7,871 research outputs found
Universality of Single Spin Asymmetries in Hard Processes
We discuss the use of time reversal symmetry in the classification of parton
correlators. Specifically, we consider the role of (small) intrinsic transverse
momenta in these correlators and the determination of the proper color gauge
link. The transverse momentum weighted correlators in hard processes can be
expressed as a product of universal gluonic pole matrix elements and gluonic
pole cross sections.Comment: Contributed paper at DIS2006, 4 page
Single spin asymmetries in hadron-hadron collisions
We study weighted azimuthal single spin asymmetries in hadron-hadron
scattering using the diagrammatic approach at leading order and assuming
factorization. The effects of the intrinsic transverse momenta of the partons
are taken into account. We show that the way in which -odd functions, such
as the Sivers function, appear in these processes does not merely involve a
sign flip when compared with semi-inclusive deep inelastic scattering, such as
in the case of the Drell-Yan process. Expressions for the weighted scattering
cross sections in terms of distribution and fragmentation functions folded with
hard cross sections are obtained by introducing modified hard cross sections,
referred to as gluonic pole cross sections.Comment: 22 pages, 4 figures; minor text modifications and some additional
reference
Single-Transverse Spin Asymmetry in Dijet Correlations at Hadron Colliders
We present a phenomenological study of the single-transverse spin asymmetry
in azimuthal correlations of two jets produced nearly "back-to-back" in pp
collisions at RHIC. We properly take into account the initial- and final-state
interactions of partons that can generate this asymmetry in QCD
hard-scattering. Using distribution functions fitted to the existing
single-spin data, we make predictions for various weighted single-spin
asymmetries in dijet correlations that are now readily testable at RHIC.Comment: 14 pages, 2 figure
TMDlib and TMDplotter: library and plotting tools for transverse-momentum-dependent parton distributions
Transverse-momentum-dependent distributions (TMDs) are central in high-energy
physics from both theoretical and phenomenological points of view. In this
manual we introduce the library, TMDlib, of fits and parameterisations for
transverse-momentum-dependent parton distribution functions (TMD PDFs) and
fragmentation functions (TMD FFs) together with an online plotting tool,
TMDplotter. We provide a description of the program components and of the
different physical frameworks the user can access via the available
parameterisations.Comment: version 2, referring to TMDlib 1.0.2 - comments and references adde
Sivers and Boer-Mulders functions in Light-Cone Quark Models
Results for the naive-time-reversal-odd quark distributions in a light-cone
quark model are presented. The final-state interaction effects are generated
via single-gluon exchange mechanism. The formalism of light-cone wave functions
is used to derive general expressions in terms of overlap of wave-function
amplitudes describing the different orbital angular momentum components of the
nucleon. In particular, the model predictions show a dominant contribution from
S- and P-wave interference in the Sivers function and a significant
contribution also from the interference of P and D waves in the Boer-Mulders
function. The favourable comparison with existing phenomenological
parametrizations motivates further applications to describe azimuthal
asymmetries in hadronic reactions.Comment: references and explanations added; version to appear in Phys. Rev.
Beam Single-Spin Asymmetry in Semi-Inclusive Deep Inelastic Scattering
We calculate, in a model, the beam spin asymmetry in semi-inclusive jet
production in deep inelastic scattering. This twist-3, -odd observable is
non-zero due to final state strong interactions. With reasonable choices for
the parameters, one finds an asymmetry of several percent, about the size seen
experimentally. We present the result both as an explicit asymmetry calculation
and as a model calculation of the new transverse-momentum dependent
distribution function .Comment: 10 pages, 6 figures; minor changes made in the discussion; version
accepted for publicatio
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
Constraints from Dust Mass and Mass Accretion Rate Measurements on Angular Momentum Transport in Protoplanetary Disks
We investigate the relation between disk mass and mass accretion rate to
constrain the mechanism of angular momentum transport in protoplanetary disks.
Dust mass and mass accretion rate in Chamaeleon I are correlated with a slope
close to linear, similar to the one recently identified in Lupus. We
investigate the effect of stellar mass and find that the intrinsic scatter
around the best-fit Mdust-Mstar and Macc-Mstar relations is uncorrelated. Disks
with a constant alpha viscosity can fit the observed relations between dust
mass, mass accretion rate, and stellar mass, but over-predict the strength of
the correlation between disk mass and mass accretion rate when using standard
initial conditions. We find two possible solutions. 1) The observed scatter in
Mdust and Macc is not primoridal, but arises from additional physical processes
or uncertainties in estimating the disk gas mass. Most likely grain growth and
radial drift affect the observable dust mass, while variability on large time
scales affects the mass accretion rates. 2) The observed scatter is primordial,
but disks have not evolved substantially at the age of Lupus and Chamaeleon I
due to a low viscosity or a large initial disk radius. More accurate estimates
of the disk mass and gas disk sizes in a large sample of protoplanetary disks,
either through direct observations of the gas or spatially resolved
multi-wavelength observations of the dust with ALMA, are needed to discriminate
between both scenarios or to constrain alternative angular momentum transport
mechanisms such as MHD disk winds.Comment: See also the paper by Lodato et a
An ALMA Survey of faint disks in the Chamaeleon I star-forming region: Why are some Class II disks so faint?
ALMA surveys of nearby star-forming regions have shown that the dust mass in
the disk is correlated with the stellar mass, but with a large scatter. This
scatter could indicate either different evolutionary paths of disks or
different initial conditions within a single cluster. We present ALMA Cycle 3
follow-up observations for 14 Class II disks that were low S/N detections or
non-detections in our Cycle 2 survey of the Myr-old Chamaeleon I
star-forming region. With 5 times better sensitivity, we detect millimeter dust
continuum emission from six more sources and increase the detection rate to
94\% (51/54) for Chamaeleon I disks around stars earlier than M3. The
stellar-disk mass scaling relation reported in \citet{pascucci2016} is
confirmed with these updated measurements. Faint outliers in the
-- plane include three non-detections (CHXR71, CHXR30A, and T54)
with dust mass upper limits of 0.2 M and three very faint disks
(CHXR20, ISO91, and T51) with dust masses M. By
investigating the SED morphology, accretion property and stellar multiplicity,
we suggest for the three millimeter non-detections that tidal interaction by a
close companion (100 AU) and internal photoevaporation may play a role in
hastening the overall disk evolution. The presence of a disk around only the
secondary star in a binary system may explain the observed stellar SEDs and low
disk masses for some systems.Comment: ApJ accepte
The role of Cahn and Sivers effects in Deep Inelastic Scattering
The role of intrinsic \bfk_\perp in inclusive and semi-inclusive Deep
Inelastic Scattering processes () is studied with exact
kinematics within QCD parton model at leading order; the dependence of the
unpolarized cross section on the azimuthal angle between the leptonic and the
hadron production planes (Cahn effect) is compared with data and used to
estimate the average values of both in quark distribution and
fragmentation functions. The resulting picture is applied to the description of
the weighted single spin asymmetry recently
measured by the HERMES collaboration at DESY; this allows to extract some
simple models for the quark Sivers functions. These are compared with the
Sivers functions which succeed in describing the data on transverse single spin
asymmetries in \pup p \to \pi X processes; the two sets of functions are not
inconsistent. The extracted Sivers functions give predictions for the COMPASS
measurement of in agreement with recent
preliminary data, while their contribution to HERMES is
computed and found to be small. Predictions for for kaon production at HERMES are also given.Comment: 21 pages, 12 figures, revtex, version published in PRD, one figure,
comments and references adde
- …