The results of the three experiments conducted by the LHC till the end of last year, during which lead ions were collided, was released by CERN yesterday, i.e. 23rd May, at the Quark Conference 2011, and there is plenty to be really excited about. In the last two weeks of 2010, LHC switched from colliding protons to colliding beams of lead ions. (For news of recent records of beam intensity and beam bunches from LHC, CERN, see here)
Peter Steinberg of Brookhaven, co-convener of the collaboration said:
The first LHC heavy-ion run was a great success for ATLAS.
Why Lead Ions?
Lead ions being heavier carry a lot more energy than protons and thus their collisions produce new heavier and exotic particles and new forms of matter, not seen in proton-proton collisions. Lead ions are produced by stripping lead atoms of all their electrons. In these collisions, scientists have detected traces of Quark Gluon Plasma, the stuff that was present in the primordial Universe.
The early universe, a few hundred microseconds old, was a soup of extreme energy density, in which quarks, gluons and leptons moved around. Quarks and gluons were believed to form a plasma like substance, called ‘Quark Gluon Plasma’. In colliding lead ions, scientists hope to recreate the plasma and study the very early universe.
What is QGP?
QGP can be theoretically made in the laboratory by heating matter upto 2×1012 K, which is roughly a hundred thousand (100,000) times hotter than the Sun’s core temperature and about a billion (1,000,000,000) times hotter than the surface of the Sun. This can only be achieved by colliding heavy ions at ultra-relativistic speeds(speeds close to that of light, or more technically, having a high Lorentz factor). Lead and gold ions have been selected by LHC. Most collisions don’t happen head on, but a few do. If enough of these head-on collisions are produced at a certain small region, they may form the exotic matter QGP, which has densities higher than those found on neutron stars. (Just a layman’s comparison: One teaspoonful of neutron star matter outweighs all the cars of the Earth put together!!) By studying the flow of this material, scientists can fit data into theory and also verify theoretical predictions.
Detectors and Achievements
QGP was first produced by the Relativistic Heavy Ion Collider (RHIC), Brookhaven, demonstrating for the first time that such dense exotic forms of matter can indeed be produced in a laboratory. ALICE, a detector at LHC, specifically designed and calibrated for very high energy collisions and for studying Quark Gluon Plasma, has confirmed the fact and has been able to study the properties of the plasma. It has verified theoretical predictions than QGP is an ideal fluid.
CMS, another detector in LHC, designed as a general purpose detector for high energy particles, have also detected the production of the W and Z boson, both critical components in the electroweak theory. CMS has also detected a marked suppression of the weakly bound states of the bottom (or, as some people prefer, beauty) quark, which scientists believe has great importance in further study of QGP.
ATLAS, one of the general purpose detectors for high energy particles alongwith CMS, has studied the macroscopic properties of the plasma, like the number and density of charged particles emerging out of it. It has also mapped out the energy and matter density, elaborating on the collision mechanisms and transport properties of the plasma.
As CERN’s director, General Rolf Heuer says:
These results from the LHC lead ion programme are already starting to bring new understanding of the primordial universe. The subtleties they are already seeing are very impressive.
LHC is now setting a benchmark in high energy physics, going where no collider has dared to go before. It would thus be appropriate to close with the following words from CMS spokesperson Guido Tonelli:
We are entering a new era of high precision studies of strongly interacting matter at the highest energies ever. By deploying the full potential of the CMS detector we are producing unambiguous signatures of this new state of matter and unravelling many of its properties.
An exciting future in high energy physics awaits us.