A recent experimental result published in Nature by the ALPHA collaboration, improves upon the direct measurement of antihydrogen charge neutrality. This is a precision test conducted in pursuit of answers to the question of why the universe today is predominantly made of matter. According to the Standard Model, that predicts a universe composed of half matter and half antimatter, antihydrogen atoms should be charge neutral.
The silicon tracker detector (courtesy: University of Liverpool).
However, experiments currently underway at the Antimatter Decelerator at CERN are all looking for physics beyond the Standard Model to address this question of baryon asymmetry by comparing measured properties of antimatter against the known equivalent quantities in matter systems. Charge neutrality is not expected to be broken in antihydrogen, and as such all current antimatter experiments assume it to be true. This measurement constitutes an empirical verification of this assumption, as well as a fundamental symmetry measurement in its own right. Even a tiny putative charge in the antihydrogen system would have significant implications for physics in general, and prosaically create systematic effects in other measurements such as those of antimatter gravitation currently in progress or proposed to take place at CERN.
Photograph of the inside of the ALPHA experiment (courtesy: APLHA).
The result presented in the paper represents a twenty-fold improvement on the bound of the antihydrogen charge achieved by using a new technique involving stochastic acceleration. In this technique, antihydrogen is first formed, trapped, and held in an atomic magnetic-minimum trap. By subjecting the trapped anti-atoms randomly to pulsed electric fields, any charged particles will eventually be accelerated out of the trapping potential. Using a comparison of trap survival with and without the acceleration fields, and comparing the expectation to simulations, the ALPHA collaboration set a new limit on antihydrogen charge as < 0.71 parts per billion (of an electron charge) at a 90% confidence level. Interestingly, this bound on antihydrogen indirectly improves the measurement of the positron charge under the assumption of charge neutrality and using the best measurements of the antiproton charge.
ALPHA is an international collaboration with over 50 researchers in 13 institutions world-wide. This study is the first precision result from the ALPHA-2 apparatus, a device focused on making precision measurements with trapped antihydrogen atoms. UK members of the collaboration play a significant role in both this particular measurement as well as the design and commissioning of ALPHA-2 in general. The group from the University of Manchester under the leadership of Dr. William Bertsche from the Cockcroft Institute designed and commissioned the Antiproton Catching Trap used to provide ultra-low energy antiprotons for the experiment; members from Swansea University under Prof. Michael Charlton developed the positron source and techniques used for this antimatter ingredient; and the group from University of Liverpool headed by Prof. Paul Nolan developed the silicon vertex detector instrumental to the efficient detection of antiproton annihilations. The ALPHA-2 apparatus is fully commissioned and is actively working towards performing the first
Further information:
M. Ahmadi, et al., “An improved limit on the charge of antihydrogen from stochastic acceleration”. Nature. 529, 373–376 (2016). doi:10.1038/nature16491.