Discussion on QCD White Paper(s) for CEPC Peter Skands (Monash U) Nonperturbative QFT remains among the most fundamental problems in physics A day will come when someone (claims to) have a solution, or at least a systematically improvable approximation (+ LHC ⟷ further refinements of phenomenological models of NP QCD) Program of high-precision QCD measurements at CEPC/FCC-ee Ultimate trial by fire for any future treatment of confinement in high-energy processes + α s measurements Basic requirements: Measure effects of order Λ QCD with high precision Disentangle different “tracers”: strangeness, baryons, mass, & spin → PID Other aspects: Fragmentation Functions, (Heavy) Flavour (Tagging), Quarkonia, (Rare) Hadron Decays, H → gg, Colour Reconnections (in Z, WW, ttbar), Power Corrections, interplay with EW and Higgs measurements, jet / particle flow calibrations, γγ collisions CEPC Workshop November 2018, IHEP, Beijing
QCD AT EE COLLIDERS ๏ QCD: (the only) unbroken Yang-Mills theory that can be compared directly with experiment. Rich structure. • CEPC / FCC-ee have tremendous potential to make decisive & detailed measurements. • End of era of testing SU(3) C → Precision determinations of α s • Theory still evolving and new questions highlighted by LHC • Confinement is still hard • LEP precision finally exhausted, almost 20 years after shutdown. • Current generation of theory models show few (albeit some) discrepancies with LEP • Within next decade: second-order-everything and next-generation hadronisation models. • + QCD in γγ collisions, interplay with EW, H, BSM, Precision Legacy for future pp collider 2 � T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
QCD AT EE COLLIDERS ๏ QCD: (the only) unbroken Yang-Mills theory that can be compared directly with experiment. Rich structure. • CEPC / FCC-ee have tremendous potential to make decisive & detailed measurements. • End of era of testing SU(3) C → Precision determinations of α s • Theory still evolving and new questions highlighted by LHC • Confinement is still hard • LEP precision finally exhausted, almost 20 years after shutdown. Current generation of theory models show few ๏ (albeit some) discrepancies with LEP • Within next decade: expect significant perturbative advances and next-generation hadronisation models. • + QCD in γγ collisions, interplay with EW, H, BSM, Precision Legacy for future pp collider 3 � T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
QCD AT EE COLLIDERS ๏ QCD: (the only) unbroken Yang-Mills theory that can be compared directly with experiment. Rich structure. • CEPC / FCC-ee have tremendous potential to Jet Algorithms Jet Substructure make decisive & detailed measurements. • End of era of testing SU(3) C → Precision Perturbative AlphaS Event Shapes QCD Extractions determinations of α s • Theory still evolving and new questions Colour Hadronisation highlighted by LHC Reconnections • Confinement is still hard MC • LEP precision finally exhausted, almost 20 Particle Correlations Particle Spectra years after shutdown. Fragmentation Functions Current generation of theory models show few ๏ (albeit some) discrepancies with LEP QCD Resummation Jet Calibrations • Within next decade: expect significant perturbative advances and next-generation Heavy Quarks hadronisation models. • + QCD in γγ collisions, interplay with EW, H, Interplay with EW, H, BSM Interplay with SppS BSM, Precision Legacy for future pp collider @ FCC-ee 4 � T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
THEMES ๏ Measure alphaS • High-Precision Z (and W) widths • High-Precision Event Shapes, Jet Rates, … (IR safe observables sensitive to alphaS) ๏ Single-Inclusive Hadron Production and Decays • Fragmentation Functions; Hadron Spectra; (+ polarisation) • Exotic /rare hadrons, quarkonium, rare decays, … • + Interplay with flavour studies (+ Interplay with DM annihilation) ๏ Understanding Confinement (Multi-hadronic / Exclusive) • In high-energy processes → hadronisation • Hadron correlations, properties with respect to global (“string”) axes • Dependence on (global and local) environment (distance to jets, hadronic density, flavours) ๏ Power Corrections / Hadronisation Corrections • Interplay with high-p T physics program • Low-Q region of event shapes, jet rates, jet substructure; jet flavour tagging, … • Crucial for alphaS measurements; also for jet calibration? 5 � T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
PRECISION α S MEASUREMENTS (see FCC-ee QCD workshops & writeups) CURRENT STATE OF THE ART: O(1%) ๏ LEP: Theory keeps evolving long after the beams are switched off • Recently, NNLO programs for 3-jet calculations τ -decays Baikov Davier Pich [Weinzierl, PRL 101, 162001 (2008)]; EERAD [Gehrmann-de-Ridder, Gehrmann, Glover, Boito ๏ Heinrich, CPC185(2014)3331] SM review HPQCD (Wilson loops) • + New resummations → new α s (m Z ) extractions HPQCD (c-c correlators) lattice Maltmann (Wilson loops) E.g., 2015 SCET-based C-parameter reanalysis PACS-CS (SF scheme) ๏ ETM (ghost-gluon vertex) N 3 LL ′ + O( α s3 ) + NPPC: α s (m Z ) = 0.1123 ± 0.0015 ๏ BBGPSV (static potent.) [Hoang, Kolodubretz, Mateu, Stewart, PRD91(2015)094018] ๏ ABM functions e + e – jets & shapes structure BBG JR • NNPDF PDG 2016 α s ( M 2 Subclass Z ) MMHT ee currently the least 0.1192 ± 0.0023 τ -decays 0 . 1187 ± 0 . 0023 ALEPH (jets&shapes) OPAL (j&s) precise subclass (due to 0.1188 ± 0.0011 lattice QCD 0 . 1184 ± 0 . 0012 JADE (j&s) large spread between 0.1156 ± 0.0021 Dissertori (3j) structure functions 0 . 1154 ± 0 . 0020 JADE (3j) individual extractions) e + e − jets & shapes 0.1169 ± 0.0034 DW (T) 0 . 1174 ± 0 . 0051 Abbate (T) 0.1151 ± 0.0028 0 . 1151 +0 . 0028 hadron collider Gehrm. (T) − 0 . 0027 Hoang 0.1196 ± 0.0030 ewk precision fits 0 . 1196 ± 0 . 0030 (C) electroweak GFitter precision fj ts hadron CMS • See also PDG QCD review and references therein collider (tt cross section) + 2016 Moriond α s review [d’Enterria]: arXiv:1606.04772 ๏ April 2016 + 2015 FCC-ee α s workshop proceedings: arXiv:1512.05194 ๏ Maximum a factor 3 further reduction possible (without FCC-ee). [Some participants believed less.] � 6 T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
PRECISION α S AT CEPC / FCC-EE (see FCC-ee QCD workshops & writeups) STATISTICS ALLOW TO AIM FOR δα s / α s < 0.1% ๏ Main Observable: ` = Γ had ng g V,f = g A,f (1 − 4 | q f | sin 2 θ W ) R 0 LO e. Γ f ∝ ( g 2 V,f + g 2 A,f ), Γ ` hile g is modified p p • QCD corrections to Γ had known to 4 th order Kuhn: Conservative QCD scale variations → O(100 keV) → δα s ~ 3 x 10 -4 ๏ Comparable with the target for CEPC / FCC-ee ๏ − | | − • Electroweak beyond LO f , sin 2 θ W → p 1 + ∆ κ f sin 2 θ W = sin 2 θ f g A,f → p 1 + ∆ ρ f g A,f , s e ff , Can be calculated (after Higgs discovery) or use measured sin 2 θ eff ๏ Mönig (Gfitter) assuming Δ m Z = 0.1 MeV, Δ Γ Z = 0.05 MeV, Δ R l = 10 -3 ๏ → δα s ~ 3 x 10 -4 ( δα s ~ 1.6 x 10 -4 without theory uncertainties) ๏ • Better-than-LEP statistics also for W → high-precision R W ratio ! Srebre & d’Enterria: huge improvement in BR(W had ) at FCC-ee (/CEPC?) ๏ Combine with expected Δ Γ W = 12 MeV from LHC (high-m T W) & factor-3 ๏ improvement in |V cs | → similar α s precision to extraction from Z decays? � 7 T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
HADRONISATION (AND LOW Z) ๏ Confinement wasn’t solved last century • Models inspired by QCD (hadronisation models) explore the non- perturbative quagmire (until it is solved and uninspired models can move in) • FFs and IR safety (power corrs) observe from a safe distance ๏ Can do track reconstruction (3 hits) down to 30-40 MeV << Λ QCD ? • Below Λ QCD → can study genuine non-perturbative dynamics • Handles: mass, strangeness, and spin. Need at least one of each meson & baryon isospin multiplet. Flavour separation crucial. (LEP |p K | > 250 MeV) • QUESTIONS: detailed mechanisms of hadron production. Is strangeness fraction constant or dynamic? Thermal vs Gaussian spectra. Debates rekindled by LHC observations of strangeness enhancement. ๏ Bonus: high(er)-precision jet calibration (particle flow) ? • Accurate knowledge (+ modeling) of particle composition & spectra 8 � T ESTI N G H A DR ONISATION M O D E L S W IT H T HE CEPC P. S KAN DS - M O N ASH U.
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