The force responsible for binding quarks together to form protons, neutrons, and atomic nuclei is known as the strong force, and it's aptly named due to its incredible strength.
This force, carried by particles called gluons, is the strongest among all the fundamental forces of nature, which include electromagnetism, the weak force, and gravity.
Interestingly, it is also the least precisely measured of these four forces. However, in a recently submitted paper to Nature Physics, the ATLAS collaboration has detailed how they harnessed the power of the Z boson, an electrically neutral carrier of the weak force, to determine the strength of the strong force with an unprecedented level of precision, achieving an uncertainty below 1%.
This measurement is important because it is described by a fundamental parameter in the Standard Model of particle physics known as the strong coupling constant. Although knowledge of this constant has improved over the years, its uncertainty is still much larger than the constants for the other fundamental forces.
A more precise measurement is needed for accurate calculations in particle physics and to answer big questions like whether all fundamental forces were once the same or if there are new, unknown forces at play.
Studying the strong force is not only vital for understanding the fundamental aspects of nature but also for addressing significant unanswered questions. For instance, could all the fundamental forces have equal strength at extremely high energies, hinting at a potential common origin? Additionally, could there be new and unknown interactions modifying the behavior of the strong force in specific processes or at certain energy levels?
In their latest examination of the strong coupling constant, the ATLAS collaboration focused on Z bosons generated during proton-proton collisions at CERN's Large Hadron Collider (LHC), operating at a collision energy of 8 TeV.
The production of Z bosons typically occurs when two quarks within the colliding protons annihilate. In this process driven by weak interactions, the strong force becomes involved through the emission of gluons from the annihilating quarks.
This gluon radiation imparts a "kick" to the Z boson, perpendicular to the collision axis, known as transverse momentum. The strength of this kick is directly linked to the strong coupling constant. By precisely measuring the distribution of Z-boson transverse momenta and comparing it with equally precise theoretical calculations, the researchers were able to determine the strong coupling constant.
In the new analysis, the ATLAS team focused on cleanly selected Z-boson decays to two leptons (electrons or muons) and measured the Z-boson transverse momentum via its decay products.
A comparison of these measurements with theoretical predictions enabled the researchers to precisely determine the strong coupling constant at the Z-boson mass scale to be 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 made by a single experiment to date.
It agrees with the current world average of experimental determinations and state-of-the-art calculations known as lattice quantum chromodynamics.
This record precision was accomplished thanks to both experimental and theoretical advances. On the experimental side, the ATLAS physicists achieved a detailed understanding of the detection efficiency and momentum calibration of the two electrons or muons originating from the Z-boson decay, which resulted in momentum precisions ranging from 0.1% to 1%.
On the theoretical side, the ATLAS researchers used, among other ingredients, cutting-edge 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 the precision.
The strength of the strong nuclear force is a key parameter of the Standard Model, yet it is only known with percent-level precision. For comparison, the electromagnetic force, which is 15 times weaker than the strong force at the energy probed by the LHC, is known with a precision better than one part in a billion.
Stefano Camarda, Physicist, CERN
Stefano Camarda concludes, “That we have now measured the strong force coupling strength at the 0.8% precision level is a spectacular achievement. It showcases the power of the LHC and the ATLAS experiment to push the precision frontier and enhance our understanding of nature.”