Context

• In an astonishing feat of metrology, physicists recently reported measuring the electron’s magnetic moment with a precision of 0.13 parts per trillion (ppt).
• The resulting measurement is 2.2 times more accurate than the previous best, recorded 14 years ago.
• More importantly, it’s the most precise test so far of a theory that has both comforted and baffled physicists – the Standard Model of particle physics – and therein lies the rub.

What is Standard Model

• The Standard Model (SM) is the theory that describes the properties of all subatomic particles, classifies them into different groups, and determines how they’re affected by three of the four fundamental forces of nature: strong-nuclear force, weak-nuclear force, and the electromagnetic force (it can’t explain gravity).
• In the 1960s, physicists used SM to predict the existence of a particle called the Higgs boson; it was finally discovered in 2012. 
• Similarly, the SM has allowed physicists to successfully predict the existence and properties of dozens of particles and is considered to be one of the most successful theories in the history of physics.
• However, it still can’t explain why the universe has more matter than antimatter, what dark matter is, or what dark energy is.

What is Electron’s magnetic moment

• The SM’s most precise prediction is of the electron’s magnetic moment. 
• Physically, the magnetic moment describes how willing an electron is to align itself in the direction of a magnetic field
• Mathematically, it’s equal to – µ/ µB. Here, µ  is the electron’s magnetic moment (measured in amperes sq.-metres) and µB is a physical constant called the Bohr magneton. Together, – µ/ µB is a dimensionless number.

https://www.thehindu.com/sci-tech/science/scientists-tested-an-electrons-response-to-a-magnetic-field-with-extreme-accuracy-why/article66511994.ece#:~:text=In%20an%20astonishing%20feat%20of,best%2C%20recorded%2014%20years%20ago.