Measurements of tt spin correlations in CMS

We present an overview of the measurements of tt spin correlations in the CMS Collaboration. We present two analyses both in the dilepton channel using proton-proton collisions at √ s = 7 TeV based on an integrated luminosity of 5.0 fb−1. The spin correlations and polarization are measured using angular asymmetries. The results are consistent with unpolarized top quarks and Standard Model spin correlation. The second analysis sets a limit on the real part of the top-quark chromo-magnetic dipole moment of −0.043 < Re(μ̂t) < 0.117 at 95 % confidence level through the measured azimuthal angle difference between the two charged leptons from tt production. 1. Spin correlation strength Due to the very short top-quark lifetime, top quarks decay before their spins decorrelate. The top-quark pair spin correlation information is therefore propagated to the decay products. A measurement of the tt spin correlation strength C allows for a stringent test of the Standard Model (SM), sensitive to the spin of the top quark and the existence of new types of strong coupling. The spin correlation strength C appears in the double angular distribution, dσ dcosθ1dcosθ2 = σ 4 (1 + α1P1cos(θ1) + α2P2cos(θ2)− Ccos(θ1)cos(θ2)) (1) where the angle θ is the angle between the direction of the top-quark decay product in the parent rest frame and the direction of the top quark with respect to the chosen basis, P1,2 is the top (anti-)quark polarization and αi is the spin analyzing power. 2. Measurement of spin correlation and top-quark polarization The presented measurement of spin correlation and top-quark polarization was performed in the dilepton channel at √ s = 7 TeV using 5.0 fb−1 of integrated luminosity [4], by means of three asymmetries shown below. The first asymmetry, in eq. 2, accesses the top-quark polarization P (assuming P1 = P2 = P) with P = 2AP in the helicity basis Ap = N [cos(θ∗ l ) > 0]−N [cos(θ∗ l ) < 0] N [cos(θ∗ l ) > 0] +N [cos(θ ∗ l ) < 0] (2) counting both the positive and negative leptons, where θ∗ l is the angle of a charged lepton in the rest frame of its parent top quark or antiquark, measured in the helicity frame. The variable ar X iv :1 41 1. 72 03 v1 [ he pex ] 2 6 N ov 2 01 4 A∆φ provides discrimination between correlated and uncorrelated tt spins with ∆φ the difference in azimuthal angle between the charged leptons A∆φ = N(∆φl+l− > π/2)−N(∆φl+l− < π/2) N(∆φl+l− > π/2) +N(∆φl+l− < π/2) (3) . The third asymmetry value, given in eq. 4, is a direct measure of the spin correlation strength with Chel = −4Ac1c2 using the helicity angle of the leptons with c1 = cos(θ∗ l+) and c2 = cos(θ ∗ l−) Ac1c2 = N(c1c2 > 0)−N(c1c2 < 0) N(c1c2 > 0) +N(c1c2 < 0) (4) . (radians) l + l φ ∆ 0 0.5 1 1.5 2 2.5 3 ) l + l φ∆ /d ( σ σ 1/ 0.2 0.25 0.3 0.35 0.4 0.45 0.5 ) unfolded kg. b Data ( Syst. uncertainty MC@NLO parton level ) t m = μ (SM, Si G. Bernreuther & Z. W. ) t m = μ (uncorrelated, Si G. Bernreuther & Z. W. = 7 TeV s at -1 CMS, 5.0 fb ) *l θ ) cos( *+ l θ cos( -1 -0.5 0 0.5 1 )* l θ ) co s( * + l θ /d (c os (