Unprecedented precision in measuring the strength of the strong force

2023-09-25 17:02:21

MADRID, 25 Sep. (EUROPA PRESS) –

Particle physicists have used the Z boson, the electrically neutral carrier of the weak force, to measure the strength of the strong force. with unprecedented uncertainty, less than 1%.

Binding quarks together to form protons, neutrons, and atomic nuclei is a force so strong it’s in the name. The strong force, carried by gluon particles, It is the strongest of all the fundamental forces of nature; the others are electromagnetism, the weak force and gravity. However, it is the least accurately measured of these four forces.

In an article sent to Nature Physics and currently available on the arXiv preprint server, scientists from the ATLAS collaboration working with the Large Hadron Collider (LHC), at the European particle acceleration laboratory CERN.

The strength of the strong force is described by a fundamental parameter in the Standard Model of particle physics called the strong coupling constant. While knowledge of the strong coupling constant has improved with measurements and theoretical developments over the years, the uncertainty about its value remains orders of magnitude greater than that of the coupling constants for the other fundamental forces. , CERN reports in a statement.

A more precise measurement of the strong coupling constant is required to improve the accuracy of theoretical calculations of particle processes involving the strong force. Important unanswered questions about nature also need to be addressed. Could all fundamental forces have the same strength at very high energy, indicating a possible common origin? Could new and unknown interactions be modifying the strong force in certain processes or in certain energies?

In its new study of the strong coupling constant, the ATLAS collaboration investigated Z bosons produced in proton-proton collisions at CERN’s Large Hadron Collider (LHC) with a collision energy of 8 TeV. Z bosons are typically produced when two quarks in colliding protons annihilate. In this weak interaction process, the strong force comes into play through gluon radiation from annihilation quarks.

This radiation gives the Z boson a “kick” transverse to the collision axis (transverse momentum). The magnitude of this kick depends on the strong coupling constant. A precise measurement of the distribution of the transverse momenta of the Z boson and a comparison with equally precise theoretical calculations of this distribution allow determining the strong coupling constant.

In the new analysis, the ATLAS team focused on clearly selected Z boson decays into two leptons (electrons or muons) and measured the Z boson’s transverse momentum through its decay products. A comparison of these measurements with theoretical predictions allowed the researchers to precisely determine that the strong coupling constant on the Z boson mass scale was 0.1183 +/- 0.0009.

With a relative uncertainty of only 0.8%, the result is the most precise determination of the strength of the strong force to date using a single experiment. It is in agreement with the current world average of experimental determinations and state-of-the-art calculations known as lattice quantum chromodynamics.

This record-breaking precision was achieved thanks to both experimental and theoretical advances. On the experimental side, ATLAS physicists gained a detailed understanding of the detection efficiency and momentum calibration of the two electrons or muons originating from the Z boson decay, resulting in timing accuracies ranging from 0.1% to 1%.

On the theoretical side, ATLAS researchers used, among other ingredients, state-of-the-art calculations of the Z boson production process that consider up to four “loops” in quantum chromodynamics. These loops represent the complexity of the calculation in terms of contributing processes. Adding more loops increases accuracy.

“The strength of the strong nuclear force is a key parameter of the Standard Model, but it is only known to a percentage precision. For comparison, the electromagnetic force, which is 15 times weaker than the strong force at the energy tested by the LHC, is known with a precision greater than one part in a billion,” says CERN physicist Stefano Camarda, a member of the analysis team.

“The fact that we have now measured the strong coupling force with a precision level of 0.8% It’s a spectacular achievement.. “It shows the power of the LHC and the ATLAS experiment to push the frontier of precision and improve our understanding of nature.”