Roberta Armillis

Conformal anomalies and the gravitational effective action: The TJJ 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 (TJJ), which is responsible for the generation of the gauge contributions to the conformal anomaly in gravity. We also present the anomalous effective action expanded in the inverse mass of the fermion as in the Euler-Heisenberg case.

Trilinear anomalous gauge interactions from intersecting branes and the neutral currents sector

We present a study of the trilinear gauge interactions in extensions of the Standard Model (SM) with several anomalous extra U(1)s, identified in various constructions, from special vacua of string theory to large extra dimensions. In these models an axion and generalized Chern-Simons interactions for anomalies cancellation are present. We derive generalized Ward identities for these vertices and discuss their structure in the Stuckelberg and Higgs-Stuckelberg phases. We give their explicit expressions in all the relevant cases, which can be used for explicit phenomenological studies of these models at the LHC.

An anomalous extra Z prime from intersecting branes with Drell–Yan and direct photons at the LHC

Abstract 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 Stuckelberg 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 small, below the size of the typical QCD corrections quantified by NNLO K -factors. Measurements at the LHC on the Z resonance in leptoproduction will be able to exclude a class of these models for variations of the cross section at the level of 4%, which is obtained at larger values of the anomalous coupling ( g B > 1 ). However, If they are not excluded, their anomalous nature is unlikely to be resolved with an inclusive NNLO analysis performed in these key processes.

Anomaly poles as common signatures of chiral and conformal anomalies

Abstract 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.

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 g−2 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.

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.

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 (J ≡ JV), to the case of mixed axial-vector/vector currents (JA). The study is performed in analogy to the case of the AVV 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 TJV JA 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.

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 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.

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. The subtraction of these anomaly poles appears necessary for the unitarization of the theory in the ultraviolet, but it may generate an infrared instability of the corresponding effective action, signalled by ghost condensation. In particular, the significance of the subtraction of the superanomaly multiplet by a pole in superspace is dubious, 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.

The Trace Anomaly and the Couplings of QED and QCD to Gravity

We report on the computation of the effective actions describing the interaction of gravity both for an abelian and a non‐abelian gauge theory, mediated by the trace anomaly.