Applied Physics Seminar

***Refreshments at 3:30pm outside Noyes 153

Abstract:

The Baryon Asymmetry Problem, which refers to the observed imbalance between matter and antimatter in the Universe, has driven significant research into the properties of antimatter. The ALPHA experiment at CERN focuses on antihydrogen, the simplest antimatter atom, which is synthesized by combining antiprotons and positrons. Antihydrogen is subsequently trapped using a combination of magnetic fields—with radial confinement provided by an octupole and axial confinement provided by a set of axially-spaced coils.

ALPHA has achieved remarkable milestones, including probing antihydrogen's spectroscopic energy levels [1,2,3], setting bounds on its charge neutrality [4], and measuring its gravitational acceleration [5]. To enhance the precision of these measurements, cooling the antihydrogen atoms to lower energies is crucial.

Laser cooling [6] is one technique that can significantly reduce the energy of antihydrogen atoms. However, this method faces limitations due to the Doppler effect and the non-uniformity of the trap's magnetic fields. An alternative approach to cooling is adiabatic expansion [7], where the volume of the magnetic trap is increased slowly relative to the anti-atom's motion. This process reduces the velocity of the antihydrogen atoms by increasing the length of their trajectories. In both cooling techniques, energy reduction is predominantly along the trap axis, with reduction in the transverse plane resulting from energy mixing due to details of the magnetic fields.

To optimize these cooling methods and improve the precision of ALPHA's experiments, diagnosing the energy of the trapped antihydrogen is essential. Traditionally, this is done using pulsed vacuum ultraviolet (VUV) laser light to excite antihydrogen to an untrappable state and then measuring the time-of-flight for annihilation with the trap walls [6]. This method nominally provides information on the transverse energy, but in principle the axial energy can be accessed by sweeping the laser detuning to determine the spectral lineshape. However, the existence of a multitude of line-broadening mechanisms prevents a straight-forward reconstruction of the axial energy. Further, this approach is time-consuming, relies on the availability of sensitive equipment, and suffers from selection effects, where only anti-atoms that intersect the laser beam are analysed.

In this work, we introduce a novel technique for measuring the energy of trapped antihydrogen atoms using radial adiabatic release of the magnetic trap. By gradually decreasing the trap depth during this release process, antihydrogen atoms annihilate with the trap walls in a manner that can be resolved both spatially and temporally. This annihilation data allows for the reconstruction of the axial and transverse energy distributions of the trapped antihydrogen atoms prior to the release.

Our experimental results demonstrate the effectiveness of this radial adiabatic release technique in diagnosing both uncooled and laser-cooled antihydrogen populations. Compared to traditional time-of-flight methods, this new technique offers faster results (within minutes), does not rely on sensitive laser equipment, and is not subject to selection effects.

Future improvements in cooling antihydrogen will involve a combination of laser cooling in a small magnetic volume followed by adiabatic expansion. Additionally, employing a partial radial adiabatic release alongside an axial expansion will further reduce the mean energy of the trapped antihydrogen population by allowing only the highest energy atoms to annihilate.

Decreases in energy achieved via laser cooling and adiabatic cooling of antihydrogen depend critically on rates of axial to transverse energy mixing. Simulations [8] make surprising predictions that distinct populations of anti-atoms exhibit different energy mixing dynamics; testing these predictions will be essential moving forward.

[1] ALPHA. Characterization of the 1S2S transition in antihydrogen. Nature, 557(7703):71–75, 2018

[2] ALPHA. Observation of the 1S–2P Lyman-α transition in antihydrogen. Nature, 561(7722):211–215, 2018

[3] ALPHA. Investigation of the fine structure of antihydrogen. Nature, 578:375–380, 2020

[4] ALPHA. An improved limit on the charge of antihydrogen from stochastic acceleration. Nature, 529(7586):373–376, 2016

[5] ALPHA. Observation of the effect of gravity on the motion of antimatter. Nature, 621(7980):716–722, 2023

[6] ALPHA. Laser cooling of antihydrogen atoms. Nature, 592(7852):35–42, 2021.

[7] ALPHA. Adiabatic Expansion Cooling of Antihydrogen, PRResearch, forthcoming

[8] A Zhong, J Fajans, and A F Zukor. Axial to transverse energy mixing dynamics in octupole-based magnetostatic antihydrogen traps. New Journal of Physics, 20(5):053003, 2018