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Probing the Standard Model Through Radiative Corrections

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Probing the Standard Model Through Radiative Corrections Scuola Normale Superiore, Pisa Classe di Scienze Probing the Standard Model through radiative corrections Thesis submitted in partial fullfillment of the requirements for the Ph. D. degree in Physics Advisor: Prof. Giuseppe Curci Candidate: Dr. Andrea Vicer´e Referees: Prof. Riccardo Barbieri Prof. Peter Weisz Anno Accademico 1991-1992 To Lucia and my Parents. Contents 1 Introduction 5 1.1 Motivations ..................................... 5 1.2 PlanoftheWork.................................. 6 I The B X γ decay 9 → s 2 Overview 10 2.1 TheCLEOresults ................................. 10 2.2 The structure of the B X γ decays ...................... 11 → s 2.2.1 Theinclusiverate ............................. 13 2.2.2 TheQCDenhancement .......................... 14 2.3 The b sγ asaprobeofnewphysics ...................... 16 → 2.3.1 TwoHiggsdoubletmodels. 16 2.3.2 b sγ intheMSSM............................ 17 → 3 Effective theories 20 3.1 Generalities .................................... 20 3.1.1 WhatisanEffectiveTheory? . 20 3.1.2 Matching .................................. 21 3.1.3 RenormalizationGroup . 22 3.2 The FCNC Effective Hamiltonian . 22 4 Renormalization Group improved perturbation theory 27 4.1 Notations for the Dimensional Regularization . .......... 27 4.1.1 The renormalization scale dependence . ..... 28 4.1.2 Theconventionalform . 31 4.1.3 ReferenceFormulas. 34 4.1.4 Finite renormalizations . 34 4.1.5 Treatment of evanescent operators . 36 4.2 TheRGimprovedperturbationtheory . ..... 37 4.2.1 LeadingLogsinQCD ........................... 38 4.2.2 Schemeandscaledependence . 39 4.2.3 Sizingthescaledependence . 40 CONTENTS 3 5 QCD corrections to the b sγ process 43 → 5.1 Reviewoftheexistingresults . ..... 43 5.2 Thesymmetrizedscheme . .. .. .. .. .. .. .. .. 44 5.2.1 On-shell and off-shell formulation . ..... 48 5.3 Thecomputation .................................. 49 5.3.1 Oneloopresults .............................. 49 5.3.2 Twoloopresults .............................. 53 5.3.3 Reduction of evanescent operators . 53 5.3.4 On-shellresults............................... 58 5.3.5 Matrix elements and scheme independence . ..... 61 5.4 Effective Hamiltonian at µ m ......................... 63 ≃ b 5.4.1 Erroranalysis................................ 65 II Heavy Top effects at LEP 67 6 Overview 68 6.1 Therenormalizationscheme . 68 6.1.1 Basicobservables.............................. 69 6.1.2 Derivedobservables .. .. .. .. .. .. .. .. 70 6.2 Heavytopeffects .................................. 71 6.2.1 Heavy top effects in the ρ parameter................... 71 6.2.2 Heavy top effects in vertices . 72 6.2.3 Boundsontopmass ............................ 73 7 Strategy to compute heavy top corrections 76 7.1 TheWardIdentities ............................... 77 7.2 Identities relevant for Z b¯b and the ρ parameter ............... 78 → 7.2.1 Formfactors ................................ 79 7.3 TheYukawamodel................................. 81 8 Heavy Top effects in the Gaugeless Limit 83 8.1 Renormalizedmodel ............................... 83 8.1.1 The renormalization constants . 86 8.2 Onelooporder ................................... 88 8.3 Two-loop order: Z b¯b .............................. 89 → 8.3.1 Cumulativeresult ............................. 92 8.4 Two loop order: ρ ................................. 93 8.5 Summaryoftheresults ............................. 96 8.5.1 Physicalobservables . 97 8.5.2 Validity of the gaugeless approximation . ...... 99 9 Conclusions and perspectives 104 9.1 An example: bounds on two Higgs doublets models . ....... 104 9.2 Futurework..................................... 105 9.3 Acknowledgments................................. 107 CONTENTS 4 A Notations 108 A.0.1 Metrics ................................... 108 A.0.2 Lagrangian and Generating functional . 110 A.0.3 QEDinMinkowskynotation . 110 A.0.4 SU(N) gauge theory in Minkowsky notation . 111 B Dirac algebra in d dimensions 113 B.1 Cliffordalgebra.................................. 113 C Standard Model 116 C.1 Generalities .................................... 116 C.1.1 Matterfields ................................ 116 C.1.2 Gaugefieldcontent............................. 117 C.1.3 Matter fields lagrangian . 117 C.1.4 Higgsfields ................................. 118 C.1.5 Symmetrybreaking ............................ 118 C.1.6 Fieldtransformations . 120 C.1.7 ThecompleteLagrangian . 121 C.2 Quantization .................................... 123 C.2.1 Nonlineargaugefixing .......................... 123 C.2.2 GhostsandSlavnovInvariance . 124 C.2.3 Feynmanrules ............................... 125 D The Background Field gauge 130 D.1 Generalities .................................... 130 D.2 CompactNotation ................................. 132 D.3 ApplicationtoStandardModel . 134 D.3.1 Modifications to Feynman Rules . 136 Chapter 1 Introduction In this thesis we are going to report on some of the topics investigated during the years at Scuola Normale Superiore, namely weak radiative decays, exemplified by the study of the so called Flavor Changing Neutral Current (FCNC) decays of the b quark, and weak radiative corrections to LEP observables, discussing the computation of the two-loop heavy top effects to the ρ parameter and to the Z b¯b decay width. → 1.1 Motivations These investigations contribute to a general research activity, aimed at testing the Standard Model at the quantum level. The Standard Model of electroweak interactions is a very successful theory, in the sense that when a process is considered which can be predicted with a good theoretical accuracy, there is full agreement with the experiment1. It appears that every known high energy process can be described at least qualitatively in the context of the SM; the common belief is that where a quantitative analysis lacks, the reason is the inadequacy of our computational techniques, not of the theory in itself. Nevertheless there are good reasons of principle to believe that the Standard Model is just an effective theory: for instance there is compelling evidence that ϕ4 models, and conse- quently the very Higgs sector of the Standard Model, is trivial. This means that these models cannot be renormalized unless the interaction term is put to zero: even if this result has been rigorously proven only for d > 4, the existing partial results speak in favour of an extension to d = 4. Leaving aside the theoretical motivations, there are aspects of the theory which are still essentially not verified. For instance the symmetry breaking mechanism is not clarified, and the Higgs sector itself lacks completely the experimental confirmation. These facts have motivated in the last years a considerable effort aimed at devising ex- tensions of the SM which may cure its theoretical shortcomings, without altering the “low energy” behavior. May be the most promising of these extensions is the Minimal Supersym- metric Standard Model. Every extended model predicts the existence of some new particles, but it is quite possible that none of these states will be detected directly in the near future: hence one has to look 1 With some possible exceptions, for instance Rb, which is found at LEP two standard deviations higher than the SM expectation, a fact however that is not a serious problem at the present level of statistical accuracy. 1.2 Plan of the Work 6 for confirmations through indirect effects. The very success of the SM implies that we cannot expect discrepancies with the experiment at the tree level, but any SM extension may reflect itself in small corrections to the observables that are already being measured. It goes without saying that in order to disentangle new physics effects it is first necessary to improve the SM predictions at the level of the present experimental accuracy. In this thesis we shall see in two explicit examples how this program is accomplished in practice. The examples chosen, the b sγ decay and the heavy top electroweak corrections, are → good representatives of a class of studies which are expected to be particularly apt to improve our understanding of the SM structure: the reason is that perturbative methods appear in these cases well suited to extract accurate theoretical predictions. 1.2 Plan of the Work The thesis is divided in two parts: in the first part we shall deal with the rare B X γ decay, → s while in the second part we shall consider the heavy top corrections to LEP parameters. We shall try to be self contained and to follow a pedagogical approach. The physical framework and the computational techniques are discussed on general grounds, as long as it is possible from first principles. In Chapter 2 we introduce from a general point of view the study of the rare B X γ → s decay, which is going to be detailed in the following Chapters. We recall the recent results of the CLEO collaboration, which has measured both the inclusive decay and one of the exclusive modes, B K⋆γ. We show how such a process is described by the Standard Model in the → context of the Operator Product Expansion, separating the short distance contributions which can be computed by perturbative techniques and essentially non perturbative quantities. The b sγ decay is unique in this respect, because the perturbative corrections are very large → and enhance the decay rate by a factor 4 ! This is not a sign of failure of perturbation ∼ theory: it only reflects the fact that the operator mixing induced by QCD corrections alters the pattern of GIM suppression. We shall see that despite these large corrections the inclusive process
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