New technique uses near-miss particle physics to peer into quantum world − two physicists explain how they are measuring wobbling tau particles

• We are two scientists who study nuclear and particle physics using CERN’s Large Hadron Collider near Geneva, Switzerland.
• Working with an international group of nuclear and particle physicists, our team realized that hidden in the data from previous studies was a remarkable and innovative experiment.
• In a new paper published in Physical Review Letters, we developed a new method with our colleagues for measuring how fast a particle called the tau wobbles.

Why measure a wobble?

• Taus are the heaviest in this family of three and the most mysterious, as they exist only for minuscule amounts of time.
• Since the 1940s, physicists have been interested in measuring magnetic moments to reveal intriguing effects in the quantum world.
• By measuring this wobble very precisely, physicists can peer into this cloud to uncover possible hints of undiscovered particles.

Testing electrons, muons and taus

• Muons and taus are much heavier but also far shorter-lived than electrons.
• They found that muons wobbled noticeably faster than Standard Model predictions, suggesting unknown particles may be appearing in the muon’s quantum cloud.
• But taus are also the hardest to see, since they live for just a millionth of the time a muon exists.

Lead ions for near-miss physics

• The particles used in this experiment were lead nuclei that had been stripped of their electrons – called lead ions.
• The electromagnetic fields of lead ions contain particles of light called photons.
• When two lead ions collide, their photons can also collide and convert all their energy into a single pair of particles.
• It goes like this: Lead ions are so small that they often miss each other in collision experiments.

First measurement of tau wobble in two decades

• Using that data, the team was also able to measure the tau magnetic moment – the first time such a measurement had been done since 2004.
• This landmark result measured the tau wobble to two decimal places.
• Much to our astonishment, this method tied the previous best measurement using only one month of data recorded in 2018.
• This increase in data will double the accuracy of the measurement of the tau magnetic moment, and improvements to analysis methods may go even further.