Measurements of charge and CP asymmetries in b-hadron decays using top-quark events collected by the ATLAS detector in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

Abstract

Same-and opposite-sign charge asymmetries are measured in lepton+jets tt events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb −1 from proton-proton collisions at a centre-of-mass energy of √ s = 8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model. Keywords: Hadron-Hadron scattering (experiments)ArXiv ePrint: 1610.07869Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP 3 .doi:10.1007/JHEP02(2017)071 The ATLAS collaboration 30 JHEP02(2017)071 IntroductionThe violation of the combined charge conjugation (C) and parity transformation (P) of particles and antiparticles implies that the laws of physics are not the same for matter and antimatter. The study of CP violation, first observed in neutral kaon decays in 1964 by Christenson, Cronin, Fitch and Turlay [1], has a long and rich history within particle physics. Since the first observation, CP violation in mixing has also been observed in neutral B d and B s mesons [2][3][4], and direct CP violation has been observed in kaon decays [5][6][7] and B-meson decays [8][9][10][11][12]. However, these observations are not sufficient to explain the matter-antimatter asymmetry in the universe [13]. In addition, a sizeable inclusive like-sign dimuon charge asymmetry (using a sample of primarily bb pairs) has been reported [14] by the D0 experiment in which an excess was observed over that predicted by the Standard Model (SM), when the measurement is interpreted in the form of CP asymmetries relevant - [18] makes it possible to perform measurements of CP asymmetries in heavy-flavour mixing and decay from top-quark decay products, as outlined in ref. [19]. All existing analyses of CP violation in B-meson decays rely on a resonant production of bb pairs or hadroproduction. The unique aspect presented in this paper is the method by which the charge of the b-quark is determined, both at production and at decay. The top quark decays before hadronisation, predominantly via t → W b. In the case where the W -boson decays leptonically, the charge of the lepton determines the charge of the produced b-quark. The b-quark hadronises and in the case that the resulting b-hadron decays semileptonically, the charge of the soft lepton determines the b-quark charge at decay. This paper exploits a soft-muon heavy-flavour tagging (SMT) algorithm [20, 21] in order to measure muons that originate from such a process, hereafter referred to as SMT muons. In principle this process may also be observed via soft electrons or soft τ leptons; however, in practice the...

CPviolation in theBsystem

“…This method, recently implemented by ATLAS [103], however results in low yields so that the measurements do not currently have competitive precision. There is no significant difference between the values of A b sl measured in the different regions, however the consistency of the binned data with the SM prediction gives a more significant deviation of 3.6σ.…”

From the bottom to the top—reconstruction oftt̄ events with deep learning

“…The added value of the work documented in this paper is threefold: We quantify the performance in reconstructing tt events with a DNN, which has a wide range of applications, as outlined above. We compare the performance to the standard approach (kinematic fitting), which has been used for many analyses at the LHC [2,3,[6][7][8][9][25][26][27][28]. We consider the simple sequential optimisation procedure that we perform and document ready-to-use for data analysis at LHC experiments.…”

T-Odd Anomalous Interactions of the Top-Quark at the Large Hadron Collider

“…The T-violating interactions of the top-quark have already been studied in the literature for a fixed CPviolating scale in the Refs. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]; for example, CPviolation at future e + e − collider in t t production is investigated in Ref. [15], Ref.…”

“…[16] considered CP-violation due to complex top-Yukawa coupling in e + e − ⟶ ht t at the future e + e − collider, Charge-asymmetries in b b pair from topquark decay were first analysed in Ref. [17], Ref. [18] studied the CP-violation using T-odd correlations in lepton plus jets channel, Ref.…”