Measurement of the (anti)proton g-factor - Status of the experiment

The measurement and comparison of the magnetic moment (or g-factor) of the proton and antiproton provide a stringent experimental test of the CPT-theorem in the baryonic sector [1]. We set up an experiment for the first selfcontained and direct high-precision measurement of the gfactor of a single isolated proton stored in a Penning trap [2]. In previous experiments the g-factor was determined indirectly, being currently known to a relative precision of 10−8 [3]. We aim to achieve a relative uncertainty of 10−9 or better. The application of the continuous Stern-Gerlach effect [4] to detect quantum jumps between the spin states of the particle offers the possibility of measuring the magnetic moment not only of a single proton but also later of a single antiproton, whose g-factor is currently known to a relative precision of only 10−3 [5]. The magnetic moment of the proton can be calculated as g = 2νL/νc, with νL being the Larmor frequency and νc the free cyclotron frequency of the stored ion. The proton motional frequencies in a Penning trap ν+, ν− and νz are measured non-destructively by detecting imagecurrents induced in the trap electrodes by the oscillatory motion of the particle. Applying the “invariance theorem” ν c = ν 2 + + ν 2 − + ν 2 z [6] the free cyclotron frequency can be calculated. The Larmor frequency can be determined from the proton spin flip resonance curve, obtained by the application of an external excitation field at the Larmor frequency. The detection of the proton spin state is based on a coupling of its magnetic moment μ to its axial oscillation frequency νz in the trap. This coupling is achieved by an inhomogeneous magnetic field component B2, the “magnetic bottle”, and results in an axial frequency shift (δνz ∝ B2μz/mpνz) according to the spin orientation. A double Penning trap setup allows the spatially separated measurement of the cyclotron frequency in a homogeneous magnetic field in the so-called precision trap and the detection of the proton spin state for the determination of the Larmor frequency in the so-called analysis trap, in which the magnetic field is strongly inhomogeneous. Comparing to other experiments, in which the same technique was used to detect the spin state of charged particles, as in [4] for the electron, the determination of the spin state of the proton constitutes a very challenging task since the ratio μz/mp is about 1.2 × 10 times smaller, so that a much stronger B2 is necessary to provide a detectable frequency shift. A new trap design was specially developed to provide a