Observing $$t{\bar{t}}Z$$ spin correlations at the LHC

Abstract

Spin correlations in the production of top-antitop quark ($$t{\bar{t}}$$ t t ¯ ) pairs at the Large Hadron Collider (LHC) are an experimentally verified prediction of the Standard Model. In this paper, we compute the full spin density matrix for $$t{\bar{t}}Z$$ t t ¯ Z production at next-to-leading order precision in QCD, for center-of-mass energies of 13 and 14 $$\text {TeV}$$ TeV . We find that the additional emission of a Z boson leads to significantly different spin correlations with respect to the $$t{\bar{t}}$$ t t ¯ case, and induces small longitudinal polarisations of the top quarks. We further propose an analysis strategy that could lead to the observation of spin correlations in $$t{\bar{t}}Z$$ t t ¯ Z events at the end of Run 3 of the LHC, or possibly earlier by combining the ATLAS and CMS datasets. In addition, we show that the pure angular information contained in the spin density matrix provides novel constraints on the dimension-6 effective field theory operators relevant to the t-Z interaction, without any reference to the total production rates.

The top quark electro-weak couplings after LHC Run 2

“…Several groups have performed combined EFT fits of the top, Higgs and electro-weak sectors [12,13], while others have explored the interplay between top quark physics and the flavour factories [14][15][16][17][18]. In addition, three recent studies explore events involving top quarks and Z bosons to further constrain some electro-weak couplings [19][20][21].…”

On two-body and three-body spin correlations in leptonic $t\bar{t}Z$ production and anomalous couplings at the LHC

“…The polarizations of top quarks, Z boson, and spin correlations of t, t and Z can be useful observables to study the t tZ interaction. There has been a lot of interest in recent days in the polarizations of top quarks , Z/W boson [36][37][38][39][40][41][42][43][44][45][46][47][48][49], including the t t spin correlation [23,[50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] for the study of new physics beyond the SM. Measurement on the top polarizations and t t spin correlations are performed earlier at the Tevatron [65][66][67][68][69][70][71] and recently at the Large Hadron Collider (LHC) by ATLAS and CMS experiments [72][73][74][75][76][77][78][79][80][81]…”

“…These searches also probe the anomalous t tZ interaction with the help of differential rates in various kinematic variables. The t tZ anomalous couplings have also been studied theoretically [63,[99][100][101][102][103][104][105][106] deriving constraints on them. A recent study [63] also discuss the possibilities of observing t t spin correlations in the t tZ production process, including the effect of anomalous t tZ interaction.…”

“…We work in the fully leptonic final state, i.e., in 4l + 2b + E T final state, for better reconstructability of the t tZ topology and the spin observables. We use the effective vertex factor parameterization for the anomalous t tZ interaction [63,107,108] to investigate them and translate their limit into the higher dimensional effective operators [63,105,107] to compare with the existing studies [90][91][92]109]. The polarizations and spin correlations provide complementary sensitivity to the anomalous t tZ interaction to the existing approaches without the binned cross section dependence.…”

The top quark electro-weak couplings after LHC Run 2

“…Several groups have performed combined EFT fits of the top, Higgs and electroweak sectors [12,13], while others have explored the interplay between top quark physics and the flavour factories [14][15][16][17][18]. In addition, three recent studies explore events involving top quarks and Z bosons to further constrain some electro-weak couplings [19][20][21].…”