In a major study conducted at the Large Hadron Collider (LHC), the CMS collaboration has explored whether top quarks – nature’s heaviest elementary particles – adhere to Einstein’s special theory of relativity.
The research marks the first attempt to test Lorentz symmetry using top-quark pairs, providing critical insights into the validity of fundamental physical laws at high energies.
Einstein’s special theory of relativity is one of the cornerstones of modern physics, forming the backbone of the Standard Model alongside quantum mechanics.
Lorentz symmetry and special relativity
A key aspect of this theory is Lorentz symmetry, which asserts that experimental results are independent of the orientation or velocity of the experimental setup within space-time. This principle has stood unchallenged for over a century.
However, certain theoretical frameworks, such as specific models of string theory, suggest that Lorentz symmetry could break down at extremely high energies.
If this were the case, experimental observations would depend on the orientation of the setup in space-time, potentially leaving detectable imprints even at the energies explored by the LHC.
Previous studies have searched for such effects but have found no evidence of Lorentz symmetry breaking in experiments conducted at colliders, including the LHC.
The CMS collaboration’s recent study represents a novel approach to searching for Lorentz symmetry breaking.
Testing Relativity at High Energies
Researchers analysed the production rates of top-quark pairs generated during proton-proton collisions at the LHC.
Top quarks, as the most massive known elementary particles, provide an ideal testing ground for high-energy phenomena.
In this study, the researchers examined whether the rate of top-quark-pair production varies with the orientation of the LHC’s proton beams in space-time.
Since Earth rotates on its axis, the direction of the beams – and consequently, the average direction of top-quark production – changes throughout the day. If Lorentz symmetry were broken, these directional changes would manifest as variations in the production rate over time.
The CMS team analysed data from the second operational run of the LHC, searching for any signs of such time-dependent variations. Their results confirmed a constant production rate, indicating that Lorentz symmetry holds and Einstein’s special relativity remains valid at these energy scales.
Using their findings, the CMS researchers set stringent limits on parameters associated with Lorentz symmetry breaking.
Limits on Lorentz symmetry breaking
These parameters, predicted to be null if the symmetry is unbroken, showed no significant deviations.
The new constraints improve upon earlier results from the Tevatron accelerator by up to a factor of 100, representing a significant advancement in precision.
The CMS study not only strengthens confidence in special relativity but also opens the door to new opportunities for testing fundamental physics.
Implications and Future Research
Data from the third operational run of the LHC, currently underway, will provide an even more detailed look at Lorentz symmetry using top quarks.
Additionally, this research paves the way for investigating other heavy particles accessible only at the LHC, such as the Higgs boson and the W and Z bosons. Exploring these particles could offer further insights into the structure of space-time and the potential limits of established theories.
The CMS collaboration’s innovative study demonstrates the enduring validity of Einstein’s special relativity, even in the high-energy realm of the LHC.
By confirming Lorentz symmetry with unprecedented precision, the researchers have solidified a foundational principle of physics while laying the groundwork for future explorations into the nature of the Universe.
Reference: A. Hayrapetyan et al, Searches for violation of Lorentz invariance in top quark pair production using dilepton events in 13 TeV proton-proton collisions, Physics Letters B (2024). DOI: 10.1016/j.physletb.2024.138979
