1,470 research outputs found
The photon angular momentum contrroversy: Resolution of a conflict between laser optics and particle physics
The claim some years ago, contrary to all textbooks, that the angular
momentum of a photon (and gluon) can be split in a gauge-invariant way into an
orbital and spin term, sparked a major controversy in the Particle Physics
community. A further cause of upset was the realization that the gluon
polarization in a nucleon, a supposedly physically meaningful quantity,
corresponds only to the gauge-variant gluon spin derived from Noether's
theorem, evaluated in a particular gauge. On the contrary, Laser Physicists
have, for decades, been happily measuring physical quantities which correspond
to orbital and spin angular momentum evaluated in a particular gauge. This
paper reconciles the two points of view.Comment: 6 pages, no figure
Parton intrinsic motion: suppression of the Collins mechanism for transverse single spin asymmetries in p(transv. polarised) p --> pion + X
We consider a general formalism to compute inclusive polarised and
unpolarised cross sections within pQCD and the factorisation scheme, taking
into account parton intrinsic motion in distribution and fragmentation
functions, as well as in the elementary dynamics. Surprisingly, the intrinsic
partonic motion, with all the correct azimuthal angular dependences, produces a
strong suppression of the transverse single spin asymmetry arising from the
Collins mechanism. As a consequence, and in contradiction with earlier claims,
the Collins mechanism is unable to explain the large asymmetries found in
p(transv. polarised) p --> pion + X at moderate to large Feynman x_F. The
Sivers effect is not suppressed.Comment: LaTeX, 21+1 pages, 1 ps figur
The angular momentum controversy: What's it all about and does it matter?
The general question, crucial to an understanding of the internal structure
of the nucleon, of how to split the total angular momentum of a photon or gluon
into spin and orbital contributions is one of the most important and
interesting challenges faced by gauge theories like Quantum Electrodynamics and
Quantum Chromodynamics. This is particularly challenging since all QED
textbooks state that such an splitting cannot be done for a photon (and a
fortiori for a gluon) in a gauge-invariant way, yet experimentalists around the
world are engaged in measuring what they believe is the gluon spin! This
question has been a subject of intense debate and controversy, ever since, in
2008, it was claimed that such a gauge-invariant split was, in fact, possible.
We explain in what sense this claim is true and how it turns out that one of
the main problems is that such a decomposition is not unique and therefore
raises the question of what is the most natural or physical choice. The
essential requirement of measurability does not solve the ambiguities and leads
us to the conclusion that the choice of a particular decomposition is
essentially a matter of taste and convenience. In this review, we provide a
pedagogical introduction to the question of angular momentum decomposition in a
gauge theory, present the main relevant decompositions and discuss in detail
several aspects of the controversies regarding the question of gauge
invariance, frame dependence, uniqueness and measurability. We stress the
physical implications of the recent developments and collect into a separate
section all the sum rules and relations which we think experimentally relevant.
We hope that such a review will make the matter amenable to a broader community
and will help to clarify the present situation.Comment: 96 pages, 11 figures, 5 tables, review prepared for Physics Report
Parton distribution functions of proton in a light-front quark-diquark model
We present the parton distribution functions (PDFs) for un- polarised,
longitudinally polarized and transversely polarized quarks in a proton using
the light-front quark diquark model. We also present the scale evolution of
PDFs and calculate axial charge and tecsor charge for and quarks at a
scale of experimental findings.Comment: XXII DAE-BRNS High Energy Physics Symposium, December 12-16, 2016,
University of Delhi, India; 4 pages, 1 figur
Some Remarks on Methods of QCD Analysis of Polarized DIS Data
The results on polarized parton densities (PDFs) obtained using different
methods of QCD analysis of the present polarized DIS data are discussed. Their
dependence on the method used in the analysis, accounting or not for the
kinematic and dynamic 1/Q^2 corrections to spin structure function g_1, is
demonstrated. It is pointed out that the precise data in the preasymptotic
region require a more careful matching of the QCD predictions to the data in
this region in order to determine the polarized PDFs correctly.Comment: 14 pages, 8 figure
General partonic structure for hadronic spin asymmetries
The high energy and large p_T inclusive polarized process, (A, S_A) + (B,
S_B) --> C + X, is considered under the assumption of a generalized QCD
factorization scheme. For the first time all transverse motions, of partons in
hadrons and of hadrons in fragmenting partons, are explicitly taken into
account; the elementary interactions are computed at leading order with
noncollinear exact kinematics, which introduces many phases in the expressions
of their helicity amplitudes. Several new spin and k_T dependent soft functions
appear and contribute to the cross sections and to spin asymmetries; we put
emphasis on their partonic interpretation, in terms of quark and gluon
polarizations inside polarized hadrons. Connections with other notations and
further information are given in some Appendices. The formal expressions for
single and double spin asymmetries are derived. The transverse single spin
asymmetry A_N, for p(transv. polarized) p --> pion + X processes is considered
in more detail, and all contributions are evaluated numerically by saturating
unknown functions with their upper positivity bounds. It is shown that the
integration of the phases arising from the noncollinear kinematics strongly
suppresses most contributions to the single spin asymmetry, leaving at work
predominantly the Sivers effect and, to a lesser extent, the Collins mechanism.Comment: RevTeX, 46 pages, 5 ps figures. v2: some clarifying comments and
appendix on kinematics added, references updated, published versio
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