ALPHA

The international ALPHA (Antihydrogen Laser Physics Apparatus) collaboration is the world’s first experiment to contain and make physical measurements of antimatter atoms.

Based at CERN near Geneva, Switzerland, ALPHA is one of two major TRIUMF-CERN collaborations, the other being the ATLAS detector on CERN’s Large Hadron Collider.

ALPHA is using the simplest antimatter atom, antihydrogen, to explore some of the biggest questions in physics and cosmology, from the precise nature of antimatter to the origins of the universe and the integration of quantum mechanics and relativity. The experiment is the century-long culmination of a global physics effort to understand the fundamental nature of antimatter.

In a famous 1928 scientific paper, the British physicist Paul Dirac predicted the existence of an “anti-electron”: a particle having the same mass as an electron but opposite charge. The positron, discovered in 1932, was the first experimentally detected antimatter.

Yet the existence of antimatter presents a conundrum that goes to the heart of the Standard Model of physics.

The current view is that, at the Big Bang, matter and antimatter were created in equal quantities. When matter and antimatter interact, they annihilate into pure energy and this is believed to be what occurred to most Big Bang matter and antimatter.

However, due presumably to subtle differences between matter and antimatter, more matter survived this annihilation, leading to the cosmos we see. Current theories to explain the cosmic matter-antimatter imbalance are based on charge-parity (CP) symmetry violation, which occurs in some subatomic reactions. However, CP-symmetry violation is considered too small to fully explain the matter-antimatter asymmetry, pointing to the possible existence of other symmetry-breaking phenomenon, including potentially in charge-parity-time reversal (CPT) symmetry.

CPT symmetry is one of the most fundamental symmetries in nature and is deeply connected to the two foundational principles of modern physics, quantum mechanics and special relativity. According to CPT symmetry, the basic properties of hydrogen and antihydrogen are identical. Thus, any measured difference between the two could indicate a violation of CPT symmetry, which in turn would imply a violation in quantum and thus would have a radical impact on the basic underpinnings of our understanding of the Universe.

However, until ALPHA, it’s been impossible to experimentally test antimatter atoms’ deeper nature. Thus, in making the world’s first physical measurements of antimatter atoms, ALPHA scientists are searching for clues to the matter/antimatter asymmetry in the nature of antihydrogen, including, crucially, its spectroscopic fingerprint, interaction with laser and microwave electromagnetic radiation and how it interacts with gravity.

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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

TRIUMF leads, and is the portal for, Canadian scientific involvement in the ALPHA collaboration with CERN. Scientists from TRIUMF and four Canadian universities make-up about one-third of ALPHA’s scientific and technical team, and TRIUMF scientist Makoto Fujiwara, one of the project’s founding scientists, is spokesperson for Canada’s ALPHA contingent. ALPHA is a collaboration of approximately 50 scientists from 15 institutions in Canada, Brazil, Denmark, Israel, Japan, Sweden, the United Kingdom and the United States.

TRIUMF contributes a powerful combination of expertise in several aspects central to ALPHA’s success, including: extreme-cryogenics; detector design and construction; and microwave and laser spectroscopy. Specific TRIUMF contributions to ALPHA include:

• TRIUMF scientists developed the conceptual design of the original ALPHA Silicon Vertex Detector which was built by collaborators at the University of Liverpool. TRIUMF also built the Silicon Vertex Detector electronics and, together with physicists from York University, developed the critical antihydrogen detection analysis software.
• In partnership with scientists from the University of Calgary, the TRIUMF-ALPHA team designed and built the advanced cryostat used in the second-generation ALPHA-2, an upgraded and completely new trap introduced into ALPHA in 2014. It enables laser access for cooling and spectroscopy of antihydrogen. TRIUMF provided the lead engineer for the ALPHA-2 upgrade.
• Working with physicists from UBC and SFU, the TRIUMF-ALPHA team has played a central role in the design, construction, application and experimental analysis of the microwave and laser spectroscopy technologies used in ALPHA experiments. This includes original conceptual proposals for the various new measurements, execution of the experiments, and the analyses of the experimental data by developing sophisticated machine-learning based techniques.
• With colleagues at the University of British Columbia, TRIUMF-ALPHA is leading the project to perform laser cooling of antihydrogen. Laser cooling is a technique that enables the cooling of atoms to the lowest possible temperature currently achievable. Application of this technique in normal atoms has revolutionized the field of atomic physics. Laser cooling of the antihydrogen atoms will increase the precision of future measurements and create opportunities for new antimatter experiments.
• Several years ago, the TRIUMF-ALPHA team proposed a new experiment to measure antihydrogen’s interaction with gravity to determine if antimatter falls in the same way as matter. Since then TRIUMF-ALPHA has played a leading role in realizing the new ALPHA-g experiment. In addition to driving the physics program, the technical contributions include designing and building the ALPHA-g detector, electronics and software system, which was shipped to CERN in summer 2018. The ALPHA-g experiment is currently under construction and is expected to perform a first commissioning run in late 2018.

Since its first success in creating and briefly containing antihydrogen atoms, the ALPHA collaboration is turning science fiction into science fact in the control and measurement of antimatter. The ALPHA science discoveries are the result of a deliberate experimental program that’s designed to lay the groundwork, piece-by-piece, for the high-precision spectroscopic, and other measurements, of antihydrogen. All of the results highlighted here have been published in the prestigious Nature journals.

2010 The ALPHA collaboration reported its first major milestone, the successful trapping of antihydrogen. During a several-week period involving 335 separate runs of the experiment involving about 10 million antiprotons and 0.7 billion positrons, the ALPHA team identified a total of 38 antiproton annihilation events consistent with the release of trapped antihydrogen. Since then, ALPHA scientists have been able to trap more than 1000 antihydrogen atoms at a time.

2011 TRIUMF scientist Makoto Fujiwara led a Nature Physics paper reporting that the ALPHA collaboration had contained antihydrogen atoms for more than 1000 seconds, or about 16 minutes, a new world record. This demonstration of long-term confinement provided ALPHA researchers with the knowledge that they could effectively contain antihydrogen for long enough to make precise measurements.

2012 In an ALPA-Canada led experiment, the collaboration demonstrated the first microwave interactions with an anti-atom. The ALPHA team zapped antihydrogen with microwaves of a frequency known to energize hydrogen. This in turn flipped the antihydrogen’s magnetic orientation, kicking it from the trap, resulting in its annihilation and detection. The experiment demonstrated that the properties of antihydrogen, like those of hydrogen, can be changed by energizing them with microwaves at a specific wavelength. The experiment was led by ALPHA-Canada physicists Walter Hardy from the University of British Columbia and Michael Hayden from Simon Fraser University.

2014 The first measurement with the upgraded ALPHA-2 apparatus was to test the electric charge neutrality of antihydrogen atoms. Charge neutrality of matter, that sum of the charges of the proton and the electron equals zero, is one of the outstanding mysteries of modern physics. Studies of the neutrality of trapped antimatter were first proposed by ALPHA-TRIUMF scientists. The measurement showed that antihydrogen is neutral at a precision better than one part per billion, making this result one of the first precision measurements with antihydrogen atoms

2016 The ALPHA team reported a major achievement: the first laser spectroscopy measurement of antihydrogen. Spectroscopy is the study of the light emitted, or absorbed, by electrons (or positrons in case of anti-atoms) as they make quantum jumps between electron (positron) orbitals.

The transition of the hydrogen electron from its ground state, the 1s orbital, to its next level, the 2s orbital is one of the most precisely studied phenomena in nature. Physicists have pinpointed its frequency to 2466061413187035 +/- 10 Hz, or a precision of 4.2 parts in a quadrillion. Thus, the ALPHA team sought to measure the 1s-2s transition in antihydrogen. They shone a laser light with exactly hydrogen’s known 1s-2s transition frequency on trapped antihydrogen atoms. If antihydrogen had the same 1s-2s transition frequency, this light would be at a resonant frequency, causing the transition and also causing the trapped antihydrogen to be lost from the trap and annihilate. Then the researchers detuned the laser by 400 kHz and zapped another batch of antihydrogen. They discovered there were almost three times as many annihilation events with the resonant frequency as compared with the detuned one, leading them to conclude that antihydrogen’s 1s-2s transition frequency is fundamentally similar to that of hydrogen with a level of precision of two-parts-in-ten billion.

This was a key result in showing that it is both possible to interact with trapped antihydrogen with laser light, and that antihydrogen has spectroscopic characteristics similar, at least for this transition, with hydrogen.

2017 The ALPHA team extended its spectroscopic studies of antihydrogen to make the first measurements of its hyperfine structure, the first ever made with an antimatter atom. Hyperfine structure refers to small shifts and splittings in the quantum energy levels of electrons due to interaction between the state of the nucleus and the electrons. The energy changes are more than million times smaller than with gross structure transitions, such as the 1s-2s transition (see 2016, above). As with the measurement of hydrogen’s 1s-2s transition, hyperfine measurements of hydrogen have been a cornerstone of 20th century physics. The ALPHA team has now extended this to antihydrogen. Using ALPHA-Canada-developed microwave technique, the team measured two spectral lines for antihydrogen and observed no difference compared to the equivalent spectral lines for hydrogen, within experimental limits.

2018 A 100-fold improvement in the precision of the 1s-2s laser spectroscopy of antihydrogen was achieved. By studying detailed spectral features of the 1s-2s transition in detail, the ALPHA team was able to determine its frequency to two parts in trillion. This is the most precise measurement ever performed on any antimatter systems. At this level of precision, the measurement has an impact on determination of the fundamental constants such as the Rydberg constant.

2018 In the latest series of measurements, the ALPHA collaboration succeeded in driving the so-called Lyman-alpha (1s-2p) transition in antihydrogen. The observation of the Lyman-alpha transition in the hydrogen by Theodore Lyman just over 100 years ago helped established quantum mechanics. However, observing the transition in antihydrogen has been challenging, due among other things to the technical difficulties with producing a special laser system. Recently, the team led by UBC-TRIUMF researchers succeeded developing the Lyman-alpha laser to drive the transition. This will open up many new possibilities in antihydrogen studies, including the measurement of the so-call Lamb shift in antihydrogen and the laser cooling of antihydrogen to ultracold temperatures.