Determination of the mass of the W boson

TL;DR

The paper analyzes how to determine the W boson mass at LEP2, detailing threshold and direct-reconstruction approaches and their respective statistical and systematic uncertainties. It combines updated theoretical inputs (cross-sections, Coulomb and ISR corrections) with collider strategies to project MW precision under LEP2 running scenarios. The work highlights the complementary roles of threshold cross-section measurements and direct mass reconstruction, while cautioning that interconnection effects (color reconnection and Bose-Einstein correlations) in hadronic WW decays could introduce non-negligible biases. Overall, it finds that MW precision at LEP2 around the 30–50 MeV range is feasible, with threshold methods offering robust cross-checks and direct reconstruction providing the strongest potential, contingent on mitigating hadronic interconnection systematics.

Abstract

Previous studies of the physics potential of LEP2 indicated that with the design luminosity of 500 inverse picobarn one may get a direct measurement of the mass of the W-boson with a precision in the range 30 - 50 MeV. This report presents an updated evaluation of the estimated error on the mass of the W-boson based on recent simulation work and improved theoretical input. The most efficient experimental methods which will be used are also described.

Figures (14)

• Figure 1: Direct measurements of $M_{{\rm W}}$ together with their average. The dashed lines correspond to the indirect limits ($\pm$ 1 $\sigma$) from a global fit in the Standard Model.
• Figure 2: A comparison of direct versus indirect determinations of $M_{{\rm t}}$ and $M_{{\rm W}}$. The contour for the indirect determination corresponds to a 70% confidence level. The dark shaded region within the contour is that compatible with the direct Higgs search limit, $\hbox{$M_{{\rm H}}$}$$> 65\; {\rm GeV}$.
• Figure 3: 70% confidence level contour plots for fits to $\epsilon$'s in units of $10^{-3}$. The outer contour is for a fit with the current error, $\Delta M_{{\rm W}} = \pm 160\; {\rm MeV}$, whereas the inner contours are for $\Delta M_{{\rm W}} = \pm 50\; {\rm MeV}$ and $\Delta M_{{\rm W}} = \pm 25\; {\rm MeV}$ respectively.
• Figure 4: Predictions for $M_{{\rm W}}$ as a function of $M_{{\rm t}}$ in the SM (solid lines) and in the MSSM (dashed lines), from Ref. holliketal. In each case the prediction is a band of values, corresponding to a variation of the model parameters consistent with current measurements and limits. An additional constraint of 'no SUSY particles at LEP2' is imposed in the MSSM calculation.
• Figure 5: The cross-section for $\hbox{${\rm e^+e^-}$}\rightarrow W^+W^-$ in various approximations: (i) Born (on-shell) cross-section, (ii) Born (off-shell) cross-section, (iii) with first order Coulomb corrections, and (iv) with initial state radiation. The parameter values are listed in Table \ref{['tab-inputpar']}.