The experiments at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland have produced many impressive results in recent years studying the nature of the fundamental particles and forces that govern the world around us. While it is better known for its function as a proton-proton collider, the LHC can also collide beams of lead ions. These collisions produce a vast array of particles to be detected, including heavy nuclei (the cores of atoms). The ALICE detector at the LHC is optimized to detect particles in these extremely active heavy ion collisions. The ALICE collaboration recently released a result which measured the ratio of electric charge to mass in certain types of heavy nuclei. By comparing this ratio in nuclei with their “anti-particle” counterparts (nuclei with the same exact mass and other physical but with opposite charge), ALICE was able to confirm with unprecedented precision that the laws of physics do not change under particular types of transformations in nature.

As is typical in physics, one can perform thought experiments where certain properties in nature are changed and imagine how the laws of physics change as a result. In particular, you can imagine three types of transformations. Consider a particle with some charge and position moving in space. The first transformation, charge conjugation (denoted with C), flips the electric charge of the particle to the opposite value (positive goes to negative and vice versa). The second transformation, parity (P), reflects the orientation of a particle in space. (Imagine what happens to your hand in a mirror – your right hand becomes your mirror self’s left hand). The third transformation, time reversal, can be seen as playing the video of the particle’s motion backward instead of forward. Many of our current theories in physics rely on the assumption that the laws of physics do not change if you apply all three of these transformations at the same time.

This so-called CPT invariance has been a subject of the tests of fundamental physics experiments for many years. The motivation for testing whether the laws of physics change under these transformations is two-fold. First, as mentioned before, many of our current theories describing how the universe works rely on the assumption of CPT symmetry. If any variations from this assumption were found, flaws in the currently well accepted descriptions of nature could be uncovered. Additionally, violation of CPT symmetry could provide hints for many currently unexplained phenomena, in particular the imbalance of matter and anti-matter in the universe. The new result from ALICE confirms the existence of this CPT invariance, allowing physicists to further constrain the theories explaining these phenomena.

Managing Correspondent: Tomo Lazovich

Media Coverage: Phys.Org

Original Paper: Nature Physics

 

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