Direct Measurement of the Total Decay Width of the Top Quark

Abstract

We present a measurement of the total decay width of the top quark using events with top-antitop quark pair candidates reconstructed in the final state with one charged lepton and four or more hadronic jets. We use the full Tevatron run II data set of ffiffi ffi s p ¼ The top quark (t) is the heaviest known elementary particle. Its large mass endows it with the largest decay width and, hence, the shortest lifetime of any of the known fermions [1]. At leading order calculation of quantum chromodynamics (QCD), the top quark decay width (À top ) depends on the top quark mass (M top ), the Fermi coupling constant (G F ), and the magnitude of the top-to-bottom quark coupling in the quark-mixing matrix (jV tb j) [2]. The next-to-leading-order calculation with QCD and electroweak corrections predicts À top ¼ 1:33 GeV at M top ¼ 172:5 GeV=c 2 with approximately 1% precision [3,4]. This is consistent with the recent next-to-nextto-leading-order calculation of À top ¼ 1:32 GeV [5]. A deviation from the standard model (SM) prediction could indicate the presence of non-SM decay channels, such as decays through a charged Higgs boson [6], the supersymmetric top quark partner [7], or a flavor-changing neutral current [8]. A direct measurement of À top provides general constraints on such processes.The D0 Collaboration has determined the width to be À top ¼ 2:00 þ0:47 À0:43 GeV in a data set corresponding to an integrated luminosity of 5:4 fb À1 , using a modeldependent, indirect measurement that assumes SM couplings [9]. The CDF Collaboration reported more model-independent measurements of the width using a direct shape comparison of the reconstructed top quark mass in data to the simulated top quark mass distributions [10,11]. The most recent measurement set an upper limit of À top < 7:6 GeV at the 95% confidence level (C.L.) with a data set corresponding to 4:3 fb À1 [11]. Even though the direct measurement is less precise than the indirect one, it probes a broader class of non-SM physics models, because the direct measurement has less dependence on the SM.This Letter reports on a direct measurement of the top quark width in p " p collisions at the Tevatron, using the full run II data set, corresponding to an integrated luminosity of 8:7 fb À1 collected with the CDF II detector [12], which is a general-purpose azimuthally and forward-backward symmetric detector surrounding the colliding beams of the Tevatron p " p collider. We not only increase statistical sensitivity using a larger sample with respect to Ref. [11], but also improve jet-energy calibrations using an artificial neural network [13].Top quarks at the Tevatron are predominantly produced in t " t pairs. We reconstruct top quark decays in the topology PRL 111, 202001 (2013) P H Y S I C A L

Signatures of top flavour-changing dark matter

“…Figure 1(left) shows the total width of the top quark (top) and the branching ratio of the above anomalous decay (bottom) as a function of the mediator mass, where we assume a massless DM. For m Z < m t , where the top quark decays into the mediator on mass-shell, the width becomes too broad to be consistent with the current bound 1 Γ t 4 GeV from Tevatron [43,44]. We note that in this parameter region the width depends only on the coupling g i3 and the mediator mass.…”

Extracting the Top-Quark Width from Nonresonant Production

“…The width of the top quark has been measured at the Tevatron and the Large Hadron Collider (LHC) using several approaches. Direct measurements, based on the reconstructed top-quark mass distribution in events with top-quark pairs were made by the CDF [4], ATLAS [5], and CMS [6] Collaborations, with Ref. [5] obtaining the most precise value of Γ t = 1.76 ± 0.33 (stat.)…”

“…Even though the top quark was discovered over 20 years ago [1,2] and its mass has been measured with a sub-percent precision [3], direct measurements of its width Γ t have an uncertainty of 50% or worse [4][5][6]. Indirect measurements of Γ t using single top-quark production are more precise, but also require additional modeling assumptions [7,8].…”

Probing the top-quark width through ratios of resonance contributions of e + e − → W + W − b b ¯ $$ {\mathrm{e}}^{+}{\mathrm{e}}^{-}\to {\mathrm{W}}^{+}{\mathrm{W}}^{-}\mathrm{b}\overline{\mathrm{b}} $$

“…We point out that though g tbW may differ from its SM value, this does not affect the validity of the assumption of equal on-and off-shell couplings discussed in section 1.2. For completeness we note that the best direct bound on Γ t is from CDF, which obtained 1.10 GeV < Γ t < 4.05 GeV at 68% confidence level [57], from template fits of the reconstructed top masses in tt events.…”