29 research outputs found
Trace Anomaly, Massless Scalars and the Gravitational Coupling of QCD
The anomalous effective action describing the coupling of gravity to a
non-abelian gauge theory can be determined by a variational solution of the
anomaly equation, as shown by Riegert long ago. It is given by a nonlocal
expression, with the nonlocal interaction determined by the Green's function of
a conformally covariant operator of fourth order. In recent works it has been
shown that this interaction is mediated by a simple pole in an expansion around
a Minkowski background, coupled in the infrared in the massless fermion limit.
This result relies on the local formulation of the original action in terms of
two auxiliary fields, one physical scalar and one ghost, which take the role of
massless composite degrees of freedom. In the gravity case, the two scalars
have provided ground in favour of some recent proposals of an infrared approach
to the solution of the dark energy problem, entirely based on the behaviour of
the vacuum energy at the QCD phase transition. As a test of this general
result, we perform a complete one-loop computation of the effective action
describing the coupling of a non-abelian gauge theory to gravity. We confirm
the appearance of an anomaly pole which contributes to the trace part of the
correlator and of extra poles in its trace-free part, in the quark and
gluon sectors, describing the coupling of the energy momentum tensor () to
two non abelian gauge currents ().Comment: 25 pages, 9 figures. Revised final version, to be published on Phys.
Rev.
Trilinear Gauge Interactions in Extensions of the Standard Model and Unitarity
We summarize recent work on the characterization of anomaly poles in
connection with the field-theory interpretation of the Green-Schwarz mechanism
of anomaly cancellation and on their effective field theories, stressing on the
properties of the anomaly vertex in two representations, the Rosenberg and the
Longitudinal/Transverse. The presence of polar amplitudes in these theories
causes a violation of unitarity at high energy which is cured by the exchange
of the axion. We comment on the possible physical implications of this
mechanism.Comment: 5 pages, 6 Figs. Presented at IFAE 2009, Bari, 15-17 April 2009,
Ital
The Trace Anomaly and the Gravitational Coupling of an Anomalous U(1)
We extend a previous computation of the TJJ correlator, involving the
energy-momentum tensor of an abelian gauge theory and two vector currents, to
the case of mixed axial-vector/vector currents. The study is performed in
analogy to the case of the vertex for the chiral anomaly. We derive the
general structure of the anomalous Ward identities and provide explicit tests
of their consistency using Dimensional Reduction. Mixed massive correlators of
the form are shown to vanish both by Ward identities and by
C-invariance. The result is characterized by the appearance of massless scalar
degrees of freedom in the coupling of chiral and vector theories to gravity,
affecting both the soft and the ultraviolet region of the vertex. This is in
agreement with previous studies of the effective action of gauge and conformal
anomalies in QED and QCD.Comment: 30 pages, 3 figure
Conformal Anomalies and the Gravitational Effective Action: The Correlator for a Dirac Fermion
We compute in linearized gravity all the contributions to the gravitational
effective action due to a virtual Dirac fermion, related to the conformal
anomaly. This requires, in perturbation theory, the identification of the
gauge-gauge-graviton vertex off mass shell, involving the correlator of the
energy-momentum tensor and two vector currents (), which is responsible
for the generation of the gauge contributions to the conformal anomaly in
gravity. We also present the anomalous effective action in the inverse mass of
the fermion as in the Euler-Heisenberg case.Comment: 47 pages, 1 figure. Revised final version, contains 1 additional
section. Accepted for pubblication on Phys. Rev.
Anomalous U(1) Models in Four and Five Dimensions and their Anomaly Poles
We analyze the role played by anomaly poles in an anomalous gauge theory by
discussing their signature in the corresponding off-shell effective action. The
origin of these contributions, in the most general kinematical case, is
elucidated by performing a complete analysis of the anomaly vertex at
perturbative level. We use two independent (but equivalent) representations:
the Rosenberg representation and the longitudinal/transverse (L/T)
parameterization, used in recent studies of of the muon and in the proof
of non-renormalization theorems of the anomaly vertex. The poles extracted from
the L/T parameterization do not couple in the infrared for generic anomalous
vertices, as in Rosenberg, but we show that they are responsible for the
violations of unitarity in the UV region, using a class of pole-dominated
amplitudes. We conclude that consistent formulations of anomalous models
require necessarily the cancellation of these polar contributions. Establishing
the UV significance of these terms provides a natural bridge between the
anomalous effective action and its completion by a nonlocal theory. Some
additional difficulties with unitarity of the mechanism of inflow in extra
dimensional models with an anomalous theory on the brane, due to the presence
of anomaly poles, are also pointed out.Comment: Revised final version, to appear on JHE
Anomaly Poles as Common Signatures of Chiral and Conformal Anomalies
One feature of the chiral anomaly, analyzed in a perturbative framework, is
the appearance of massless poles which account for it. They are identified by a
spectral analysis of the anomaly graph and are usually interpreted as being of
an infrared origin. Recent investigations show that their presence is not just
confined in the infrared, but that they appear in the effective action under
the most general kinematical conditions, even if they decouple in the infrared.
Further studies reveal that they are responsible for the non-unitary behaviour
of these theories in the ultraviolet (UV) region. We extend this analysis to
the case of the conformal anomaly, showing that the effective action describing
the interaction of gauge fields with gravity is characterized by anomaly poles
that give the entire anomaly and are decoupled in the infrared (IR), in
complete analogy with the chiral case. This complements a related analysis by
Giannotti and Mottola on the trace anomaly in gravity, in which an anomaly pole
has been identified in the corresponding correlator using dispersion theory in
the IR. Our extension is based on an exact computation of the off-shell
correlation function involving an energy-momentum tensor and two vector
currents (the gauge-gauge-graviton vertex) which is responsible for the
appearance of the anomaly.Comment: 13 pages, 4 figures revised final version, to appear on Phys. Lett.
Axions and Anomaly-Mediated Interactions: The Green-Schwarz and Wess-Zumino Vertices at Higher Orders and g-2 of the muon
We present a study of the mechanism of anomaly cancellation using only
transverse invariant amplitudes on anomaly diagrams at higher perturbative
orders. The method is the realization of the Green-Schwarz (GS) mechanism at
field theory level, which restores the Ward identities by a subtraction of the
anomaly pole. Some of the properties of the GS vertex are analyzed both in the
context of unitarity and of the organization of the related perturbative
expansion. We investigate the role played by the GS and the Wess-Zumino
vertices in the anomalous magnetic moment of the muon and in the hyperfine
splitting of muonium, which are processes that can be accompanied by the
exchange of a virtual anomalous extra Z-prime and an axion-like particle.Comment: 51 Pages, 31 Figures, Revised final version to appear on JHE
An Anomalous Extra Z Prime from Intersecting Branes with Drell-Yan and Direct Photons at the LHC
We quantify the impact of gauge anomalies at the Large Hadron Collider by
studying the invariant mass distributions in Drell-Yan and in double prompt
photon, using an extension of the Standard Model characterized by an additional
anomalous U(1) derived from intersecting branes. The approach is rather general
and applies to any anomalous abelian gauge current. Anomalies are cancelled
using either the Wess-Zumino mechanism with suitable Peccei-Quinn-like
interactions and a Stueckelberg axion, or by the Green-Schwarz mechanism. We
compare predictions for the corresponding extra Z-prime to anomaly-free
realizations such as those involving U(1)_{B-L}. We identify the leading
anomalous corrections to both channels, which contribute at higher orders, and
compare them against the next-to-next-to-leading order (NNLO) QCD background.
Anomalous effects in these inclusive observables are found to be very small,
far below the percent level and below the size of the typical QCD corrections
quantified by NNLO K-factors.Comment: 46 pages, 36 figures, comments and citations adde
The Search for Extra Neutral Currents at the LHC: QCD and Anomalous Gauge Interactions
Extensions of the Standard Model with extra neutral currents due to
additional anomalous abelian gauge factors are considered. We summarize the
main features of the effective action associated to these theories. They are
characterized by an axion-like particle (the {\em axi-higgs}) which can be
(almost) massless, with its mass generated non-perturbatively in the QCD vacuum
as for an ordinary Peccei-Quinn axion, but that can also mix with the scalars
of the Higgs sector, becoming a heavy axion. We briefly describe the interplay
between the electroweak and the QCD sectors in these types of theories, which
emerge either from special vacua of string/brane theory; from partial
decoupling of a heavier fermion sector or from an anomaly inflow in the context
of models with extra dimensions.Comment: Talk given at QCD@work 2007, Martina Franca, Italy, 16-20 June 2007,
5 pages, 2 figures. To be published in the American Institute of Physics
(AIP) conference proceeding
Comments on Anomaly Cancellations by Pole Subtractions and Ghost Instabilities with Gravity
We investigate some aspects of anomaly cancellation realized by the
subtraction of an anomaly pole, stressing on some of its properties in
superspace. In a local formulation these subtractions can be described in terms
of a physical scalar, an axion and related ghosts. They appear to be necessary
for the unitarization of the theory in the ultraviolet, but they may generate
an infrared instability of the corresponding effective action, signalled by
ghost condensation. In particular the subtraction of the superanomaly multiplet
by a pole in superspace is of dubious significance, due to the different nature
of the chiral and conformal anomalies. In turn, this may set more stringent
constraints on the coupling of supersymmetric theories to gravity.Comment: 18 pages. Revised version. To appear in "Classical and Quantum
Gravity