Scientists at the Relativistic Heavy Ion Collider (RHIC) have captured new evidence of how the quark-gluon plasma – an exotic state of matter thought to have existed just after the Big Bang – responds when struck by jets of energetic particles.
Using precision photon-jet measurements, researchers observed a dramatic “sideways splash” effect inside this primordial soup, offering a deeper glimpse into one of the most extreme substances in the Universe.
These findings not only unlock new clues about the early cosmos but also challenge long-held assumptions about the plasma’s perfect fluidity and energy dynamics.
What is quark-gluon plasma?
The quark-gluon plasma (QGP) is the ultra-hot, dense medium that filled the Universe just after the Big Bang.
At that moment, fundamental particles like quarks and gluons – normally bound inside protons and neutrons – roamed freely in a soupy, frictionless fluid. Understanding this state of matter is crucial to uncovering how the visible Universe, including atoms and galaxies, eventually formed.
To recreate and study the QGP, researchers use RHIC, a powerful particle accelerator at the U.S. Department of Energy’s Brookhaven National Laboratory.
By smashing together gold nuclei at near-light speed, they briefly melt the atomic components into a quark-gluon plasma that lasts less than a trillionth of a trillionth of a second.
Reconstructing jets: A new window into the plasma
The new RHIC data focuses on high-energy jets – cascades of particles that emerge from violent collisions – and their interaction with the QGP.
Until now, most studies centred on “jet quenching,” where jets appear weakened or scattered after travelling through the plasma. However, these studies often examined only the highest-energy particles, offering an incomplete picture.
In this new analysis by RHIC’s STAR Collaboration, scientists have, for the first time, reconstructed entire jets that appear back-to-back with photons – particles of light. Because photons do not interact with the quark-gluon plasma, they serve as a precise reference for the jet’s original energy.
By comparing the energy and spread of these jets with and without the presence of QGP, scientists can effectively use the jet as a probe to illuminate the characteristics of the plasma – similar to how X-rays reveal structures inside the human body.
Splashes and sideways energy: A broader cone of discovery
One of the most groundbreaking revelations from the study is the observation of how energy disperses when a jet interacts with the QGP.
By reconstructing jets within various angular cones – from narrow to wide – scientists found that the energy was more spread out in collisions involving QGP. This suggests that as jets pass through the plasma, they shed energy not directly forward but sideways.
The sideways distribution, often referred to as the splash effect, resembles how a bicycle wheel splashes water when hitting a puddle. The jet loses energy by exciting the surrounding plasma, which in turn creates a broad wave of particles detectable in a wider angular field.
This evidence clearly shows that the so-called “missing” energy isn’t lost but redistributed by interactions within the plasma.
The role of direct photons and advanced algorithms
Detecting this subtle sideways splash required sophisticated data analysis and precise particle tracking.
The key was identifying “direct photons” – those emitted directly from the collision event, as opposed to those generated by later decay processes. These photons act like timestamps, emerging at the same moment and energy level as their partner jet.
Using a finely tuned photon-identification algorithm and advanced statistical techniques, the team was able to isolate these direct photon events.
From there, they reconstructed associated jets and analysed how they behaved differently in collisions that produced the quark-gluon plasma compared to those that didn’t, such as simpler proton-proton interactions.
Implications for the nature of the QGP
These findings are not just about better detection – they offer profound implications for our understanding of QGP’s physical properties.
Notably, the discovery that most of the lost jet energy can be recovered within a 30-degree cone implies a limit to how far the plasma is disturbed by the jet, shedding light on the viscosity and density of the QGP.
QGP is often described as a nearly perfect fluid with almost no resistance to flow. These new observations support that description and may help define more precise values for its viscosity, one of the most important characteristics in fluid dynamics.
Moreover, understanding how energy loss depends on the jet’s path through the plasma opens the door to mapping the QGP in greater detail. Scientists aim to determine whether the jet’s travel distance or interaction strength plays a bigger role in energy dissipation.
By continuing to refine techniques for jet and photon tracking, researchers are turning the RHIC into a time machine of sorts – probing conditions from the first microseconds of the Universe.
As researchers push the boundaries of high-energy physics, quark-gluon plasma remains one of the most exciting frontiers – a fleeting but revealing relic of the Universe’s fiery birth.
