Symmetries make the world go round, but so do asymmetries. A case in point is an asymmetry known as charge-parity asymmetry, which is required to explain why matter vastly outnumbers antimatter in the Universe even though both forms of matter should have been created in equal amounts in the Big Bang.
ATLAS (left) and CMS (right) candidate events for a Higgs boson decaying into a pair of tau leptons. Image credit: CERN.
The Standard Model of particle physics includes sources of charge-parity (CP) asymmetry, and some of these sources have been confirmed in experiments.
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.
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.
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 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.
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.
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.
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.
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).
After 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.
The results are consistent with the Standard Model within the present measurement precision.
“More data will allow us to either confirm this conclusion or spot CP asymmetry in the Higgs boson-tau lepton interaction, which would have a profound impact on our understanding of the history of the Universe,” the physicists said.
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ATLAS Collaboration. 2022. Measuring CP properties of Higgs boson interactions with τ leptons with the ATLAS detector. ATLAS-CONF-2022-032
A. Tumasyan et al. (CMS Collaboration). Analysis of the CP structure of the Yukawa coupling between the Higgs boson and τ leptons in proton-proton collisions at s√=13 TeV. J. High Energ. Phys 2022 (12); doi: 10.1007/JHEP06(2022)012
