How Physicists Hunt for Cracks in the Universe's Symmetry express
In the ATLAS detector at the Large Hadron Collider, protons collide with enormous energy, and among the debris, a Higgs boson is sometimes born — the particle that gives mass to other particles. Physicists gathered data on how this boson is produced and what it decays into: through vector boson fusion, and into pairs of tau leptons, W bosons, photons, Z bosons, and b quarks. Combining all these channels, they checked whether CP symmetry holds: that is, whether the Higgs boson behaves the same when particles are swapped for antiparticles and mirrored.
CP violation is one of the biggest mysteries in physics: it could explain why the Universe has almost no antimatter, even though the Big Bang should have produced equal amounts of particles and antiparticles. But ATLAS data showed no signs of such violation. Instead, they set the tightest limits to date on three parameters that describe possible violation in theories beyond the Standard Model. These parameters are like tuning knobs: if they are not zero, the symmetry is broken.
Now physicists know that these "knobs" can only be turned by a tiny angle, if at all. This narrows the field for new theories and brings us closer to understanding why the world is made of matter, not antimatter. The next step is to accumulate more data and probe other processes involving the Higgs boson.
Original abstract
as written by the authors arXiv:2603.20117
A combination of measurements of the CP properties of Higgs boson interactions with electroweak gauge bosons is presented, using 140 fb$$^{-1}$$ of proton-proton collisions at $$\sqrt{s} = 13$$ TeV recorded by the ATLAS detector. Results from VBF $$H\to\tau\tau/WW^{*}/\gamma\gamma$$, inclusive $$H\to ZZ^{*}$$, and $$WH,H\to b\bar{b}$$ channels are combined. No evidence of CP violation is observed, and constrains on the CP-violating operators in the Standard Model Effective Field Theory framework are set in the Warsaw basis. The results from the combination improve by over 40$$\% $$ on previous individual limits on $$c_{H\tilde{W}}$$ and, for the first time, simultaneous constraints on three coefficients $$c_{H\tilde{W}}$$, $$c_{H\tilde{B}}$$, and $$c_{H\tilde{W}B}$$ are set. These limits are the most stringent constraints to date on the relevant Wilson coefficients in the SMEFT framework with minimum model dependence.
If CP symmetry were violated just a bit more, the Universe might have no matter left for stars and planets — everything would have annihilated in the first moments after the Big Bang.