Function, the Iterated Function Being That Involved in the Solution of the Two-Loop /9-Function Equation
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Durham E-Theses A convergent reformulation of perturbative QCD Alves(1), RicardoJooGaio How to cite: Alves(1), RicardoJooGaio(2000)A convergent reformulation of perturbative QCD, Durhamtheses, DurhamUniversity.AvailableatDurhamE− T hesesOnline : http : //etheses.dur.ac.uk/4259/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk A Convergent Reformulation of perturbative QCD Ricardo Joao Gaio Alves^ Centre for Particle Theory University of Durham October 2, 2000 A thesis submitted to the University of Durham for the degree of Doctor of Philosophy The copyright of this thesis rests with the aiitlior. No quotation from it should be puhlislicd in any form, includinj; Electronic and the Internet, without the author's prior written consent. All information derived from this thesis must he acknowlcdj;ed appropriately. ^The author was the recipient of a scholarship from Programa Praxis XXI-JNICT- Portugal. 3 JUL 2Q01 Abstract We present and explore a new formulation of perturbative QCD based not on the renormalised coupling but on the dimensional transmutation parameter of the theory and the property of asymptotic scaling. The approach yields a continued function, the iterated function being that involved in the solution of the two-loop /9-function equation. In the so-called large-b limit the continued function reduces to a con• tinued fraction and the successive approximants are diagonal Pade approximants. We investigate numerically the convergence of successive approximants using the leading-6 approximation, motivated by renormalons, to model the all-orders result. We consider the Adler D-function of vacuum polarisation, the Polarised Bjorken and Gross-Llewellyn Smith sum rules, the (unpolarised) Bjorken sum rule, and the Minkowskian quantities Rr and the R-ratio of e+e" annihilation. In contrast to di• agonal Pade approximants the truncated continued function method gives remark• ably stable large-order approximants in cases where infra-red renormalon effects are important. We also use the new approach to determine the QCD fundamental pa• rameters from the Rr and the R-r&tio measurements, where we find A~^=516±i8 MeV (which yields as(fi= rrir) =0.360110^1)^ and Aj^=299i^ MeV (which yields as{fj, = mzo) — 0.1218 ± 0.0004), respectively. The evolution of the former value to the rrizo energy results in as{fx= m^o) = 0.123 ± 0.002. These values are in line with other determinations available in the literature. We implement the Complete Renormalisation Group Improvement (CORGI) scheme throughout all the calculations. We report on how the mathematical concept of Stieltjes series can be used to assess the convergence of Pade approximants of perturbative series. We find that the combinations of UV renormalons which occur in perturbative QCD may or may not be Stieltjes series depending on the renormalisation scheme used. Acknowledgements J first wish to thank my supervisor Chris Maxwell, for his guidance, and for the patience he had to clarify my doubts over the most meticulous details of our research, during the three years of our productive collaboration. I acknowledge the receipt of a money grant from the Physics department to cater for supervisory meetings (at CERN) during the second semester of the first year. My thanks to Peter Watson go for reading part of the manuscript and actually discussing it, and for having introduced me to FeynDiagram. He was also able to convince me that however dire my situation was, his circumstances were much worse, and this was priceless on many occasions. I am thankful to Stephen Burby for help with Latex and Mathematica and for being willing to join me in numerous discussions about Physics. My thanks to Carlo Oleari go for his encouragement and are proportional to the amount of commentaries he made after reading part of the manuscript. I also thank Abolfazl Mirjalili for a few discussions about Physics. I am grateful to all the people I shared an office with and which actually made an effort to provide the placid working environment that is necessary for research, and vital for such a stressful task as writing a Ph.D. Thesis: John Campbell, Matthias Heyssler, Tom Weidig, Matthew Slater, Stephen Burby, and Athanasios Koukout- sakis. I am also grateful to Nigel Glover in this area. I also wish to express my gratitude to all the people who gave me some human support during the harsh and miserable times I spent in Durham. Specially and foremost Mana Mariana, Mae Maria, Pai Abilio, Catarina and Tiago. Let me also mention the uplifting friendship of Nathan Doble, James Ingham, Pedro Castelo, Bruno Mendes, Guilherme Neves, Joao Correia (who also tipped me off as to what Mathematica 4.0 was actually useful for), John Outhwaite, and also all those chaps who fell flat on their backs at Judo (they are too numerous to mention). Last but not least let me thank Alison Welch, Anna Dezeuze, loanna Bella, Irazema Contreras, Therese Personn, and others, for having given me some motivation. Without all this people, what was probably the longest writing-up period in the history of the Department of Physics would certainly have been much more difficult to bring to an end. Declaration I declare that no material in this thesis has previously been submitted for a degree at this or any other university. The research reported in this thesis has been carried out in collaboration with Dr. C.J. Maxwell. The publication of the material contained in Chapters 5 and 6 is in preparation. The copyright of this thesis rests with the author. Contents Elements of QCD 11 1.1 The QCD Lagrangian 11 1.1.1 The QCD free Lagrangian 12 1.1.2 Local gauge invariance 13 1.1.3 Gauge-fixing and ghost Lagrangian 14 1.2 The Feynman rules 15 1.3 Renormalisation 16 1.3.1 One example of regularisation at work: radiative corrections to the gluon propagator 18 1.3.2 The renormalisation group 20 1.3.3 The /3-function equation and the one loop running coupling . 23 1.3.4 Exact solutions of the renormalisation group fS-function equa• tion at two-loop order 25 1.3.5 The renormalisation group ^-function equation at three loops order 27 1.4 Renormalisation-scheme dependence 28 1.4.1 Labelling the renormalisation scheme 29 1.4.2 The NLO renormalisation-scheme invariant 30 1.4.3 The physical scale 31 1.4.4 The Principle of Minimal Sensitivity 32 1.4.5 The ELM scale 35 1.4.6 The effective charge scheme 36 1.4.7 Renormalisation-scheme invariants 37 1.4.8 An alternative set of RS-invariants 39 1.4.9 Complete Renormalisation Group Improvement 40 Large-Orders in Perturbation Theory 43 2.1 Why perturbation theory will diverge 43 2.1.1 Dyson's argument for the divergence of QED perturbation the• ory ...' 43 2.1.2 Extension of Dyson's argument to include fermions 46 2.1.3 Counting Feynman diagrams: (f)'^ field theory as an example . 47 2.2 Asymptotic series 48 2.2.1 The Borel method of resummation 50 2.2.2 The balance between the resummation method and the ana- lyticity domain 55 2.3 Pade approximants 56 2.3.1 Pade approximants as continued fractions 58 2.3.2 Singularities and the convergence of Pade approximants .... 59 2.3.3 Some examples of Pade approximants at work 60 2.4 Stieltjes series and Stieltjes functions 65 2.4.1 Hamburger functions 70 2.4.2 Stieltjes functions 71 Renormalons 74 3.1 The Adler function ' . 75 3.1.1 The first few Borel plane singularities of the Adler function . 78 3.1.2 The full singularity structure of the Adler function in the Borel plane 81 3.1.3 Instantons 85 3.1.4 The Adler function in the Hamburger representation 86 3.2 Motivating the leading-6 approximation 89 3.2.1 The large b limit and the renormalised coupling 91 3.2.2 Comparison between leading-6 estimates and exact coefficients 93 3.2.3 The leading-6 approximation in operator analysis 94 3.3 Power corrections 97 3.3.1 The Operator Product Expansion 98 3.3.2 Perturbative series and phenomenology 100 4 A reformulation of QCD perturbation theory 103 4.1 Derivation of the truncated continued function method 104 4.2 CORGI in the leading-6 approximation 108 4.3 Study of one example on the quest for a proof of convergence 110 4.4 Summary 114 5 Reformulated perturbative QCD for some euclidean observables 115 5.1 The Adler function revisited 116 5.1.1 The CORGI series for the Adler function D{a) 117 5.1.2 The Borel sum of the Adler function D{a) 118 5.1.3 Pade approximants and QCD 120 • 5.1.4 Numerical results for the Adler function D{a) 122 5.2 Deep inelastic scattering (DIS) sum rules 126 5.2.1 Polarised Bjorken sum rule (PBjSR) 126 5.2.2 Gross-Llewellyn Smith sum rule (GLSSR) 127 5.2.3 Large-order coefficients and Borel plane singularities of the PBjSR and GLSSR K{a) 127 5.2.4 Numerical results for the PBjSR and GLSSR K(a) 129 5.3 Unpolarised Bjorken sum rule (BjSR) 131 5.3.1 Numerical results for the BjSR U{a) 133 5.4 Summary and conclusions 135 6 Reformulated perturbative QCD for contour-improved observables 136 6.1 The r decay ratio Rr 137 6.1.1 The experimental Rr 138 6.2 The R-ratio decay rate 138 6.2.1 The experimental R-ratio at the Z° energy 139 6.3 The contour integral representation of Minkowski observables 139 6.3.1 The Rr as a contour integral 139 6.3.2 Power corrections to the i?^.