## Project Details

### Description

Overview
This proposal is to measure the electron and positron magnetic moments using a quantum cyclotron.
The latter is a single elementary particle whose lowest quantum states can be resolved using quantum
non-demolition measurements while it is suspend for months at a time at a temperature of 0.1 K. A new
measurement approach will rely upon a recently demonstrated capacity to couple and uncouple the quantum
cyclotron and its cryogenic detector during a measurement. New lineshape calculations show how to evade
detector backaction in the decoupled con�guration to measure the electron magnetic moment ten times more
accurately. The positron moment could be measured 150 times more accurately because it has been less well
measured. The positron and electron will thus be compared at least 150 times more precisely.
For the �rst time, the quantum cyclotron will be coupled to a 200 MHz, \quantum-limited" SQUID
detector which will heat the particle much less, will increase the detection e�ciency, and which will operate
more robustly than conventional transistor detectors. Such detectors operate only in a small magnetic �eld
while the suspended particle that must be nearby requires a very large �eld. What makes this possible for
the �rst time is a new solenoid that "actively shielded" so the detector is only 40 cm from the particle, along
with being "self-shielded" to shield out external magnetic �eld
uctuations. A new cryogenic system being
tested includes this solenoid and a dilution refrigerator is scheduled to be arrive and be installed just before
the new grant period. A unique gas 3He NMR probe has been demonstrated at cryogenic temperatures is
ready for tuning the spatial homogeneity of the new solenoid.
The last measurement that made a signi�cantly more precise measurement of the electron magnetic
moment, still widely celebrated and used for BSM limits, took 20 years to complete using new methods
developed one PhD thesis at a time. There is some reason for optimism that a similar advance could take
place during the next grant period.
Intellectual Merit
The electron and positron magnetic moment measurements will be the most precise measurements of
a property of elementary particles. These measurements test the most precise prediction of the Standard
Model of Particle Physics (SM), which is the size of the electron magnetic moment (�) in Bohr magnetons
(�B). Crucial elements of the standard model (SM) are tested by this most precise confrontation of the-
ory and experiment are Dirac theory, quantum electrodynamics (QED), hadronic contributions, and weak
contributions. Owing to its fundamental CPT symmetry invariance, the SM also predicts that the electron
and positron moments are equal in magnitude but opposite in sign. The proposed measurements will be the
most stringent test of CPT invariance with a lepton system.
There has been a long standing agreement between the SM prediction and measurement, to a remarkable
precision of 1 part in 1012. However, an intriguing 2.4 standard deviation discrepancy recently emerged at a
higher precision. The uncertainty is not small enough so that the discrepancy is a discovery of new physics
beyond the standard model (BSM), but it has stimulated theoretical investigation of BSM possibilities. The
new measurements will test whether the discrepancy holds up when the uncertainty is greatly reduced.
Broader Impacts
The most stringent tests of the Standard Model of Particle Physics (SM), the fundamental theoretical
description of physical reality, will come from the p

Status | Active |
---|---|

Effective start/end date | 9/1/21 → 8/31/26 |

### Funding

- National Science Foundation (PHY-2110565)

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