2023-09-29 06:00:04
This result demonstrates the LHC’s ability to push the limits of precision further and improve our understanding of nature.
It is this which binds quarks together to form protons, neutrons and atomic nuclei; it is so intense that it is called the strong force (or the strong interaction). Carried by particles called gluons, it is the most powerful of all the fundamental forces of nature – the other three being electromagnetism, the weak force and the gravitation. At the same time, of these four forces, it is the one that is the least precisely measured.
The ATLAS experiment at CERN (Image: CERN)
In a article which has just been submitted for publication to the journal Nature Physics, the collaborationATLAS explains how she used the Z boson, an electrically neutral particle carrier of the weak force, to determine the intensity of the strong force with an uncertainty of less than 1%, which constitutes a new level of precision.
To describe the intensity of the strong force, the Standard Model of Particle Physics use a setting called the strong interaction coupling constant. Even though our understanding of this constant has improved over the years through measurements and theoretical advances, the uncertainty attached to its value is still orders of magnitude greater than that of the coupling constants of other fundamental forces. However, we need a more precise measurement in order to improve the theoretical calculations of the processes involving the strong force and to move forward on other questions: could all the fundamental forces have the same intensity at very high energy, which would suggest that they could have a common origin? Could new, unknown interactions modify the strong force in certain processes or at certain energies?
In its new study on the strong interaction coupling constant, the ATLAS collaboration analyzed Z bosons produced in proton-proton collisions atLarge Hadron Collider (LHC) at an energy of collision of 8 TeV. Most often, Z bosons are produced when, in proton collisions, two quarks annihilate each other. In this process, which involves the weak force, the strong force comes into play through the radiation of gluons from quarks undergoing annihilation. This radiation gives the Z boson a transverse momentum relative to the collision axis. The magnitude of this effect depends on the coupling constant of the strong interaction. By precisely measuring the distribution of the transverse momenta of the Z boson and comparing these measurements to equally precise theoretical calculations of this distribution, it is possible to determine this constant.
New value of the strong interaction coupling constant measured by ATLAS compared to other measurements.
Image: ATLAS/CERN
In this new analysis, the ATLAS team focused on certain decays of the Z boson into two leptons (electrons or muons), chosen for their sharpness, and measured the transverse momentum of the Z boson via the products of its disintegrations. By comparing these measurements to theoretical predictions, scientists were able to accurately determine that the coupling constant of the strong interaction on the mass scale of the Z boson is 0.1183 ± 0.0009. With a relative uncertainty of only 0.8%, this result constitutes to date the most precise determination of the intensity of the strong force made by a single experiment. It agrees with the current world average of experimental results and calculations of quantum chromodynamics on network (see the graph below).
This record precision was made possible thanks to advances on the experimental and theoretical levels. On the experimental side, ATLAS scientists managed to precisely understand the detection efficiency and the calibration of the momentum of the two electrons or muons resulting from the decay of the Z boson, which made it possible to obtain, for pulse measurement, accuracy ranging from 0.1% to 1%. On the theory side, ATLAS scientists used, among other things, cutting-edge calculations of the Z boson production process that take into account up to four “loops” in quantum chromodynamics. These loops correspond in the calculations to the complexity intermediate processes that contribute to the phenomenon. By adding loops, we increase precision.
“The strong nuclear force is a key parameter of the Standard Model, but it is only known with percent accuracy. For comparison, the electromagnetic forcewhich is 15 times less intense than the strong force at the energy studied by the LHC, is known with a precision greater than a billionth, underlines Stefano Camarda, physicist at CERN and member of the team that carried out this measurement. Having measured the coupling intensity of the strong force at a precision level of 0.8% is a real feat. It is an illustration of the capacity of the LHC and the ATLAS experiment to further push the limits of precision and improve our understanding of nature.”
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