Muon g − 2, current experimental status and future prospects

The muon gyromagnetic anomaly aµ has been measured with a precision δaµ/aµ = 540 ppb using magic-momentum muon decays recorded up to 2001 by the E821 BNL experiment. Two projects aim at significantly improving that experimental precision: the E989 Collaboration at Fermilab plans to collect 21 times the BNL statistics and to improve by a factor four the uncertainty, the E34 Collaboration is designing a new experiment at J-PARC with a novel approach based on the production, injection and storage of ultra-cold low-energy muons. E34 aims at matching the BNL precision in a first phase, and to provide a significantly higher precision measurement in a second phase.