Last April Nature Communications published a paper [1] outlining a new technique in measuring the effect of gravity on neutral antimatter specifically regarding the trapping of antihydrogen. This recent experimental achievement has lead to a range of new possibilities in exploring fundamental physical phenomena, including most recently a paper published today in Nature Communications [2], signifying the charge neutrality of antihydrogen.

Antimatter is predicted to have the same properties as matter aside from having an opposite charge. The universe is composed with a larger proportion of matter. One of the biggest questions in physics is to understand why there is such a significant asymmetry of matter and antimatter.

The ALPHA experiment has been trapping antihydrogen (a positron orbiting an antiproton) since 2010 [3] holding antihydrogen for questioning for up to 1000 seconds (~15 mins) [4]. By applying an electric field of 50 Volts per metre across their trap they are able to deduce the total charge of the antiatom by the deflection induced as they turn off the trap. To date, some ~500 atoms of anti-hydrogen have been trapped in the ALPHA experiment, meaning applying macroscopic methods for studying the charge of normal matter are not suitable.

Charge neutrality has been deduced from antihydrogen with a value of (-1.3 ±1.1 ±0.4) x 10-8 e,[i] a value consistent with zero; an accuracy the equivalent of measuring the distance between Anfield and Everton to 0.01 mm. Although this result comes as no surprise, since hydrogen atoms are electrically neutral, it is the first time that the charge of an antiatom has been measured to such a high precision.

In order to observe single antiatoms in the experiment a Silicon Vertex Detector (Annihilation Detector, shown below) is used, which can ‘see’ the tracks of particles produced during the annihilation of antiatoms (antihydrogen) with normal atoms (the walls of the atom trap).  The Liverpool built Silicon Vertex Detector had the central role in detecting the deflection of particles.

 

A diagram of the antihydrogen production and trapping region of the ALPHA apparatus [2]

A diagram of the antihydrogen production and trapping region of the ALPHA apparatus [3]

The ALPHA experiment is a collaboration of about 50 scientists from 16 universities and institutions from around the world, with the experiment itself placed at the Antiproton Decelerator at CERN. Each institution complements each other bringing a range of expertise and specialities from various fields of physics to the group. Liverpool’s contribution to this group includes this very Silicon Vertex Detector designed at the University and developed by a subgroup including 5 scientists and 3 engineers lead by Professor Paul Nolan from the Department of Physics at Liverpool.

The silicon detector was built and tested at the Liverpool Semiconductor Detector Centre, a multimillion pound facility located in the Oliver Lodge clean room on campus. (More information regarding the clean room facilities can be found here: http://www.liv.ac.uk/particle-physics/infrastructure/lsdc/). The air in the centre is practically clean of all dust, and kept at a constant temperature to insure there is no damage when the sensors are exposed. Special overalls and hair nets are required, no, they haven’t just got of shift at Greggs.

In addition to the development of the Silicon Vertex Detector, the clean room at Liverpool has been busy of the last few years, not only working with ALPHA at CERN, they have also build the VELO modules for the LHCb experiment, the Endcap C for ATLAS along with many other projects at various labs around the world.

Detector build in progress. Team members from left to right, Joseph McKenna (UoL), Andrea Capra (Visiting from YorkU, Canada) and Petteri Pusa (UoL). With the ALPHA Silicon Vertex Detector (Annihilation Detector).

Detector build in progress. Team members from left to right, Joseph McKenna (UoL), Andrea Capra (Visiting from YorkU, Canada) and Petteri Pusa (UoL). With the ALPHA Silicon Vertex Detector (Annihilation Detector).

 

“It has been hard work but extremely rewarding to have been able to do my PhD with the ALPHA collaboration at Liverpool University, I am looking forward to August 2014 when the AD physics program resumes, oh and finishing my thesis this year too”, tells Joseph McKenna (pictured in the figure above) who is in his final stage with his PhD thesis on the ALPHA experiment. Special thanks to Joseph, whom we met last year [5], for helping out a huge deal with this article.

For any readers interested in more information about the experiment, find more at the collaborations website: http://alpha.web.cern.ch/

[i] plus or minus numbers representing statistical and systematic uncertainties on the measurement.


[1] C. Amole et. al, “Description and first application of a new technique to measure the gravitational mass of antihydrogen,” Nature Communications, vol. 4, no. 1785, 2013.
[2] C. Amole et al, “An experimental limit on the charge of antihydrogen.” Nature Communications, vol. 5,  no. 3955, 2014
[3] G. B. Andresen et. al, “Trapped Antihydrogen,” Nature, vol. 468, no. 673, 2010.
[4] G. B. Andresen et. al, “Confinement of antihydrogen for 1000 seconds,” Nature Physics, vol. 7, no. 558, 2011.
[5] P. Paudyal, “LSMedia,” 30 April 2013. Available: http://www.thesphinx.co.uk/2013/04/joseph-mckenna-and-an-insight-into-the-world-of-antihydrogen/.