9 November 2010

CERN collides heavy ions at record energies

The experiments at CERN have entered a new phase and first results pouring in. Up until this point CERN has been colliding protons in the giant particle accelerator, LHC. In the new round of experiments lead ions, 208 times heavier than protons, are being smashed together with tremendous energy. Already in the first collisions researchers see exciting new phenomena with the ALICE detector, an experiment that Danish researchers from the Niels Bohr Institute and the Discovery Center are centrally involved in.

The figure shows one of the first collisions between
two lead ions in the Large Hadron Collider (LHC)
at CERN, identified in the ALICE detector Monday,
November 8, at 11:29.

The experiments were initiated Thursday with the injection of lead ions into the accelerator and subsequent circulation of bunches of lead ions in the 27 km long subterranean LHC accelerator.  Lead is a heavy element whose nucleus contains 208 of two types of particles, protons and neutrons. The lead ions are accelerated to high kinetic energy in the LHC accelerator. In the first collisions, which took place at 11:30AM on Monday, November 8, 2010, the total collision energy was 2750 GeV *208 (1 GeV= 1 Billion electron volts), almost 15 times higher than previously possible.  

The forerunner of the LHC, the RHIC accelerator at Brookhaven National Laboratory in New York state, USA, makes it possible to collide gold ions (197Au) at an energy of 200 GeV *197. The Danish group from the Niels Bohr Institute has also had a leading role in the BRAHMS experiment at RHIC. The RHIC experiments have shown that, in the collisions, a state consisting of quarks and gluons (the fundamental particles that neutrons and protons are made up of) is formed, in exact replication of the state the universe in the very first microseconds after the Big Bang. In the collisions, a ’plasma’ of quarks and gluons was formed with properties of a perfect liquid – a kind of ’primordial soup’, as it will have looked instants before quarks and gluons became trapped forever inside the protons and neutrons.

The figure shows the tracks measured in the ALICE detector at LHC at CERN from a lead-lead collision Monday morning.
It has already been established that several thousand new
particles and antiparticles are formed in the collisions, which can be seen directly from the number of tracks with positive
and negative curvature in ALICE’s magnetic field.

"The huge jump in energy we are making at LHC opens up a whole new research horizon and allows us, under Earthly conditions, to create a state at much higher temperature than was possible at RHIC and thus to  recreate  a state  the Universe was in at at  a much earlier time after the Big Bang. Before the fluid-like state we believe that there was a state which was rather like a gas and in which quarks and gluons hardly interacted, a state known as ‘asymptotic freedom’. In the LHC we will create and study the quark-gluon plasma copiously and under controlled conditions, thereby significantly increasing our understanding of the strong nuclear force, and in particular, finding out how the confinement of quarks takes place,” explains Jens Jørgen Gaardhøje, professor in experimental particle physics at the Niels Bohr Institute and the Danish National Research Foundation center Discovery, and project leader of the ALICE experiment at LHC in Switzerland.

In the first collision between lead ions, on Monday morning, many thousands of particles and antiparticles were formed in each collision. The ALICE experiment is designed to be able to handle up to 80,000 new particles per collision.  The experimental teams  are already working intensely on  interpreting the first measurements. The experiments with lead-on-lead  collisions will continue until December 6.

”We are looking greatly forward to a very interesting measuring program in the coming month and especially in  the years  years, with even higher energies and intensities. Experience tells us that new and unexpected phenomena normally turn up when you can increase the energy by a factor of 10 or more,” says Jens Jørgen Gaardhøje.

A central collision between two lead ions (208Pb) with a total kinetic energy of approximately
500 TEV measured in ALICE.  This is the highest collision energy ever measured. The left side
shows the overall event. The right side shows a narrow slice through the detector. The number of
tracks indicates that this event produced approximately 10,000 particles and antiparticles.