Physicists from the CMS Collaboration at CERN’s Large Hadron Collider (LHC) have successfully measured the effective leptonic electroweak mixing angle. The results were presented at the annual general meeting. Rencontre de Morion Conference is the most accurate measurement ever made at the Hadron Collider and is in good agreement with predictions from the Standard Model of particle physics.
Installation of CMS beam pipe. Image credit: CERN/CMS Collaboration.
The Standard Model is the most accurate description of particles and their interactions to date.
Precise measurements of parameters, combined with precise theoretical calculations, provide incredible predictive power that allows us to identify phenomena even before we directly observe them.
In this way, the model has succeeded in constraining the masses of the W and Z particles, the top quark, and recently the Higgs boson.
Once these particles are discovered, these predictions serve as a consistency check on the model, allowing physicists to explore the limits of the theory’s validity.
At the same time, precise measurements of the properties of these particles provide a powerful tool for exploring new phenomena beyond the standard model, so-called “new physics.” This is because new phenomena appear as mismatches between different measured and calculated quantities.
The electroweak mixing angle is a key element of these consistency checks. This is a fundamental parameter of the Standard Model and determines how unified electroweak interactions give rise to electromagnetic and weak interactions through a process known as electroweak symmetry breaking.
At the same time, we mathematically connect the masses of the W and Z bosons that transmit weak interactions.
Therefore, measurements of W, Z, or mixed angles provide a good experimental cross-check of the model.
The two most accurate measurements of the weak mixing angle were made by experiments at CERN’s LEP collider and by the SLD experiment at the Stanford Linear Accelerator Center (SLAC).
These values have puzzled physicists for more than a decade because they don’t agree with each other.
The new results are in good agreement with standard model predictions and are a step towards resolving the discrepancy between standard model predictions and measurements of LEP and SLD.
“This result shows that precision physics can be performed at the Hadron Collider,” said Dr. Patricia McBride, spokesperson for the CMS Collaboration.
“The analysis had to deal with the challenging environment of LHC Run 2, with an average of 35 simultaneous proton-proton collisions.”
“This paves the way for even more precise physics, where more than five times as many proton pairs collide simultaneously at the high-luminosity LHC.”
Precise testing of Standard Model parameters is a legacy of electron-positron collider such as CERN’s LEP, which operated until 2000 in the tunnel that now houses the LHC.
Electron-positron collisions provide a clean environment ideal for such high-precision measurements.
Proton-proton collisions at the LHC are more challenging for this type of research, even though the ATLAS, CMS, and LHCb experiments have already yielded numerous new ultra-high-precision measurements.
This challenge is primarily due to the vast background from physical processes other than those studied, and the fact that protons, unlike electrons, are not subatomic particles.
With the new results, it seemed impossible to reach accuracy similar to that of the electron-positron collider, but now it has been achieved.
The measurements presented by CMS physicists 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.−1 or about 11 billion collisions.
“The mixing angle is obtained through analysis of the angular distribution in collisions in which pairs of electrons or muons are produced,” the researchers said.
“This is the most accurate measurement ever made at the Hadron Collider and improves on previous measurements by ATLAS, CMS, and LHCb.”
Source: www.sci.news
