ALPHA experiment

The Antihydrogen Laser Physics Apparatus (ALPHA), also known as AD-5, is an experiment at CERN's Antiproton Decelerator, designed to trap antihydrogen in a magnetic trap in order to study its atomic spectra. The ultimate goal of the experiment is to test CPT symmetry through comparing the respective spectra of hydrogen and antihydrogen.[1] Scientists taking part in ALPHA include former members of the ATHENA experiment (AD-1), the first to produce cold antihydrogen in 2002.

Antiproton decelerator
(AD)
ELENAExtra low energy antiproton ring – further decelerates antiprotons coming from AD
AD experiments
ATHENAAD-1 Antihydrogen production and precision experiments
ATRAPAD-2 Cold antihydrogen for precise laser spectroscopy
ASACUSAAD-3 Atomic spectroscopy and collisions with antiprotons
ACEAD-4 Antiproton cell experiment
ALPHAAD-5 Antihydrogen laser physics apparatus
AEgISAD-6 Antihydrogen experiment gravity interferometry spectroscopy
GBARAD-7 Gravitational behaviour of anti-hydrogen at rest
BASEAD-8 Baryon antibaryon symmetry experiment
PUMAAD-9 Antiproton unstable matter annihilation
ALPHA experiment

On 27 September 2023, ALPHA collaborators published findings suggesting that antimatter interacts with gravity in a way similar to regular matter, supporting a prediction of the weak equivalence principle.[2][3]

Experimental setup

Working with antimatter presents several experimental challenges. Magnetic traps—wherein neutral atoms are trapped using their magnetic moments—are required to keep antimatter from annihilating with matter, but are notoriously weak. Only atoms with kinetic energies equivalent to less than one kelvin may be trapped. The ATHENA and ATRAP (AD-2) projects produced antihydrogen by merging cold plasmas of positrons and antiprotons. While this method has been quite successful, it creates antimatter atoms with kinetic energies too large to be trapped. Moreover, to do laser spectroscopy on these antimatter atoms, they need to be in their ground state, something that does not appear to be the case for the majority of antimatter atoms created with this technique.

Antiprotons are received from the antiproton decelerator and are 'mixed' with positrons from a specially-designed positron accumulator in a versatile Penning trap. The central region where the mixing and thus antihydrogen formation takes place is surrounded by a superconducting octupole magnet and two axially separated short solenoid "mirror-coils" to form a "minimum-B" magnetic trap. Once trapped, antihydrogen can be subjected to study, and the measurements compared to those of hydrogen.

Antihydrogen detection

In order to detect trapped antihydrogen, ALPHA also includes a 'silicon vertex detector': a cylindrical detector composed of three layers of silicon strips. Each strip acts as a detector for the charged particles passing through. By recording how the strips are excited, ALPHA can reconstruct the traces of particles traveling through the detector. When an antiproton annihilates, the process typically results in the emission of 3 or 4 charged pions. By reconstructing their traces through the detector, the location of the annihilation can be determined. These traces are quite distinct from those of cosmic rays also detected, but due to their high energy they pass straight through the detector.

To confirm successful trapping, the ALPHA magnet that creates the minimum B-field was designed to allow rapid and repeated de-energizing. The decay of current during de-energization has a characteristic duration of 9 ms, orders of magnitude faster than similar systems. In theory, the fast turn-off speed and the ability to suppress false cosmic rays signals allows ALPHA to detect the release of single antihydrogen atoms during de-energization.

Cooling antihydrogen

One of the main challenges of working with antihydrogen is cooling it enough to be able to trap it. Antiprotons and positrons are not easily cooled to cryogenic temperatures, so in order to do this ALPHA has implemented a well known technique from atomic physics known as evaporative cooling.[4] State-of-the art minimum-B traps such as the one ALPHA uses have depths of order 1 Kelvin.

Results

A preliminary experiment conducted in 2013 found that the gravitational mass of antihydrogen atoms was between −65 and 110 times their inertial mass, leaving considerable room for refinement using larger numbers of colder antihydrogen atoms.[5]

ALPHA has succeeded in the laser cooling antihydrogen atoms, a technique known as that was first demonstrated on normal matter in 1978.[6][7][8]

On 27 September 2023, the ALPHA team published a paper supporting the prediction that the gravitational interaction of antimatter is similar to that of regular matter. For the weak equivalence principle of general relativity to be correct, it is required that the two substances display identical gravitational properties.[2][3] The findings rule out a 'repulsive [antigravity]', as previously theorized by some in the field.

Collaborators

ALPHA collaborators include the following institutions:

References
  1. Madsen, N. (2010). "Cold antihydrogen: a new frontier in fundamental physics". Philosophical Transactions of the Royal Society A. 368 (1924): 3671–82. Bibcode:2010RSPTA.368.3671M. doi:10.1098/rsta.2010.0026. PMID 20603376. S2CID 12748830. Archived from the original on 2020-03-29. Retrieved 2021-07-22.
  2. Overbye, Dennis (27 September 2023). "Nothing's the Matter With Antimatter, New Experiment Confirms - Consider it good news, physicists say: "The opposite result would have had big implications."". The New York Times. Archived from the original on 27 September 2023. Retrieved 28 September 2023.
  3. Anderson, E. K. (27 September 2023). "Observation of the effect of gravity on the motion of antimatter". Nature. 621 (7980): 716–722. doi:10.1038/s41586-023-06527-1. PMID 37758891. Archived from the original on 27 September 2023. Retrieved 28 September 2023.
  4. Grossman, Lisa (2010). "The Coolest Antiprotons". Physics. American Physical Society. 26. Archived from the original on 4 July 2010. Retrieved 2010-07-02.
  5. The ALPHA Collaboration & A. E. Charman (2013). "Description and first application of a new technique to measure the gravitational mass of antihydrogen". Nature Communications. 4: 1785. Bibcode:2013NatCo...4.1785A. doi:10.1038/ncomms2787. PMC 3644108. PMID 23653197. Article number: 1785.
  6. Baker, C. J.; Bertsche, W.; Capra, A.; Carruth, C.; Cesar, C. L.; Charlton, M.; Christensen, A.; Collister, R.; Mathad, A. Cridland; Eriksson, S.; Evans, A. (2021). "Laser cooling of antihydrogen atoms". Nature. 592 (7852): 35–42. Bibcode:2021Natur.592...35B. doi:10.1038/s41586-021-03289-6. ISSN 1476-4687. PMC 8012212. PMID 33790445.
  7. Wineland, D. J.; Drullinger, R. E.; Walls, F. L. (1978). "Radiation-Pressure Cooling of Bound Resonant Absorbers". Physical Review Letters. 40 (25): 1639–1642. Bibcode:1978PhRvL..40.1639W. doi:10.1103/PhysRevLett.40.1639. ISSN 0031-9007.
  8. Neuhauser, W.; Hohenstatt, M.; Toschek, P.; Dehmelt, H. (1978). "Optical-Sideband Cooling of Visible Atom Cloud Confined in Parabolic Well". Physical Review Letters. 41 (4): 233–236. Bibcode:1978PhRvL..41..233N. doi:10.1103/PhysRevLett.41.233. ISSN 0031-9007. Archived from the original on 2023-09-29. Retrieved 2021-07-22.

Record for ALPHA experiment on INSPIRE-HEP

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