ATLAS measurement supports lepton universality

Particle-physics collaborations are always on the lookout for discrepancies between their measurements and the standard model’s predictions. Deviations can help point the researchers in the right direction (see, for example, Physics Today, June 2021, page 14 ). Researchers were therefore excited when a working group combed through data from four earlier experiments performed at CERN’s now-dismantled Large Electron–Positron Collider (LEP) and found that the results were inconsistent with the standard model’s assertion of lepton universality, albeit with a probability of less than 1%.

All three leptonic generations—electronic, muonic, and tauonic—supposedly have the same coupling to weak force–mediating W bosons. So when a W boson decays, it should be equally likely to produce any one of the leptons, along with its associated antineutrino. Several experiments at Fermilab and CERN have confirmed that W bosons generate electrons and muons at the same rate. But the LEP data showed that tauons were produced slightly more often than muons; the ratio of their production rates was R(τ/μ) = 1.070 ± 0.026. Other experiments studying particles that contain bottom quarks have seen hints of the same problem.

Now the ATLAS collaboration has collected and analyzed data at the Large Hadron Collider (LHC) that resolves the apparent disagreement. The precision of the collaboration’s measurement is twice that of the LEP result, and the value, R(τ/μ) = 0.992 ± 0.013, agrees with the standard-model prediction of unity.

The experiment exploited the fact that the LHC’s proton–proton collisions produce a large number of top–antitop quark pairs. A top quark nearly always decays into a W boson and a bottom quark, so the researchers had easy access to many W bosons whose decays they could observe. Some of the W bosons directly produced muons, whereas others produced intermediate tauons that later decayed into muons. Because of their different origins, the muons formed two populations whose signals in the detector could be differentiated by the particles’ impact parameters and transverse momenta. The ATLAS researchers analyzed tens of thousands of W-boson decays for each type of lepton, compared with only a couple thousand each in the LEP data, and counted how many took each path.

On the whole, data now support the standard model’s prediction of lepton universality in W-boson decays. But the search continues: Hints of lepton universality violations have also been seen in beauty-meson decays at significance levels that are starting to draw attention from high-energy physicists. (ATLAS collaboration, Nat. Phys., 2021, doi:10.1038/s41567-021-01236-w .)