Computer model of heavy-ion collisions inside the ALICE experiment.CERN/LHC
After colliding lead ions at close to the speed of light, physicists at the Large Hadron Collider (LHC) using the ALICE detector have discovered the Universe acted like a fluid in the moments immediately after the Big Bang. Also, the ATLAS and CMS detectors have observed a phenomenon known as “jet quenching” for the first time.
Until recently, the LHC only accelerated protons and collided them inside the particle accelerator mainly to search for the infamous Higgs boson and other exotic particles. But earlier this month, heavier lead ions were injected into the LHC. This is when the quantum party really got started.
For three weeks, lead ions have raced around the accelerator ring at relativistic speeds, crashing head-on with other lead ions traveling in the opposite direction.
Lead ions are significantly bigger than protons, so they carry more energy. When they collide, they release so much energy that physicists often refer to the lead-lead collisions as “micro-Big Bangs.”
Each ion collision can, quite literally, recreate the conditions just after the Big Bang, some 13.75 billion years ago.
For a brief moment, these mini-Big Bangs flashed up to an estimated temperature of 10 trillion degrees Kelvin (that’s more than 500,000 times hotter than the center of the sun), giving the ALICE detector a peek into how matter would have acted right at the Universe’s superheated birth.
Quarks and Gluons, That’s All
It is already known that high-energy collisions in particle accelerators can produce a strange, primordial state of matter. A “quark-gluon plasma” can be created if the collisions are energetic enough, a state of matter that existed during the high-energy conditions just after the Big Bang.
During this time, the Universe would have been so hot and energetic that the particles making up the elements we know today were unable to form, leaving the constituents to float “free” as a primordial soup.
Quarks and gluons were only able to condense into larger particles when universal energy conditions were low enough. Hadrons (i.e. particles made from quarks; including baryons like neutrons and protons) were only allowed to form 10-6 seconds after the Big Bang.
But what was the nature of this quark-gluon primordial soup? Was it a gas or a liquid? What was the Universe actually like 10-6 seconds after the Big Bang (apart from being really, really hot)?
The ALICE experiment has confirmed the quark-gluon plasma is a ultra-low viscosity liquid at these energies. From this finding, physicists now know the newborn Universe acted as a perfect fluid.
Although the U.S. Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has already carried out similar experiments to arrive at similar conclusions, the LHC has done it at much higher energies.
This result has surprised many scientists who predicted the LHC would generate a plasma that acted more like a gas than a liquid, but the results recently published show strong interactions within this primordial soup that resemble a perfect fluid.
The “Big Bang Machine”
Although it’s only been three weeks, the lead-lead collision experiments in the LHC have already ruled out some theories about how the early Universe behaved.
“With nuclear collisions, the LHC has become a fantastic ‘Big Bang’ machine,” said ALICE spokesperson Jürgen Schukraft. “In some respects, the quark-gluon matter looks familiar, still the ideal liquid seen at RHIC, but we’re also starting to see glimpses of something new.”
Other observations in the CMS and ATLAS detectors have provided a fascinating look at how this primordial matter interacts with itself.
Immediately after lead ions collided, jets were created by the quarks and gluons blasting away from the micro-Big Bangs. By monitoring how these jets formed, physicists were able to see how the intensely chaotic turmoil evolved.
During proton-proton collisions, these jets are very basic and often form in pairs. In ion-ion collisions, many more particles are generated, producing a huge number of jets. As they tangle together, jets lose energy through interactions scientists are only just beginning to understand. This loss of energy is known as “jet quenching.”
I think we are only just witnessing the tip of the iceberg insofar as LHC discoveries, but as we collide particles at higher and higher energies, we peel back the history of the Universe one microsecond at a time.
Image: The aftermath of a lead ion collision, paths of newborn particles tracked by the ALICE detector (CERN/ALICE)