Transcript What do we (think) we know?

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
v3
v2
As 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 forwardbackward 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.