The results are consistent with the Standard Model within the present measurement precision. In theories that extend the Standard Model, however, the angle may deviate from zero and the interaction may be partially or fully CP asymmetric depending on the angle (an angle of -90 or +90 degrees corresponds to a fully CP-asymmetric interaction, whereas any angle in between, except 0 degrees, corresponds to a partially CP-asymmetric interaction).Īfter analyzing their samples of Higgs boson decays into tau leptons, the ATLAS team obtained a mixing angle of 9 ± 16 degrees and the CMS team −1 ± 19 degrees, both of which exclude a fully CP-asymmetric Higgs boson-tau lepton interaction with a statistical significance of about three standard deviations. In the Standard Model, the mixing angle is zero and thus the interaction is CP symmetric, meaning that it remains the same under a transformation that swaps a particle with the mirror image of its antiparticle. The scientists then analyzed this decay’s kinematics, which depends on an angle, called the mixing angle, that quantifies the amount of CP asymmetry in the interaction between the Higgs boson and the tau lepton.
To search for this asymmetry, the ATLAS and CMS teams first looked for Higgs bosons decaying into pairs of tau leptons in proton-proton collision data recorded by the experiments during the second run of the LHC. In their latest studies, they searched for this asymmetry in the interaction between the Higgs boson and the tau lepton, a heavier version of the electron. The researchers had previously searched for - and found no signs of - CP asymmetry in the interactions of the Higgs boson with other bosons as well as with the heaviest known fundamental particle, the top quark. In two independent investigations, physicists from ATLAS and CMS collaborations at CERN’s Large Hadron Collider (LHC) turned to the Higgs boson to see if this unique particle hides a new, unknown source of CP asymmetry. However, these sources collectively generate an amount of CP asymmetry that is far too small to account for the matter-antimatter imbalance in the Universe, prompting physicists to look for new sources of CP asymmetry. The Standard Model of particle physics includes sources of charge-parity (CP) asymmetry, and some of these sources have been confirmed in experiments. ATLAS (left) and CMS (right) candidate events for a Higgs boson decaying into a pair of tau leptons.
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