CERN’s ATLAS detector
Claudia Marceloni and Max Brice/CERN
Researchers have made the most precise measurements to date of the strong forces that bind the particles that make up protons and neutrons. Although it is the most powerful of all the fundamental forces of nature, its strength is more uncertain than any other force. Measuring it accurately is key to understanding the nature of the world around us.
The other fundamental forces, namely gravity, electromagnetism, and the weak force, all weaken as the particles they act on move away. But strong forces become even more powerful. This causes special effects that neutralize it, making it difficult to measure directly.
“It’s the only way to observe strong forces indirectly,” he says. Stefano Camarda At the CERN particle physics laboratory near Geneva, Switzerland. “This measurement is particularly difficult, and improvements since the mid-80s have been very slow.”
Camarda and his colleagues used the ATLAS experiment at the Large Hadron Collider (LHC) to dramatically improve accuracy and reduce the relative uncertainty in the force strength to 0.8 percent. “This measurement represents a two- to three-fold improvement over the best previous experimental measurements,” he says. Alberto Belloni at the University of Maryland.
The researchers measured the strong force by colliding pairs of protons. A particle called the Z boson. If there is no force mediating the interaction between the protons, the last she-Z particle will be in a stopped state. However, the strong force gave this particle a small “shock”. The resulting momentum depends on the magnitude of the strong force.
It is important to study the value of the strong force because it is one of the largest sources of uncertainty remaining in the Standard Model of particle physics. “Everything we measure at the LHC and the predictions we calculate depends on the value of what is strong. [force]” says Camarda. Without reducing the strong force uncertainty, he says, it will be difficult to determine whether the LHC has discovered evidence of physics beyond the Standard Model.
This powerful force is also important in understanding the fate of the universe. There is a small possibility that the universe will eventually come to an end through a phenomenon called vacuum collapse. In vacuum collapse, quantum fluctuations create a tiny bubble of unusual spacetime called a pure vacuum, which then rapidly grows to engulf the entire universe. “The probability of the universe disappearing inside a quantum bubble is extremely low,” Camarda says. “But there is uncertainty in this statement, and that uncertainty is caused by the value of this power.”
Even with this new measurement, our knowledge of strong forces still does not extend to accurate calculations of other fundamental forces. And because measurements are so difficult, even if we had better data, it’s unlikely we’ll reach the same accuracy anytime soon. However, CERN has a proposal for a new collider aimed at studying Z particles. Once it’s built, we may eventually reach that level of accuracy.
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