CERN creates mini big bangs
On November 2010, the Large Hadron Collider (LHC), operated by CERN under the French-Swiss border near Geneva, successfully created an initial series of mini big bangs by smashing together lead ions.
First collisions of lead ions
First collisions of lead ions seen by the ALICE experiment on 7 November 2010 recorded by the Inner Tracking System. The lines are the reconstructed particle trajectories, and the colour scale indicates the energy of the particles.
The collisions produce atomic-scale dense fireballs and result in almost unimaginable temperatures of about 10 trillion °C – a million times hotter than the centre of the Sun. In the extreme conditions, the colliding ions break down into the quarks and gluons that make up the protons and neutrons of the nuclei in a mixture known as quark-gluon plasma.
In a press release from Geneva on 8 November 2010, CERN (Conseil Européen pour la Recherche Nucléaire, or European Council for Nuclear Research) wrote that the transition from routinely colliding protons in the 27 km-round LHC to colliding the comparatively heavy lead ions (lead atoms with electrons removed) was achieved in 4 days.
“The speed of the transition to lead ions is a sign of the maturity of the LHC,” says CERN Director General Rolf Heuer. “The machine is running like clockwork after just a few months of routine operation.”
Operating the LHC with lead ions is similar to operating the machine with protons. For lead ions, as for protons before them, the procedure started with threading a single bunch of ions round the ring in one direction before repeating the process for the other direction. Once circulating bunches had been established, they could be accelerated to the full energy of 2.76TeV per nucleon for each lead ion in the beam bunches. This energy is 14 times higher than ever achieved for heavy ion collisions.
This total energy per collision is also much higher than for proton beams because lead ions contain 82 protons and 126 neutrons. When the beams were lined up for collision, 208 protons and neutrons were smashing into 208 other protons and neutrons heading in the opposite direction.
As the lead collision data was accumulated, the number of circulating beam bunches was increased to 113 bunches in each direction. By the end of the lead ion collision runs on 6 December 2010, the CMS detector had accumulated approximately 60 million collisions. The LHC at CERN is now shut down for a scheduled ‘technical stop’ of a couple of months over the Northern Hemisphere winter.
CMS experiments
There were 3 experiments recording data from these lead ion collisions. These are known as ALICE, ATLAS and CMS.
A team of New Zealand physicists, led by nuclear physicist Dr David Krofcheck at Auckland University, is involved in the Compact Muon Solenoid (CMS) experiments.
Z⁰ particles found in heavy ion collisions
Z0 particles interact only weakly with quarks in nuclear matter, so they act as ‘standard candles’ that escape unscathed from the hot nuclear matter produced in lead ion collisions.
The Compact Muon Solenoid (CMS) is a particle physics detector built on the LHC. It detects a wide range of particles and phenomena produced in high-energy proton-proton and heavy ion collisions. Approximately 3,600 people from 183 scientific institutes representing 38 countries form the CMS collaboration that built and now operates the detector. It is located in an underground cavern at Cessy in France, just across the border from Geneva.
The CMS group hopes to learn more about the origin of mass, extra dimensions of space, the reasons for the imbalance of matter and antimatter observed in the Universe, and dark matter.
These are important unanswered problems in particle and nuclear physics. We are hoping the LHC-based experiments will answer our questions about the fundamental properties of matter.
Dr David Krofcheck
The New Zealand CMS group has been involved in studying proton collisions from the LHC, and they are now examining the data from the recent collisions between counter-rotating beams of heavy lead nuclei.
In particular, they hope to learn more about the plasma the Universe was made of a millionth of a second after the Big Bang, 13.7 billion years ago, and to better understand ‘strong force’ – the force that binds the nuclei of atoms together or, in other words, sticks all the quarks together into larger objects such as protons and neutrons.
The University of Auckland's specific role has been to develop and perform tests of the Beam Radiation Monitoring detectors and to develop analysis programmes for proton-proton and heavy ion reactions. Also, a CMS Data Grid computing site is in full operation using KAREN – the Kiwi Advanced Research and Education Network at the University campus.
Understanding quark-gluon plasma
“208 protons and neutrons colliding with 208 other protons and neutrons will give you a lot of quarks colliding at the same time, which should allow us to see the ‘herd behaviour’ of the quarks. I’m very interested in this collective motion of quarks and the collective flow of quark-gluon plasma as it tells us about the nature of a very strong nuclear force,” says Dr Krofcheck.
One new result from the CMS experiment is the first observation of Zo particles ever found in heavy ion collisions. These particles interact only weakly with quarks in nuclear matter, so they act as ‘standard candles’ that escape unscathed from the hot nuclear matter produced in lead ion collisions.
‘Jet quenching’ effects
Studying ‘jet quenching’ can better explain how dense the quark-gluon plasma may be and how particles lose their energy while traversing the plasma of the early Universe.
The CMS and ATLAS detector data has also revealed unprecedented ‘jet quenching’ effects in lead ion collisions. These are collisions in which 2 high energy sprays of particles leave the collision in exactly opposite directions. In some cases, one of these sprays of particles, or ‘jets’, loses a tremendous amount of energy to the quark-gluon plasma. “By studying jet quenching, we can better understand how dense the quark-gluon plasma may be and how particles lose their energy while traversing the plasma of the early Universe,” says Dr. Krofcheck.
Early results, measured by ALICE, show that about 18,000 particles are produced from head-on collisions of lead ions. When the collisions are not head-on (rather more like glancing blows), the physicists have noted the quark-gluon plasma created flows like a liquid rather than a gas as some had predicted.
Activity ideas
This article is about the Large Hadron Collider and Dr David Krofcheck’s related research. Your students may be interested in watching these video clips below in which David discusses what his research involves and why he finds it so fascinating.
