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.