by Panos Charitos. Published: 13 October 2012

Charm quarks have come to the front stage at the Quark Matter 2012 conference that was 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 ALICE experiment has presented 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, was dominated by the presentation of high statistics data on the production of particles containing charm quarks: the so-called “charmed particles”. ALICE Matters met Federico Antinori, Physics Coordinator of the ALICE experiment, during the conference and asked him about the importance of charm quarks in studying the quark – gluon plasma.


Federico Antinori in the ALICE control room with colleagues

A.M. What is special about charm quarks?

F.A. 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. Charm quarks are found in hadrons such as J/? or D mesons and constitute their building blocks.

A.M. Are they rare?

F.A. No, actually at the LHC, they are abundantly produced: tens of charm-anti-charm pairs can be created in a single nucleus-nucleus collision, but they are still very interesting. 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.


Lead-Lead collision in ALICE

A.M. How are charm particles detected in ALICE?

F.A. 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, such as the ALICE ITS.

A.M. Since they are produced in pairs I am wondering how do they interact with anti-charm anti-quarks.

F.A. Indeed one of the most interesting cases for ALICE Physicists is when 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 particle. We are very excited, since we currently have some indications about this process. This can only happen if these giants are almost slowed down to speeds corresponding to the thermal agitation of the plasma.

A.M. And why are charm quarks particularly important for the study of the QGP? Do they open a new window compared to our previous understanding?

F.A. Yes, definitely. Charm can tell us a lot about what goes on inside the QGP. Another example: by measuring tiny differences in the production of charmed particles along and across the orientation of the original impact of the two nuclei, we have observed tantalising signs that such a “thermalisation” phenomenon might indeed occur. As the plasma explodes following the collision, 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.

A.M. Where do we go from here?

F.A. This is actually just the beginning: we have more lead-lead data that we are still looking at and proton-proton data (essential to provide the reference behaviour against which any Pb-Pb anomaly has to be analysed) are still being collected. In addition, a crucial control experiment with proton-lead collisions is scheduled for the beginning of next year: it will increase tremendously the precision with which we know how many charm-anticharm pairs are produced when lead ions collide, further enhancing the accuracy with which we can describe the properties of the plasma!