Cusp trap

We proposed a cusp trap in which antihydrogen Rydberg states may not only be synthesized at low temperatures but also extract them as a beam for ground state hyperfine splitting measurement. A theoretical calculation also shows most of antihydrogen atoms to cascade down to the ground state in CUSP trap for subsequent use in a variety of experiments. The cusp trap consists of a magnetic quadrupole (cusp) field formed by a pair of superconducting solenoids (antihelmholtz coil) together with an electrostatic field.

Since the ground state of antihydrogen has infinite lifetime, its high precision spectroscopy will give unprecedented accuracies in terms of CPT symmetry tests. In the case of hydrogen, the ground-state hyperfine splitting (GS-HFS) frequency \nu_HF has been measured in a classic series of experiments which began in the 1930’s with relatively simple atomic beam experiments, and culminated with maser experiments in the early 1970s which ultimately achieved a relative precision of order 1e−12. For the antihydrogen atom, a measurement of \nu_HF with precision equal to that achieved in the hydrogen case some fifty years ago would constitute a commensurately precise test of CPT symmetry. It may also be interpreted in terms of the gravitational interaction of antimatter. To the leading order, the GS-HFS of antihydrogen is proportional to the spin magnetic moment of the antiproton, ⃗μp, which is experimentally known only at the level of 0.3%. Below the level of several ppm accuracy, νHF also depends on the electric and magnetic form factors of the antiproton. The measurements of νHF(H) to a relative accuracy of better than 10&super{−6}; as discussed in this letter will therefore yield an improvement of the value of \mu_p by three orders of magnitude, and give some insight into the structure of the antiproton. Furthermore, the only existing phenomenological extension of the standard model that includes CPT violations (the standard model extension - SME - of Kostelecky’s group) predicts that CPT violation in the 1S-2S transition is cancelled in first order, while for the hyperfine structure it is a leading-order effect. In addition, the parameters introduced by Kostelecky et al. have the dimension of energy (or frequency). Therefore, by measuring a relatively small quantity on an energy scale (like the 1.4 GHz GS-HFS splitting), a smaller relative accuracy is needed to reach the same absolute precision for a CPT test.

The key objective for either laser or microwave spectroscopy of antihydrogen atoms is to prepare an adequate number of them in the 1S ground state at low temperature and to confine these in a neutral atom trap. The principal mechanisms for antihydrogen atom synthesis are three body recombination processes in a high density, low temperature plasma. This inevitably results in the high-Rydberg state atoms which are not suitable for spectroscopic purposes.

Concept of the CUSP trap

The CUSP trap consists of the combination of a superconducting anti-Helmholz coil and a stack of ring electrodes. The CUSP trap provides the minimum B field configuration but still maintaining axial symmetry. Due to this symmetry, the CUSP trap realizes stable handlings of both antiprotons and positrons such as trapping, cooling, and mixing.

MCEO configuration

In the MCEO (Magnet Cusp and Electic Octupole fiels) configuration, magnetic cusp and electric octupole fields are superimposed. Although a neutral plasma confined in the magnetic cusp is hydromagnetically very stable, particles can still escape along the magnetic-field lines. In the case of non-neutral plasma, this leakage can be “plugged” by applying an electric octupole field, as illustrated in the figure. The MCEO therefore possesses advantageous plasma stabilization properties in addition to simply confining the non-neutral particle plasma, and experiments have already shown that it is well able to confine an electron plasma, with confinement time proportional to the square of the magnetic-field strength and a plasma density distribution that is nearly parabolic in the central region.

Nested well configuration

So-called nested well configuration is also adopted as shown in the above figure. The confinement well for positron plasma is nested in the well for antiproton cloud. This configuration has an advantage to make a spin polarized beam, that is, the polarization of 50K antihydrogen beam amounts to about 30% when they are synthesized near the maximum magnetic field region in the cusp magnetic field.

Cusp trap as a polarized antihydrogen beam souce

Formed antihydrogen atoms in LFS (low field seeking) states are preferentially focused along the axis whereas those in the HFS (high field seeking) states are defocused, resulting in the formation of an intesity enhanced spin-polarized antihydrogen beam. This is a novel feature of the CUSP trap. By using this antihydrogen beam, we adopt Rabi-technique for the microwave spectroscopy.

References

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