Abstract
Measurements of the top quark polarization and top quark pair ($\mathrm{t\bar{t}}$) spin correlations are presented using events containing two oppositely charged leptons (e$^+$e$^-$, e$^\pm\mu^\mp$, or $\mu^+\mu^-$) produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The data were recorded by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$^{-1}$. A set of parton-level normalized differential cross sections, sensitive to each of the independent coefficients of the spin-dependent parts of the $\mathrm{t\bar{t}}$ production density matrix, is measured for the first time at 13 TeV. The measured distributions and extracted coefficients are compared with standard model predictions from simulations at next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD), and from NLO QCD calculations including electroweak corrections. All measurements are found to be consistent with the expectations of the standard model. The normalized differential cross sections are used in fits to constrain the anomalous chromomagnetic and chromoelectric dipole moments of the top quark to $-$0.24 $<C_\text{tG}/\Lambda^{2}$ $<$ 0.07 TeV$^{-2}$ and $-$0.33 $< C^{I}_\text{tG}/\Lambda^{2}$ $<$ 0.20 TeV$^{-2}$, respectively, at 95% confidence level.
Highlights
The top quark is the heaviest known fundamental particle and has a lifetime on the order of 10−25 s [1]
In the standard model (SM), a small longitudinal polarization arises from electroweak (EW) corrections, while a small transverse polarization comes from absorptive terms at one loop
The uncertainty arising from the imperfect modeling of the b tagging efficiency is determined by varying the measured scale factors (SFs) within their uncertainties, both globally and depending on the pT and η of the b jets
Summary
The top quark is the heaviest known fundamental particle and has a lifetime on the order of 10−25 s [1]. Differential tt cross sections corresponding to a subset of the coefficients, and other observables sensitive to the top quark polarization and tt spin correlations, have pbeffieffi n measured by the ATLAS and CMS Collaborations at s 1⁄4 7, 8, and 13 TeV [7,8,9,10,11]. In this analysis, each coefficient is extracted from a measured normalized differential tt cross section, using the same event selection and reconstruction as described in Ref. We set limits on contributions from these operators using simultaneous fits to the measured normalized differential cross sections, including constraints on the chromomagnetic and chromoelectric dipole moments of the top quark
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