98 research outputs found
The Angular Momentum and g_1^p Sum Rules for the Proton
The gauge invariant operator formulation of the angular momentum sum rule
for the proton is presented and contrasted with the sum
rule for the first moment of the polarised structure function . The
decoupling of the axial charge from the angular momentum sum rule is
highlighted and the possible QCD field-theoretic basis for an angular momentum
sum rule of the form {1\over2} = {1\over2}\D q + \D g + L_q + L_g is
critically discussed.Comment: 5 pages, LaTeX, uses espcrc2.sty; invited talk at QCD00, Montpellier,
July 200
The effects of an extra U(1) axial condensate on the radiative decay eta' --> gamma gamma at finite temperature
Supported by recent lattice results, we consider a scenario in which a
U(1)-breaking condensate survives across the chiral transition in QCD. This
scenario has important consequences on the pseudoscalar-meson sector, which can
be studied using an effective Lagrangian model. In particular, generalizing the
results obtained in a previous paper (where the zero-temperature case was
considered), we study the effects of this U(1) chiral condensate on the
radiative decay eta' --> gamma gamma at finite temperature.Comment: 15 pages, LaTeX fil
A Local Effective Action for Photon-Gravity Interactions
Quantum phenomena such as vacuum polarisation in curved spacetime induce
interactions between photons and gravity with quite striking consequences,
including the violation of the strong equivalence principle and the apparent
prediction of `superluminal' photon propagation. These quantum interactions can
be encoded in an effective action. In this paper, we extend previous results on
the effective action for QED in curved spacetime due to Barvinsky, Vilkovisky
and others and present a new, local effective action valid to all orders in a
derivative expansion, as required for a full analysis of the quantum theory of
high-frequency photon propagation in gravitational fields.Comment: 22 pages, 9 figures, harvmac Te
Radiatively-induced gravitational leptogenesis
AbstractWe demonstrate how loop effects in gravitational backgrounds lead to a difference in the propagation of matter and antimatter, and show this is forbidden in flat space due to CPT and translation invariance. This mechanism, which is naturally present in beyond the standard model (BSM) theories exhibiting C and CP violation, generates a curvature-dependent chemical potential for leptons in the low-energy effective Lagrangian, allowing a matter–antimatter asymmetry to be generated in thermodynamic equilibrium, below the BSM scale
Target independence of the `proton spin' effect
Recent work by the author in collaboration with S. Narison and G. Veneziano
on the EMC-SMC-SLAC `proton spin' effect is reviewed. This uses a novel
approach to deep inelastic scattering in which the matrix elements arising from
the OPE are factorised into composite operator propagators and proper vertices.
For polarised scattering, the composite operator propagator is equated
to the square root of the first moment of the QCD topological susceptibility,
. We evaluate using QCD spectral sum
rules and find a significant suppression relative to its OZI expectation. This
is identified as the source of the violation of the Ellis-Jaffe sum rule for
the first moment of the polarised proton structure function . Our
predictions, and
, are in excellent agreement with the new SMC
data. This supports our earlier conjecture that the suppression in the flavour
singlet component of the first moment of is a target-independent
feature of QCD related to the anomaly and is not a special property of
the proton structure.Comment: 6 pages, SWAT 94-4
Radiative Decays, the Topological Susceptibility and the Witten-Veneziano Mass Formula
The formulae describing the radiative decays \eta'(\eta)\rta\c\c in QCD
beyond the chiral limit are derived. The modifications of the conventional PCAC
formulae due to the gluonic contribution to the axial anomaly in the flavour
singlet channel are precisely described. The decay constants are found to
satisfy a modified Dashen formula which generalises the Witten--Veneziano
formula for the mass of the . Combining these results, it is shown how
the topological susceptibility in QCD with massive, dynamical quarks may be
extracted from measurements of \eta'(\eta)\rta\c\c.Comment: TeX, 21 pages, no figures, uses harvma
The `Proton Spin' Effect - Theoretical Status '97
The theoretical status of the `proton spin' effect is reviewed. The
conventional QCD parton model analysis of polarised DIS is compared with a
complementary approach, the composite operator propagator-vertex (CPV) method,
each of which provides its own insight into the origin of the observed
suppression in the first moment of . The current status of both
experiment and non-perturbative calculations is summarised. The future role of
semi-inclusive DIS experiments, in both the current and target fragmentation
regions, is described.Comment: Review talk at QCD97 Montpellier, July 1997. 12 pages, LaTeX, incl.
13 figures. Typo corrected in eq(36
Target Fragmentation in Semi-Inclusive DIS: Fracture Functions, Cut Vertices and the OPE
We discuss semi-inclusive Deep Inelastic Scattering (DIS) in the z -> 1
limit, in particular the relationship between fracture functions, generalised
cut vertices and Green functions of the composite operators arising in the OPE.
The implications, in the spin-polarised case, for testing whether the "proton
spin" effect is target-independent are explored. Explicit calculations in
(phi^3)_6 theory are presented which are consistent with our observations.Comment: 22 pages, 25 figures, LaTeX 2e; uses graphics packag
Pseudoscalar Meson Decay Constants and Couplings, the Witten-Veneziano Formula beyond large N_c, and the Topological Susceptibility
The QCD formulae for the radiative decays \eta,\eta'\to\c\c, and the
corresponding Dashen--Gell-Mann--Oakes--Renner relations, differ from
conventional PCAC results due to the gluonic axial anomaly. This
introduces a critical dependence on the gluon topological susceptibility. In
this paper, we revisit our earlier theoretical analysis of radiative
pseudoscalar decays and the DGMOR relations and extract explicit experimental
values for the decay constants. This is our main result. The flavour singlet
DGMOR relation is the generalisation of the Witten-Veneziano formula beyond
large , so we are able to give a quantitative assessment of the
realisation of the expansion in the sector of QCD.
Applications to other aspects of physics, including the relation with
the first moment sum rule for the polarised photon structure function g_1^\c,
are highlighted. The Goldberger-Treiman relation is extended to
accommodate SU(3) flavour breaking and the implications of a more precise
measurement of the and -nucleon couplings are discussed. A
comparison with the existing literature on pseudoscalar meson decay constants
using large- chiral Lagrangians is also made.Comment: 27 pages, 4 figure
Superluminality and UV Completion
The idea that the existence of a consistent UV completion satisfying the
fundamental axioms of local quantum field theory or string theory may impose
positivity constraints on the couplings of the leading irrelevant operators in
a low-energy effective field theory is critically discussed. Violation of these
constraints implies superluminal propagation, in the sense that the
low-frequency limit of the phase velocity exceeds . It is
explained why causality is related not to but to the
high-frequency limit and how these are related by the
Kramers-Kronig dispersion relation, depending on the sign of the imaginary part
of the refractive index \Ima n(\w) which is normally assumed positive.
Superluminal propagation and its relation to UV completion is investigated in
detail in three theories: QED in a background electromagnetic field, where the
full dispersion relation for n(\w) is evaluated numerically for the first
time and the role of the null energy condition T_{\m\n}k^\m k^\n \ge 0 is
highlighted; QED in a background gravitational field, where examples of
superluminal low-frequency phase velocities arise in violation of the
positivity constraints; and light propagation in coupled laser-atom
\L-systems exhibiting Raman gain lines with \Ima n(\w) < 0. The possibility
that a negative \Ima n(\w) must occur in quantum field theories involving
gravity to avoid causality violation, and the implications for the relation of
IR effective field theories to their UV completion, are carefully analysed.Comment: 42 pages, 14 figure
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