Evaluation of the LEP Centre-of-Mass Energy Above the W-Pair Production Threshold

TL;DR

This paper details the methods and precision achieved in measuring the LEP2 centre-of-mass energy above the W-pair production threshold, crucial for accurate W-boson mass determination.

Contribution

It presents a calibration and measurement technique for LEP2 beam energy using NMR probes and flux loops, achieving a 25 MeV precision in 1997 data.

Findings

Beam energy determined with 25 MeV precision in 1997

Systematic uncertainties dominated by flux loop linearity test

Potential for improved energy measurement precision outlined

Abstract

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the the centre-of-mass energies at each interaction point. In addition the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.