Tau leptons are fundamental particles in the lepton family, similar to electrons and muons, but with unique properties that make them particularly challenging to study. Like other leptons, they have a half-integer spin, but they are significantly heavier and have extremely short lifetimes, decaying rapidly into other particles. These characteristics limit opportunities for direct observation and detailed analysis.

The Standard Model of particle physics describes the fundamental particles and forces, along with the mathematical framework that governs their interactions. According to quantum electrodynamics (QED), a component of the Standard Model, protons in high-energy environments can emit photons (γ), which can then fuse to create a pair of tau leptons (ττ⁻):    γ γ → ττ

Using QED equations, scientists have previously calculated the probability of this process, how the tau leptons would be produced, and how often it should occur at specific energies. While muons have been extensively studied in proton collisions, tau leptons have remained more elusive due to their short lifetimes.

In a major breakthrough, researchers at CERN have used data from the CMS detector at the Large Hadron Collider (LHC) to make the first measurement of tau lepton pair production via photon-photon fusion in proton-proton collisions. Previously, this phenomenon had only been observed in lead-ion (PbPb) collisions by the ATLAS and CMS collaborations. In those cases, the photons were generated by the strong electromagnetic fields of the heavy nuclei, within a highly complex environment filled with many particles and background noise. In contrast, proton-proton collisions are much cleaner but also much rarer, making the detection of photon-induced tau production a greater technical challenge.

Notably, the team were able to distinguish QED photon collisions from QCD (Quantum Chromodynamics) collisions by the lack of the underlying event. They demonstrated tau particles were being produced without other nearby tracks (paths left by particles) using the excellent vertex resolution of their pixel detector. To verify the technique, the researchers did careful studies of the same processes in muon pair production and developed corrections to apply to the tau lepton processes.

Demonstrating tau pair production in proton-proton collisions not only confirms theoretical predictions but also opens a new avenue for studying tau leptons in high-energy environments. This breakthrough enhances our understanding of lepton interactions and provides a valuable tool for testing the Standard Model with greater precision.