Top quark mass measurements at and above threshold at CLIC

Abstract

We present a study of the expected precision of the top quark mass determination, measured at a linear e + e − collider based on CLIC technology. GEANT4-based detector simulation and full event reconstruction including realistic physics and beam-induced background levels are used. Two different techniques to measure the top mass are studied: The direct reconstruction of the invariant mass of the top quark decay products and the measurement of the mass together with the strong coupling constant in a threshold scan, in both cases including first studies of expected systematic uncertainties. For the direct reconstruction, experimental uncertainties around 100 MeV are achieved, which are at present not matched by a theoretical understanding on a similar level. With a threshold scan, total uncertainties of around 100 MeV are achieved, including theoretical uncertainties in a well-defined top mass scheme. For the threshold scan, the precision at ILC is also studied to provide a comparison of the two linear collider technologies.

First look at the physics case of TLEP

“…The study makes use of a multi-parameter fit of m top , Γ top , λ top and α s to the top cross section, the top momentum distributions, and the forward-backward asymmetry. When constraining the value of α s (m Z ) to its currently measured value, the study obtained sta ±0.0007, the current uncertainty on this quantity), and from the knowledge of the beamenergy spectrum: a 20% uncertainty of the RMS width of the main luminosity peak would result in top mass uncertainties of approximately 75 MeV, far in excess of the statistical uncertainty [71]. The expected TLEP statistical uncertainties are summarized in table 11.…”

Probing the top-quark width through ratios of resonance contributions of e + e − → W + W − b b ¯ $$ {\mathrm{e}}^{+}{\mathrm{e}}^{-}\to {\mathrm{W}}^{+}{\mathrm{W}}^{-}\mathrm{b}\overline{\mathrm{b}} $$

“…Refs. [59][60][61] report a projected accuracy of 20-30 MeV on Γ t from top-quark pair production measurements at threshold. In the continuum, due to the fine-resolution detectors and the cleaner environment at a linear collider, performing fits of the invariant-mass lineshape (reconstructed via the decay products of the top quark) provides a realistic method to precisely determine the top-quark width.…”

“…3 We will comment on this extraction method in section 2.4, however we highlight that refs. [61,62] estimate that, using reconstruction of the invariant mass at a linear collider, Γ t can be determined with a precision of 60-220 MeV for √ s = 500 GeV and an integrated luminosity of 100 fb −1 .…”

“…This consists of a simple fit of a BreitWigner (BW) function to the reconstructed mass peak M (W + , J b ) or M (W − , Jb) (see for example [61]). The method is usually applied to samples of simulated events where the underlying hard process is on-shell top-quark pair production e + e − → tt.…”

“…Until these new tools are utilised it should be kept in mind that template or BW-fitting extractions of the width based on simulations using LO top-quark decays (such as those performed for example in refs. [61,62]) ought to include a potentially important systematic error due to these missing higher-order effects. We note that there is a corresponding, but smaller, uncertainty of about 200 MeV in the extraction of m t .…”