UC Davis physicists working at the Large Hadron Collider at CERN in Geneva, Switzerland, recently completed a measurement that sets an upper limit on the size of one of the smallest particles known, the tau lepton. 

Leptons include the familiar electron, the heavier muon (ubiquitous in cosmic rays) and the tau, which is heavier than a proton. These particles are believed to be fundamental and have no size at all and no substructure. But if they are made from smaller things, this could be visible if they are produced in an excited state in the high energy proton-proton collisions at the LHC.  

UC Davis postdoctoral researcher Frank Jensen and graduate students Troy Welton and Benjamin Barton, under the supervision of Professor John Conway, used data from the Compact Muon Solenoid experiment at the LHC to look for collisions in which a pair of tau leptons was produced, with one in an excited state which decays immediately to a normal tau and a high energy gamma ray. 

After scouring billions of recorded collisions for excited taus, only a small number of candidate events remained. These were explained by ordinary background events, leaving no sign of any excited tau leptons. 

The result, presented at the LHC Physics conference in Boston this week, can be expressed as a limit on how large the tau lepton can be: it is smaller than 4 x 10^-20 meters, a nearly unimaginably small size, 2.5 billion times smaller than a hydrogen atom. If a hydrogen atom was the size of the earth, the tau lepton would be smaller than a pea in comparison. 

With new data being collected every day at CERN, future searches for excited taus will probe even smaller size scales.