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M. R. Pennington

Publications and source records attributed to M. R. Pennington.

At least 37 records · Page 2Linked to original sources
arXiv2011

Glimpsing Colour in a World of Black and White

The past 40 years have taught us that nucleons are built of constituents that carry colour charges with interactions governed by Quantum Chromodynamics (QCD). How experiments (past, present and future) at Jefferson Lab probe colourless nuclei to map out these internal colour degrees of freedom is presented. When combined with theoretical calculations, these will paint a picture of how the confinement of quarks and gluons, and the structure of the QCD vacuum, determine the properties of all (light) strongly interacting states.

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arXiv2011

Chiral Perturbation Theory, the ${1/N_c}$ expansion and Regge behaviour determine the structure of the lightest scalar meson

The leading $1/N_c$ behaviour of Unitarised Chiral Perturbation Theory distinguishes the nature of the $ρ$ and the $σ$. At one loop order the $ρ$ is a ${\bar q}q$ meson, while the $σ$ is not. However, semi-local duality between resonances and Regge behaviour cannot be satisfied for larger $N_c$, if such a distinction holds. While the $σ$ at $N_c=3$ is inevitably dominated by its di-pion component, Unitarised Chiral Perturbation Theory beyond one loop order reveals that as $N_c$ increases above 6-8, the $σ$ has a sub-dominant ${\bar q}q$ fraction up at 1.2 GeV. Remarkably this ensures semi-local duality is fulfilled for the range of $N_c \lesssim 15$, where the unitarisation procedure adopted applies.

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arXiv2011

Exciting Baryons: now and in the future

This is the final talk of NSTAR2011 conference. It is not a summary talk, but rather a looking forward to what still needs to be done in excited baryon physics. In particular, we need to hone our tools connecting experimental inputs with QCD. At present we rely on models that often have doubtful connections with the underlying theory, and this needs to be dramatically improved, if we are to reach definitive conclusions about the relevant degrees of freedom of excited baryons. Conclusions that we want to have by NSTAR2021.

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arXiv2011

The nature of the lightest scalar meson, its Nc behaviour and semi-local duality

One-loop unitarized Chiral Perturbation Theory (UChPT) calculations, suggest a different Nc behaviour for the sigma or f0(600) and rho(770) mesons: while the rho meson becomes narrower with Nc, as expected for a qqbar meson, the f0(600) contribution to the total cross section below 1 GeV becomes less and less important. Here we review our recent work where we have shown, by means of finite energy sum rules, that a different Nc behaviour for these resonances may lead to a conflict with semi-local duality for large Nc, since local duality requires a cancellation between the f0(600) and rho(770) amplitudes. However, UChPT calculations also suggest a subdominant qqbar component for the f0(600) with a mass above 1 GeV and this can restore semi-local duality, as we show.

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arXiv2011

Strong Coupling Continuum QCD

The Schwinger-Dyson, Bethe-Salpeter system of equations are the link between coloured quarks and gluons, and colourless hadrons and their properties. This talk reviews some aspects of these studies from the infrared behaviour of ghosts to the prediction of electromagnetic form-factors.

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arXiv2010

Physics with the KLOE-2 experiment at the upgraded DA$ϕ$NE

Investigation at a $ϕ$--factory can shed light on several debated issues in particle physics. We discuss: i) recent theoretical development and experimental progress in kaon physics relevant for the Standard Model tests in the flavor sector, ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled kaon states, iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and eta/eta$^\prime$ mesons, iv) the contribution to understand the nature of light scalar mesons, and v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the $e^+ e^-$ physics in the continuum with the measurements of (multi)hadronic cross sections and the study of gamma gamma processes.

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arXiv2010

Light $O^{++}$ Mesons: Scalargators in Florida

Light scalar mesons abound in hadron processes, like the alligators in the Florida Everglades. Moreover, scalars are intimately tied to the vacuum structure of QCD. They are the product of many decays. Consequently, a rich source of recent information about them has come from experiments producing heavy flavour mesons. Indeed, scalars will continue to dominate many of the processes to be studied at forthcoming facilities like BESIII in Beijing, FAIR at GSI Darmstadt and the GlueX experiment at JLab, making an understanding (or at least an excellent and theoretically consistent description) essential for the physics missions of these facilities.

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arXiv2010

Structure of the lightest scalar meson from the 1/Nc expansion of Unitarized Chiral Perturbation Theory and Regge Theory

One-loop unitarized Chiral Perturbation Theory (UChPT) calculations, suggest a different Nc behaviour for the sigma or f_0(600) and rho(770) mesons: while the rho meson becomes narrower with Nc, as is expected for a q-qbar meson, the sigma becomes broader, and its contribution to the total cross section is less and less important. On the other hand, local duality requires a cancellation between the sigma and rho amplitudes, but if there is a different Nc behaviour for them, there is a possible contradiction between the Inverse Amplitude Method (IAM) and local duality for large Nc. However, next to next to leading order UChPT calculations suggested a subdominant q-qbar component for the sigma with a mass around 1.2 GeV. In this work, we show that this subdominant q-qbar component is indeed needed to ensure local duality.

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arXiv2009

Strong Coupling QCD in the Continuum

A short overview of studies of strong coupling problems in hadron physics is presented. The Schwinger-Dyson/Bethe-Salpeter approach in the continuum allows the perturbative regime to be connected to the world of strong coupling that controls confinement and chiral dynamics that in turn determines the properties of light hadrons with just one scale, {\it viz.} $Λ_{QCD}$, and with the addition of current masses the properties of heavy flavours too.

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arXiv2009

Building the Full Fermion-Photon Vertex of QED by Imposing Multiplicative Renormalizability of the Schwinger-Dyson Equations for the Fermion and Photon Propagators

In principle, calculation of a full Green's function in any field theory requires knowledge of the infinite set of multi-point Green's functions, unless one can find some way of truncating the corresponding Schwinger-Dyson equations. For the fermion and boson propagators in QED this requires an {\it ansatz} for the full three point vertex. Here we illustrate how the properties of gauge invariance, gauge covariance and multiplicative renormalizability impose severe constraints on this fermion-boson interaction, allowing a consistent truncation of the propagator equations. We demonstrate how these conditions imply that the 3-point vertex {\bf in the propagator equations} is largely determined by the behaviour of the fermion propagator itself and not by knowledge of the many higher point functions. We give an explicit form for the fermion-photon vertex, which in the fermion and photon propagator fulfills these constraints to all orders in leading logarithms for massless QED, and accords with the weak coupling limit in perturbation theory at ${\cal O}(α)$. This provides the first attempt to deduce non-perturbative Feynman rules for strong physics calculations of propagators in massless QED that ensures a more consistent truncation of the 2-point Schwinger-Dyson equations. The generalisation to next-to-leading order and masses will be described in a longer publication.

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arXiv2008

Amplitude Analysis of High Statistics Results on $γγ\toπ^+π^-$ and the Two Photon Width of Isoscalar States

We perform an Amplitude Analysis of the world published data on $γγ\toπ^+π^-$ and $π^0π^0$. These are dominated in statistics by the recently published results from Belle on the charged pion channel. Nevertheless, having only limited angular information, a range of solutions remain possible. We present two solutions with $Γ(f_0(980)\toγγ) = 0.42$ and 0.10 keV, and $Γ(f_2(1270)\toγγ) = 3.14 \pm 0.20$ and $3.82 \pm 0.30$ keV, respectively: the former being the solution favoured by $χ^2$, the latter at the edge of acceptability. Models of the structure of the $f_0(980)$ predict two photon widths to be between 0.2 and 0.6 keV, depending on its composition as mainly ${\bar K}K$, ${\bar s}s$ or ${\bar{qq}}qq$. Presently available data cannot yet distinguish unambiguously between these predictions. However, we show how forthcoming results on $γγ\toπ^0π^0$ can not only discriminate between, but also refine, these classes of partial wave solutions.

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arXiv2007

How adding zero to the complex relation between production and scattering amplitudes found by van Beveren and Rupp gives the expected real relation

If a hadronic production process is dominated by two body final state interactions, the amplitude $A$ for the production process can be expanded as a sum of the scattering amplitudes $T$ for the relevant two body channels. Van Beveren and Rupp have claimed that the unitarity relation ${\rm {Im}} A= T^†A$ can be satisfied if the coefficients in this expansion are complex. We demonstrate that the coefficients have to be real if the scattering amplitudes $T$ satisfy unitarity. Van Beveren and Rupp have merely written real coefficients as a sum of complex numbers.

hep-ph↗
arXiv2007

Structure of the Scalars

The PDG Tables list more scalar mesons than can fit into one quark model nonet: indeed, even more than can belong to two multiplets. Consequently, some of these must be states beyond the quark model. So which of these is ${\bar q}q$ or ${\bar{qq}}qq$ or multi-meson molecule or largely glue? How experiment can help us distinguish between these possibilities is discussed.

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arXiv2007

Location, correlation, radiation: where is the $σ$, what is its structure and what is its coupling to photons?

Scalar mesons are a key expression of the infrared regime of QCD. The lightest of these is the $σ$. Now that its pole in the complex energy plane has been precisely located, we can ask whether this state is transiently ${\bar q}q$ or ${\bar {qq}} qq$ or a multi-meson molecule or largely glue? The two photon decay of the $σ$ can, in principle, discriminate between these possibilities. We review here how the $γγ\toπ^+π^-$, $π^0π^0$ cross-sections can be accurately computed. The result not only agrees with experiment, but definitively fixes the radiative coupling of the $σ$. This equates to a two photon width of $(4.1 \pm 0.3)$ keV, which accords with the simple non-relativistic quark model expectation for a ${\bar u}u, {\bar d}d$ scalar. Nevertheless, robust predictions from relativistic strong coupling QCD are required for each of the possible compositions before we can be sure which one really delivers the determined $γγ$ coupling.

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arXiv2007

Dalitz plot analysis of the D+ to K-pi+pi+ decay in the FOCUS experiment

Using data collected by the high energy photoproduction experiment FOCUS at Fermilab we performed a Dalitz plot analysis of the Cabibbo favored decay D+ to K-pi+ pi+. This study uses 53653 Dalitz-plot events with a signal fraction of ~ 97%, and represents the highest statistics, most complete Dalitz plot analysis for this channel. Results are presented and discussed using two different formalisms. The first is a simple sum of Breit--Wigner functions with freely fitted masses and widths. It is the model traditionally adopted and serves as comparison with the already published analyses. The second uses a K-matrix approach for the dominant S-wave, in which the parameters are fixed by first fitting Kpi scattering data and continued to threshold by Chiral Perturbation Theory. We show that the Dalitz plot distribution for this decay is consistent with the assumption of two body dominance of the final state interactions and the description of these interactions is in agreement with other data on the Kpi final state.

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arXiv2007

Decay channels and charmonium mass-shifts

The discovery in the last few years of the $X, Y$ and $Z$ states of the extended charmonium family has highlighted the importance of the closeness of decay channels to an understanding of these mesons. We aid this debate by illustrating a simple calculational procedure for including the effect of open and nearby closed channels.

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