Top-quark  production in E+E- at the NLC

The aim of this study is to understand the reconstruction of top quarks at the NLC using the calorimeter information (ECAL/HCAL) only and the energy-flow algorithms (EFA). This should help to explore different options for the design of a general-purpose detector at the NLC.

First, look at the

Presentation (by S.Chekanov, J.Repond)

at  the Chicago NLC meeting (January, 2002)

Here is the short summary:

a) W-boson and top-mass reconstruction:
It was verified that the resolution on the reconstruction of the top-quark and W-boson masses in the process:

using the EFA is by a factor 2 better than for a pure calorimeter mass reconstruction. In this comparison, we used an "idealized" detector, i.e. we applied a Gaussian smearing for energies of particles generated with PYTHIA, assuming that the resolution on the particle directions has much less impact on the top-mass reconstruction. To simulate the NLC calorimeter, we applied ~13% energy smearing for photons and leptons ("ECAL") and ~32% smearing for all other particles ("HCAL"). For an "idealized" energy-flow algorithm, we applied the ECAL/HCAL smearing only for neutral particles (gammas, K0_L etc.). We use the true energies for charged particles, assuming an excellent momentum resolution for the tracking system. This result represents a limit on the possible improvement of the top reconstruction using the EFA, when the hadronic jets are used for the reconstruction of the all-hadronic top-decay decay channel.

b) Jet energy resolutions:
As it has been shown, the improvement on the mass resolutions discussed above is due to a better energy resolution on the hadronic jets used to reconstruct the invariant masses of W-bosons and top quarks. Figure 1a and Figure 1b show the jet resolutions defined with respect to the partonic jets. In case of the ECAL/HCAL smearing the jet energy resolution is approximately 55%/sqrt(E), if the true partonic jets are used as the reference.

In case of the EFA-type of smearing, the jet energy resolution is almost by factor 2 better than in the case of ECAL/HCAL smearings. This can be clearly seen from Figure 1b . However, such an improvement is difficult to see in Figure 1a , since the fits are rather poor for jets with the energy spreads dominated by the underlying physics and/or the jet algorithm (hadron jets, EFA, ECAL etc). In fact, for the hadron level the fits require a negative value for the constant terms, therefore the resoltion shown for hadrons in Figure 1a is overestimated. This problem can also be seen in Figure 1b , which shows a clear non-constant behavior for the hadronic, EFA and ECAL jets. Also, let us note that all these numbers refer to jets from top decays, which are known to be rather complicated due to a significant contribution from heavy flavors, tau-leptons and neutrinos (b-initiated jets are also included in these numbers).

Part 2: Uncertainties on the top-quark mass determination at the NLC

The aim is to understand the uncertainties on the top mass measurements coming from an incomplete knowledge of various effects related to the hadronic final state, which are presently  can only be simulated with  some degree of arbitrariness using Monte Carlo (MC) model. Using various MC tuning from the LEP experiments, it vas estimated that the uncertainties on the top-mass measurements in the fully hadronic decays are approximately within +/-340 MeV range,  while for the semi-leptonic decays, the uncertainties are smaller and within +/-250 MeV range.

The paper draft ANL-HEP-PR-02-4 (accepted by Eur. J. Phys. C.) is available from this PS file