Top FCNC induced by a Z′ boson

Abstract

We consider a Z model where the Z boson couples only to up and top quarks. As a simple setup, one can consider a family-dependent U (1) symmetry, under which only right-handed up-type quarks are charged. After symmetry breaking, the right-handed quarks mix with each other and top flavor-changing neutral currents (FCNCs) mediated by the Z boson can be generated at tree level. We take into account several processes to probe the top FCNCs, and find that the same sign top quark pair production or the triple top quark production is the most capable of testing the top FCNCs. We also consider a few non-FCNC processes to probe the Z boson, for example, dijet production at the LHC. arXiv:1910.05925v1 [hep-ph] 14 Oct 2019investigate the FCNCs through the production of the top quark. In the case of top-Higgs FCNCs via t-q-H (q = u or c) couplings, Atwood et al.[8] discussed three processes, pp → tt(tt), pp → tj(tj), and pp → tjH(tjH), where j denotes a jet and found that the last process is the most capable of yielding the best upper bound on the top FCNCs.In this work, we consider top FCNCs which are generated by a new gauge boson, Z , heavier than the top quark. In search for FCNCs we take into account four FCNC processes, same sign top quark pair production, pp → tt (tt), single top quark production, pp → tj(tj), radiative Z production, pp → tZ j(tZ j), and triple top quark production, pp → ttt (ttt). We calculate cross sections for the four FCNC processes and compare them with each other. We search for the best FCNC process that yields the strongest upper bound on the top FCNC coupling. Since the FCNC couplings are related to non-FCNC couplings, such as t-t-Z coupling, we also consider non-FCNC processes, dijet production, pp → jj, top quark pair production, pp → tt, and four top quark production, pp → tttt to compare them with FCNC processes. Furthermore, since the triple top quark production involves both FCNC and non-FCNC couplings, the constraints from the non-FCNC processes must be considered [9]. This paper is organized as follows. In Sec. 2, we introduce an effective model which provides FCNCs through a Z exchange. As an example of the UV complete model, we consider a U (1) model in which only right-handed up-type quarks are charged. In Sec. 3, we perform numerical analyses for the FCNC processes as well as the non-FCNC processes. Finally, we summarize our results in Sec. 4.

Topping-up multilepton plus b-jets anomalies at the LHC with a Z′ boson

“…Other processes that could be tested in these channels (see section 3.2) are either numerically irrelevant such as same-sign top-quark pair production (tt, t t) [27], or chirality suppressed such as radiative Z production (tZ j, tZ j) [27,29]. Note that these results can be fairly specific to our model, see e.g.…”

“…High-energy collider physics is sensitive both to inclusive on-shell production of the Z + X and to non-resonant behaviour, such as the re-scaling of different top-physics crosssections [22][23][24]27]. As mentioned before, taking g uu , g cc ≈ 0 makes our model insensitive to many of them.…”

“…In particular, a very similar set-up has been implemented in ref. [27], albeit in a different mass range and assuming a hidden sector that increases the width-to-mass JHEP05(2021)125…”

Probing top changing neutral Higgs couplings at colliders

“…m k Same-sign top was advocated 70 to appear with even 1 fb −1 LHC data due to some Tevatron "anomaly", but this was quickly ruled out. 71,72 Same-sign top production has also been advocated recently with flavor-changing Z , 73,74 as well as neutral scalar φ exchange. 73 l This is in part because SM 4t cross section 75,78 at ∼ 12 fb is an order of magnitude larger than 3t in SM, hence can be measured sooner if SM holds true.…”

“…We would advocate, 68 however, that "triple-top" search is more informative with a larger dataset. New physics triple-top search has been advocated in topcolorassisted technicolor model, 101 usual 2HDM with Z 2 symmetry, 102, 103 a literal effective tqh coupling, 104,105 flavor-changing Z , 74 and also using EFT approach with four-fermi operators involving triple-top. [106][107][108] Our sub-TeV exotic Higgs bosons have dimension-4 couplings (Sec.…”