Physicists from Yale University and Duke University have uncovered that many particles analyzed in particle accelerators, which were thought to reflect the conditions of the early universe, actually originate from subsequent reactions rather than the initial primordial "soup" of matter. This discovery, detailed in the journal Physics Letters B, challenges earlier assumptions regarding the early universe's composition and underscores the complexities involved in studying its origins.
The artist’s depiction of the spray of particles arising from the collision of two heavy atoms. As the hot subatomic soup cools, newly formed particles shower off into space. Image Credit: Joseph Dominicus Lap
The Science
The early universe was 250,000 times hotter than the core of the sun, a temperature far beyond what is needed to form protons and neutrons, which are the building blocks of matter. To study these extreme conditions, scientists use particle accelerators to collide atoms at nearly the speed of light. By analyzing the resulting burst of particles, researchers can gain insights into the formation of matter.
These particles can originate from the primordial quark-gluon plasma or from subsequent interactions. These later interactions began just one microsecond after the Big Bang when quark-based composite particles started to interact. Recent calculations reveal that up to 70 % of some observed particles result from these subsequent reactions rather than from those that occurred during the earliest moments of the universe.
The Impact
This finding enhances our understanding of the origins of matter by clarifying how much of the matter in the universe was created in the initial moments after the Big Bang compared to how much resulted from later reactions as the universe expanded.
The discovery suggests that a significant portion of the matter around us formed later than previously anticipated. To accurately interpret collider experiments, scientists need to account for particles originating from these later reactions. Only particles formed during the primordial quark-gluon plasma provide insights into the universe's early conditions. The new calculations indicate that the proportion of particles resulting from these later reactions is much higher than previously thought.
Summary
In the 1990s, physicists discovered that certain particles become abundant as a result of subsequent reactions that follow the initial formation of the universe. Among these particles are D mesons, which can interact to create a rare particle known as charmonium.
There was initially little agreement on the significance of this phenomenon. Due to the rarity of charmonium, measuring it proved challenging. However, recent experiments have provided data on the production rates of charmonium and D mesons in particle colliders.
Researchers from Yale and Duke universities utilized this data to reassess the impact of these interactions, revealing that the effect is far more substantial than previously anticipated. Over 70 % of the charmonium detected may arise from such reactions.
As the primordial soup of subatomic particles cools, it expands rapidly, creating a fiery ball. This process occurs in less than a hundredth of the time it takes for light to travel across an atom. Given the speed of this expansion, scientists have struggled to fully understand its details.
The new calculations suggest that precise knowledge of this expansion process may not be necessary for understanding the production of charmonium. The collisions in question produce a considerable amount of charmonium regardless of the specifics of the fireball's expansion. This advancement brings scientists closer to uncovering the fundamental origins of matter.
Journal Reference:
Lap, D. J., et al. (2023) Hadronic J/ψ regeneration in Pb+Pb collisions. Physics Letters B. doi.org/10.1016/j.physletb.2023.138246.