A “Little Bang” arrives at the LHC

In November, the Large Hadron Collider (LHC) at CERN began its first heavy-ion run, producing lead-lead collisions with the highest center of mass energy ever achieved. Now, a pair of papers appearing in Physical Review Letters, from the ALICE [1] and ATLAS [2] experiments at the LHC, presents a first glimpse of what new information these high-energy collisions will offer about the quark-gluon plasma—the state of matter believed to have filled the universe at the time of the Big Bang. The ALICE results strongly indicate that the quark-gluon plasma remains a nearly ideal liquid, as seen earlier at the Relativistic Heavy Ion Collider (RHIC), even at significantly higher energies. Complementing this work, the ATLAS team has shown that even very high energy jets of particles emitted from the collision lose a large fraction of their energy into the quark-gluon plasma (and are sometimes completely dissipated), a sign that the quarks and gluons are strongly interacting with the hotter plasma.

Phys. Rev. Lett. 105, 252302 (2010) – Published December 13, 2010

Sorting superfluidity from Bose-Einstein condensation in atomic gases

One of the neatest formulations of the concept of superfluidity involves the response of the fluid to rotation in the so-called “rotating bucket experiment”: while the normal component of the fluid is dragged by the bucket, the superfluid component is almost unaffected by the rotating walls . This idea was first put into practice in 1946 by Andronikashvili using a torsional oscillator and a bulk three-dimensional sample of liquid helium the appearance of a superfluid is detected by the drop in the moment of inertia . Interesting measurements of the reduced moment of inertia of atomic Bose-Einstein condensates have been performed by looking at the frequency of the so-called scissors mode in an anisotropic trap and at the time evolution of the shape of an expanding condensate after releasing the trap .

The definition of superfluid fraction can be formulated in a formal and quantitative way in terms of the response of the fluid to an external vector field . If placed in a rotating trap, neutral atoms behave in fact as if they were subject to a constant magnetic field parallel to the rotation axis; in this picture, the absence of response to rotation is the superfluid analog of the Meissner effect of superconductors in which magnetic fields are excluded from the material. Along these lines, it was soon recognized that the study of the response of the gas to artificial magnetic fields may offer a much wider range of experimental possibilities to investigate superfluidity.

Nigel R. Cooper and Zoran Hadzibabic

Phys. Rev. Lett. 104, 030401 (2010) – Published January 19, 2010