The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has recently shed light on the extreme conditions of the early universe by colliding particles at near-light speeds, effectively simulating the environment just moments after the Big Bang. Researchers have been studying the formation of quark-gluon plasma (QGP) – a state of matter believed to have existed shortly after the universe came into existence.
A study published in Nature Communications details the analysis of data collected from the STAR detector, which has measured the temperature of the QGP during various stages of gold ion collisions. As gold nuclei collide, they turn into a high-energy state, allowing scientists to observe how quarks and gluons come together to form familiar protons and neutrons as the medium cools.
For the first time, the temperature of the QGP was directly measured using pairs of electrons and their corresponding antiparticles, known as positrons. This method has proven crucial for understanding the dynamics of the QGP, with temperatures peaking at about 3.3 trillion degrees Celsius – roughly 220,000 times hotter than the center of the sun. As the QGP cooled, measurements indicated that temperatures before transitioning into conventional nuclear matter were recorded at half of this peak value.
The STAR collaboration employed a unique technique to monitor the emissions of electron-positron pairs, known as dileptons, which occur infrequently during particle collisions. Identifying these rare events is challenging, but they provide a precise 'thermometer' for measuring QGP temperature due to their insensitivity to background variations that can affect results.
Significant advancements have been made, including pinpointing the phase transitions that occur as the QGP cools. The findings reveal that all measured temperatures at various collision energies align with the expected transition point, an encouraging result for the research team.
While previous studies at RHIC and CERN's Large Hadron Collider have utilized photon emissions to ascertain QGP temperatures, the new dilepton approach offers a clearer perspective. Unlike photons, dileptons maintain their invariant mass even as the medium expands, providing a steady benchmark for temperature measurements throughout different stages of the collisions.
Recent trial data from two energy settings of 27 and 54.5 GeV has set the groundwork for future analyses at higher energy levels of 200 GeV. By comparing findings with similar data from other nuclear physics institutions, the RHIC team aims to explore the conditions that mirror those of the universe’s infancy, enhancing the understanding of the fundamental forces governing matter.
Ongoing research at facilities like RHIC, supported by the Department of Energy and various scientific organizations, continues to seek answers to questions about the early universe and the fundamental nature of matter. As new measurements and methodologies develop, physicists are poised to delve further into the complexities of the QGP and its implications for our understanding of cosmic history.