Coherent Showers in Decays of Coloured Resonances Helen Brooks & - PowerPoint PPT Presentation

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 Parton Showers and Resummation Vienna, 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.

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” 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.