Patrick Janot edited Observations.tex  about 9 years ago

Commit id: 6744b5ba77dfc5018fffccd7edd7064a481a8932

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The first striking observation is that an increase of the  centre-of-mass energy far beyond the top-pair production threshold  is not particularly useful relevant  to improve the precision on the top-quark electroweak couplings. For four out of fiveof these  couplings, the optimum is reach actually reached  for $\sqrt{s} \simeq 360$\,GeV. The expected  precision then  degrades by up to a factor four with 500\,${\rm fb}^{-1}$ at $\sqrt{s} = 500$\,GeV. It can also be noted that a very decent precision is alredy reached for $\sqrt{s} = 350$\,GeV. The second observation is that the precision reached for these four couplings is at the per-mil level, level of the per mil,  and that the tt$\gamma$ and the ttZ couplings can be determined independently with this precision without the need of initial polarization. (It was in fact  checked that, with this method, incoming beam polarization would actually significantly degrade the statistical power of the coupling determination, by up to one order of magnitude.) It is only for $F_{1A}^Z$ that an a moderate  increase of the centre-of-mass energy could would  improve the precision by a factor of two, from 2\% at $\sqrt{s} = 360$\,GeV to 1\% at $\sqrt{s} = 420$\,GeV, an energy at which the single-top production would need to be included as a background to the study. There are, however, many other observables to be studied in a ${\rm t\bar t}$ event, beyond the energies and angles of the leptons. It was noticed, for example, that a factor of two improvement could be obtained $\sigma(F_{1A}^Z)$  at $\sqrt{s} = 360$\,GeV by including the energy and angular distributions of the b quarks, quarks in the covariance matrix,  but this remark is beyond the scope of the present study.