A parts-per-billion measurement of the antiproton magnetic moment

Abstract

Precise comparisons of the fundamental properties of matterantimatter conjugates provide sensitive tests of charge-parity-time (CPT) invariance 1 , which is an important symmetry that rests on basic assumptions of the standard model of particle physics. Experiments on mesons 2 , leptons 3,4 and baryons 5,6 have compared different properties of matter-antimatter conjugates with fractional uncertainties at the parts-per-billion level or better. One specific quantity, however, has so far only been known to a fractional uncertainty at the parts-per-million level 7,8 : the magnetic moment of the antiproton, μ p . The extraordinary difficulty in measuring μ p with high precision is caused by its intrinsic smallness; for example, it is 660 times smaller than the magnetic moment of the positron 3 . Here we report a high-precision measurement of μ p in units of the nuclear magneton μ N with a fractional precision of 1.5 parts per billion (68% confidence level). We use a two-particle spectroscopy method in an advanced cryogenic multi-Penning trap system. Our result μ p = −2.7928473441(42)μ N (where the number in parentheses represents the 68% confidence interval on the last digits of the value) improves the precision of the previous best μ p measurement 8 by a factor of approximately 350. The measured value is consistent with the proton magnetic moment 9 , μ p = 2.792847350(9)μ N , and is in agreement with CPT invariance. Consequently, this measurement constrains the magnitude of certain CPT-violating effects 10 to below 1.8 × 10 −24 gigaelectronvolts, and a possible splitting of the protonantiproton magnetic moments by CPT-odd dimension-five interactions to below 6 × 10 −12 Bohr magnetons 11 .Within the physics programme at the Antiproton Decelerator of CERN, the properties of protons and antiprotons 5,6 , antiprotons and electrons 12 , and hydrogen 13 and antihydrogen 14,15 are compared with high precision. Such experiments, including those described here, provide stringent tests of CPT invariance. Our presented antiproton magnetic moment measurement reaches a fractional precision of 1.5 parts per billion (p.p.b.) at 68% confidence level, enabled by our new measurement scheme. Compared to the double-Penning trap technique 16 used in the measurement of the proton magnetic moment 9 , this new method eliminates the need for cyclotron cooling in each measurement cycle and increases the sampling rate.Our technique uses a hot cyclotron antiproton for measurements of the cyclotron frequency ν c , and a cold Larmor antiproton to determine the Larmor frequency ν L . By evaluating the ratio of the frequencies measured in the same magnetic field, the magnetic moment of the antiproton (in units of the nuclear magneton, the g-factor) ν ν μ μN is obtained. With this new technique we have improved the precision of the previous best antiproton magnetic moment measurement 8 by a factor of approximately 350 (Fig. 1a).Our experiment 17 is located in the Antiproton Decelerator facility, which provides bunches of 30 million antiprotons at a...

Lorentz and CPT Tests Using Penning Traps

“…where Q is a general 4 × 4 spinor matrix involving the covariant derivative iD µ and the electromagnetic field tensor F αβ ≡ ∂ α A β − ∂ β A α . The hermiticity of the Lagrange density (2) guarantees that Q satisfies condition Q = γ 0 Q † γ 0 . In the limit of the free Dirac fermion with A α = 0, Ref.…”

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“…(XZ) e , and b (YZ) e has been achieved, but also that new coefficients b (XY) F,e and b * XX F,e − b * YY F,e have been constrained. In the proton sector, the limits on the tilde coefficients have been improved by factors of up to three compared to the existing results [2]. The constraints on the tilde coefficients that are not sensitive to the corresponding work are left blank in Table 1.…”

Sympathetic cooling of protons and antiprotons with a common endcap Penning trap

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Hyperfine spectroscopy of hydrogen and antihydrogen in ASACUSA

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