Colour & Precision Top Physics Peter Skands (Monash University) - PowerPoint PPT Presentation

Colour & Precision Top Physics Peter Skands (Monash University) Perturbative aspects of top physics The top quark mass Top quark modelling at colliders A new approach to coherence Non-perturbative aspects of top physics Collective effects in pp collisions? Quo Vadis? VINCIA AIP Summer Meeting VINCIA RMIT, December, 2019

<latexit sha1_base64="md98SCkERFzdPFk85/1vlp4eCs=">AB+HicbVDJSgNBEK1xjXHJqEcvjUHwFGZc0GPQi8cIZoHMEHo6PZMmPQvdNUIc8iVePCji1U/x5t/YWQ6a+KDg8V4VfWCTAqNjvNtrayurW9slrbK2zu7exV7/6Cl01wx3mSpTFUnoJpLkfAmCpS8kylO40DydjC8nfjtR60SJMHGXcj2mUiFAwikbq2ZUoIh6mBL2AqgLHPbvq1JwpyDJx56QKczR69pfXT1ke8wSZpFp3XSdDv6AKBZN8XPZyzTPKhjTiXUMTGnPtF9PDx+TEKH0SpspUgmSq/p4oaKz1KA5MZ0xoBe9ifif180xvPYLkWQ58oTNFoW5JNPTQqkLxRnKEeGUKaEuZWwAVWUocmqbEJwF19eJq2zmnteu7y/qNZv5nGU4AiO4RcuI63EDmsAgh2d4hTfryXqx3q2PWeuKNZ85hD+wPn8AiV+TBg=</latexit> <latexit sha1_base64="HXvisY5ZKQgitkCchUKesQVy60Y=">AB/nicbVDLSsNAFJ34rPUVFVduBovgqiQ+0GXRjcsK9gFNKJPpB06maQzN0IJAX/FjQtF3Pod7vwbp20W2npg4Mw593LvPUEiuAbH+baWldW19ZLG+XNre2dXtv6njVFHWoLGIVTsgmgkuWQM4CNZOFCNRIFgrGN5O/NYjU5rH8gHGCfMj0pc85JSAkbr24cgLiMpGOfYgxjD9QN61K07VmQIvErcgFVSg3rW/vF5M04hJoIJo3XGdBPyMKOBUsLzspZolhA5Jn3UMlSRi2s+m6+f4xCg9HMbKPAl4qv7uyEik9TgKTGVEYKDnvYn4n9dJIbz2My6TFJiks0FhKvDkUpMF7nHFKIixIYQqbnbFdEAUoWASK5sQ3PmTF0nzrOqeVy/vLyq1myKOEjpCx+gUuegK1dAdqMGoihDz+gVvVlP1ov1bn3MSpesoucA/YH1+QOLcpXf</latexit> The Top Quark s ๏ Heaviest particle in the SM e wn elementary particle: Jet • m t ~ 170 GeV/c 2 ~ m Au e q • Lifetime: 10 -24 s ( Γ t ~ 1.5 GeV) . s • Mainly pair produced at colliders: ¯ q g b W + s gg → t ¯ q → t ¯ t q ¯ t Dominates at LHC Dominated at Tevatron t h s • Complicated (cascade) decays: ! p ¯ p p t → ¯ t → bW + ¯ bW − ! y s ! { q 0 , `⌫ } W → { q ¯ ¯ t ! → s ! ๏ quarks → jets ๏ b-quarks → b-jets ¯ W – b s b-quarks → b-jets l 2 Complex multi-body final states n (+ hadronisation) ➜ highly nontrivial to t ¯ • ν measure mass with high precision (<1%) e Illustration from: P Skands, Nature 514 (2014) 174 2 � P E T ER S K A ND S

The Top Quark Mass Bezrukov et al.’12; Degrassi et al.’13; + several more recent works ๏ ➤ Top-Higgs Yukawa coupling SM probably has metastable vacuum • Gateway to new physics • + SM vacuum stability ๏ Definition (from corrections to Higgs potential, assuming no NP) • For this talk, pole mass ~ Breit- Wigner mass ~ MC mass • Important to resolve “renormalon ambiguity” ≲ 100 MeV; not the subject of this talk. “this particular CMS result is mostly ๏ Recent Measurements sensitive to uncertainties coming from the theoretical knowledge of the top • Running of top quark mass quark in Quantum Chromodynamics” CMS-TOP-19-007 • Γ t = 1.9 ± 0.5 GeV ATLAS-CONF-2019-038 • LHC Δ m t ~ 50 MeV ~ 0.3% See eg LHCTopWG Twiki page 3 � P E T ER S K A ND S

<latexit sha1_base64="Lj0k2nEoBvldSUhT0uvWUeRSoY=">AB7nicbVBNS8NAEJ3Ur1q/qh69LBbBU0m0oseiF48V7Ae0oWy2m3bpZhN2J0IJ/RFePCji1d/jzX/jts1BWx8MPN6bYWZekEh0HW/ncLa+sbmVnG7tLO7t39QPjxqmTjVjDdZLGPdCajhUijeRIGSdxLNaRI3g7GdzO/cS1EbF6xEnC/YgOlQgFo2ildi+gOsNpv1xq+4cZJV4OalAjka/NUbxCyNuEImqTFdz03Qz6hGwSflnqp4QlYzrkXUsVjbjxs/m5U3JmlQEJY21LIZmrvycyGhkziQLbGVEcmWVvJv7ndVMb/xMqCRFrthiUZhKgjGZ/U4GQnOGcmIJZVrYWwkbU0Z2oRKNgRv+eV0rqoepfVq4dapX6bx1GEziFc/DgGupwDw1oAoMxPMrvDmJ8+K8Ox+L1oKTzxzDHzifP6Aoj8Q=</latexit> <latexit sha1_base64="t6XaytdIsHwdU4AeCNDSjPNP5sM=">AB6HicbVDLSgNBEOz1GeMr6tHLYBA8hV0f6DHoxWMC5gHJEmYns8mY2dlplcIS7AiwdFvPpJ3vwbJ8keNLGgoajqprsrSKQw6Lrfzsrq2vrGZmGruL2zu7dfOjhsmjVjDdYLGPdDqjhUijeQIGStxPNaRI3gpGd1O/9cS1EbF6wHC/YgOlAgFo2ilOvZKZbfizkCWiZeTMuSo9Upf3X7M0ogrZJIa0/HcBP2MahRM8kmxmxqeUDaiA96xVNGIGz+bHTohp1bpkzDWthSmfp7IqORMeMosJ0RxaFZ9Kbif14nxfDGz4RKUuSKzReFqSQYk+nXpC80ZyjHlCmhb2VsCHVlKHNpmhD8BZfXibN84p3UbmqX5art3kcBTiGEzgD6hCvdQgwYw4PAMr/DmPDovzrvzMW9dcfKZI/gD5/MH4m2M/w=</latexit> What top quarks look like In theory m t e p t X Monte Carlo Event Generators: j “Pythia", “Herwig”, … j Decays, showers, hadronisation, … p X ¯ t µ If you are measuring the top quark mass, E T you want to know: how accurately is this transfer function known/modelled? In practice ➜ want “good physics” under the hood. + good validations (preferably in-situ). 4 � P E T ER S K A ND S

The Physics of Hadronic Jets ๏ More than just a (fixed-order perturbative) expansion in α s Bremsstrahlung : accelerated particles radiate ⟷ • Infinite-order perturbative structures of indefinite particle most of my research number ⟷ universal amplitude structures in QFT Confinement (strong gluon fields) ⟷ Hadronization phase transition ⟷ quantum-classical correspondence. Non- perturbative physics. String dynamics. String breaks. Hadrons ⟷ Spectroscopy (incl excited and exotic states) , lattice QCD, (rare) decays, mixing, light nuclei. Hadron beams → multiparton interactions, diffraction, … � 5 P E T ER S K A ND S

Types of Bremsstrahlung Showers ๏ Parton Showers are based on iterated 1 → 2 splittings • Each parton undergoes a sequence of splittings 2 2 • Exact in limit that one diagram dominates: collinear + splittings; good starting point for describing jets Some interference effects can be included via “angular ๏ ordering” or “dipole functions” (~partitioned interference terms) (E,p) conservation achieved via (ambiguous) recoil effects ๏ ๏ At Monash, we develop an Antenna Shower , in which splittings are fundamentally 2 → 3 (+ working on 2 → 4…) 2 • Evolution in terms of colour dipoles/antennae + + Intrinsically coherent (to leading power of 1/N C2 ~ 10%) ๏ + Manifestly Lorentz invariant kinematics with local (E,p) cons. ๏ (+ Markovian/Invertible: important for future applications) Includes dipole interference ๏ 6 � P E T ER S K A ND S

Modelling Top Pair Production and Decay VINCIA ๏ In limit Γ t ~ 0, factorise production and decay • These stages are showered independently (regardless of which type of shower) m t < Q evol < Q cut Bremsstrahlung Showers √ s < Q evol < Q cut (perturbative) w o fl r u o l o c F R ⊗ IF colour flow II colour flow I: initial F: final R: resonance IF colour flow ⊗ PRODUCTION DECAY(S) • Production ISR + FSR shower • Resonance-Decay FSR shower preserves Breit-Wigner shape • preserves Breit-Wigner shape • 7 � P E T ER S K A ND S M O NA S H U.

Interference between production and decay? VINCIA ๏ Would modify BW shape. • But expect small effects. Cutoff of perturbative shower Q cut ~ 1 GeV ; Γ t ~ 1.5 GeV (in SM); Interference only from scales 1 GeV < Q < 1.5 GeV m t < Q evol < Q cut √ s < Q evol < Q cut w fl o u r o o l c I F w o fl r u o l o c F R ⊗ IF colour flow II colour flow I: initial F: final R: resonance IF colour flow ⊗ ๏ ➤ Ignored in narrow-width approximation (eg PYTHIA). Production showered to Q cut , decay as well. ๏ An e + e - study found Δ m t < 50 MeV but not repeated for LHC (to my knowledge) ๏ Khoze, Sjöstrand, Phys.Lett. B328 (1994) 466 though see Ravasio et al, Eur.Phys.J. C78 (2018) no.6, 458 � 8 P E T ER S K A ND S M O NA S H U.

Shower Ambiguities: Coherence VINCIA ๏ Default “Pythia” showers not fully coherent for “IF” or “RF” flows • All initial-state partons treated as II. (Some coherence by rapidity ~angular vetos) • All final-state partons treated as FF. (MECs ➤ 1st emission in top decay correct; + b mass corrections for all emissions.) w o fl r u o l o c F R Recoils and phase space ⊗ IF colour flow Recoils and Recoils and II colour flow phase space phase space I: initial F: final R: resonance IF colour flow ⊗ • RF not coherent from 2 nd emission onwards. (So eg Powheg does not help.) • Issues for soft wide-angle, recoil effects, and some phase-space effects. 9 � P E T ER S K A ND S M O NA S H U.

Coherence in VINCIA VINCIA Brooks, Skands, Phys.Rev. D100 (2019) no.7, 076006 ARXIV:1907.08980 ๏ Explicit IF and (recently) RF antennae • Based on coherent dipole-antenna patterns, with full t and b mass effects. • Collective recoils for RF emissions: coherent radiation recoils against “crossed” top • + VINCIA now integrated within PYTHIA 8.301 w o fl r u o l o c F R ⊗ IF colour flow II colour flow I: initial F: final R: resonance IF colour flow ⊗ ๏ + Under development (with H. Brooks, R. Verheyen, C. Preuss) ( ) Interleaved resonance decays ➤ interference between production and decays. ๏ Matrix-Element Merging & Iterated ME Corrections. (So far it is a pure shower.) ๏ Automated uncertainty variations (in the same style as internal Pythia 8 ones). ๏ Electroweak showers, second-order antenna functions, … ๏ � 10 P E T ER S K A ND S M O NA S H U.

Prime Motivation: Top Quark Mass VINCIA Slide from H. Brooks Ravasio et al, Eur.Phys.J. C78 (2018) no.6, 458 arXiv:1801.03944 “... the very minimal message that can be drawn from our work is that, in order to assess a meaningful theoretical error in top-mass measurements, the use of di ff erent shower models, associated with di ff erent NLO+PS generators, is mandatory.” 11 � P E T ER S K A ND S M O NA S H U.