Coherent Showers in Decays of Coloured Resonances Helen Brooks & Peter Skands (Monash University) A new shower model based on “Resonance- Final” antennae (with mass- and helicity-dependence) VINCIA VINCIA log 10 ( a RF g/qq s AK ) as a function of θ jk in A COM frame 90 � 135 � 45 � a n n e t n a F R ⊗ IF antenna 6 5 4 3 2 1 180 � 0 � II antenna log(E/GeV) = 0.0 log(E/GeV) = 0.2 log(E/GeV) = 0.4 log(E/GeV) = 0.6 log(E/GeV) = 0.8 log(E/GeV) = 1.0 IF antenna 225 � 315 � log(E/GeV) = 1.2 ⊗ log(E/GeV) = 1.4 log(E/GeV) = 1.6 270 � PRODUCTION log(E/GeV) = 1.8 DECAY(S) RF ANTENNA PATTERN HEP Seminar Lund, June 2019
Coherence in Resonance Decays VINCIA ๏ In narrow width approximation, • Factorise production and decay of resonances; • These stages are showered independently. Goal is to shower the resonance-final m t < Q evol < Q cut antenna in decay √ s < Q evol < Q cut coherently, without modifying the a n n e t n a F R invariant mass of ⊗ IF antenna the resonance, II antenna needed for resonance-aware IF antenna ⊗ matching. PRODUCTION DECAY(S) Note: interference between production and decay will occur at scales < Γ ; not the topic of this talk � 2 H E LEN B RO O K S & P E TE R S K A N D S M O NA S H U.
Prime Motivation: Top Quark Mass VINCIA 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.” 3 � H E LEN B RO O K S & P E TE R S K A N D S M O NA S H U.
Uncertainties VINCIA ๏ Fixed-order accuracy ( μ R ) + PDFs ( μ F ) + matching/merging ( e.g. h damp ) ๏ Parton shower ambiguities from logarithmic accuracy • → Estimate by comparing different shower architectures + systematic parametric variations ๏ • → To reduce , need systematic improvements: At LL / LC: coherence & “optimised” choices (for muR, evolution scale, ๏ recoil strategies, …) Beyond LL / LC: genuine subleading colour (beyond optimised LC) and ๏ higher-order corrections to shower kernels (beyond optimised LL) ๏ + Mass Effects, Finite-Width Effects, Polarisation Effects ๏ + Non-perturbative: Renormalon pole mass ambiguity ≲ Λ QCD , ( ) colour-reconnections, ︎ MPI, beam remnant treatment, hadronisation, hadron rescattering, hadron and τ decays, … 4 � H E LEN B RO O K S & P E TE R S K A N D S M O NA S H U.
Dipoles vs Antennae (in resonance decays) VINCIA * ๏ Dipole showers ๏ Antenna Showers • Each branching has a well-defined • Agnostic as to who is the radiator; “radiator” and a “recoiler”, with smooth transition in kinematics distinct kinematics maps. Interpolates between collinear limits ๏ • Neglect contribution from • Coherence built in; cannot neglect resonance as radiator (partition resonance’s contribution can even become negative). • Recoil strategy relates to antenna • In principle free to choose recoiler, factorisation e.g. W in t → W b kinematics X t − t b ? b t → b W: t → b W : Top sits at rest (does not radiate) Antenna between bottom and crossed top. Bottom quark radiates; recoils against the Kinematics map with X = W ⟹ W acquires recoil only other final-state parton, W. More branchings: unambiguous. Parton colour- More branchings : ambiguous what recoiler connected to top participates in the RF antenna; to use for parton colour-connected to top rest = X collectively acquire the recoil. * Note: the original dipole shower, ARIADNE, is of the type I here call “antenna shower” 5 � H E LEN B RO O K S & P E TE R S K A N D S M O NA S H U.
Current Status of Resonance Decay Showers VINCIA Slide from H. Brooks Shower Type Decay shower? Coherence? * ( ) Pythia 8 [hep-ph/0010012] Dipole 3 7 *Via ME corrections [hep-ph/0408302] Sherpa [1412.6478] Catani-Seymour ( 3 ) 7 (no RF dipole) (production only) Herwig 7 ( ˜ q ) Angular-ordered 3 3 [1810.06493] Herwig 7 (dip) Catani-Seymour ( 3 ) ( 3 ) (no RF dipole) (on-shell only) [1810.06493] Vincia - NEW! Antenna 3 3 * ✔ ? Dire? Dipole ✔ ? *: not completely sure about status 6 � H E LEN B RO O K S & P E TE R S K A N D S M O NA S H U.
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