Measurement of the W boson polarisation in tt¯tt¯ events from pp collisions at s√s = 8 TeV in the lepton + jets channel with ATLAS

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  • External authors:
  • Sarah Barnes
  • Rafal Bielski
  • Brian Cox
  • Cinzia Da Via
  • Nick Dann
  • Giulio Forcolin
  • Alessandra Forti
  • Julia Iturbe Ponce
  • Xingguo Li
  • Stephen Marsden
  • Jiri Masik
  • Stephen Menary
  • Thomas Neep
  • Rustem Ospanov
  • Joleen Pater
  • Arnaud Pin
  • Darren Price
  • Yang Qin
  • John Raine
  • Rhys Roberts
  • Savanna Shaw
  • Lee Tomlinson
  • Stephen Watts
  • Fabian Wilk
  • Martin Woudstra
  • Terence Wyatt
  • The ATLAS Collaboration

Abstract

This paper presents a measurement of the polarisation of W bosons from tt¯tt¯ decays, reconstructed in events with one high- p T p T lepton and at least four jets. Data from pp collisions at the LHC were collected at s√s = 8 TeV and correspond to an integrated luminosity of 20.2 fb −1−1 . The angle θ∗θ∗ between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cosθ∗cos⁡θ∗ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F0= 0.709 ± 0.019F0= 0.709 ± 0.019 , FL= 0.299 ± 0.015FL= 0.299 ± 0.015 and FR= −0.008 ± 0.014FR= −0.008 ± 0.014 , and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.

Bibliographical metadata

Original languageEnglish
Article number264
JournalEuropean Physical Journal C. Particles and Fields
Volume77
Issue number264
Early online date26 Apr 2017
DOIs
Publication statusPublished - 26 Apr 2017