Thinking and Living In Three Dimensions at Last Asymmetric Ion Collisions at RHIC Paul Stankus, ORNL PHENIX FOCUS Feb 21, Run 12

Once upon a time …..all heavy-ion collisions were asymmetric: AGS: O+Au, Si+Au SPS: O+Pb, S+Pb As soon as heavy beams were available, highest- A symmetric collisions were preferred: Simple kinematics Highest energy/participant per fm 2 So why go back to asymmetric collisions at RHIC?

Changing the (2-D) Geometry d 2 N Part /dxdy What’s new: odd-harmonic asymmetry in the 2-D/midrapidity density pattern

Copper Gold N_Participant Pressure gradients? Jet opacity? The tie between hydrodynamical flow and jet-medium interactions is even more important, and more revealing, when symmetry is reduced 4

Asymmetric Ion Collisions: N Part Au+Au 194 Cu+Au Si+Au 2867 Au+Au Cu+Au Si+Au Central b=0 (Simple optical model with spherical nuclei)

+0.3 Au+Au Cu+Au Si+Au WA80 PRC 44 (1991) 2736 Shifting the CMS PHOBOS d+Au at RHIC Phys Rev C83, (2011) S+A at SPS Fixed-Target WA80 PRC 44 (1991) 2736

“His pattern indicates two- dimensional thinking.” Typical participant energy deposit initial condition 7

8 Longitudinal momenta balance along center line Non-zero net longitudinal momentum off center line What about local longitudinal momentum density?

9 Off-axis energy density pushed to higher/lower rapidity

Transverse profiles of initially-deposited energy density Rapidity or Longitudinal Space Assuming no rapidity dependence of transverse profile With rapidity dependence based on local longitudinal momentum density 10

Why is this picture wrong? Forward and backward rapidity profiles should have opposite signed odd moments…. …but north ϕ 3 positively correlated with South ϕ 3 ; (indicates dominance of long-Δη fluctuations?). PHENIX PPG098 PRL 105, (2010) Asymmetric collisions amplify longitudinal momentum asymmetry and also add a non- zero average v3 moment.

Two initial populations: Collisions Sites where hard-scattering, high-Q 2 objects are first formed; these later propagate through the medium Participants Site of initial energy deposit, 3-D distributions still unclear; these evolves as the medium Both need to be pictured in 3 dimensions! Asymmetric collisions will change both profiles

Example: J/Psi v1, v2 at forward and backward rapidities From R. Hollis, HHJ PWG Feb 16

Question: What will the J/Psi pattern vs azimuth look like? At forward/backward rapidities in the Cu-going and Au-going directions?

Back to the small picture Operational issues: > Main triggering on min-bias Cu+Au probably OK (AN: CH.C., A B.) > How to define centrality? What do we expect BBCN vs BBCS to look like? Or ZDCN vs ZDCS? Non-trivial, some simulations needed? Analyses of immediate interest: * Hadronic flow v1, v2, v3 at the mid-plane, vs pT * J/Psi yield muon arm North vs South (vs Central) * Calorimetric flow v1, v2, v3 at high rapidities (MPC) * Higher pT hadrons/jets quenching pattern vs phi

Summary (so far…): Asymmetric collisions will break down the forward- backward and rotational symmetries of symmetric systems The initial distributions of hard scattering sites and bulk energy deposit will be greatly modified in 3-D Measurements of flow and quenching at the mid- plane will sensitively test lower-symmetry energy deposition patterns Measurements of flow and quenching at forward/backward rapidities will stringently test 3-D models of energy deposit, hydrodynamics and parton-medium interaction