Modeling Uncertainties on the Z Boson Background in the Context of High Precision W Boson Mass Measurements

TL;DR

This paper examines how uncertainties in modeling the Z boson background affect the precision measurement of the W boson mass, finding potential shifts up to 8 MeV but not enough to explain the CDF anomaly.

Contribution

It introduces a reweighting method using updated predictions to assess the impact of Z background modeling uncertainties on W mass measurements.

Findings

Z background modeling uncertainties can cause up to 8 MeV shift in m_W

The shift from background mismodeling is larger than previously estimated

The effect is insufficient to account for the CDF W mass discrepancy

Abstract

The precise determination of the $W$ boson mass, $m_W$, is a cornerstone of electroweak precision tests and a critical input to global fits of the Standard Model (SM). While most existing measurements of $m_W$ are in agreement with SM predictions, the recent result from the CDF collaboration exhibits a significant deviation from the SM prediction but also exceeding $4\sigma$ from the global average of all other high precision measurements. This prompts renewed scrutiny of potential sources of systematic bias. In this study, we investigate the modeling of the $Z$ boson background, which is particularly relevant in the muon decay channel of the $W$ boson, where undetected muons can mimic missing transverse momentum. Using updated theoretical predictions and high-precision simulations, we construct reweighted templates to evaluate the impact of possible mismodeling in the $Z$ background. Our analysis suggests that such effects can induce a shift in the extracted $m_W$ value of up to 8~MeV in the muon decay channel. While this shift is non-negligible and exceeds the originally quoted uncertainty from this source, it remains insufficient to explain the full discrepancy observed in the CDF measurement.