image: In this plot, θ denotes the polar scattering angle between the directions of the outgoing Z and the initial state e⁻, and ϕ∗ is defined as the angle between the scattering plane and the decay plane of the final state Z boson in the center-of-mass frame of e⁻e⁺.
Credit: ©Science China Press
In the hunt for new physics beyond the Standard Model, scientists are now looking closely at an unusual class of particle interactions called neutral triple gauge couplings (nTGCs). These interactions do not appear in the present Standard Model (SM) and only show up when we consider more subtle, higher-order effects. Even more intriguingly, some of these interactions could break CP symmetry—a tiny imbalance that might help explain why the universe contains more matter than anti-matter.
A team of researchers from Shanghai Jiao Tong University, Peking University, and King’s College London and CERN recently revisited how physicists describe these rare interactions. They found that the conventional method used in earlier studies—based on so-called “form factors”—was not fully compatible with the gauge symmetry of the Standard Model. This inconsistency could lead to incorrect predictions, especially at very high energies. To fix this, the authors developed a new, symmetry-respecting formulation of these interactions, ensuring that they match properly with the Standard Model Effective Field Theory, that is the model-independent framework used to capture indirect signs of new physics.
With this improved description, the team calculated in detail how CP-violating nTGCs would affect the process e⁺e⁻→Zγ, where an electron and a positron collide to produce a Z boson and a photon. They showed that these exotic interactions would produce distinctive signatures through the angular distributions of the Z boson’s decay products—essentially a sinusoidal “wiggle” tied to how the particle decay plane is oriented. By dividing this angular space cleverly and combining many decay channels, they devised a set of observables that greatly sharpen the sensitivity to these tiny effects.
Using these observables, the researchers estimated how well future electron–positron colliders can probe signatures of new physics. These machines could probe new physics at energy scales between 1 and 13 TeV, with polarized beams improving the reach by up to a factor of 2.6. This study demonstrates that the future high energy e⁺e⁻ colliders under planning can outperform the LHC for probing these couplings, while a future 100-TeV proton-proton collider would ultimately go even further.
Journal
Science China Physics Mechanics and Astronomy
Method of Research
Experimental study