The CMS experiment at CERN measures a key parameter of the Standard Model

Last week, at the annual Rencontres de Moriond conference, the CMS collaboration presented a measurement of the effective leptonic electroweak mixing angle. The result is the most precise measurement made at the Hadron Collider to date and matches well with the predictions of the Standard Model.

The Standard Model of particle physics is the most accurate description to date of particles and their interactions. Precise measurement of its parameters, combined with precise theoretical calculations, yields superb predictive power that allows events to be determined even before they are directly observed.

In this way, the model successfully controlled the masses of the W and Z bosons (discovered at CERN in 1983), the top quark (discovered at Fermilab in 1995) and, most recently, the Higgs boson (discovered at CERN in 2012). . , Once these particles were discovered, these predictions became consistency checks for the model, allowing physicists to explore the limits of the theory’s validity.

At the same time, precise measurement of the properties of these particles is a powerful tool for discovering new phenomena beyond the Standard Model – the so-called “new physics” – because new phenomena manifest themselves as discrepancies between various measured and calculated quantities. Will do.

The electroweak mixing angle is a key element of these consistency checks. It is a fundamental parameter of the Standard Model, which determines how the unified electroweak interaction gives rise to the electromagnetic and weak interactions through a process called electroweak symmetry breaking. Also, it mathematically ties together the masses of the W and Z bosons that transmit the weak interaction. So, measurement of w, z or mixing angle provides a good experimental cross-check of the model.

The two most precise measurements of the weak mixing angle were made by experiments at the CERN LEP Collider and the SLD experiment at the Stanford Linear Accelerator Center (SLAC). These values ​​disagree with each other, which has puzzled physicists for more than a decade. The new result matches well with the Standard Model predictions and is a step towards resolving the discrepancy between the latter and the LEP and SLD measurements.

“This result shows that accurate physics can be done at the Hadron Collider,” says CMS spokesperson Patricia McBride. “The analysis had to handle the challenging environment of LHC Run 2, with an average of 35 simultaneous proton-proton collisions. This paves the way for more precise physics at the high-luminosity LHC, where five times more proton pairs will collide simultaneously. “

Precise tests of Standard Model parameters are a legacy of electron–positron colliders such as CERN’s LEP, which operated until the year 2000 in the tunnel that now houses the LHC. Electron-positron collisions provide an ideal clean environment for such high-precision measurements.

Proton–proton collisions at the LHC are more challenging for such studies, even though the ATLAS, CMS and LHCb experiments have already provided many new ultra-precise measurements. The challenge is primarily due to the vast background of physics processes other than the one being studied and the fact that protons, unlike electrons, are not elementary particles.

Thanks to this new result, reaching the same accuracy as the Electron-Positron Collider seemed like an impossible task, but it has now been achieved.

The measurements presented by CMS use a sample of proton–proton collisions collected from 2016 to 2018 at a center of mass energy of 13 TeV and a total integrated luminosity of 137 fB.^-1That means about 11,000 million collisions.

The mixing angle is obtained through analysis of the angular distribution in collisions where pairs of electrons or muons are produced. This is the most precise measurement made at the Hadron Collider to date, superior to previous measurements at ATLAS, CMS and the LHCb.