Unifying Searches for New Physics with Precision Measurements of the W Boson Mass

Abstract

The Standard Model (SM) of particle physics has been extremely successful in describing the interactions of electromagnetic, weak nuclear, and strong nuclear forces. Yet, there are both unexplained phenomena and experimentally observed tensions with the SM, motivating searches for new physics (NP). Collider experiments typically perform two kinds of analyses: direct searches for new physics and precision measurements of SM observables. For example, experimental collaborations use collider data to search for NP particles like the heavy superpartners of the SM particles, whose observation would be clear evidence of supersymmetry (SUSY). These direct searches often consider kinematic regions where the SM background is small. This strategy is unable to probe regions of the NP parameter space where the SM background is dominant. The same collaborations also measure the masses of SM particles, which not only serve as consistency tests of the SM, but can also probe effects of NP. In 2022, the Collider Detector at Fermilab (CDF) collaboration published the most precise measurement of the $W$ boson mass: $m_W$ = 80433.5 $\pm$ 9.4 MeV. This measurement is in $7\sigma$ significance tension with the SM prediction via the electroweak (EW) fit, $m_W^{\rm pred.}$ = 80354 $\pm$ 7 MeV. Many extensions to the SM can affect the prediction of $m_W$ with indirect effects of heavy NP. However, in 2023, the ATLAS re-measurement of the $W$ boson mass, $m_W$ = 80360 $\pm$ 16 MeV, was found to be consistent with the SM prediction. Both collaborations found a high-precision agreement between the measured kinematic distributions and the SM prediction of the kinematic distributions for their corresponding extracted $m_W$. We propose using the precision measurements of $m_W$ to directly probe NP contributing to the same final state used to measure $m_W$: a single charged lepton $\ell$ and missing transverse energy $\met$. This strategy is independent of modifying the EW fit, which tests indirect effects of NP on the predicted value of $m_W$. Any NP producing $\ell+\met$ which modifies the kinematic distributions used to extract $m_W$ can be probed with this method. With this strategy, since these distributions are used to search for NP while measuring $m_W$, a simultaneous fit of NP and SM parameters is required, thus unifying searches and measurements. This simultaneous fitting can induce a bias in the measured $m_W$, but only to a limited extent for our considered models. We consider three categories of NP which can be probed: ($i$) modified decay of $W$ bosons; ($ii$) modified production of $W$ bosons; and ($iii$) $\ell+\met$ scenarios without an on-shell $W$ boson. We also show that models whose signals extend beyond the kinematic region used to measure $m_W$ can be probed in an intermediate kinematic region. Our results highlight that new physics can still be discovered at the LHC, including light new physics, via SM precision measurements. Additionally, anticipated improvements in precision SM measurements at the High Luminosity LHC further enables new searches for physics Beyond the Standard Model (BSM).