A noisy train station at rush hour. Thousands of people hurry past, voices merging into a dull roar. Somewhere in the crowd, two people try to shake hands with a special, barely visible gesture. Spotting such a handshake is nearly impossible. That's the kind of challenge physicists faced at the Large Hadron Collider when they set out to search for new particles in the mass range from 20 to 60 GeV (for comparison, the proton mass is about 1 GeV).
This range used to be a 'blind spot': signals from light particles drown in the background like a whisper at a rock concert. To hear the whisper, the researchers built a special 'hearing aid'—an algorithm that picks out the tracks of tau leptons from the data mess. The tau lepton is a heavy cousin of the electron that lives only for an instant and decays into a shower of other particles. Catching such a shower, especially when the tau lepton is born almost at rest, is a task bordering on the fantastic.
The new approach worked: physicists not only heard the whisper but also made out the words. They managed to register the production of the well-known Upsilon mesons, which decay into pairs of tau leptons. The signal was so clean that its significance above the background reached 5.8 sigma: that's like flipping a coin and getting heads 20 times in a row. But no new, uninvited guests were found in this mass range.
The absence of a discovery is also a result. It maps out a boundary beyond which new particles cannot hide: their production followed by decay into two tau leptons must occur less often than 40–400 events per hundred billion collisions. Now we know where not to look, and we can focus on other hiding places. And the noisy train station has become a bit clearer, and this is just the beginning.
as written by the authors arXiv:2605.25103
An inclusive search is presented for spin-zero bosons decaying to $$\tau\tau$$ in a previously unexplored mass range between 20 and 60 GeV using proton-proton collision data at $$\sqrt{s}$$ = 13.6 TeV, corresponding to an integrated luminosity of 61.9 fb$$^{-1}$$ recorded by CMS in 2022$$-$$2023. A high-rate trigger stream in combination with a novel low-momentum hadronic tau reconstruction algorithm have enabled a measurement of the $$\Upsilon$$(1S,2S,3S) $$\to$$ $$\tau^+\tau^-$$ process in the challenging environment of a hadron collider, with a 5.8$$\sigma$$ significance above background and production cross section of 3.5 $$\pm$$ 0.7 (stat) $$\pm$$ 0.7 (syst) nb for visible rapidity $$\lvert y_{\text{vis}}\rvert$$ $$\lt$$ 1.2 and visible transverse momentum $$p_\mathrm{T}^\text{vis}$$ $$\gt$$ 15 GeV. No significant excess above the standard model background is observed. Upper limits on the product of the spin-zero resonance production cross section and branching fraction to $$\tau\tau$$ are set at 95% confidence level between 40 and 400 pb.
The tau lepton was discovered in 1975 and was the first representative of the third, heaviest generation of particles. Its discovery was so unexpected that Nobel laureate Isidor Rabi, upon learning about the muon (a second-generation particle), exclaimed, 'Who ordered that?'—and then along came the tau.
In Liu Cixin's novel 'The Three-Body Problem,' aliens use protons unfolded in space to interfere with particle accelerators. Real physicists, in contrast, are learning to extract ever more subtle signals from proton collisions.