8,008 research outputs found
Color entanglement for azimuthal asymmetries in the Drell-Yan process
In the resummation of collinear gluons emitted together with active partons
from the hadrons in the Drell-Yan process (DY) effects of color entanglement
become important when the transverse directions are taken into account. It is
then no longer possible to write the cross section as the convolution of two
soft correlators and a hard part. We show that the color entanglement
introduces additional color factors that must be taken into account in the
extraction of transverse momentum dependent parton distribution functions (TMD
PDFs) from azimuthal asymmetries. Examples where such effects matter are the
extraction of the double Sivers and double Boer-Mulders asymmetries.
Furthermore, we will argue why this color entanglement is a basic ingredient
already in the tree-level description of azimuthal asymmetries.Comment: 5 pages, minor corrections and updated reference
Operator analysis of -widths of TMDs
Transverse momentum dependent (TMD) parton distribution functions (PDFs),
TMDs for short, are defined as the Fourier transform of matrix elements of
nonlocal combinations of quark and gluon fields. The nonlocality is bridged by
gauge links, which for TMDs have characteristic paths (future or past
pointing), giving rise to a process dependence that breaks universality. It is
possible, however, to construct sets of universal TMDs of which in a given
process particular combinations are needed with calculable, process-dependent,
coefficients. This occurs for both T-odd and T-even TMDs, including also the
{\it unpolarized} quark and gluon TMDs. This extends the by now well-known
example of T-odd TMDs that appear with opposite sign in single-spin azimuthal
asymmetries in semi-inclusive deep inelastic scattering or in the Drell-Yan
process. In this paper we analyze the cases where TMDs enter multiplied by
products of two transverse momenta, which includes besides the -broadening
observable, also instances with rank two structures. To experimentally
demonstrate the process dependence of the latter cases requires measurements of
second harmonic azimuthal asymmetries, while the -broadening will require
measurements of processes beyond semi-inclusive deep inelastic scattering or
the Drell-Yan process. Furthermore, we propose specific quantities that will
allow for theoretical studies of the process dependence of TMDs using lattice
QCD calculations.Comment: 10 pages, no figures; expanded discussions, matches version accepted
by JHE
Universality of TMD distribution functions of definite rank
Transverse momentum dependent (TMD) distribution and fragmentation functions
are described as Fourier transforms of matrix elementscontaining nonlocal
combinations of quark and gluon fields. These matrix elements also contain a
gauge link operator with a process dependent path, of which the process
dependence that can be traced back to the color flow in the process. Expanding
into irreducible tensors built from the transverse momenta p_\st, we can
define a universal set of TMD correlators of definite rank with a well-defined
operator structure.Comment: 6 pages, to be published in proceedings of the Third Worshop on the
QCD Structure of the Nucleon (QCD N'12), Bilbao, Spain, 22-26 October 201
Universality of TMD correlators
In a high-energy scattering process with hadrons in the initial state, color
is involved. Transverse momentum dependent distribution functions (TMDs)
describe the quark and gluon distributions in these hadrons in momentum space
with the inclusion of transverse directions. Apart from the (anti)-quarks and
gluons that are involved in the hard scattering process, additional gluon
emissions by the hadrons have to be taken into account as well, giving rise to
Wilson lines or gauge links. The TMDs involved are sensitive to the process
under consideration and hence potentially nonuniversal due to these Wilson line
interactions with the hard process; different hard processes give rise to
different Wilson line structures. We will show that in practice only a finite
number of universal TMDs have to be considered, which come in different linear
combinations depending on the hard process under consideration, ensuring a
generalized universality. For quarks this gives rise to three Pretzelocity
functions, whereas for gluons a richer structure of functions arises.Comment: 6 pages, presented by the first author at the 4th International
Workshop on Transverse Polarization Phenomena in Hard Processes (Transversity
2014), June 9-13, 2014, Chia, Italy. To appear in EPJ Web of Conference
Single spin asymmetries from a single Wilson loop
We study the leading-power gluon transverse momentum dependent distributions
(TMDs) of relevance to the study of asymmetries in the scattering off
transversely polarized hadrons. Next-to-leading-order perturbative calculations
of these TMDs show that at large transverse momentum they have common dynamical
origins, but that in the limit of small longitudinal momentum fraction only
one origin remains. We find that in this limit only the dipole-type gluon TMDs
survive and become identical to each other. At small they are all given by
the expectation value of a single Wilson loop inside the transversely polarized
hadron, the so-called spin-dependent odderon. This universal origin of
transverse spin asymmetries at small is of importance to current and future
experimental studies, paving the way to a better understanding of the role of
gluons in the three-dimensional structure of spin-polarized protons.Comment: 5 pages, 1 figure. Various changes to the text, additional
references, conclusions unchanged, version accepted for publication in
Physical Review Letter
Polarization effects in double open-charm production at LHCb
Double open-charm production is one of the most promising channels to
disentangle single from double parton scattering (DPS) and study different
properties of DPS. Several studies of the DPS contributions have been made. A
missing ingredient so far has been the study of polarization effects, arising
from spin correlations between the two partons inside an unpolarized proton. We
investigate the impact polarization has on the double open-charm cross section.
We show that the longitudinally polarized gluons can give significant
contributions to the cross section, but for most of the considered kinematic
region only have a moderate effect on the shape. We compare our findings to the
LHCb data in the D0D0 final state, identify observables where polarization does
have an impact on the distribution of the final state particles, and suggest
measurements which could lead to first experimental indications of, or limits
on, polarization in DPS.Comment: 16 pages, 13 figure
Effects of TMD evolution and partonic flavor on annihilation into hadrons
We calculate the transverse momentum dependence in the production of two
back-to-back hadrons in electron-positron annihilations at the medium/large
energy scales of BES-III and BELLE experiments. We use the parameters of the
transverse-momentum-dependent (TMD) fragmentation functions that were recently
extracted from the semi-inclusive deep-inelastic-scattering multiplicities at
low energy from HERMES. TMD evolution is applied according to different
approaches and using different parameters for the nonperturbative part of the
evolution kernel, thus exploring the sensitivity of our results to these
different choices and to the flavor dependence of parton fragmentation
functions. We discuss how experimental measurements could discriminate among
the various scenarios.Comment: 33 pages, 10 composite figures, JHEP style fil
Scattered light mapping of protoplanetary disks
High-contrast scattered light observations have revealed the surface
morphology of several dozens of protoplanetary disks at optical and
near-infrared wavelengths. Inclined disks offer the opportunity to measure part
of the phase function of the dust grains that reside in the disk surface which
is essential for our understanding of protoplanetary dust properties and the
early stages of planet formation. We aim to construct a method which takes into
account how the flaring shape of the scattering surface of an (optically thick)
protoplanetary disk projects onto the image plane of the observer. This allows
us to map physical quantities (scattering radius and scattering angle) onto
scattered light images and retrieve stellar irradiation corrected (r^2-scaled)
images and dust phase functions. We apply the method on archival polarized
intensity images of the protoplanetary disk around HD 100546 that were obtained
with VLT/SPHERE in R'-band and VLT/NACO in H- and Ks-band. The brightest side
of the r^2-scaled R'-band polarized intensity image of HD 100546 changes from
the far to the near side of the disk when a flaring instead of a geometrically
flat disk surface is used for the r^2-scaling. The decrease in polarized
surface brightness in the scattering angle range of ~40-70 deg is likely a
result of the dust phase function and degree of polarization which peak in
different scattering angle regimes. The derived phase functions show part of a
forward scattering peak which indicates that large, aggregate dust grains
dominate the scattering opacity in the disk surface. Projection effects of a
protoplanetary disk surface need to be taken into account to correctly
interpret scattered light images. Applying the correct scaling for the
correction of stellar irradiation is crucial for the interpretation of the
images and the derivation of the dust properties in the disk surface layer.Comment: Accepted for publication in A&A, 6 pages, 3 figure
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