4/15/2020
W. A. Horowitz
University of Cape Town
October 4, 2010
With many thanks to Brian Cole, Miklos Gyulassy,
Ulrich Heinz, Jiangyong Jia, and Yuri Kovchegov
EIC at the INT 1

• Huge low-p
T v
2
• Huge high-p
T suppression
20-30%
Hirano et al., PRC77, 2008
– Described by hydro with low viscosity
4/15/2020 EIC at the INT
Y. Akiba for the PHENIX collaboration ,
PLB630, 2005
– p
0 R
AA described by pQCD
2

• Want to characterize the QGP
– Can’t directly measure
• Use indirect tools
• Is QGP:
– Most perfect fluid ever created/studied?
– Can one use strongly and/or weakly coupled field theory methods?
• pQCD vs. AdS/CFT
• Enormous influence of geometry
4/15/2020 EIC at the INT 3

Spacetime Evolution of a HI Collision
• At RHIC t = -
 t = 0 t = 1 fm/c t = 3 fm/c t = 4 fm/c t = +

Initial State Initial Overlap Thermalization QGP Hadronization Hadron Gas
– Nontrivial to learn about QGP through HIC
4/15/2020 EIC at the INT 4

• Qualitative picture:
Anisotropic initial geometry => anisotropic flow
4/15/2020 EIC at the INT 5

2
• Hydro
– Early therm.
–  m
T mn = 0
– Equation of State
(EOS)
– Ideal: h
/s = 0
– v
2
: 2 nd Fourier coef of particle spectrum:
4/15/2020 EIC at the INT 6

• Viscosity reduces elliptic flow
– Naive pQCD => h
/s ~ 1
– Naive AdS/CFT => h
/s ~ 1/4 p
Shear Viscosity, Wikipedia
4/15/2020
=> Strongly coupled medium?
EIC at the INT
Luzum and Romatschke,
Phys.Rev.C78:034915,2008
7

• Poorly constrained initial geom => 100% uncertainty in viscosity
T Hirano, et al.,
Phys.Lett.B636:299-304,2006
– KLN CGC breaks down at edge of nuclear overlap
• Whole effect comes from edges!
4/15/2020
• Experimental constraints needed!
EIC at the INT 8

T
• Tomography in medicine
One can learn a lot from a single probe … and even more with multiple probes
4/15/2020
PET Scan http://www.fas.org/irp/imint/docs/rst/Intro/P art2_26d.html
EIC at the INT
SPECT-CT Scan uses internal g photons and external X-rays
9

• Requires wellcontrolled theory of:
– production of rare, highp
T probes
• g, u, d, s, c, b
– in-medium E-loss
– hadronization
• Requires precision measurements of decay fragments f
Invert attenuation pattern => measure medium properties p
T
, g
, e -
4/15/2020 EIC at the INT 10

• Learn about E-loss mechanism
– Most direct probe of DOF
4/15/2020 EIC at the INT 11

T
Naively: if medium has no effect, then R
AA
= 1
Common variables used are transverse momentum, p
T
, and angle with respect to the reaction plane, f
Fourier expand R
AA
: f p
T
, g
, e -
4/15/2020 EIC at the INT 12

• Bremsstrahlung Radiation
– Weakly-coupled plasma
• Medium organizes into Debye-screened centers
– T ~ 250 MeV, g ~ 2
• m
~ gT ~ 0.5 GeV
• l mfp
~ 1/g 2 T ~ 1 fm
• R
Au
~ 6 fm
– 1/ m
<< l mfp
<< L
• mult. coh. em.
Gyulassy, Levai, and Vitev, NPB571 (200)
– LPM dp
T
/dt ~ -LT 3 log(p
T
/M q
)
4/15/2020 EIC at the INT
– Bethe-Heitler dp
T
/dt ~ -(T 3 /M q
2 ) p
T
13

(circa 2005)
Y. Akiba for the PHENIX collaboration ,
PLB630, 2005
– Consistency:
R
AA
( h
)~R
AA
( p
)
– Null Control:
R
AA
( g
)~1
– GLV Prediction: Theory~Data for reasonable fixed L~5 fm and dN g
/dy~dN p
/dy
4/15/2020 EIC at the INT 14

• Mass of quarks should be important
– Expect heavy quarks to lose less energy
• Non-photonic electrons
(NPE) surprisingly suppressed
– Decay fragments of c and b quarks
PHENIX NPE e -
Djordjevic,et al. PLB632 (2006)
4/15/2020 UT Colloquium 15

• Appreciable!
• Finite time effects small
4/15/2020
Mustafa, PRC72 (2005)
UT Colloquium
Adil, Gyulassy, WAH, Wicks, PRC75 (2007)
16

• Lack of qual. simultaneous understanding
– p
0 , h
, g
R
AA
– e R
AA
, v
2 well described, BUT is not, even with elastic loss
PHENIX p
0
9.5 GeV!
30-40% Centrality p
0 R
AA
Wicks et al.
pQCD assumes M << E:
4/15/2020 b E-loss not under control
EIC at the INT
Death of pQCD at RHIC?
17

• Model heavy quark jet energy loss by embedding string in AdS space dp
T
/dt = m p
T m
= pl 1/2
T 2 /2M q
– Similar to Bethe-Heitler dp
T
/dt ~ -(T 3 /M q
2 ) p
T
J Friess, S Gubser, G Michalogiorgakis, S Pufu, Phys Rev D75 (2007)
– Very different from LPM dp
T
/dt ~ -LT 3 log(p
T
/M q
)
4/15/2020 EIC at the INT 18

• String drag: qualitative agreement
4/15/2020 EIC at the INT
WAH, PhD Thesis
19

WAH, in preparation
4/15/2020
D
L g therm
~ E 1/3
D
L q therm
~ (2E) 1/3
EIC at the INT
Renk and Marquet, PLB685, 2010
20

High-p
T and HIC Spacetime Evolution
• Jet sees full spacetime evolution t = -
 t = 0
JET t = 1 fm/c t = 3 fm/c t = 4 fm/c t = +

Initial State Initial Overlap Thermalization QGP Hadronization
– Affected by:
• Initial geometry
• Thermalization process
• E-loss in cold, hadronic matter
4/15/2020 EIC at the INT
Hadron Gas
21

T
2
• Effects of geom. on, e.g. v
2
, might be quite large
– CGC vs. KLN and rotating RP
– Effect not large enough
4/15/2020
WAH and J Jia, in preparation
Need experimental constraints on initial geometry!
EIC at the INT 22

• Hot spots can be • For simple E-loss not huge a large effect
– NEXUS calculation for
10% most central top
RHIC energy event
4/15/2020 EIC at CUA
Jia and Wei, arXiv:1005.0645
– Important for fluc., opacity exp.?
23

• Hydro evolution may amplify fluctuations
4/15/2020
Gyulassy, Rischke, Zhang, NPA613, 1997
EIC at CUA 24

• EIC could give experimental handle on initial geometry
– Recall e + A diffraction exps. on A at rest
4/15/2020
Hahn, Ravenhall, and Hofstadter, Phys Rev 101 (1956)
UT Colloquium 25

-3 e
• Coherent vector meson production in e + A e’ g
*
J/ y
Must reject incoherent collisions at ~100%
A A’
2 gluon exchange
=> mean & correlations
Also DVCS and
Incoherent production
4/15/2020
10 6 J/ y
Events
UT Colloquium
Caldwell and Kowalski, PRC 81 (2010)
26

• Large fraction of jet lifetime spent in hadronic matter
– QGP lasts ~ 2 fm
– Typical jet pathlength ~ 5 fm
• Measure in EIC
4/15/2020
Accardi, QM Italia
EIC at the INT
Accardi et al.,
Riv.Nuovo Cim.32, 2010
27

• Tantalizing physics discoveries at RHIC
– Large low-p
T v
2
• nearly perfect strongly coupled fluid (?)
– Large high-p
T suppression
• weakly coupled quasiparticle plasma (?)
• Qualitative understanding of QGP physics requires constraints on full spacetime evolution of HI collision
• Exciting eA physics opportunities for AA
– Knowledge of: r
(b), thermalization, cold matter E-loss a qualitative advance of HI via EIC
4/15/2020 EIC at the INT 28