ATLAS provides first measurement of the W-boson width at the LHC

5 April 2024 | By

The discovery of the Higgs boson in 2012 slotted in the final missing piece of the Standard Model puzzle. Yet, it left lingering questions. What lies beyond this framework? Where are the new particles solving the Universe's remaining mysteries, such as the nature of Dark Matter or the origin of the matter–antimatter asymmetry?

Figure 1: Comparison of the measured W-boson width with the Standard-Model predictions derived from a global electroweak fit, as well as with measurements from the LEP and Tevatron colliders. The vertical grey band illustrates the Standard-Model prediction, while the black dots and their associated horizontal bands represent the published experimental results. (Image: ATLAS Collaboration/CERN)

One parameter that may hold clues about new phenomena is the “width” (ΓW) of the W boson, one of the force-carrying particles of the weak force. A particle’s width is directly related to its lifetime and describes how it decays. If the W boson decays ​​into any unknown particles, these would influence the measured width. As its value is precisely predicted by the Standard Model, any deviation from prediction could indicate the presence of new physics phenomena.

In a groundbreaking new result, the ATLAS Collaboration has measured the W-boson width for the first time at the Large Hadron Collider (LHC). The W-boson width had previously been measured at CERN’s Large Electron-Positron (LEP) collider and Fermilab’s Tevatron collider, yielding an average value of 2085 ± 42 MeV, consistent with the Standard-Model prediction of 2088 ± 1 MeV. Using data collected during Run 1 of the LHC, the ATLAS Collaboration measured the W-boson width to be 2202 ± 47 MeV. This is the most precise single-experiment measurement to date, and is consistent with the Standard-Model prediction within about 2.4 standard deviations, given the uncertainty of the measurement (see Figure 1).

The ATLAS Collaboration has measured the W-boson width for the first time at the Large Hadron Collider, providing the most precise single-experiment measurement to date.

This remarkable result was achieved by analysing the kinematic spectra of W-boson decays into an electron or a muon and their corresponding neutrino. This required physicists to calibrate the detector response to accurately reconstruct the transverse momenta of the electrons and muons. The distribution of the transverse mass in these events was of particular interest, as its shape is sensitive to the values of the W-boson mass and the width (see Figure 2).

Figure 2: Distribution of the transverse mass in W→μν events simulated with the W-boson mass and width values of mW=80399 MeV and ΓW=2085 MeV. The ratio panels represent the relative effect of varying these parameters by ±60 MeV and ±200 MeV, respectively. (Image: ATLAS Collaboration/CERN)

However, achieving such high precision isn’t solely the outcome of a single exceptional analysis – it also requires the confluence of several high-precision results. For instance, an accurate understanding of W-boson production in proton-proton collisions was essential, and researchers relied on a combination of theoretical predictions validated by various measurements of W and Z boson differential cross sections. Also crucial to this measurement were Parton Distribution Functions (PDFs), which describe the inner structure of the proton. ATLAS physicists incorporated and tested modern PDFs derived from fits to data from different particle physics experiments.

The ATLAS Collaboration measured the W-boson width simultaneously with the W-boson mass using an improved statistical method that adjusts the systematic uncertainties along with the values of the W-boson mass and width. This method allowed part of the uncertainties to be constrained by the dataset itself, leading to better systematic control. The updated measurement of the W-boson mass is 80366.5 ± 15.9 MeV, which supersedes the previous ATLAS measurement using the same data. The measured values of mW and ΓW are consistent with the Standard Model predictions.

Future measurements of the W-boson width and mass using larger ATLAS datasets are expected to reduce statistical and experimental uncertainties. Concurrently, advancements in theoretical predictions and a more refined understanding of PDFs will help reduce theoretical uncertainties. As their measurements become ever more precise, physicists will be able to conduct yet more stringent tests of the Standard Model's consistency and probe for the potential existence of new particles or forces.

About the event display: View of a 7 TeV collision event in which a candidate W boson decays into a muon and a neutrino. Starting from the centre of the ATLAS detector, the reconstructed tracks of the charged particles in the Inner Detector (ID) are shown as orange lines. The energy deposits in the calorimeters are shown as yellow boxes. The identified muon is shown as a longer red line. The missing transverse momentum is shown by a green dashed line.

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