Einstein’s Theory Faces Its Heaviest Challenge Yet – and It Still Holds Up

Researchers at the Large Hadron Collider tested whether top quarks, the most massive known elementary particles, comply with Einstein’s theory of relativity.

Despite theories suggesting potential deviations at high energies, the experiments confirmed that Lorentz symmetry remains intact, offering no evidence of variation in particle behavior due to the experiment’s orientation or the time of day.

In a groundbreaking study at the Large Hadron Collider (LHC), the CMS collaboration has investigated whether top quarks follow the rules of Einstein’s special theory of relativity.

Einstein’s special relativity, along with quantum mechanics, forms the foundation of the Standard Model of particle physics. A key principle of this theory is Lorentz symmetry, which states that experimental results should remain the same regardless of the experiment’s speed or orientation in space.

While special relativity has consistently proven accurate, some theories — such as certain models of string theory — suggest that it might break down at extremely high energies. In such cases, experimental results could start to depend on the experiment’s direction in space-time. Although signs of this Lorentz symmetry breaking might appear at the energy levels reached by the LHC, previous studies at the LHC and other colliders have found no evidence of it so far.

In its recent study, the CMS collaboration searched for Lorentz symmetry breaking at the LHC using pairs of top quarks – the most massive elementary particles known. In this case, a dependence on the orientation of the experiment would mean that the rate at which top-quark pairs are produced in proton-proton collisions at the LHC would vary with time.

More precisely, since Earth rotates around its axis, the direction of the LHC proton beams and the average direction of top quarks produced in collisions at the center of the CMS experiment also change depending on the time of the day. As a consequence, and if there is a preferential direction in space-time, the top-quark-pair production rate would vary with the time of the day. Hence, finding a deviation from a constant rate would amount to discovering a preferential direction in space-time.

The new CMS result, which is based on data from the second run of the LHC, agrees with a constant rate, meaning that Lorentz symmetry is not broken and Einstein’s special relativity remains valid. The CMS researchers used the result to set limits on the magnitude of parameters that are predicted to be null when the symmetry holds. The limits obtained improve by up to a factor of 100 upon results from a previous search for Lorentz symmetry breaking at the former Tevatron accelerator.

The results pave the way for future searches for Lorentz symmetry breaking based on top-quark data from the third run of the LHC. They also open the door to scrutiny of processes involving other heavy particles that can only be investigated at the LHC, such as the Higgs boson and the W and Z bosons.