Advancements in comprehending the flighty, transient positronium particle have been made by physicist teams at CERN and the University of Tokyo.
The breakthrough now allows for deeper studies on antimatter, substances with subatomic particles exhibiting opposing electrical properties to those in ordinary matter.
Positronium: Unique Structure, Laser Cooling Breakthroughs
Positronium constitutes a unique system, composed of an electron and its antiparticle, a positron, forming an "exotic atom." Its orbital dynamics and energy levels closely resemble those of a hydrogen atom, yet due to the lower reduced mass, the associated spectral line frequencies are less than half of hydrogen's.
The ground state of positronium, akin to hydrogen, exhibits two potential configurations based on the relative spins of the electron and positron. Predictions regarding positronium date back to Stjepan Mohorovičić in 1934, who referred to it as "electrum." Carl Anderson is also credited with predicting its existence in 1932, with Martin Deutsch experimentally discovering it in 1951, naming it positronium.
Positronium holds the title of the lightest known particle system, marked by extreme instability. The collision of matter and antimatter leads to mutual annihilation, generating a flash of radiation. Positronium self-annihilates in a mere 142 billionths of a second, emitting gamma rays.
However, experimental studies involving positronium encounter challenges due to its rapid and varied velocities. Cooling positronium emerges as a solution, slowing its particles for more precise property measurements. Implementing this is no easy feat, as common methods involve time-consuming processes that alter velocity distributions.
Laser cooling, proposed in 1988, is a temperature reduction method where particles absorb and emit photons. Laser light, tuned to the particle's energy level, causes it to absorb and re-emit photons, altering its momentum and slowing it down. This breakthrough holds promise for enhanced studies on positronium, enabling accurate measurements of its properties.
Advancements in Positronium Studies by AEgIS and University of Tokyo Teams
The AEgIS collaboration successfully utilized a laser tailored to meet experimental specifications, achieving a temperature reduction of over half in a cloud of positronium. This exotic particle assembly comprises an electron and its antiparticle, the positron.
Simultaneously, physicist Kenji Shu's team at the High Energy Accelerator Research Organization in Japan, affiliated with the University of Tokyo, employed chirp cooling techniques to decrease the temperature of their positronium cloud to approximately one Kelvin (-272 °C), significantly slowing electron and positron velocities.
Both teams utilized distinct laser cooling methods to reduce velocity distributions and lower temperatures. AEgIS employed broadband laser cooling, targeting a broad velocity spectrum, achieving a temperature reduction from 380 Kelvin to 170 Kelvin (106 °C to -103 °C).
Meanwhile, Shu's team utilized chirp cooling, adjusting the laser to match particle deceleration, narrowing the distribution of particle movements in their sample.
The study of antimatter is crucial, aiming to unravel the mystery of its disproportionate distribution compared to matter in the universe's formation. Investigating whether antimatter behaves similarly to matter provides insights into this cosmic puzzle.
Physicists also aspire to create a Bose-Einstein condensate of positronium, a high-density particle cloud at just above absolute zero, potentially generating coherent gamma-ray light for revealing atomic nucleus structures.
The AEgIS collaboration's work, documented in Physical Review Letters, opens avenues for fundamental and applied research, particularly in utilizing a positronium Bose-Einstein condensate to produce coherent gamma-ray light, facilitating exploration into the finest structures of the universe's atomic composition.
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