Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of √ s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb −1 , are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t → bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 × 10 −4 (2.4 × 10 −4 ) on the t → uZ branching ratio and 2.4 × 10 −4 (3.2 × 10 −4 ) on the t → cZ branching ratio, constituting the most stringent limits to date.

Constraining four-fermion operators using rare top decays

“…Analyses devoted for other experimental signatures might be sensitive to rare top decays mediated by four-fermion operators, though. Thus, in this article we consider the latest and most constraining search for t → Zj to date [31], and demonstrate that it can already constrain the scale of contact interactions contributing to non-resonant t → + − j beyond the TeV. This possibility has been previously pointed out in the literature; see e.g.…”

“…In short terms, this analysis demands three light leptons (either electrons or muons), two of them same-flavour opposite-sign (SFOS), as well as exactly one b-tagged jet and at least two more light jets. (The b-tagging efficiency is reported to be 0.77, while the misstag rates P(c → b) and P(j → b) are 0.16 and 0.0075 respectively [31]. )…”

“…Although the leptons resulting from the top decay via the contact interactions do not always reconstruct a Z boson, we will show that these kind of searches do still have a large sensitivity. To this end, we consider the latest ATLAS search [31] for FCNC top quark decays at LHC13, i.e. √ s = 13 TeV and L = 36 fb −1 .…”

Fingerprinting the top quark FCNC via anomalous Ztq couplings at the LHeC

“…Subsequently, from the non-observance of FCNC they put the following bounds: [13], excluding BR(t → Zq) > 0.05% at the 95% confidence level. The latest ATLAS search at √ s = 13 TeV [14] with a luminosity of 36.1 fb −1 sets BR(t → Zu(c)) < 1.7(2.4) × 10 −4 . The event considered for investigation was pp → tt, (t → Zq(u, c),t → W + b) with both Z, W -bosons decay leptonically.…”

Higgs Quark Flavor Violation: Simplified Models and Status of General Two-Higgs-Doublet Model

“…Limits on flavour changing couplings of the top quark to the Z boson are also quite stringent (see Refs. [13,[17][18][19][20][21]):…”