Physicists with the CMS Collaboration at CERN’s Large Hadron Collider have tested whether top quarks adhere to Albert Einstein’s special theory of relativity.
Installation of the CMS beam pipe. Image credit: CERN / CMS Collaboration.
Along with quantum mechanics, Albert Einstein’s special theory of relativity serves as the basis of the Standard Model of particle physics.
At its heart is a concept called Lorentz symmetry: experimental results are independent of the orientation or the speed of the experiment with which they are taken.
Special relativity has stood the test of time. However, some theories, including particular models of string theory, predict that, at very high energies, special relativity will no longer work and experimental observations will depend on the orientation of the experiment in space-time.
Remnants of such Lorentz symmetry breaking could be observable at lower energies, such as at the energies of the Large Hadron Collider (LHC), but despite previous efforts, they have not been found at the LHC or other colliders.
In a new study, the CMS physicists 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,” they said.
“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 centre 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 team’s new 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 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,” the scientists said.
“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.”
The study was published in the October 2024 issue of the journal Physics Letters B.
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CMS Collaboration. 2024. Searches for violation of Lorentz invariance in top quark pair production using dilepton events in 13 TeV proton-proton collisions. Physics Letters B 857: 138979; doi: 10.1016/j.physletb.2024.138979
