Two Quick HEP Hits

Toponium

It is often said that top quarks don't hadronize, but that isn't actually what the Standard Model says. 

Instead, top quarks decay so quickly that it is highly improbable, but not impossible, for a top quark hadron to form. If a top quark anti-top quark meson, called Toponium, forms quickly enough, there can be a top quark hadron. Its properties and likelihood of forming under particular conditions are well described in the Standard Model and experimental measurements of it could allow for much higher precision determinations of the top quark mass. This could be measured at future colliders.

We explore toponium, the smallest known quantum bound state of a top quark and its antiparticle, bound by the strong force. With a Bohr radius of 8×10^−18~m and a lifetime of 2.5×10^−25 s, toponium uniquely probes microphysics. Unlike all other hadrons, it is governed by ultraviolet freedom, exhibiting feeble interactions at distances much smaller than 10^−15 m, rather than infrared slavery that characterizes powerful interactions at approximately 10^−15~m. This distinction offers novel insights into quantum chromodynamics. 

Our analysis reveals a toponium signal exceeding five standard deviations in the distribution of the cross section ratio between e+e−→bb¯ and e+e−→qq¯ (q=b, c, s, d, u), based on 400~fb−1 {(1 fb = 10^−43 m2)} of data collected at around 341~GeV, driven by quantum interference. This discovery enables a top quark mass measurement with an uncertainty reduced by a factor of ten compared to current precision levels. 

Detection prospects at the Circular Electron Positron Collider or the Future Circular Lepton Collider underscore their potential to revolutionize our understanding of quantum mechanics.

Jing-Hang Fu, et al., "Toponium: the smallest bound state and simplest hadron in quantum mechanics" arXiv:2412.11254 (December 15, 2024).

Currently, the top quark mass is known to a precision of about 300 MeV. This method could reduce the uncertainty to about 30 MeV.

The smallest branching fraction every observed at five sigma

This measurement predicted that a hadron decay that was expected really happened with the expected frequency. But the extreme experimental achievement of definitively observing such a rare decay (which happens once in about 10 billion decays of a positively charged kaon and required about 400 billion K+ decays to confirm at a five sigma level) is what is notable in this case. Since a K+ is only one of many possible products of a collision, it took many trillions of collisions over six years, overall, to produce this result.

A measurement of the K+→π+νν¯ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016--2018 data taking. Combining NA62 data collected in 2016--2022, a measurement of B(K+→π+νν¯)=(13.0+3.3−3.0)×10^−11 is reported. With 51 signal candidates observed and an expected background of 18+3−2 events, B(K+→π+νν¯) becomes the smallest branching ratio measured with a signal significance above 5σ.

NA62 Collaboration, "Observation of the K+→π+νν¯ decay and measurement of its branching ratio" arXiv:2412.12015 (December 16, 2024).