CERN observes melted nuclear matter for the first time

November 29th, 2010

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This is a picture of lead-lead collision measured in the ATLAS detector at CERN. The collision contains two powerful "jets" of particles. The new result of ATLAS shows that these can "melt" at the temperatures achieved in the lead collisions of the LHC. Credit: CERN

The experiments at CERN have entered a new phase and have obtained the first results. In the giant particle accelerator, LHC, collisions have until now been with protons. The new experiments have been made with lead ions, 208 times heavier than a proton, that are collided at colossal energies. Danish scientists from the Niels Bohr Institute take part in the ATLAS experiment, where new results show, for the first time that the constituents of nuclei can melt. This shows how we think the world looked around a billionth of a second after the Big Bang. The results are published in Physical Review Letter.

The ATLAS experiment at the LHC is built to explore the fundamental nature of the universe. This exploration is carried out in two ways. One way is using lead ion collisions where the properties of nuclear matter are are studied at extreme temperatures. The other is to use proton collisions, which permit detailed studies, not only of matter, but also also directly probe the fundamental forces of Nature, which govern the evolution of the universe.

Since Thursday November 4th CERN has been circulating bunches of lead ions in the 27 km (17 miles) underground LHC accelerator. Lead is a heavy element. The nucleus contains 208 particles of two varieties: protons and neutrons. The lead ions are accelerated to very high energies in the LHC accelerator.

Scientists of the Discovery center at the Niels Bohr Institute, University of Copenhagen have taken an active part in the build-up of the ATLAS experiment right from the beginning, 20 years ago. The experiment is now for the first time observing directly the effect on ordinary nucleons such as protons and neutrons when they traverse the hot primordial soup of the collisions.

At the first collisions that took place at 11.30 on Monday November 8th 2010, the combined energy of the collision reached 2750 GeV *208 (1 GeV= 1 billion electron Volt), 15 times the highest energy so far achieved anywhere.

"What we see is a collision between two lead ions in the ATLAS detector. We se two powerful 'jets', splashes of particles emitted into the detector at the moment of collision", Rasmus Mackeprang explains. As a scientist at the Niels Bohr Institute he can follow the experiments at CERN and immediately see when interesting results come out.

The protons and neutrons are 'dissolved' into their smaller constituents: quarks and gluons that fly away from the collision region. They will each form a tight 'shower' of new particles (such as protons and neutrons) that hit the detector at high energy. This behavior is well known and has been observed in different contexts since the 1970's.

What is new is, that at this picture ceases to be valid at the temperatures achievable at the LHC. Measurements show for the first time how some collisions are 'missing' jets that should be there. This means that only one jet from the collision survives and can be observed while the other one melts on its way through the collision region.

"This is the first time we directly observe the nuclei melt and dissolve into the state you might call the primordial soup of the universe" a very happy physicist, Rasmus Mackeprang explains. He adds that this feels as if being on a journey through time to the beginning of the universe.

Provided by University of Copenhagen