Mainz 2014 – wissenschaftliches Programm

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T: Fachverband Teilchenphysik

T 61: Top-Quarks: Single Top

T 61.9: Vortrag

Dienstag, 25. März 2014, 18:45–19:00, P102

Search for CP violation in single top quarks events with the ATLAS detector at LHC — •Maria Moreno Llacer — II.Physics Institute, University of Goettingen, Germany — Instituto de Fisica Corpuscular (IFIC), Valencia, Spain

This contribution presents a search for CP violation in the decay of polarised top quarks produced via electroweak interactions by probing the couplings of the top quark in the Wtb vertex. In an effective operator framework this vertex can be written as L_Wtb= −g/√2u_b(γ^µ(V_LP_L+V_RP_R)+iσ^µνq_ν/M_W(g_LP_L+g_RP_R))u_tW_µ being V_L, V_R, g_L and g_R the left and right-handed vector and tensor couplings. At leading order in the Standard Model, V_L is almost one and the others vanish. Non-zero values of V_R, g_L and g_R (anomalous couplings) can give hints for new physics beyond the Standard Model. The so-called W boson helicity fractions probe the presence of non-zero real anomalous couplings, but they do not contain all the information from the top quark decay, in particular regarding complex phases which would imply that the top quark decay has a CP violation component. For polarised top quarks, such as those produced via electroweak interaction at the LHC (single top quark production), it is worth defining the normal N and transverse T polarisation fractions, orthogonal to its momentum. The most interesting fact is that the forward-backward asymmetry A_FB^N in the angular distribution cosθ^N, where θ^N is the angle between the direction of the lepton in the W boson rest frame and the direction N, is very sensitive to the imaginary part of the coupling g_R.