Grassmannian Geometry of Scattering Amplitudes

Cambridge University Press 978-1-107-08658-6 - Grassmannian Geometry of Scattering Amplitudes Nima Arkani-Hamed, Jacob Bourjaily, Freddy Cachazo, Alexander Goncharov, Alexander Postnikov and Jaroslav Trnka Frontmatter More information

GRASSMANNIAN GEOMETRY OF SCATTERING AMPLITUDES

Outlining a revolutionary reformulation of the foundations of perturbative quantum field theory, this book is a self-contained and authoritative analysis of the application of this new formulation to the case of planar, maximally supersymmetric Yang-Mills theory. The book begins by deriving connections between scattering amplitudes and Grassmannian geometry from first principles before introducing novel physical and mathematical ideas in a systematic manner accessible to both physicists and mathematicians. The principle players in this process are on-shell functions which are closely related to certain sub-strata of Grassmannian manifolds called positroids—in terms of which the classification of on-shell functions and their relations becomes combinatorially manifest. This is an essential introduction to the geometry and combinatorics of the positroid stratification of the Grassmannian and an ideal text for advanced students and researchers working in the areas of field theory, high energy physics, and the broader fields of mathematical physics.

Nima Arkani-Hamed is Professor of Physics at the Institute for Advanced Study, Princeton, New Jersey, USA.

Jacob Bourjaily is Assistant Professor of Physics at the Niels Bohr International Academy and Discovery Center at the University of Copenhagen, Copenhagen, Denmark.

Freddy Cachazo is the Gluskin Sheff Freeman Dyson Chair in Theoretical Physics at the Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada.

Alexander Goncharov is Professor of Mathematics at Yale University, New Haven, Connecticut, USA.

Alexander Postnikov is Professor of Applied Mathematics and Algebraic Combi- natorics at the Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

Jaroslav Trnka is Assistant Professor of Physics at the University of California, Davis, California, USA.

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GRASSMANNIAN GEOMETRY OF SCATTERING AMPLITUDES

NIMA ARKANI-HAMED Institute for Advanced Study

JACOB BOURJAILY Niels Bohr Institute

FREDDY CACHAZO Perimeter Institute

ALEXANDER GONCHAROV Yale University

ALEXANDER POSTNIKOV Massachusetts Institute of Technology

JAROSLAV TRNKA University of California, Davis

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www.cambridge.org Information on this title: www.cambridge.org/9781107086586 c Nima Arkani-Hamed, Jacob Bourjaily, Freddy Cachazo, Alexander Goncharov, Alexander Postnikov, and Jaroslav Trnka 2016 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2016 Printed in the United Kingdom by TJ International Ltd. Padstow Cornwall A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication data Names: Arkani-Hamed, Nima. Title: Grassmannian geometry of scattering amplitudes / Nima Arkani-Hamed (Institute for Advanced Study) [and five others]. Description: New York : Cambridge University Press, 2016. | Includes bibliographical references and index. Identifiers: LCCN 2015046557 | ISBN 9781107086586 (hardback) Subjects: LCSH: Quantum field theory. | Particles (Nuclear physics) | Graph theory. | Combinatorial analysis. | Geometry, Algebraic. Classification: LCC QC174.45.G67 2016 | DDC 516.3/5–dc23 LC record available at http://lccn.loc.gov/2015046557 ISBN 978-1-107-08658-6 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

Contents

Acknowledgments page ix 1 Introduction 1 2 Introduction to on-shell functions and diagrams 5 2.1 On-shell particles, functions, and kinematical data 5 2.2 Scattering amplitudes and their amalgamations 8 2.3 On-shell building blocks: the massless three-particle S-matrix 10 2.4 On-shell supersymmetry and (maximally) supersymmetric theories 14 2.5 Building up diagrams with “BCFW-bridges” 18 2.6 On-shell recursion for all-loop amplitudes 20 2.7 Physical equivalences among on-shell diagrams 23 3 Permutations and scattering amplitudes 29 3.1 Combinatorial descriptions of scattering processes 29 3.2 The BCFW-bridge construction of representative graphs 33 4 From on-shell diagrams to the Grassmannian 37 4.1 The Grassmannian of k-planes in n dimensions, G(k,n) 37 4.2 Grassmannian description of kinematical data—λ and λ˜ 40 4.3 Grassmannian representation of on-shell diagrams 41 4.4 Amalgamation of on-shell diagrams 45 4.5 “Boundary measurements” and canonical coordinates 49 4.6 Coordinate transformations induced by moves and reduction 52 4.7 Relation to composite leading singularities 56 5 Conﬁgurations of vectors and the positive Grassmannian 60 5.1 The geometry and combinatorics of the positroid stratiﬁcation 60 5.2 Canonical coordinates and the equivalence of permutation labeling 66

vi Contents

5.3 Positroid cells and the positive part of the Grassmannian 69 5.4 Canonically positive coordinates for positroids 73 6 Boundary configurations, graphs, and permutations 77 6.1 Physical singularities and positroid boundaries 77 6.2 Boundary configurations: combinatorics and stratification 78 6.3 (Combinatorial) polytopes in the Grassmannian 79 6.4 Approaching boundaries in canonical coordinates 81 7 The invariant top-form and the positroid stratification 83 7.1 Equivalence with the canonical positroid volume form 85 8 (Super-)conformal and dual conformal invariance 88 8.1 The Grassmannian geometry of momentum conservation 88 8.2 Twistor space and the superconformality of on-shell forms 90 8.3 Momentum-twistors and dual super-conformal invariance 92 9 Positive diffeomorphisms and Yangian invariance 97 10 The kinematical support of physical on-shell forms 101 10.1 Kinematical support of NMHV Yangian-invariants 102 10.2 Kinematical support for one-dimensional kinematics 103 10.3 A general combinatorial test of kinematical support 103 11 Homological identities among Yangian-invariants 107 11.1 Homological identities in the Grassmannian 108 12 (Relatively) orienting canonical coordinate charts on positroids configurations 113 12.1 Comparing the orientations of canonical coordinate charts 115 12.2 Accessing boundary configurations from the canonical atlas 118 13 Classification of Yangian-invariants and their relations 121 14 The Yang–Baxter relation and ABJM theories 127 14.1 The on-shell avatar of the Yang–Baxter relation 127 14.2 ABJM theories 130 15 On-shell diagrams for theories with N < 4 supersymmetries 136 16 Dual graphs and cluster algebras 141 16.1 The ‘dual’ of an on-shell diagram 141 16.2 Cluster algebras: seeds, mutations, and cluster coordinates 146 16.3 Cluster amalgamation 150 16.4 (Brief) overview of cluster structures in physics 152 17 On-shell representations of scattering amplitudes 155 17.1 (Diagrammatic) proof of the BCFW recursion relations 156 17.2 The structure of (tree) amplitudes in the Grassmannian 159

Contents vii

17.3 Canonical coordinates for loop integrands 164 17.4 The transcendentality of loop amplitudes 173 18 Outlook 178 References 183 Index 192

Acknowledgments

This book represents the synthesis of several years of work and was originally made available on the arXiv as reference [1]. There are many people without whom this book would not have been possible, or without whose comments would not be nearly as comprehensive as we hope that it is. Many of the mathematical topics discussed were greatly improved through early discussions with Pierre Deligne, Bob MacPherson, and Mark Goresky at the Institute for Advanced Study; and for the physics topics discussed, we owe an enormous debt of gratitude for discussions with Andrew Hodges and Simon Caron-Huot. For helpful comments regarding early versions of the manuscript, we are grateful to Simon Caron-Huot, Henriette Elvang, Sebastian Franco, Yu-tin Huang, Lionel Mason, Marcus Spradlin, Cristian Vergu, Congkao Wen, and Lauren Williams. This work was made possible through the hospitality of the School of Natural Sciences at the Institute for Advanced Study, and was supported in part by grants from the National Science Foundation DMS-1100147 (A.P.), DMS-1059129 (A.G.), and PHY-0756966 (J.T.), the Harvard Society of Fellows and a grant from the Harvard Milton Fund (J.B.), the NSERC of Canada and MEDT of Ontario (F.C.), and U.S. Department of Energy contracts DE-FG02-91ER40654 (N.A.-H.) and DE-FG02-91ER40654 (J.B.). The authors are also grateful to the Aspen Public Library for their hospitality.