612 research outputs found
Wilson Lines off the Light-cone in TMD PDFs
Transverse Momentum Dependent (TMD) parton distribution functions (PDFs) also
take into account the transverse momentum () of the partons. The
-integrated analogues can be linked directly to quark and gluon matrix
elements using the operator product expansion in QCD, involving operators of
definite twist. TMDs also involve operators of higher twist, which are not
suppressed by powers of the hard scale, however. Taking into account gauge
links that no longer are along the light-cone, one finds that new distribution
functions arise. They appear at leading order in the description of azimuthal
asymmetries in high-energy scattering processes. In analogy to the collinear
operator expansion, we define a universal set of TMDs of definite rank and
point out the importance for phenomenology.Comment: 12 pages, presented by the first author at the Light-Cone Conference
2013, May 20-24, 2013, Skiathos, Greece. To be published in Few Body System
Nuclear absorption of Charmoniums in pA and AA collisions
We have analysed the latest NA50 data on production in pA and AA
collisions. The production is assumed to be a two step process, (i)
formation of pairs, perturbatively calculable, and (ii) formation of
from the pair, a non-perturbative process, which is conviniently
parametrized. In a nuclear medium, as the pair passes through the
nuclear medium, it gain relative square momentum and some of the pairs can gain
enough square momentum to cross the threshold for open charm meson, leading to
suppression in nuclear medium. Few parameters of the model were fixed from the
latest high statistics NA50 pA and NA38 SU total cross sectional data.
The model then reproduces the centrality dependence of over Drell-Yan
ration in 200 GeV/c S+U and 158 GeV/c Pb+Pb collisions. We also discuss the
centrality dependence of suppression at RHIC energy.Comment: 4 pages including 3 figures, Revised version, to be published in
Phys.Rev.
The Construction of Gauge-Links in Arbitrary Hard Processes
Transverse momentum dependent parton distribution and fragmentation functions
are described by hadronic matrix elements of bilocal products of field
operators off the light-cone. These bilocal products contain gauge-links, as
required by gauge-invariance. The gauge-links are path-ordered exponentials
connecting the field operators along a certain integration path. This
integration path is process-dependent, depending specifically on the
short-distance partonic subprocess. In this paper we present the technical
details needed in the calculation of the gauge-links and a calculational scheme
is provided to obtain the gauge-invariant distribution and fragmentation
correlators corresponding to a given partonic subprocess
suppression in Pb+Pb collisions and broadening
We have analysed the NA50 data, on the centrality dependence of
broadening of 's, in Pb+Pb collisions, at the CERN-SPS. The data were
analysed in a QCD based model, where 's are suppressed in 'nuclear'
medium. Without any free parameter, the model could explain the NA50
broadening data. The data were also analysed in a QGP based threshold model,
where suppression is 100% above a critical density. The QGP based
model could not explain the NA50 broadening data. We have also predicted
the centrality dependence of suppression and broadening at RHIC
energy. Both the models, the QGP based threshold model and the QCD based
nuclear absorption model, predict broadening very close to each other.Comment: The paper was completely revised. The conclusion is also changed. 5
pages, 4 figure
Progressive transformation of a flux rope to an ICME
The solar wind conditions at one astronomical unit (AU) can be strongly
disturbed by the interplanetary coronal mass ejections (ICMEs). A subset,
called magnetic clouds (MCs), is formed by twisted flux ropes that transport an
important amount of magnetic flux and helicity which is released in CMEs. At 1
AU from the Sun, the magnetic structure of MCs is generally modeled neglecting
their expansion during the spacecraft crossing. However, in some cases, MCs
present a significant expansion. We present here an analysis of the huge and
significantly expanding MC observed by the Wind spacecraft during 9 and 10
November, 2004. After determining an approximated orientation for the flux rope
using the minimum variance method, we precise the orientation of the cloud axis
relating its front and rear magnetic discontinuities using a direct method.
This method takes into account the conservation of the azimuthal magnetic flux
between the in- and out-bound branches, and is valid for a finite impact
parameter (i.e., not necessarily a small distance between the spacecraft
trajectory and the cloud axis). Moreover, using the direct method, we find that
the ICME is formed by a flux rope (MC) followed by an extended coherent
magnetic region. These observations are interpreted considering the existence
of a previous larger flux rope, which partially reconnected with its
environment in the front. These findings imply that the ejected flux rope is
progressively peeled by reconnection and transformed to the observed ICME (with
a remnant flux rope in the front part).Comment: Solar Physics (in press
Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works
The detection of exolife is one of the goals of very ambitious future space
missions that aim to take direct images of Earth-like planets. While
associations of simple molecules present in the planet's atmosphere (,
, etc.) have been identified as possible global biomarkers, we
review here the detectability of a signature of life from the planet's surface,
i.e. the green vegetation. The vegetation reflectance has indeed a specific
spectrum, with a sharp edge around 700 nm, known as the "Vegetation Red Edge"
(VRE). Moreover vegetation covers a large surface of emerged lands, from
tropical evergreen forest to shrub tundra. Thus considering it as a potential
global biomarker is relevant. Earthshine allows to observe the Earth as a
distant planet, i.e. without spatial resolution. Since 2001, Earthshine
observations have been used by several authors to test and quantify the
detectability of the VRE in the Earth spectrum. The egetation spectral
signature is detected as a small 'positive shift' of a few percents above the
continuum, starting at 700 nm. This signature appears in most spectra, and its
strength is correlated with the Earth's phase (visible land versus visible
ocean). The observations show that detecting the VRE on Earth requires a
photometric relative accuracy of 1% or better. Detecting something equivalent
on an Earth-like planet will therefore remain challenging, moreover considering
the possibility of mineral artifacts and the question of 'red edge'
universality in the Universe.Comment: Invited talk in "Strategies for Life Detection" (ISSI Bern, 24-28
April 2006) to appear in a hardcopy volume of the ISSI Space Science Series,
Eds, J. Bada et al., and also in an issue of Space Science Reviews. 13 pages,
8 figures, 1 tabl
Non-perturbative effects and the resummed Higgs transverse momentum distribution at the LHC
We investigate the form of the non-perturbative parameterization in both the
impact parameter (b) space and transverse momentum (p_T) space resummation
formalisms for the transverse momentum distribution of single massive bosons
produced at hadron colliders. We propose to analyse data on Upsilon
hadroproduction as a means of studying the non-perturbative contribution in
processes with two gluons in the initial state. We also discuss the theoretical
errors on the resummed Higgs transverse momentum distribution at the LHC
arising from the non-perturbative contribution.Comment: 22 pages, 10 figure
J/psi suppression in heavy ion collisions by quark momentum diffusion
The momentum diffusion effect of the quark pair due to the multiple
scattering in a nuclear medium is studied to explain the observed J/psi yields
in SPS experiments. The resulting suppression is found to be insufficient to
reproduce the J/psi yield in Pb-Pb collisions at SPS energy.Comment: 2 pages (1 figure
Transient Magnetic and Doppler Features Related to the White-light Flares in NOAA 10486
Rapidly moving transient features have been detected in magnetic and Doppler
images of super-active region NOAA 10486 during the X17/4B flare of 28 October
2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely
energetic white-light events. The transient features appeared during impulsive
phases of the flares and moved with speeds ranging from 30 to 50 km s.
These features were located near the previously reported compact acoustic
\cite{Donea05} and seismic sources \cite{Zharkova07}. We examine the origin of
these features and their relationship with various aspects of the flares, {\it
viz.}, hard X-ray emission sources and flare kernels observed at different
layers - (i) photosphere (white-light continuum), (ii) chromosphere (H
6563\AA), (iii) temperature minimum region (UV 1600\AA), and (iv) transition
region (UV 284\AA).Comment: 26 pages, 13 figures, 2 tables, accepted for publication in Solar
Physic
Light-cone QCD Sum Rules for the Baryon Electromagnetic Form Factors and its magnetic moment
We present the light-cone QCD sum rules up to twist 6 for the electromagnetic
form factors of the baryon. To estimate the magnetic moment of the
baryon, the magnetic form factor is fitted by the dipole formula. The numerical
value of our estimation is , which is in
accordance with the experimental data and the existing theoretical results. We
find that it is twist 4 but not the leading twist distribution amplitudes that
dominate the results.Comment: 13 page, 7 figures, accepted for publication in Euro. Phys. J.
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