CERN-PH-TH-2015-299, CoEPP-MN-15-13 High-precision αs measurements from LHC to FCC-ee Workshop Proceedings, CERN, Geneva, 12{13 October 2015 Editors David d'Enterria (CERN), Peter Z. Skands (Monash) Authors S. Alekhin (U. Hamburg), A. Banfi (U. Sussex), S. Bethke (MPI, M¨unchen), J. Bl¨umlein(DESY), K.G. Chetyrkin (KIT, Karlsruhe), D. d'Enterria (CERN), G. Dissertori (ETH Zurich), X. Garcia i Tormo (Bern), A. H. Hoang (U. Wien), M. Klasen (U. M¨unster), T. Klijnsma (ETH Zurich), S. Kluth (T.U. M¨unchen), J.-L. Kneur (U. Montpellier 2), B.A. Kniehl (U. Hamburg), D. W. Kolodrubetz (MIT), J. K¨uhn(KIT, Karlsruhe), P. Mackenzie (Fermilab), B. Malaescu (LPNHE, Paris), V. Mateu (U. Wien/U. A. Madrid), L. Mihaila (KIT, Karlsruhe), S. Moch (U. Hamburg), K. M¨onig(DESY), R. P´erez-Ramos(Paris), A. Pich (U. Val`encia),J. Pires (U. Milano/MPP Munich), K. Rabbertz (KIT, Karlsruhe), G. P. Salam (CERN), F. Sannino (CP3-Origins, Odense), J. Soto i Riera (U. Barcelona), M. Srebre (U. Ljubljana), I. W. Stewart (MIT) Abstract This document provides a writeup of all contributions to the workshop on \High precision measurements of αs: From LHC to FCC-ee" held at CERN, Oct. 12{13, 2015. The workshop explored in depth the latest developments on the determination of the QCD coupling αs from 15 methods where high precision measurements are (or will be) available. Those include low-energy observables: (i) lattice QCD, (ii) pion decay factor, (iii) quarkonia and (iv) τ decays, (v) soft parton-to-hadron fragmentation functions; as well as high-energy observables: (vi) global fits of parton distribution functions, (vii) hard parton-to-hadron fragmentation functions, (viii) jets in e±p DIS and γ-p photoproduction, (ix) photon structure function in γ-γ, (x) event shapes + and (xi) jet cross sections in e e− collisions, (xii) W boson and (xiii) Z boson decays, and (xiv) jets and (xv) top-quark cross sections in proton-(anti)proton collisions. The current status arXiv:1512.05194v1 [hep-ph] 16 Dec 2015 of the theoretical and experimental uncertainties associated to each extraction method, the improvements expected from LHC data in the coming years, and future perspectives achievable + 1 in e e− collisions at the Future Circular Collider (FCC-ee) with 1 100 ab− integrated O − luminosities yielding 1012 Z bosons and jets, and 108 W bosons and τ leptons, are thoroughly reviewed. The current uncertainty of the (preliminary) 2015 strong coupling world-average value, α (m2 ) = 0.1177 0.0013, is about 1%. Some participants believed this may be reduced s Z ± by a factor of three in the near future by including novel high-precision observables, although this opinion was not universally shared. At the FCC-ee facility, a factor of ten reduction in the αs uncertainty should be possible, mostly thanks to the huge Z and W data samples available. This document is dedicated to the memory of Guido Altarelli. Speakers A. Banfi (U. Sussex), S. Bethke (MPI, M¨unchen), J. Bl¨umlein (DESY), D. d'Enterria (CERN), X. Garcia i Tormo (Bern), A. Hoang (U. Wien), B.A. Kniehl (U. Hamburg), M. Klasen (U. M¨unster),S. Kluth (T.U. M¨unchen), J.-L. Kneur (U. Montpellier 2), J. K¨uhn(KIT, Karlsruhe), P. Mackenzie (Fermilab), B. Malaescu (LPNHE, Paris) L. Mihaila (KIT), A. Mitov (Cambridge), K. M¨onig(DESY), R.P´erez-Ramos(Paris), A. Pich (U. Val`encia),J. Pires (U. Milano, MPP Munich), K. Rabbertz (KIT, Karlsruhe), G. Salam (CERN), F. Sannino (CP3-Origins, Odense), P.Z. Skands (Monash), J. Soto i Riera (U. Barcelona), M. Srebre (U. Ljubljana) Additional Participants A. Ali (DESY), S. Amoroso (CERN), A. Blondel (U. Gen`eve), M. Gonz´alez-Alonso(IPN, Lyon), C. Gracios (Puebla), K. Hamacher (Bergische Univ. Wuppertal), R. Hern´andez-Pinto (IFIC, Val`encia), P. Janot (CERN), M. Klute (MIT), I. Kolbe (U. Cape-Town), A. Larkoski (MIT/Harvard), J. Llorente-Merino (Univ. Aut´onomaMadrid), G. Luisoni (CERN), B. Meiring (U. Cape-Town), S. Menke (MPI, M¨unchen), R. Morad (U. Cape-Town), A.N. Rasoanaivo (U. Cape-Town), P. Telles-Rebello (CBPF, Rio de Janeiro) 2 1 Introduction The strong coupling αs is one of the fundamental parameters of the Standard Model (SM), setting the scale of the strength of the strong interaction theoretically described by Quantum Chromody- namics (QCD). Its measured (2014) value amounts to α (m2 ) = 0.1185 0.0006 at the reference Z s Z ± pole mass scale. Given its current δα (m2 )/α (m2 ) 0.5% uncertainty|orders of magnitude larger s Z s Z ≈ than that of the gravitational (δG=G 10−5), Fermi (δG =G 10−8), and QED (δα/α 10−10) ≈ F F ≈ ≈ couplings|the strong coupling is the least precisely known of all fundamental constants in nature. Improving our knowledge of αs is a prerequisite to reduce the theoretical uncertainties in the cal- culations of all high-precision perturbative QCD (pQCD) processes whose cross sections or decay rates depend on higher-order powers of αs, as is the case for virtually all those measured at the LHC. In the Higgs sector, in particular, the uncertainty on αs is currently the second major contrib- utor (after the bottom mass) to the parametric uncertainties of its dominant H bb partial decay. ! The same applies for the extraction of the charm Yukawa coupling via future H cc¯ measurements. ! The workshop \High-precision αs measurements from LHC to FCC-ee" was held at CERN, October 12{13, 2015, as part of the FCC-ee QCD and γ-γ physics working group activities in the context of the preparation of the FCC-ee Conceptual Design Report in 2016. The meeting brought together experts from several different fields to explore in depth the latest developments on the determination of the QCD coupling αs from the key categories where high precision measurements are (or will be) available, and put its emphasis on the following issues: What is the current state-of-the-art of each one of the α determination methods, from the • s theoretical and experimental points of view? What is the current size of the theoretical (missing higher-order QCD and electroweak cor- • rections, power corrections, hadronization corrections,...) and experimental uncertainties associated to each measurement? What is the expected α uncertainty in 10 years from now thanks to the ongoing (or ex- • s ∼ pected) theoretical developments, plus 1 ab−1 collected p-p data at 14 TeV at the LHC? O What are the improvements expected to be brought about by e+e− collisions at the FCC-ee • (ps = 91, 160, 240 and 350 GeV) with 1 100 ab−1 integrated luminosities yielding 1012 O − Z bosons and jets, and 108 W bosons and τ leptons? What are the systematic errors that the FCC-ee detectors should target in order to match the • expected statistical precision, or where that is not possible, what are the important theoretical targets that should be met or exceeded? With those goals in mind, the workshop was organized along four broad sessions: 1. An introductory session, presenting the motivations of the workshop, the current status of the world average of the strong coupling, the impact of αs on Higgs cross sections and branching ratios, and on new physics constraints. 2. A session dedicated to αs determination at low energy including results from: lattice QCD, pion decay factor, τ decay, QQ decays, and soft parton-to-hadron fragmentation functions. 3. A session dedicated to αs determination at higher energy scales including: global fits of parton distribution functions, hard parton-to-hadron fragmentation functions, jets in deep-inelastic scattering and photoproduction in e±-p collisions, e+e− event shapes, e+e− jets, hadronic Z and W decays, σ(e+e− hadrons), and the SM electroweak fit,... ! 3 4. Recent experimental and theoretical results and plans for αs measurements at the LHC via top-quark pair and jets cross sections. One important goal of the workshop was to facilitate discussion between the different groups, and in particular to give speakers the opportunity to explain details that one would normally not be able to present at a conference, but which have an important impact on the analyses. There were about 50 physicists who took part in the workshop, and 24 talks were presented. Slides as well as background reference materials are available on the conference website http://indico.cern.ch/e/alphas2015 The sessions and talks in the workshop program were organized as follows: Introduction • { \Introduction and goals of the workshop", D. d'Enterria and P.Z. Skands { \World Summary of αs (2015)", S. Bethke { \αs and physics beyond the Standard Model", F. Sannino { \Impact of αs on Higgs production and decay uncertainties", L. Mihaila Measurements of α at low energy scales: • s { \αs from lattice QCD", P. Mackenzie { \αs from the QCD static energy", X. Garcia i Tormo { \αs from pion decay factor", J.-L. Kneur { \αs from hadronic tau decays", A. Pich { \αs from hadronic quarkonia decays", J. Soto i Riera { \αs from soft parton-to-hadron fragmentation functions", R. P´erez-Ramos Measurements of α at high energy scales: • s { \αs from global fits of parton distribution functions", J. Bl¨umlein { \αs from jets in DIS and photoproduction", M. Klasen { \αs from scaling violations of hard parton-to-hadron fragmentation functions", B.A. Kniehl + − { \αs from e e event shapes", S. Kluth + − { \αs from e e C-parameter event shape", A. Hoang + − { \αs from e e jet cross sections", A. Banfi { \αs from hadronic Z decays and from the full electroweak fit”, K.