QCD @ FCC-ee 1 s t F C C P h y s i c s W o r k s h o p , 1 6 - 2 0 J a n 2 0 1 7 , C E R N Peter Skands (Monash University) & David d’Enterria (CERN) WHAT IS QUARK On behalf of the FCC-ee working group “QCD & gamma-gamma physics” AND IS IT HEALTHY? (Condensed from the contributions to the 2015 and 2016 QCD@FCC-ee workshops, with thanks to all participants)
QCD AT EE COLLIDERS ๏ QCD: (the only) unbroken Yang-Mills theory that can be compared directly with experiment. Rich structure. • End of era of testing SU(3) C → Precision determinations of α s • Understanding jet (sub)structure • Testing models of confinement and (non-perturbative) QCD effects • Monte Carlo tuning & constraints • Fragmentation Functions • QCD in γγ collisions • Interplay with EW, H, BSM @ FCC-ee • Precision Legacy for FCC-hh 2 P e t e r S k a n d s M o n a s h U n i v e r s i t y
QCD AT EE COLLIDERS ๏ QCD: (the only) unbroken Yang-Mills theory that can be compared directly with experiment. Rich structure. • End of era of testing SU(3) C → Precision determinations of α s • Understanding jet (sub)structure • Testing models of confinement and (non-perturbative) QCD effects • Monte Carlo tuning & constraints • Fragmentation Functions • QCD in γγ collisions • Interplay with EW, H, BSM @ FCC-ee • Precision Legacy for FCC-hh 3 P e t e r S k a n d s M o n a s h U n i v e r s i t y
QCD WG ACTIVITIES (+ RESOURCES) ๏ High-precision α s measurements from LHC to FCC-ee Oct 2015: Slides on indico.cern.ch/event/392530 ๏ Proceedings at arXiv:1512.05194 ๏ ๏ Parton Radiation and Fragmentation from LHC to FCC-ee Nov 2016: Slides on indico.cern.ch/event/557400 ๏ Proceedings to appear on arXiv soon ๏ ๏ FCC-ee γγ session at Photon 2017 (CERN) May 22-26 2017: https://indico.cern.ch/event/604619/ ๏ Join the WG to receive notifications ๏ Join QCD WG at http://CERN.ch/FCC-ee (join us, subscribe) + Let us know about any studies you have done that pertain to QCD @ FCC-ee 4 P e t e r S k a n d s M o n a s h U n i v e r s i t y
FRAGMENTATION FUNCTIONS S. Moch (& others): field now moving towards NNLO accuracy: 1% errors (or better) ๏ FFs from Belle to FCC-ee [A. Vossen] + + - - World Data (Sel.) for e World Data (Sel.) for e e e ± ± +X Production +X Production → → π π • Precision of TH and EXP big advantage ) 13 S s 10 L D 9 1 G e V FCC-ee? ( 1 12 × 0 c( Complementary to pp and SIDIS 12 ) 10 D E L P ๏ H I × 9 1 G 11 e 10 V ( /dz 5 × 1 10 A × 0 L E • Evolution: ) P H 9 10 1 G 10 e V ( 3 × 9 1 0 × ) σ 9 T A 10 S d S O Belle has FCC-ee like stats at 10 GeV. 3 4 G e V , 4 4 G tot.had. e V ๏ 8 ( 10 7 × 7 1 0 × Evolution ) T P C Scaling 2 9 FCC-ee: very fine binning all the way to G e 7 V 10 ( 2 × 1 6 0 × ) ๏ σ 6 10 z=1 with 1% |p| resolution (expected) 1/ t h i s m e a s . 5 , B 10 e l l e 1 1 G C e L V E ( O 0 1 × . 0 0 • Flavour structure for FFs of hyperons 4 G ) e 4 V 10 ( 3 0 × 0 0 ) A R G 3 U S 10 9 G and other hadrons that are difficult to e V , 1 0 G e V ( 1 2 5 × 10 0 ) R o reconstruct in pp and SIDIS. n a n e t a 10 l . 3 G e V ( 1 × ) 1 Will depend on Particle Identification 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ๏ capabilities. z • Low Z: Higher ee energy (than Belle) → smaller mass effects at low z. 3 tracker hits down to 30-40 MeV allows to reach z = 10 -3 (ln(z) = -7) ๏ Kluth: if needed, could get O(LEP) sample in ~ 1 minute running with lower B-field ๏ • gluon FFs, heavy-quark FFs, p T dependence in hadron + jet, polarisation,… 5 P e t e r S k a n d s M o n a s h U n i v e r s i t y
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 ๏ Expect 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. [Next slide] ๏ Bonus: high-precision jet calibration (particle flow) • Accurate knowledge (+ modeling) of particle composition & spectra 6 P e t e r S k a n d s M o n a s h U n i v e r s i t y
STRANGENESS ENHANCEMENTS (IN PP) ๏ ALICE: clear enhancement of ) + π strangeness with (pp) event multiplicity + 0 2K − S π 0 Ratio of yields to ( 2K − 1 S 10 • Especially for multi-strange baryons + ( 2) Λ Λ × No corresponding enhancement for protons ๏ + ( 2) Λ Λ × (not shown here but is in ALICE paper) → must really be a strangeness effect + − + ( 6) Ξ Ξ × ๏ + − + ( 6) Ξ Ξ × • Measurements of phi now underway ๏ Jet universality: jets at LHC modelled 2 − 10 + − Ω + Ω ( × 16) the same as jets at LEP + − + ( 16) Ω Ω × • → Flat line ! (cf PYTHIA) ALICE pp, s = 7 TeV • Some models anticipated the effect! p-Pb, s = 5.02 TeV NN Pb-Pb, s = 2.76 TeV DIPSY (high-tension overlapping strings) NN ๏ [1] PYTHIA8 EPOS (thermal hydrodynamic “core”) DIPSY [2] ๏ [3] EPOS LHC • Is it thermal? Or stringy? (or both?) ALICE, arXiv:1606.07424 − 3 10 3 2 • Basic check in ee → WW: two strings 10 10 10 d N /d 〈 η 〉 D.D. Chinellato – 38th International Conference on High ch | η |< 0.5 D.D. Chinellato – 38th International Conference on High Energy Physics (LEP: total Ω rate only known to ± 20%) 7 P e t e r S k a n d s M o n a s h U n i v e r s i t y
COLOUR RECONNECTIONS T. Sjöstrand, W. Metzger, S. Kluth, C. Bierlich ๏ At LEP 2: hot topic (by QCD standards) : ’string drag’ effect on W mass • Non-zero effect convincingly demonstrated at LEP-2 No-CR excluded at 99.5% CL [Phys.Rept. 532 (2013) 119] W W ๏ LC But not much detailed (differential) information ๏ O (1) • Thousand times more WW at FCC-ee • Sjöstrand: turn the W mass problem around; use huge sample of semi-leptonic events to measure m W Γ W � Λ QCD • → use as constraint to measure CR in hadronic WW CR ✓ 1 ๏ Has become even hotter topic at LHC ◆ ∼ O N 2 C • It appears jet universality is under heavy attack. ⊗ kinematics Fundamental to understanding & modeling hadronisation Follow-up studies now underway at LHC. ๏ + Overlaps → interactions? ๏ High-stats ee → other side of story increased tensions (strangeness)? • Also relevant in (hadronic) ee → tt, and Z → 4 jets breakdown of string picture? Overviews of recent models: arXiv:1507.02091 , arXiv:1603.05298 8 P e t e r S k a n d s M o n a s h U n i v e r s i t y
OTHER PARTICLE CORRELATIONS Octet neutralisation? (zero-charge gluon jet Leading baryons in g jets? with rapidity gaps) → neutrals (discriminates between string/cluster models) Colour reconnections, glueballs, … high-E baryons q ¯ ¯ ¯ ¯ ¯ ¯ ¯ q qq q q q s s q q q q q How local? How local? How local? ๏ Further precision non-perturbative aspects • Baryon-Antibaryon correlations: how local is hadronisation? Kluth: both OPAL measurements were statistics-limited; would reach ๏ OPAL systematics at 10 8 Z decays ( → 10 9 with improved systematics?) • + Strangeness correlations, p T , spin/helicity correlations (“screwiness”?) • Bose-Einstein Correlations & Fermi-Dirac Correlations Identical baryons! (pp, ΛΛ ) ; highly non-local in string picture ๏ W. Metzger emphasised remaining Fermi-Dirac radius puzzle: correlations at LEP across ๏ multiple experiments & for both pp and ΛΛ → 0.1 fm << r p (MC dependent? Were p Λ cross checks ever done? see EPJC 52 (2007) 113 ) 9 P e t e r S k a n d s M o n a s h U n i v e r s i t y
JET (SUB)STRUCTURE : WHAT IS QUARK? ๏ LEP: 45-GeV quark jet fragmentation → What is gluon? • Inclusive: gluon FF only appears at NLO (similar to gluon PDF at HERA) • 3-jet events. Game of low sensitivity (3 rd jet) vs low statistics (Z → bbg) (Initially only “symmetric” events; compare q vs g jets directly in data) ๏ • Expect naive C A /C F ratios between quarks and gluons [next slide] Many subtleties. Coherent radiation → no ‘independent fragmentation’, ๏ especially at large angles. Parton-level “gluon” only meaningful at LO. ๏ … and is it healthy? • Note: highly relevant interplay with Q/G sep @ LHC & FCC-hh: S/B • Language evolved: Just like “a jet” is inherently ambiguous,“quark- like” or “gluon-like” jets are ambiguous concepts See Les Houches arXiv:1605.04692 Define taggers (adjective: “q/g-LIKE”) using only final-state observables ๏ Optimise tagger(s) using clean (theory) references, like X->qq vs X->gg ๏ 10 P e t e r S k a n d s M o n a s h U n i v e r s i t y
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