ALICE measures the shape ofhead-on lead-lead collisions

摘要

One the many surprises to have emerged from studies of heavy-ion collisions at Brookhaven's Relativistic Heavy Ion Collider (RHIC) and now at CERN's LHC concerns the extreme fluidity of the dense matter of the nuclear fireball produced. This has traditionally been studied experimentally by measuring the second harmonic of the azimuthal distribution of emitted particles with respect to the plane of nuclear impact. Known as v2, this observable is remarkably large, saturating expectations fromhydrodynamic models, suggesting that the so-called quark-gluon plasma is one of the most perfect fluids in nature (CERN Courier April 2011 p7). Many assumed that the matter in the elliptical nuclear overlap region becomes smooth upon thermalization, rendering the Fourier coefficients other than v_2 negligible in comparison.