Summary of other reasonable theoretical ideas Paul Romatschke Thank the organizer for giving me this remarkable title for the talk. The fact that my talk is the last before the conference dinner confirmed by suscipions about the nature of this talk, so I decided to rise up to the challenge. My understanding of my charge in this talk is summarized in the next slide. &

…but my client’s idea is perfectly reasonable, your honor!

“other”: no comments will be made on other ongoing trials… CGC!

I’ll defend myself It’s much more fun to be the devil than the devil’s advocate!

How to “fake” flow

Definition: To “fake” flow: produce results that look like they had come from hydrodynamics, when they in fact had not

List of known suspects accused of faking flow: Hadron cascades (UrQMD,…) AMPT The escape mechanism Free streaming+hadronization Different kinds of flow: radial, elliptic, v3, … Some easier to fake than others

Many more very reasonable theoretical ideas… …but my time in this court today does not allow me to discuss them!

Suspect #1: Hadron Cascades [1508.06160] The first trial suspect we discuss in this court today is that of … Mass ordering (“radial flow”) Elliptic flow

Suspect #1: Hadron Cascades Hadronic interactions alone can generate some “radial flow” (mass-ordering) in p+A, but not enough (?) Hadronic interactions alone can generate some elliptic “flow” in p+A, but not enough (?) Hadronic interactions generate some v3, but not enough Large non-flow effects in p+A (c2{4}>0)!

Suspect #2: AMPT Mass ordering (“radial flow”): 1501.06880 σ=1.5mb [1501.06880] Mass ordering (“radial flow”): 1501.06880 Elliptic flow & triangular flow: 1501.06880, 1503.03655

Suspect #2: AMPT AMPT can generate elliptic “flow” in p+A in agreement with data even with almost no interaction (σ=1.5mb) Same for radial & triangular flow Works even for Cu+Au [1509.07784] ! ? !? See Jamie Nagle’s talk on Tue!

Suspect #3: Escape mechanism [1502.05572] Surface bias effect, w/o interactions! Elliptic flow

Suspect #3: Escape mechanism Anisotropic “flow” signal from surface emission bias Contributes to v2, v3; significant if parton interactions small See Zi-Wei Lin’s talk on Wed!

Suspect #3: Free-streaming 1504.02529 radial flow equal or larger than hydro Elliptic flow, Triangular flow, HBT, …

The defense would like to present a little more detail…. I’ll defend myself It’s much more fun to be the devil than the devil’s advocate! The defense would like to present a little more detail….

“Flow” in free-streaming 3D case, initial condition: Exact FS solution Energy-Momentum tensor

Decomposition If a local rest frame exists (not restricted to hydro!) Solve for ur in Free-streaming case: “Flow” without interactions! See U. Heinz’ talk on Mon!

FS flow similar to hydro flow 1504.02529

The importance of freeze-out Without interactions, particle pT does not change If there is a change in d.o.fs, such as hadronization, we generically use switching hypersurfaces to get final particle spectra (Cooper-Frye) Because ur is non-vanishing in FS, Cooper-Frye leads to a change in pT

Free-streaming: flow in p+Pb 1504.02529

Free-streaming: flow in Pb+Pb 1504.02529

Free-streaming: HBT in Pb+Pb 1504.02529 Unequal particles? I don’t know. Need to calculate.

Free-streaming: mass-ordering in Pb+Pb 1504.02529

If final particles originate from constant T hypersurface, you get radial flow no matter what (hydro, FS, AMPT,…)

Does radial flow ever turn off? 1504.02529 Radial flow proportional to the lifetime in the “hot” phase. Does turn off if no cell exceeds QCD phase transition temperature.

Also: small v2, v3,… without hydro when you have a hadronic afterburner. Let me tell you about it…

In this case, your honor, I will summarize…

Summary Different “reasonable” ideas for explaining exp’ data in p+A collisions One common finding: can “fake” radial flow, mass ordering and HBT simply by having a T=const freeze-out surface You can even get this from free-streaming! Radial flow, mass ordering and HBT are not indicative of the presence of hydrodynamics in p+A or even A+A!

I like hydro, but it’s really fun to be the devil and pick it apart I like hydro, but it’s really fun to be the devil and pick it apart. We might learn a lot about QCD this way.

Bonus Material

Equation of State (EoS) Massive particle EoS with Z dofs Used in both hydro and free-streaming model m,Z chosen to latch onto hadronic EoS at TS=170 MeV (“freeze-out”)

Equation of State (EoS)

Initial Conditions (1/2) Event-by-Event MonteCarlo-Glauber MC position of nucleons using pdfs (WS for A, vanHulthen/GFMC for d/3He) Positions (x,y) of nucleons undergoing at least one collision recorded (“participants”) Assumed to contribute to energy density at (x,y) in form of a Gaussian with width 0.4 fm Total energy density is sum over all Gaussians

Initial Conditions (2/2) Assume Tab(τ=τ0,x,y) to be isotropic Then energy density fixes Tab For simplicity, velocities and viscous stresses at τ=τ0 set to zero

Hydro evolution Solved numerically in 2+1d using vh2+1 Cooper-Frye freeze-out for cells reaching T=170 MeV

Free-streaming evolution Analytic solution: Cooper-Frye freeze-out for cells reaching T=170 MeV

Hadron Cascade Once all cells have cooled below T=170 MeV, evolution simulated using hadron cascade (B3D) All stable particle resonances up to M=2.2 GeV Conversion fully matching Tab incl. viscous stresses (not only ideal fluid part) Typically 100,000 B3D events per hydro/FS event Particle spectra, vn calculated from B3D events

Importance of correct switching 1504.02529

Radial flow in p+A

Radial flow in p+A

Radial flow in p+A

Radial flow in p+A Finding: radial flow in almost ideal hydro (η/s=0.08) and free-streaming (η/s=∞) essentially identical for p+A

Radial flow in A+A

Radial flow Conclusions Finding 1: radial flow in viscous hydro (η/s=0.08) almost the same as in free-streaming (η/s=∞) for p+A Finding 2: radial flow in free-streaming larger than in almost ideal hydro My conclusion: observation of radial flow is no indication of hydro phase

Elliptic flow in p+A

Elliptic flow in p+A

Elliptic flow in p+A

Elliptic flow in p+A Finding 1: there is some elliptic flow generated in free-streaming (η/s=∞) + hadron cascade dynamics in p+A Finding 2: the amount of elliptic flow in free-streaming (η/s=∞) + hadron cascade dynamics is only about 1/3 of that in almost ideal hydro (η/s=0.08) +hadron cascade in p+A

Elliptic flow in A+A

Elliptic flow Conclusions Finding 1: big differences of elliptic flow between viscous hydro (η/s=0.08) and free-streaming (η/s=∞) for p+A and AA My conclusion: observation of large elliptic flow is evidence for hydro phase

Triangular flow in p+A

Triangular flow in p+A

Triangular flow in p+A

Triangular flow in p+A Finding: triangular flow in free-streaming (η/s=∞) +hadron cascade similar to (p+Pb) or larger than (d+Au, 3He+Au) in almost ideal hydro (η/s=0.08)+cascade

Triangular flow in A+A

Triangular flow Conclusions Finding 1: triangular flow in viscous hydro (η/s=0.08) almost the same as in free-streaming (η/s=∞) for p+Pb Finding 2: triangular flow in free-streaming larger than in almost ideal hydro for d+Au and 3He+Au at RHIC My conclusion: v3,v4 measurements in p+A can easily be generated by hydro-imposters; do not use as viscometer!

HBT radii in p+A

HBT radii in p+A

HBT radii in p+A

HBT Radii Conclusions Finding 1:HBT Radii are essentially identical in viscous hydro (η/s=0.08) and free-streaming (η/s=∞) dynamics in p+A and also A+A My conclusion: measurements of HBT Radii are not sensitive to the presence or absence of a hydro phase

Remarks about 1504.02529 Apples-to-apples comparison of models for p+A collisions (hydro vs. free-streaming) Radial flow, HBT radii are essentially the same for hydro and FS; do not use as evidence for hydro phase v3 is similar in hydro/FS in p+A; do not use as ‘viscometer’ v2 is the gold standard: hard to fake without hydro; “large” v2 is evidence for hydro phase