Search for flavor changing neutral currents via quark–gluon couplings in single top quark production using 2.3 fb−1 of collisions (original) (raw)
Related papers
2019
Our main aim in this paper is to present detailed studies to probe the top quark flavor changing neutral current (FCNC) interactions at tqg, tqγ, tqH and tqZ(σ µν , γµ) vertices in the triple-top signal pp → tt ¯ t (¯ tt ¯ t) at the high energy proposal of Large Hadron Collider (HE-LHC) and future circular hadron-hadron collider (FCC-hh). To this end, we investigate the production of three top quarks which arises from the FCNC couplings taken into account the fast simulation at √ s = 27 TeV of HE-LHC and 100 TeV of FCC-hh considering the integrated luminosities of 10, 15 and 20 ab −1. All the relevant backgrounds are considered in a cut based analysis to obtain the limits on the anomalous couplings and the corresponding branching ratios. The obtained exclusion limits on the coupling strengths and the branching ratios are summarized and compared in details with the results in the literature, namely the most recent direct LHC experimental limits and HL-LHC projections as well. We show that, for higher energy phase of LHC, a dedicated search for the top quark FCNC couplings can achieve much better sensitivities to the triple-top signal than other top quark production scenarios. We found that the limits for the branching ratios of tqg and tqH transitions could reach an impressive sensitivity and the obtained 95% CL limits are at least three orders of magnitude better than the current LHC experimental results as well as the existing projections of HL-LHC. CONTENTS
Search for Top-Quark Production via Flavor-Changing Neutral Currents inW+1Jet Events at CDF
Physical Review Letters, 2009
We report on a search for the non-standard-model process u(c) + g → t using pp collision data collected by the CDF II detector corresponding to 2.2 fb −1. The candidate events are classified as signal-like or background-like by an artificial neural network. The observed discriminant distribution yields no evidence for FCNC top-quark production, resulting in an upper limit on the production cross section σ(u(c) + g → t) < 1.8 pb at the 95% C.L. Using theoretical predictions we convert the cross-section limit to upper limits on FCNC branching ratios: B(t → u+g) < 3.9×10 −4 and B(t → c + g) < 5.7 × 10 −3 .
Search for top quark FCNC couplings in Z′ models at the LHC and CLIC
The European Physical Journal C, 2010
The top quark is the heaviest particle to date discovered, with a mass close to the electroweak symmetry breaking scale. It is expected that the top quark would be sensitive to the new physics at the TeV scale. One of the most important aspects of the top quark physics can be the investigation of the possible anomalous couplings. Here, we study the top quark flavor changing neutral current (FCNC) couplings via the extra gauge boson Z ′ at the Large Hadron Collider (LHC) and the Compact Linear Collider (CLIC) energies. We calculate the total cross sections for the signal and the corresponding Standard Model (SM) background processes. For an FCNC mixing parameter x = 0.2 and the sequential Z ′ mass of 1 TeV, we find the single top quark FCNC production cross sections 0.38(1.76) fb at the LHC with √ s pp = 7(14) TeV, respectively. For the resonance production of sequential Z ′ boson and decays to single top quark at the Compact Linear Collider (CLIC) energies, including the initial state radiation and beamstrahlung effects, we find the cross section 27.96(0.91) fb at √ s e + e − = 1(3) TeV, respectively. We make the analysis to investigate the parameter space (mixing-mass) through various Z ′ models. It is shown that the results benefit from the flavor tagging.
Evidence for production of single top quarks
Physical Review D, 2008
We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron p " p collider. The standard model predicts that the electroweak interaction can produce a top quark together with an antibottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a 0:9 fb À1 data set that have an electron or muon and missing transverse energy from the decay of a W boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W þ jets and t " t events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of ðp " p ! tb þ X; tqb þ XÞ ¼ 4:7 AE 1:3 pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa quark mixing matrix element that describes the Wtb coupling and find jV tb f L 1 j ¼ 1:31 þ0:25 À0:21 , where f L 1 is a generic vector coupling. This model-independent measurement translates into 0:68 < jV tb j 1 at the 95% C.L. in the standard model.
Physics Letters B, 2010
We present a search for flavor changing neutral currents via quark–gluon couplings in a sample of single top quark final states corresponding to 2.3 fb−1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider. We select events containing a single top quark candidates with an additional jet, and obtain separation between signal and background using Bayesian neural networks. We find consistency between background expectation and observed data, and set limits on flavor changing neutral current gluon couplings of the top quark to up quarks (tgu) and charm quarks (tgc ). The cross section limits at the 95% C.L. are σtgu<0.20 pbσtgu<0.20 pb and σtgc<0.27 pbσtgc<0.27 pb. These correspond to limits on the top quark decay branching fractions of B(t→gu)<2.0×10−4B(t→gu)<2.0×10−4 and B(t→gc)<3.9×10−3B(t→gc)<3.9×10−3.