A new perspective of the STAR detector at RHIC, seen through crystal ball refraction photography. The photo was a finalist for the Photowalk of the Brookhaven National Laboratory in 2018. Source: Joe Caggiano

The leading theory of the beginning of the universe is the Big Bang, which states that the universe existed as a singularity 14 billion years ago, a one-dimensional point with a large number of fundamental particles. Extremely high heat and energy caused it to bloat and then expand into the cosmos as we know it – and the expansion continues today.

The first result of the Big Bang was an intense hot and energetic liquid that lasted for only microseconds and was about 10 billion degrees Fahrenheit (5.5 billion Celsius). This liquid contained nothing less than the building blocks of all matter. As the universe cooled, the particles disintegrated or united, which led to … well, everything.

Quark-Gluon-Plasma (QGP) is the name for this mysterious substance, which is so named because it consists of quarks – the fundamental particles – and gluons, which the physicist Rosi J. Reed describes as "what quarks talk to each other" ,

Scientists like Reed, an assistant professor at Lehigh University's Institute of Physics, whose research includes experimental high-energy physics, can not go back to study how the universe began. Thus, they re-create the circumstances by colliding heavy ions like gold at near-light speed, creating an environment 100,000 times hotter than the sun's interior. The collision mimics how the quark-gluon plasma became matter after the Big Bang, but the other way round: The heat melts the protons and neutrons of the ions, releasing the quarks and gluons hidden inside.

Currently, there are only two accelerators worldwide that are capable of colliding heavy ions – and only one in the USA: the Relativistic Heavy Ion Collider (RHIC) of the Brookhaven National Lab. It is about three hours drive from Lehigh in Long Island, New York.

Reed is part of the STAR Collaboration, an international group of scientists and engineers who are conducting experiments on the Solenoidal Tracker by RHIC (STAR). The STAR detector is massive and actually consists of many detectors. It's the size of a house and weighs 1,200 tons. STAR's specialty is the tracking of the thousands of particles that are generated by each ion collision at RHIC in search of signatures of quark-gluon plasma.

"When you do experiments, there are two" buttons "that we can change: the species – like gold on gold or proton on proton – and the collision energy," says Reed. "We can accelerate the ions differently to achieve a different energy-to-mass ratio."

With the various STAR detectors, the team collides ions at different collision energies. The goal is to map the quark-gluon plasma tracing diagram or the various transition points as the material changes under different pressure and temperature conditions. The mapping of the Phark-Gluon plasma phase diagram also maps the strong nuclear force, also known as Quantum Chromodynamics (QCD). This is the force that holds positively charged protons together.

The photo was a winner of the 2018 Photowalk of the Brookhaven National Laboratory. Picture credits: Steven Schreiber

"At the center of an ion, there are a number of protons and neutrons," explains Reed. "These are positively charged and should repel, but there is a 'strong force' that holds them together – strong enough to overcome their tendency to break up."

The understanding of the quark-gluon plasma tracing diagram and the location and existence of the phase transition between plasma and normal matter are fundamental, says Reed.

"It's a unique opportunity to experience how one of the four fundamental forces of nature works at temperature and energy densities similar to those that existed just seconds after the Big Bang," says Reed.

Upgrade of the RHIC detectors for better representation of the "strong force"

The STAR team uses a Beam Energy Scan (BES) to perform the assignment of phase transitions. During the first part of the project, known as BES-I, the team collected observable evidence with "intriguing results." Reed presented these results at the 5th Joint Meeting of the APS Department of Nuclear Physics and the Physical Society of Japan in October 2018 in Hawaii in a paper titled: "Testing Quark Gluon Plasma Thresholds with Energy and Arsenal Scans." RHIC. "

However, limited statistics, acceptance, and insufficient resolution of the event level did not allow any clear conclusions for a discovery. The second phase of the project, known as BES-II, is under way and includes an enhancement to Reed working with STAR team members: an upgrade to the Event Plan Detector. Collaborators include scientists in Brookhaven and Ohio State University.

The STAR team plans to conduct further experiments and collect data with the new 2019 and 2020 Event Plan detectors. According to Reed, the new detector will determine exactly where the collision takes place and will help to characterize the collision.

"It will also help improve the measurement capabilities of all other detectors," says Reed.

The STAR collaboration is expected to conduct its next experiments at RHIC in March 2019.

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Lehigh University