For the first time, physicists in the US have trapped a single positron such that its lowest-energy quantum states can be fully resolved. Gerald Gabrielse and colleagues at Northwestern University did this by creating a device that they call a “one-positron quantum cyclotron”. The team hopes that future studies could measure the positron’s magnetic moment with unprecedented precision. This could pave the way for the an extremely stringent test of a fundamental symmetry of the Standard Model of particle physics.
The Standard Model has been extraordinarily successful at describing fundamental particles and how they interact with each other. However, the Standard Model is not able to explain one of the most obvious things about the observable universe: that it contains much more matter than antimatter.
“The great mystery of the Standard Model is its ability to make extremely precise predictions of what can be measured, while being inconsistent with basic features of the universe,” Gabrielse explains.
As a result, physicists are constantly testing the Standard Model, looking for situations where it falls short on predicting the properties of particles – with the hope of discovering new physics.
Precision particle physics
The conventional way to do this testing is to collide particles at ever higher energies in the hope of spotting deviations from what is predicted by the Standard Model. More recently however, physicists have also been doing low-energy experiments that involve trapping particles using magnetic and electric fields and making exquisitely precise measurements of properties such as the magnetic moment.
One important tenet of the Standard Model is CPT invariance. It considers three hypothetical transformations of the universe. These are charge conjugation (C), in which every particle is swapped for its antiparticle; parity (P), in which everything is flipped into its mirror image; and time reversal (T), in which time runs backwards.
According to the Standard Model, a universe subjected to all three transformations at once would obey exactly the same laws of physics as our own. If this were the case, every particle must be the perfect twin of its corresponding antiparticle: with identical masses, and magnetic moments equal in size but opposite in sign.
To develop a test of CPT invariance for the the electron and its antiparticle, the positron, Gabrielse’s team trap a single positron in a device called a Penning trap. Inside, a strong magnetic field puts the positron into a circular path. The trap is cooled to 300 mK in order to place the positron in its lowest-energy quantum states.
“One-quantum jumps”
Gabrielse explains how this is done, “High speed positrons from the decay of a sodium nucleus are slowed and captured in an environment that is so cold that they can be observed making one-quantum jumps between their lowest quantum states”.
The team showed that the positron can be held in its cyclotron ground state, and can also be excited to higher-energy states using a driving electromagnetic force. This includes transitions between the particle’s spin states. These low energy states are defined by the size of the positron’s cyclotron orbit and by the relative orientation of the positron’s magnetic moment to the cyclotron’s magnetic field.
Measuring the energy difference between spin states reveals the size of the positron’s magnetic moment. The team believes that future single-positron cyclotron measurements will provide a value for the magnetic moment to one part in 1013.
“The one-positron quantum cyclotron should enable the first quantum measurements of the positron magnetic moment,” Gabrielse says.
If CPT invariance holds, the result should exactly match the size of the electron’s magnetic moment, which has already been measured to 1 part in 10¹³ using a one-electron quantum cyclotron. By bringing the positron up to the electron’s level of precision, Gabrielse and colleagues hope to deliver the most stringent test yet of CPT invariance for leptons, perhaps offering a new route towards observations that may finally prove to break the Standard Model’s predictions.
The research is described in a paper that has been accepted for publication in Physical Review Letters.