What do we (think) we know?

At present, the best evidence we have for (fast) local equilibration is the large observed azimuthal anisotropy (elliptic flow) and the agreement with hydrodynamic calculations that assume local equilibration.

Attracting a lot of attention recently: the claim that the thermalized QGP (or sQGP) state shows very low viscosity, e.g. “most perfect fluid ever”, etc.

But, proving low viscosity quantitatively is challenging; need to line up as much information as possible.

Also, we still don’t have a very good idea of how the earliest locally-equilibrated state is created -- potentially new physics!

A Modest Proposal

Propose: Asymmetric A+B nuclear collisions provide a much fuller constraint on hydrodynamic models.

A+A collisions are too symmetric, all odd cosine terms vanish for production at mid-rapidity. But v1 and v3 will return at mid-rapidity in A+B collisions.

A+B also breaks symmetry in longitudinal direction, which may shed additional light on equilibration mechanism.

Needed: A Sensitivity Study

v2

v3As a general statement, viscosity is a dissipative effect and all dissipative effects tend to drive local equilibrium towards global equilibrium. So higher viscosity means lower v2, and also lower v3, v4, etc. (v1 is a special case, since it obeys a sum rule).

Also as a general statement, higher wavenumber inhomogeneities tend to be more affected by local dissipative effects. So, as viscosity is raised above zero, is v3 affected more than v2? Can a combined measurement of v2 and v3 versus centrality constrain viscosity better than just v2 versus centrality? These questions can be

answered, at some level, even with a classical 2+1 viscous hydro calculation.

“His Pattern Indicates Two-Dimensional Thinking”

Stopping is intimately related to equilibration. Breaking the forward-backward degeneracy opens two new observables, offset and shape symmetry, which can better constrain any model of equilibration and longitudinal flow than in A+A.

How an asymmetric system “feels” its CMS motion is a non-trivial problem! Landau worried about it; shouldn’t you?

Also, azimuthal anisotropy versus rapidity further constrains hydrodynamics, especially with maximally broken A+B symmetry.

For the RHIC II case

Reasoning via hydrodynamic descriptions is very promising for EOS investigation; initial equilibration remains unknown but potentially involves new and fundamental physics.

A program of asymmetric ion A+B running can provide more information than symmetric A+A to constrain both hydrodynamical descriptions and models of initial equilibration. A+B needs to be in the RHIC future at level comparable to Run 4/5 statistics.

Takes advantage of RHIC flexibility; better than LHC?

Large rapidity coverages of PHOBOS and BRAHMS are needed for true 3-D picture. If they are not available, then this capability needs to be recovered in STAR/PHENIX upgrades.