arXiv subjects

G. M. Shore

Publications and source records attributed to G. M. Shore.

At least 19 recordsLinked to original sources

arXiv2022

Testing Fundamental Physics in Antihydrogen Experiments

The recent advent of high precision antihydrogen spectroscopy opens the way to stringent experimental tests of the fundamental principles underlying particle physics and general relativity (GR), such as Lorentz and CPT invariance and the Einstein Equivalence Principle (EEP), on pure antimatter systems. In this paper, the nature and implications of these tests is investigated, with special reference to the ALPHA antihydrogen programme at CERN. This is underpinned by a theoretical review of the role of antiparticles, causality and fundamental symmetries in relativistic quantum field theory (QFT) and the theory of time measurement in GR. Low-energy effective theories which break Lorentz and CPT symmetry, or the Strong Equivalence Principle (SEP), are then introduced, together with a review of several `fifth force' scenarios involving new long-range forces which would effectively violate the universality of free-fall, or Weak Equivalence Principle (WEPff). The possible role of CPT violation in determining the matter-antimatter asymmetry of the Universe is discussed. Explicit calculations are given for the dependence on possible Lorentz and CPT violating couplings of the transition frequencies amongst the 1S, 2S and 2P hyperfine states measured in the magnetic field of the ALPHA trap and the resulting bounds are compared with existing limits. An analysis of the implications for the EEP of current free-fall and spectroscopic measurements with antihydrogen is presented and existing and potential bounds on WEPff and the universality of clocks (WEPc) are derived, together with constraints on fifth forces. Future prospects for high-precision antihydrogen spectroscopy, free-fall and gravitational redshift experiments, and anti-atom matter-wave interferometry are described and experimental possibilities involving other antimatter species are briefly outlined.

hep-ph↗

arXiv2017

Leptogenesis and gravity: baryon asymmetry without decays

A popular class of theories attributes the matter-antimatter asymmetry of the Universe to CP-violating decays of super-heavy BSM particles in the Early Universe. Recently, we discovered a new source of leptogenesis in these models, namely that the same Yukawa phases which provide the CP violation for decays, combined with curved-spacetime loop effects, lead to an entirely new gravitational mechanism for generating an asymmetry, driven by the expansion of the Universe and independent of the departure of the heavy particles from equilibrium. In this Letter, we build on previous work by analysing the full Boltzmann equation, exploring the full parameter space of the theory and studying the time-evolution of the asymmetry. Remarkably, we find regions of parameter space where decays play no part at all, and where the baryon asymmetry of the Universe is determined solely by gravitational effects.

hep-ph↗

arXiv2015

Radiatively-induced gravitational leptogenesis

We 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.

hep-ph↗

arXiv2012

Polarized Structure Functions and Two-Photon Physics at Super-B

The potential of polarized, high-luminosity, moderate-energy e+e- colliders for performing unique measurements in fundamental QCD is described, with particular reference to the proposed Super-B facility. An extensive programme of 2-photon physics is proposed, focusing on measurements of the polarized photon structure functions g1 and g2 and pseudoscalar meson transition functions. The experimental requirements for Super-B to make the first measurement of the first moment sum rule for the off-shell polarized photon structure function g1(x,Q^2;K^2) are described in detail. Cross-section formulae and experimental issues for investigations of NLO and higher-twist effects in g1 and g2 together with exclusive 2-photon meson production are presented. This programme of QCD studies complements the core mission of Super-B as a high-luminosity B factory investigating flavour physics and rare processes signaling new physics beyond the standard model.

hep-ph↗

arXiv2007

The U(1)_A Anomaly and QCD Phenomenology

The role of the $U(1)_A$ anomaly in QCD phenomenology is reviewed, focusing on the relation between quark dynamics and gluon topology. Topics covered include a generalisation of the Witten-Veneziano formula for the mass of the $η'$, the determination of pseudoscalar meson decay constants, radiative pseudoscalar decays and the $U(1)_A$ Goldberger-Treiman relation. Sum rules are derived for the proton and photon structure functions $g_1^p$ and $g_1^\c$ measured in polarised deep-inelastic scattering. The first moment sum rule for $g_1^p$ (the `proton spin' problem) is confronted with new data from COMPASS and HERMES on the deuteron structure function and shown to be quantitatively explained in terms of topological charge screening. Proposals for experiments on semi-inclusive DIS and polarised two-photon physics at future $ep$ and high-luminosity $e^+ e^-$ colliders are discussed.

hep-ph↗

arXiv2007

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 $v_{\rm ph}(0)$ exceeds $c$. It is explained why causality is related not to $v_{\rm ph}(0)$ but to the high-frequency limit $v_{\rm ph}(\infty)$ 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 $Ł$-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.

hep-th↗

arXiv2006

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 $η,η'\to\c\c$, and the corresponding Dashen--Gell-Mann--Oakes--Renner relations, differ from conventional PCAC results due to the gluonic $U(1)_A$ 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 $N_c$, so we are able to give a quantitative assessment of the realisation of the $1/N_c$ expansion in the $U(1)_A$ sector of QCD. Applications to other aspects of $η'$ physics, including the relation with the first moment sum rule for the polarised photon structure function $g_1^\c$, are highlighted. The $U(1)_A$ Goldberger-Treiman relation is extended to accommodate SU(3) flavour breaking and the implications of a more precise measurement of the $η$ and $η'$-nucleon couplings are discussed. A comparison with the existing literature on pseudoscalar meson decay constants using large-$N_c$ chiral Lagrangians is also made.

hep-ph↗

arXiv2004

The Polarised Photon $g_1^γ$ Sum Rule at the Linear Collider and High Luminosity B Factories

The sum rule for the first moment of the polarised (virtual) photon structure function $g_1^γ(x,Q^2;K^2)$ is revisited in the light of proposals for future $e^+ e^-$ colliders. The sum rule exhibits an array of phenomena characteristic of QCD: for real photons ($K^2=0$) electromagnetic gauge invariance constrains the first moment to vanish; the limit for asymptotic photon virtuality ($m_ρ^2 \ll K^2 \ll Q^2$) is governed by the electromagnetic $U_A(1)$ axial anomaly and the approach to asymptopia by the gluonic anomaly; for intermediate values of $K^2$, it reflects the realisation of chiral symmetry and is determined by the off-shell radiative couplings of the pseudoscalar mesons; finally, like many polarisation phenomena in QCD, the first moment of $g_1^γ$ involves the gluon topological susceptibility. In this paper, we review the original sum rule proposed by Narison, Shore and Veneziano and extend the relation with pseudoscalar mesons. The possibility of measuring the sum rule in future polarised $e^+ e^-$ colliders is then considered in detail, focusing on the International Linear Collider (ILC) and high luminosity $B$ factories. We conclude that all the above features of the sum rule should be accessible at a polarised collider with the characteristics of SuperKEKB.

hep-ph↗

arXiv2003

Causality and Superluminal Light

The causal properties of curved spacetime, which underpin our sense of time in gravitational theories, are defined by the null cones of the spacetime metric. In classical general relativity, it is assumed that these coincide with the light cones determined by the physical propagation of light rays. However, the quantum vacuum acts as a dispersive medium for the propagation of light, since vacuum polarisation in QED induces interactions which effectively violate the strong equivalence principle (SEP). For low frequencies the phenomenon of gravitational birefringence occurs and indeed, for some metrics and polarisations, photons may acquire {\it superluminal} phase velocities. In this article, we review some of the remarkable features of SEP violating superluminal propagation in curved spacetime and discuss recent progress on the issue of dispersion, explaining why it is the high-frequency limit of the phase velocity that determines the characteristics of the effective wave equation and thus the physical causal structure.

gr-qc↗

arXiv2002

Constructing Time Machines

The existence of time machines, understood as spacetime constructions exhibiting physically realised closed timelike curves (CTCs), would raise fundamental problems with causality and challenge our current understanding of classical and quantum theories of gravity. In this paper, we investigate three proposals for time machines which share some common features: cosmic strings in relative motion, where the conical spacetime appears to allow CTCs; colliding gravitational shock waves, which in Aichelburg-Sexl coordinates imply discontinuous geodesics; and the superluminal propagation of light in gravitational radiation metrics in a modified electrodynamics featuring violations of the strong equivalence principle. While we show that ultimately none of these constructions creates a working time machine, their study illustrates the subtle levels at which causal self-consistency imposes itself, and we consider what intuition can be drawn from these examples for future theories.

gr-qc↗

arXiv2002

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.

gr-qc↗

arXiv2002

Faster than Light Photons in Gravitational Fields II - Dispersion and Vacuum Polarisation

Vacuum polarisation in QED in a background gravitational field induces interactions which effectively violate the strong equivalence principle and affect the propagation of light. In the low frequency limit, Drummond and Hathrell have shown that this mechanism leads to superluminal photon velocities. To confront this phenomenon with causality, however, it is necessary to extend the calculation of the phase velocity $\vp(\w)$ to high frequencies, since it is $\vp(\infty)$ which determines the characteristics of the effective wave equation and thus the causal structure. In this paper, we use a recently constructed expression, valid to all orders in a derivative expansion, for the effective action of QED in curved spacetime to determine the frequency dependence of the phase velocity and investigate whether superluminal velocities indeed persist in the high frequency limit.

gr-qc↗

arXiv2001

$η'(η)\toγγ$: A Tale of Two Anomalies

The radiative decays of the pseudoscalar mesons $P=(π^0,η,η')$ are of special interest as they provide an experimental window on the electromagnetic and colour chiral $U_A(1)$ anomalies. $π^0\toγγ$ is well described by the electromagnetic $U_A(1)$ anomaly under the assumption that $π^0$ is a Goldstone boson for spontaneously broken chiral symmetry, but the analogous results for $η'(η)\toγγ$ are complicated by the colour $U_A(1)$ anomaly in QCD. This paper reviews the theory of $η'(η)\toγγ$ decays, emphasising the role of the colour $U_A(1)$ anomaly and the renormalisation group. The relation to the Witten-Veneziano mass formula for the $η'$ and the QCD topological susceptibility is derived. The implications for the phenomenological analysis of $P\toγγ$ are described and a proposal is made for a comprehensive re-analysis of existing data on $P\toγγ$, $P\to Vγ$, where $V=(ρ,ω, ϕ)$ is a light $1^-$ meson, $η'(η)\to ππγ$, $ψ\toη'(η)γ$, etc. Other applications, to the muon anomalous magnetic moment $g_\m-2$ and the determination of the photon structure function $g_1^γ$ in polarised deep inelastic scattering, are also briefly discussed.

hep-ph↗

arXiv2000

Accelerating Photons with Gravitational Radiation

The nature of superluminal photon propagation in the gravitational field describing radiation from a time-dependent, isolated source (the Bondi-Sachs metric) is considered in an effective theory which includes interactions which violate the strong equivalence principle. Such interactions are, for example, generated by vacuum polarisation in conventional QED in curved spacetime. The relation of the resulting light-cone modifications to the Peeling Theorem for the Bondi-Sachs spacetime is explained.

gr-qc↗

arXiv2000

The Angular Momentum and g_1^p Sum Rules for the Proton

The gauge invariant operator formulation of the angular momentum sum rule ${1\over2} = J_q + J_g$ for the proton is presented and contrasted with the sum rule for the first moment of the polarised structure function $g_1^p$. The decoupling of the axial charge $a^0$ 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.

hep-ph↗

arXiv2000

The Gauge-Invariant Angular Momentum Sum-Rule for the Proton

We give a gauge-invariant treatment of the angular momentum sum-rule for the proton in terms of matrix elements of three gauge-invariant, local composite operators. These matrix elements are decomposed into three independent form factors, one of which is the flavour singlet axial charge. The other two are interpreted as total quark and gluon angular momentum. We further show that the axial charge cancels out of the sum-rule. The general form of the renormalisation mixing of the three operators is written down and also determined to one loop from which the scale dependence and mixing of the form factors is derived. We relate these results to a previous parton model calculation by defining the parton model quantities in terms of the three form factors. We also mention how the form factors can be measured in experiments.

hep-ph↗

arXiv1999

Correlation Functions of the Energy Momentum Tensor on Spaces of Constant Curvature

An analysis of one and two point functions of the energy momentum tensor on homogeneous spaces of constant curvature is undertaken. The possibility of proving a $c$-theorem in this framework is discussed, in particular in relation to the coefficients $c,a$, which appear in the energy momentum tensor trace on general curved backgrounds in four dimensions. Ward identities relating the correlation functions are derived and explicit expressions are obtained for free scalar, spinor field theories in general dimensions and also free vector fields in dimension four. A natural geometric formalism which is independent of any choice of coordinates is used and the role of conformal symmetries on such constant curvature spaces is analysed. The results are shown to be constrained by the operator product expansion. For negative curvature the spectral representation, involving unitary positive energy representations of $O(d-1,2)$, for two point functions of vector currents is derived in detail and extended to the energy momentum tensor by analogy. It is demonstrated that, at non coincident points, the two point functions are not related to $a$ in any direct fashion and there is no straightforward demonstration obtainable in this framework of irreversibility under renormalisation group flow of any function of the couplings for four dimensional field theories which reduces to $a$ at fixed points.

hep-th↗

arXiv1999

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.

hep-ph↗