the kinematics of the dense gas of starless and protostellar cores
traced by the N2D+(2-1), N2H+(1-0), DCO+(2-1), and H13CO+(1-0) transitions
along the L1495 filament and the kinematic links between the cores and the
surrounding molecular cloud.
We measure velocity dispersions, local and total velocity gradients and
estimate the specific angular momenta of 13 dense cores in the four transitions
using the on-the-fly observations with the IRAM 30 m antenna. To study a
possible connection to the filament gas, we use the fit results of the
C18O(1-0) survey performed by Hacar et al. (2013).
All cores show similar properties along the 10 pc-long filament. N2D+(2-1)
shows the most centrally concentrated structure, followed by N2H+(1-0) and
DCO+(2-1), which show similar spatial extent, and H13CO+(1-0). The non-thermal
contribution to the velocity dispersion increases from higher to lower density
tracers. The change of magnitude and direction of the total velocity gradients
depending on the tracer used indicates that internal motions change at
different depths within the cloud. N2D+ and N2H+ show smaller gradients than
the lower density tracers DCO+ and H13CO+, implying a loss of specific angular
momentum at small scales. At the level of cloud-core transition, the core's
external envelope traced by DCO+ and H13CO+ is spinning up, consistent with
conservation of angular momentum during core contraction. C18O traces the more
extended cloud material whose kinematics is not affected by the presence of
dense cores. The decrease in specific angular momentum towards the centres of
the cores shows the importance of local magnetic fields to the small scale
dynamics of the cores. The random distributions of angles between the total
velocity gradient and large scale magnetic field suggests that the magnetic
fields may become important only in the high density gas within dense cores.Comment: Accepted for publication in A&A. The abstract is shortene
Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?
The origin of striations aligned along the local magnetic field direction in
the translucent envelope of the Taurus molecular cloud is examined with new
observations of 12CO and 13CO J=2-1 emission obtained with the 10~m
submillimeter telescope of the Arizona Radio Observatory. These data identify a
periodic pattern of excess blue and redshifted emission that is responsible for
the striations. For both 12CO and 13CO, spatial variations of the J=2-1 to
J=1-0 line ratio are small and are not spatially correlated with the striation
locations. A medium comprised of unresolved CO emitting substructures (cells)
with a beam area filling factor less than unity at any velocity is required to
explain the average line ratios and brightness temperatures. We propose that
the striations result from the modulation of velocities and the beam filling
factor of the cells as a result of either the Kelvin-Helmholtz instability or
magnetosonic waves propagating through the envelope of the Taurus molecular
cloud. Both processes are likely common features in molecular clouds that are
sub-Alfvenic and may explain low column density, cirrus-like features similarly
aligned with the magnetic field observed throughout the interstellar medium in
far-infrared surveys of dust emission.Comment: 11 pages, 4 figures. Accepted for publication in MNRA
Fit to the Bjorken, Ellis-Jaffe and Gross-Llewellyn-Smith sum rules in a renormalon based approach
We study the large order behaviour in perturbation theory of the Bjorken,
Ellis-Jaffe and Gross-Llewellyn-Smith sum rules. In particular, we consider
their first infrared renormalons, for which we obtain their analytic structure
with logarithmic accuracy and also an approximate determination of their
normalization constant. Estimates of higher order terms of the perturbative
series are given. The Renormalon subtracted scheme is worked out for these
observables and compared with experimental data. Overall, good agreement with
experiment is found. This allows us to obtain {\hat a}_0 and some higher-twist
non-perturbative constants from experiment: {\hat a}_0=0.141\pm 0.089;
f_{3,RS}(1 GeV)=-0.124^{+0.137}_{-0.142} GeV^2.Comment: 23 pages, 18 figures, one reference added, journal versio
Deep inelastic scattering and factorization in the 't Hooft Model
We study in detail deep inelastic scattering in the 't Hooft model. We are
able to analytically check current conservation and to obtain analytic
expressions for the matrix elements with relative precision O(1/Q^2) for 1-x >>
\beta^2/Q^2. This allows us to compute the electron-meson differential cross
section and its moments with 1/Q^2 precision. For the former we find maximal
violations of quark-hadron duality, as it is expected for a large N_c analysis.
For the latter we find violations of the operator product expansion at
next-to-leading order in the 1/Q^2 expansion.Comment: 55 pages, 16 figure
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