Probing tq Z anomalous couplings in the trilepton signal at the HL-LHC,HE-LHC,and FCC-hh

We investigate the prospect of discovering the Flavour Changing Neutral Current(FCNC)tqZ couplings via two production processes yielding trilepton signals:top quark pair production pp→tt with one top quark decaying to the Z boson and one light jet and the anomalous single top quark plus Z boson production process pp→tZ.We study these channels at various successors of the Large Hadron Collider(LHC),i.e,the approved High-Luminosity LHC(HL-LHC)as well as the proposed High-Energy LHC(HE-LHC)and Future Circular Collider in hadron-hadron mode(FCC-hh).We perform a full simulation for the signals and the relevant Standard Model(SM)backgrounds and obtain limits on the Branching Ratios(BRs)of t→qZ(q=u,c),eventually yielding a trilepton final state through the decay modes t→bW+→bl^(+)V_(l)and Z→l^(+)l^(-).The upper limits on these FCNC BRs at 95%Confidence Level(CL)are obtained at the HL-LHC with√s=14 TeV and 3 ab^(-1),at the HE-LHC with√s=27TeV and 15 ab^(-1),and at the FCC-hh with√s=100 TeV and 30ab^(-1).

A general analysis of Wtb anomalous couplings

We investigate new physics effects on the Wtb effective couplings in a model-independent framework. The new physics effects can be parametrized by four independent couplings, f1^L, f1^R, f2^L and f2^R. We further introduce a set of parameters x0, xm, xp and x5 which exhibit a linear relation to the single top production cross sections. Using recent data for the t-channel single top production cross section at, tW associated production cross section crtw, s-channel single top production cross section δs, and W-helicity fractions F0, FL and FR collected at the 8 TeV LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our globM fitting results show that the top quark effective couplings are strongly correlated. We show that （i） improving the measurements of δt and δtw is important in constraining the correlation of （f1^R, f2^R） and （f2^L,f2^R）; （ii） f1^L and f2^R are anti-correlated, and are sensitive to all the four experiments; （iii） f1^R and f2^L are also anti-correlated, and are sensitive to the F0 and FL measurements; （iv） the correlation between f2^L and f2^R is sensitive to the precision of the δt, δtw and F0 measurements. The effective Wtb couplings are studied in three kinds of new physics models： the G（221） = SU（2）1 SU（2）2 U（1）x models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson＇s mass （Mw,） is less than several hundred GeV, but the constraint is loose if Mw^t 〉 1 TeV. Fhrthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.

Constraints on anomalous quartic gauge couplings via Wγjj production at the LHC

Vector boson scattering at the Large Hadron Collider(LHC)is sensitive to anomalous quartic gauge couplings(aQGCs).In this study,we investigate the aQGC contribution to Wγjj production at the LHC with√s=13 TeV in the context of an effective field theory(EFT).The unitarity bound is applied as a cut on the energy scale of this production process,which is found to have significant suppressive effects on signals.To enhance the statistical significance,we analyze the kinematic and polarization features of the aQGC signals in detail.We find that the polarization effects induced by aQGCs are unique and can discriminate the signals from the SM backgrounds well.With the proposed event selection strategy,we obtain the constraints on the coefficients of dimension-8 operators with current luminosity.The results indicate that the process pp→Wγjj is powerful for searching for the OM2,3,4,5 and OT5,6,7 operators.

Polarized and unpolarized off-shell H^(*)→ZZ→4■ decay above the 2m_(z) threshold

An analysis of the off-shell H^(*)→Zz-■_(1)■_(1)■_(2)■_(2) decay width is presented for both unpolarized and polarized Z gauge bosons in the scenario with the most general H^(*)ZZ vertex function,which is expressed in terms of two CP-even(bz and cz)and one CP-odd(bz)anomalous couplings.The SM contributions to the H^(*)ZZ coupling up to the one-loop level are also included.Explicit analytic results for the unpolarized and polarized H^(*)-Zz-■_(1)■_(1)■_(2)■_(2) square amplitudes and the four-body phase space are presented,out of which several obseryable quantities can be obtained straightforwardly.Regarding numerical analysis,a cross-check was performed via MadGraph5_aMC@NLO,where our model was implemented with the aid of FeynRules.We then considered the most stringent bounds on anomalous complex H^(*)ZZ couplings and analyzed the effects of the polarizations of the Z gauge bosons through the polarized H^(*)-ZZ→■_(1)■_(1)■_(2)■_(2) decay width as well as left-right and forward-backward asymmetries,which were found to be sensitive to new-physics effects.Particular focus was put on the effects of the absorptive parts of the anomalous H^(*)ZZ couplings,which have been largely overlooked up to now in LHC analyses.It was found that the studied observable quantities,particularly the left-right asymmetries,can be helpful to search for effects of CP-violation in the H^(*)ZZ coupling and set bounds on the absorptive parts.For completeness,we also analyzed the case of unpolarized Z gauge bosons.

Anomalous H→ZZ→4ℓ decay and its interference effects on gluon–gluon contribution at the LHC

We calculate the spinor helicity amplitudes of anomalous H→ZZ→4ℓdecay.After embedding these analytic formulas into the MCFM package,we study the interference effects between the anomalous gg→H→ZZ→4ℓprocess and the SM processes,which are indispensable in the Higgs off-shell region.Subsequently,the constraints on the anomalous couplings are estimated using LHC experimental data.