As the antimatter doppelgänger of hydrogen, the first element of the Periodic Table, antihydrogen consists of an antiproton and a positron, as compared with the proton and electron that make up hydrogen. ALPHA uses a step-wise process to create, trap, cool (or slow down), measure and detect antihydrogen.
- The first ALPHA step is to produce and contain antiprotons and positrons.
ALPHA’s positrons are collected from the beta-decay of the radioactive isotope sodium 22 (22Na) and electromagnetically directed to create a beam of positrons in a vacuum. Extreme vacuum technology at every step of the process is crucial to ALPHA’s success; if antimatter interacts with its matter counterpart, they immediately annihilate into pure energy.
CERN is the only laboratory in the world that has mastered the technology to produce high quality beams of antiprotons, a process that requires a very powerful accelerator. Antiprotons are created by firing a pulse of very energetic protons (about 26 GeV) into a stationary target. The resulting nuclear reactions produce a soup of particle fragments, including antiprotons. These are collected by a machine called the antiproton decelerator, which dramatically slows the antiprotons, and delivers them to the ALPHA experiment.
- The second ALPHA step is the trapping and cryogenic cooling of positrons and antiprotons.
To create antihydrogen, the antiprotons and positrons must be combined under just the right conditions. To start, this is done by injecting them into opposite ends of a cryogenically cooled Penning trap, held in an ultra-high vacuum chamber. Since antiprotons and positrons have the opposite electrical charges, the Penning trap uses electric fields to contain them axially, or length-wise, in different locations – or electrical valleys – within the ALPHA chamber.
- The third ALPHA step is to create antihydrogen by merging the positrons and antiprotons.
To create antihydrogen, ALPHA scientists alter the electric fields to merge the antiprotons and positrons. Mixing millions of positrons and tens of thousands of antiprotons results in the formation of thousands of antihydrogen atoms.
The success of APHA’s antihydrogen creation depends on having ultra-cold antiprotons and positrons. The entire inner part of ALPHA is cryogenically cooled using liquid helium to near 4.2 Kelvin (-268.9 °C) using a TRIUMF-created cryogenic system. This extreme cold is essential because it slows the positrons and antiprotons’ movement, which increases the chance that they’ll bond when they come into contact. The low temperature also increases the chance that the newly-synthesized antihydrogen can be captured since only the slowest-moving anti-atoms can be trapped in the magnetic trap
- The forth step is to trap antihydrogen in a magnetic trap.
To contain the neutral antihydrogen atoms, ALPHA uses a magnetic trap. An antihydrogen atom, although electrically neutral, has a slight magnetic moment and thus responds to a magnetic force like a tiny bar magnet. The ALPHA magnetic bottle is created by two sets of powerful magnets, in a vacuum chamber, oriented so that they create a magnetic field gradient which is strongest on the outside and at ends and weakest in the middle. In this way, the antihydrogen atoms are trapped in the magnetic center of ALPHA and thus can be contained without annihilating by coming into contact with the apparatus. With improvements to experimental technique, the number of the trapped antihydrogen atoms in ALPHA has dramatically increased from the start of the experiment from just a few to more than 1000 atoms at a time.
- Detection of antihydrogen through detecting annihilations.
One of the most notable challenges with ALPHA is that scientists don’t know whether they’ve succeeded in making and measuring antihydrogen until after they’ve destroyed it.
This is because it’s only through detecting the annihilation of antihydrogen that they know they’ve created and contained it.
In early days, the process of identifying annihilation events was greatly aided by a unique feature of the ALPHA magnets: the current could be shut-off in less than a hundredth-of-a-second. Thus, the antihydrogen atoms escaped the magnetic field and annihilated with the matter of the apparatus over a very short time period. This gave researchers a very precise temporal window during which they knew that detections might be antihydrogen. This helped distinguish antihydrogen annihilation products from cosmic ray detections. In the past few years, the trapping rates have increased so dramatically that the antihydrogen atoms can be efficiently detected even when the magnetic bottle is shut off much more slowly.
To detect antimatter annihilation events, ALPHA is surrounded by a TRIUMF-inspired a triple-layer Silicon Vertex Detector (SVD). The SVD is analogous to a 3D camera for antimatter annihilation: it captures both the annihilation products; and enables researchers to reconstruct the location of annihilation.
To do this, the ALPHA magnets are shut-off causing the antihydrogen to hit the apparatus itself and annihilate. The annihilation of an antiproton results in a variety of products including a number of energetic pions. Each pion passes through the ALPHA trap and the SVD, leaving a tiny amount of energy in each of its three thin silicon sensor layers, enabling ALPHA scientists to identify a pion’s trajectory. ALPHA scientists extend these tracks backwards and use the intersection of the pion tracks to determine the annihilation’s spatial location, or vertex.
- Measurement of antihydrogen’s physical characteristics.
Finally, ALPHA researchers use lasers and microwaves to study antihydrogen’s fundamental nature. The upcoming ALPHA-g project will drop antihydrogen to, for the first time, study the gravitational properties of antimatter.