Speaker
Description
The BASE experiment at CERN investigates the fundamental properties of protons and antiprotons to perform tests of the fundamental charge (C), parity (P), and time (T) reversal invariance in the baryon sector. With its cryogenic multi-Penning-trap system, the magnetic moment of the antiproton has been measured with a fractional precision of 1.5 ppb [1]. Our current statistical measurement precision, 20 times better than in [1], is limited by systematic uncertainties, particularly due to magnetic field fluctuations and inhomogeneities. Accordingly, two technical improvements have been implemented in the experiment: the superconducting joints of our persistent magnetic shielding system [2] and the introduction of a ferro-magnetic compensation ring, to locally tune the most important trap (PT) for precision frequency measurements to high magnetic homogeneity.
Our trap stack is placed inside a superconducting magnet that provides a field of 1.945 T. To conduct our g-factor measurements via the continuous Stern Gerlach Effect [3], a magnetic bottle is created by using ferro magnetic CoFe as a ring electrode of the analysis trap [4]. This introduces residual magnetic field inhomogeneities at the precision trap (PT), where a homogeneous magnetic field is required. To control the magnetic inhomogeneities at the PT, four superconducting coils are used. By loading a certain current into the coils, the linear and quadratic components of the magnetic field can, in principle, be tuned to zero. However, because of the external magnetic field at the superconducting joints [5], the coils could maintain too small current for linear gradient compensation. We show that shielding the joints with Mu-metal increased the current limit by a factor of about 5. In addition, a compensation ring was implemented in the experiment. A ring made of cobalt-ferrite alloy is placed near the precision trap to generate a magnetic slope that compensates for the residual field inhomogeneity. The geometry of the ring was selected by analytical calculations, FEM simulations and measurements. Combining the features of the ring with the larger coil tuning range, the linear gradient of the PT was tuned to zero with superconducting coils.
Recent results incorporating these improvements will be summarized and presented.
[1] Smorra, C., Sellner, S., Borchert, M. et al. A parts-per-billion measurement of the antiproton magnetic moment, Nature 550, 371–374 (2017)
[2] Jack A. Devlin et al. Superconducting Solenoid System with Adjustable Shielding Factor for Precision Measurements of the Properties of the Antiproton, Phys. Rev. Applied 12, 044012 (2019)
[3] H. Dehmelt, Continuous Stern-Gerlach effect: Principle and idealized apparatus, Proc. Natl. Acad. Sci. U.S.A. 83 (8) 2291-2294 (1986)
[4] C. Smorra et al. Observation of individual spin quantum transitions of a single antiproton, Physics Letters B 769, 1-6 (2017)
[5] W.A.Fietz, M.R.Beasley and J.Silcox, Magnetization of Superconducting Nb-25%Zr Wire, Phys. Rev. 136, A335 (1964)
| Academic level | PhD student |
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