ALICE at Quark Matter 2012
The Charmed Plasma
Charm quarks will come to the front stage at the Quark Matter 2012 conference, the 23rd edition of this prestigious series of international conferences, that will be held in Washington from the 13th to the 18th of August, dedicated to the study of the Quark-Gluon Plasma (QGP), a state of matter present in the early Universe during the first millionths of a second after the Big Bang. In order to recreate such extreme conditions, the world’s most powerful accelerators are used to collide heavy nuclei accelerated at almost light speed. The most powerful such machine is the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) near Geneva.
The ALICE experiment, built and operated at the LHC by a collaboration of over 1000 physicists from 36 countries, will be presenting 40 different communications at the conference, covering all the important areas of QGP investigation. This rich release of results, unprecedented at the Quark Matter Conference, will be dominated by the presentation of high statistics data on the production of particles containing charm quarks: the so-called “charmed particles”.
Charm quarks are giants, heavier than a hydrogen atom, 250 times heavier than the first family quarks (the up quark and the down quark) that dominate the plasma, and more than ten times heavier than their lighter partner in the second family: the strange quark.
They are produced in pairs - each charm quark accompanied by an anti-charm quark - in the initial impact between the two colliding nuclei.
At the LHC, they are abundantly produced: tens of charm-anti-charm pairs can be created in a single nucleus-nucleus collision.
When these cannon balls traverse the plasma, they interact with it, experiencing a formidable drag. They emerge from the collision significantly decelerated, “dressed” together with other quarks into charmed particles, offering to scientists a unique tool to probe the plasma properties.
Although their production at the LHC is significant, charmed particles are still elusive: they live only a few tenths of a trillionth of a second, travelling just a few tenths of a millimetre before disintegrating into charmless particles. They can only be detected using sophisticated high precision sensors positioned just outside of the accelerator’s vacuum tube.
ALICE Physicists are also very excited about indications that in some cases, at the end of its ride through the plasma, a charm quark encounters an anti-charm anti-quark and the two emerge together as a charm-anti-charm particles. This can only happen if these giants are almost slowed down to speeds corresponding to the thermal agitation of the plasma.
And indeed, by sophisticated measurements of tiny differences in the production of charmed particles along and across the orientation of the original impact of the two nuclei, ALICE physicists have observed tantalising signs that that such a “thermalisation” phenomenon might indeed occur: as the plasma explodes following the collision, the plasma particles are pushed out more strongly in the direction along the orientation of the original impact, and there are hints in the data that charm particles could be pushed around in the same way, sharing and revealing the underlying properties of the fluid.
“This is only the start of the adventure”, said Paolo Giubellino, Spokesperson of the ALICE Collaboration, “with more data still being analysed and a crucial control experiment scheduled for February next year, we are closer than ever to unravelling the mysterious properties of the primordial state of the Universe: the Quark-Gluon Plasma.”
Read some articles on recent results from ALICE in ALICE Matters, the ALICE online newsletter:
Read some relevant interviews in ALICE Matters: