CERN Experiments Detect Signs of the Universe’s Primordial Matter

All four major LHC experiments have found new evidence that collisions between oxygen and neon may produce the extreme state of matter that existed during the first microseconds after the Big Bang.

Inside the Large Hadron Collider (LHC), collisions between relatively light oxygen and neon nuclei may be producing matter from the earliest moments of the Universe. One year after the collider’s first oxygen runs, all four major LHC experiments, ALICE, ATLAS, CMS, and LHCb, have reported signs of quark–gluon plasma (QGP).

QGP forms under immense pressure at temperatures more than 100,000 times hotter than the center of the Sun. In these conditions, composite particles break apart into quarks and the gluons that normally bind them together. This state of matter is thought to have filled the Universe during the first millionths of a second after the Big Bang. Nearly 14 billion years later, physicists can briefly recreate it through high-energy nuclear collisions at the LHC.

Light ions overturn an old assumption

Physicists once believed that QGP could form only when very heavy ions such as lead collided. A lead ion is more than 200 times heavier than the protons routinely used at the LHC, making lead collisions an obvious way to generate the necessary heat and pressure.

Recent evidence has steadily weakened that assumption. Earlier this year, the ALICE Collaboration, which specializes in investigating QGP, reported another sign of the plasma in proton–proton and proton–lead collisions. Last year, the four LHC experiments also found the first indications that oxygen–oxygen collisions might create it.

After examining the data more closely, the experiments have now identified several signals consistent with QGP formation in both oxygen–oxygen and neon–neon collisions.

Energy loss reveals a dense medium

One important clue comes from what happens to fast-moving quarks and gluons as they travel through the hot, dense plasma. They lose energy during the journey, an effect known as parton energy loss.

The ATLAS Collaboration detected this effect through an imbalance between pairs of particle jets produced in oxygen–oxygen and neon–neon collisions. The imbalance grew stronger in more head-on (central) collisions, where the greater volume of QGP would be expected to drain more energy from the particles passing through it.

Preliminary ATLAS measurements of charged particles recoiling from photons showed the same relationship with collision centrality. That pattern is also consistent with particles losing energy while crossing the plasma.

Particle suppression strengthens the case

ALICE, CMS and LHCb searched for parton energy loss in another way by measuring whether the collisions produced fewer energetic particles than expected.

CMS observed suppressed charged-particle production in oxygen–oxygen and neon–neon collisions compared with proton–proton collisions. This reduction points to parton energy loss and supports the presence of QGP in collisions involving light ions.

In another study, LHCb examined particles made from one charm quark and one light quark. These particles were more strongly suppressed in neon–neon collisions than in oxygen–oxygen collisions. That difference matches expectations for parton energy loss because the heavier neon system should create a larger volume of QGP.

Other processes can also reduce particle production, so ALICE performed a separate comparison to isolate the effect. The collaboration measured neutral pions in oxygen–oxygen and proton–oxygen collisions and found unambiguous evidence of parton energy loss in the oxygen–oxygen data.

Independent signals point to QGP

Researchers also looked for changes in short-lived particles made from a heavy quark and its antiquark. These pairs can bind together with different strengths, and QGP should suppress the more weakly bound states more readily than the stronger ones.

CMS found evidence of this pattern among upsilon mesons, particles containing a bottom quark and its antiquark, by comparing oxygen–oxygen with neon–neon collisions. LHCb reported preliminary evidence for the same type of suppression using proton–oxygen and oxygen–oxygen data.

ALICE has also released preliminary findings involving the directions in which particles emerged from oxygen–oxygen collisions. Particles made of three quarks (baryons) showed stronger directional emission than particles made of two quarks (mesons), a pattern called anisotropic flow.

The leading explanation is that QGP transfers its collective motion to particles created in the collision at intermediate momenta. Because baryons contain one more quark than mesons, they can inherit a greater share of that flow.

Researchers are continuing to search the light-ion collision data for additional evidence of QGP. At the same time, the LHC is being upgraded into the more powerful High-Luminosity LHC, which will allow physicists to examine this extreme state of matter in even greater detail.

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