In the first study of its kind at the Large Hadron Collider (LHC), the CMS collaboration has tested whether top quarks adhere to Einstein's special theory of relativity.
Along with quantum mechanics, 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 LHC, but despite previous efforts, they have not been found at the LHC or other colliders.
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 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 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.
