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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 five
of 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.