7,503 research outputs found
The 3D entangled structure of the proton; transverse degrees of freedom in QCD, momenta, spins and more
Light-front quantized quark and gluon states (partons) play a dominant role
in high energy scattering processes. Initial state hadrons are mixed ensembles
of partons, while produced pure partonic states appear as mixed ensembles of
hadrons. The transition from collinear hard physics to the 3D structure
including partonic transverse momenta is related to confinement which links
color and spatial degrees of freedom. We outline ideas on emergent symmetries
in the Standard Model and their connection to the 3D structure of hadrons.
Wilson loops, including those with light-like Wilson lines such as used in the
studies of transverse momentum dependent distribution functions (TMDs) may play
a crucial role here, establishing a direct link between transverse spatial
degrees of freedom and gluonic degrees of freedom.Comment: 8 pages, invited talk presented at the Lightcone 2017 Workshop, 18-22
Sep 2017, Mumbai, India; prepared for proceedings to be published in Few Body
Physic
Spin Physics and Transverse Structure
Spin is a welcome complication in the study of partonic structure that has
led to new insights, even if theoretically and experimentally not all dust has
settled, in particular on quark flavor dependence and gluon spin. At the same
time it opened new questions on angular momentum and effects of transverse
structure. In this talk the focus is on the role of the transverse momenta of
partons. Like for collinear parton distribution functions (PDFs), we are also
in the case of transverse momentum dependent (TMD) PDFs, talking about forward
matrix elements. TMD PDFs (or in short TMDs) extend collinear PDFs with only
spin-spin correlations to PDFs that include spin-momentum correlations,
including also time-reversal-odd (T-odd) correlations, relevant for the
description of single spin asymmetries. In this way TMDs open up new ways of
studying the spin structure. Their operator structure within QCD, however, is
more complex leading to various ways of breaking of universality.Comment: 8 pages, to appear in proceedings of DIS2015 (XXIII International
Workshop on Deep-Inelastic Scattering and Related Subjects, 27 April - 1 May,
2015, Dallas, USA
Current fragmentation in semiinclusive leptoproduction
Current fragmentation in semiinclusive deep inelastic leptoproduction offers,
besides refinement of inclusive measurements such as flavor separation and
access to the chiral-odd quark distribution functions ,
the possibility to investigate intrinsic transverse momentum of hadrons via
azimuthal asymmetries.Comment: 14 pages, Latex using aipproc.sty and epsfig.sty; plenary talk given
at the Second Workshop on Physics with an Electron Polarized light-Ion
Collider (EPIC), Sept. 14-16, 2000, MIT, Cambridge, US
Sivers Effect Asymmetries in Hadronic Collisions
We argue that weighted azimuthal single spin asymmetries in back-to-back jet
or pion production in polarized proton-proton scattering can be written as
convolutions of universal distribution and fragmentation functions with gluonic
pole cross sections as hard functions. Gluonic pole cross sections are
gauge-invariant weighted sums of Feynman diagrams. The weight factors are a
direct consequence of the (diagram-dependence of) gauge links. The best known
consequence of the gauge links is the generation of the Sivers effect that is a
source for single-spin asymmetries. Moreover, due to the dependence of the
gauge links on the color-flow of the hard diagram the Sivers effect in SIDIS
enters with opposite sign as it does in Drell-Yan scattering. The weight
factors in the gluonic pole cross sections are the appropriate generalizations
to more complicated processes of this relative sign difference. Furthermore, it
is argued that the gluon-Sivers effect appears in twofold.Comment: Contribution to the 17th International Spin Physics Symposium
(SPIN2006), Kyoto, Japan, Oct. 2-7, 200
Gluonic Pole Cross Sections and Single Spin Asymmetries in Hadron-Hadron Scattering
The gauge-links connecting the parton field operators in the hadronic matrix
elements appearing in the transverse momentum dependent distribution functions
give rise to T-odd effects. Due to the process-dependence of the gauge-links
the T-odd distribution functions appear with different pre-factors. A
consequence is that in the description of single spin asymmetries the parton
distribution and fragmentation functions are convoluted with gluonic pole cross
sections rather than the basic partonic cross sections. In this paper we
calculate the gluonic pole cross sections encountered in single spin
asymmetries in hadron-hadron scattering. The case of back-to-back pion
production in polarized proton-proton scattering is worked out explicitly. It
is shown how T-odd gluon distribution functions originating from gluonic pole
matrix elements appear in twofold.Comment: v2: includes explicit definitions of polarized cross sections + minor
corrections; to appear in JHE
Non-universality of transverse momentum dependent parton distribution functions
In the field theoretical description of hadronic scattering processes, single
transverse-spin asymmetries arise due to gluon initial and final state
interactions. These interactions lead to process dependent Wilson lines in the
operator definitions of transverse momentum dependent parton distribution
functions. In particular for hadron-hadron scattering processes with hadronic
final states this has important ramifications for possible factorization
formulas in terms of (non)universal TMD parton distribution functions. In this
paper we will systematically separate the universality-breaking parts of the
TMD parton correlators from the universal T-even and T-odd parts. This might
play an important role in future factorization studies for these processes. We
also show that such factorization theorems will (amongst others) involve the
gluonic pole cross sections, which have previously been shown to describe the
hard partonic scattering in weighted spin asymmetries.Comment: v2: some textual changes in the paper and corrections in references,
to appear in Nucl. Phys.
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