Speaker
Description
In the era of ever-increased precision measurements of the fundamental properties of antihydrogen at the ALPHA experiment at CERN, knowledge of the antihydrogen energy distribution has become vital; for example, it provided a significant contribution to the uncertainty in the first direct measurement of the gravitational acceleration of antihydrogen [1]. Increased precision in ALPHA's experiments can be achieved by reducing antihydrogen energy, but measuring the extent of achieved cooling is essential for optimization and simulation-experiment comparison. In recently demonstrated techniques of Doppler laser cooling [2] and adiabatic expansion cooling [3] of trapped antihydrogen, energy reduction is predominantly along the trap axis, with reduction in the transverse plane resulting from energy mixing due to details of the trap magnetic fields [4]; highlighting the importance of knowledge of energy in both axial and transverse dimensions. Further, in some antihydrogen spectroscopic experiments, the dominating lineshape broadening effect depends on the axial energy [5], whereas for others it depends on the transverse energy [6].
Currently, antihydrogen energy is measured using pulsed laser light to excite a transition to an untrappable state [2], and measuring the time taken for the anti-atom to annihilate with the trap wall. This time-of-flight method gives access to the transverse energy; 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, measuring the energy using this method takes several hours and relies on the availability of the pulsed laser light.
In ALPHA, antihydrogen is confined in an Ioffe-Pritchard magnetic trap, where an octupole provides radial confinement and solenoids on the left and right of the trapping region provide axial confinement. We demonstrate a technique to measure the axial and transverse energies of a population of trapped antihydrogen atoms via a controlled rampdown of the octupole magnetic field (keeping the solenoid fields static). During this process, the trap depth gradually decreases, resulting in annihilations on the trap walls that can be resolved in space and time by ALPHA's silicon vertex detector. This technique relies on the principle that lower energy antihydrogen can be confined by a shallower magnetic trap, causing these anti-atoms to annihilate later in time. However, we show that a correlation of the axial and radial confining potentials means the axial annihilation location can be combined with the annihilation time to enable a measurement of both the axial and transverse antihydrogen energy just before annihilation. Since this energy differs from energy prior to release due to radial adiabatic expansion cooling, we present a modified version of a technique in Ref. [7], in which we use simulations of the octupole rampdown technique to reconstruct the axial and transverse energy prior to release using annihilation information.
We present experimental annihilation-data of both uncooled and laser-cooled antihydrogen atoms undergoing an octupole rampdown, and compare these results to simulations of the experimental procedure. This annihilation data is used to reconstruct the axial and transverse energy distributions of the experimental antihydrogen populations in typical confining potentials, showcasing the temperatures reached using ALPHA's current laser cooling methods. Finally, we compare the reconstructed transverse energy to the equivalent quantity determined using existing time-of-flight methods. This work makes it possible to measure distributions of both components of the antihydrogen energy in 10s of seconds.
[1] E. K. Anderson, C. J. Baker, W. Bertsche, N. M. Bhatt, G. Bonomi, A. Capra, I. Carli, C. L. Cesar, M. Charlton, Christensen, et al. (The ALPHA Collaboration), Nature 621, 716 (2023).
[2] C. J. Baker, W. Bertsche, A. Capra, C. Carruth, C. L. Cesar, M. Charlton, A. Christensen, R. Collister, A. C. Mathad, et al., Nature 592, 35 (2021).
[3] D. Hodgkinson, On the Dynamics of Adiabatically Cooled Antihydrogen in an Octupole-Based Ioffe-Pritchard Magnetic Trap, Ph.D. thesis, The University of Manchester (2022).
[4] M. Zhong, J. Fajans, and A. F. Zukor, New Journal of Physics 20, 053003 (2018).
[5] M. Ahmadi, B. X. R. Alves, C. J. Baker, W. Bertsche, A. Capra, C. Carruth, C. L. Cesar, M. Charlton, S. Cohen, Collister, et al. (ALPHA Collaboration), Nature 578, 375 (2020).
[6] M. Ahmadi, B. Alves, C. Baker, W. Bertsche, A. Capra, C. Carruth, C. Cesar, M. Charlton, S. Cohen, R. Collister, et al. (ALPHA Collaboration), Nature 557, 71 (2018).
[7] C. Amole, G. Andresen, M. Ashkezari, M. Baquero-Ruiz, W. Bertsche, E. Butler, C. Cesar, S. Chapman, M. Charlton, A. Deller, et al. (The ALPHA Collaboration), New Journal of Physics 14, 015010 (2012).
