1. W. A. Horowitz The Ohio State University March 12, 2010
Successes, Failures, and Uncertainties in Jet Physics in Heavy Ion Collisions
W. A. Horowitz
The Ohio State University
March 12, 2010
With many thanks to Brian Cole, Miklos Gyulassy, Ulrich Heinz, and Yuri Kovchegov
12/5/2018
Wayne State University Seminar

2. QCD: Theory of the Strong Force
ALEPH, PLB284, (1992)
PDG
Running as
-b-fcn
SU(Nc = 3)
Nf(E)
Nf(RHIC) ≈ 2.5
Griffiths Particle Physics
12/5/2018
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3. Bulk QCD and Phase Diagram
Long Range Plan, 2008
12/5/2018
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4. Methods of QCD Calculation I: Lattice
Long Range Plan, 2008
Kaczmarek and Zantow, PRD71 (2005)
Davies et al. (HPQCD), PRL92 (2004)
All momenta
Euclidean correlators
12/5/2018
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5. Methods of QCD Calculation II: pQCD
Jäger et al., PRD67 (2003)
d’Enterria,
Any quantity
Small coupling (large momenta only)
12/5/2018
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6. Methods of QCD Calculation III: AdS(?)
Maldacena conjecture: SYM in d  IIB in d+1
Gubser, QM09
Next up, experiments!
All quantities
Nc → ∞
SYM, not QCD: b = 0
Probably not good approx. for p+p; maybe A+A?
12/5/2018
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7. Wayne State University Seminar
Why High-pT Jets?
Tomography in medicine
One can learn a lot from a single probe…
and even more with multiple probes
SPECT-CT Scan uses internal g photons and external X-rays
PET Scan
12/5/2018
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8. Wayne State University Seminar
Tomography in QGP
Requires well-controlled theory of:
production of rare, high-pT probes
g, u, d, s, c, b
in-medium E-loss
hadronization
Requires precision measurements of decay fragments pT f
, g, e-
Invert attenuation pattern => measure medium properties
12/5/2018
Wayne State University Seminar

9. Wayne State University Seminar
QGP Energy Loss
Learn about E-loss mechanism
Most direct probe of DOF
AdS/CFT Picture
pQCD Picture
12/5/2018
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10. Wayne State University Seminar
High-pT Observables
Naively: if medium has no effect, then RAA = 1
Common variables used are transverse momentum, pT, and angle with respect to the reaction plane, f pT f
, g, e-
Fourier expand RAA:
12/5/2018
Wayne State University Seminar

11. Wayne State University Seminar
pQCD Rad Picture
Bremsstrahlung Radiation
Weakly-coupled plasma
Medium organizes into Debye-screened centers
T ~ 250 MeV, g ~ 2
m ~ gT ~ 0.5 GeV
lmfp ~ 1/g2T ~ 1 fm
RAu ~ 6 fm
1/m << lmfp << L
mult. coh. em.
LPM
dpT/dt ~ -LT3 log(pT/Mq)
Bethe-Heitler
dpT/dt ~ -(T3/Mq2) pT
12/5/2018
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12. Wayne State University Seminar
pQCD Success at RHIC:
(circa 2005)
Y. Akiba for the PHENIX collaboration, hep-ex/
Consistency: RAA(h)~RAA(p)
Null Control: RAA(g)~1
GLV Prediction: Theory~Data for reasonable fixed L~5 fm and dNg/dy~dNp/dy
12/5/2018
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13. Trouble for Rad E-Loss Picturev2 e-
WAH, Acta Phys.Hung.A27 (2006)
Anticorrelated; have e- come in later e-
Djordjevic, Gyulassy, Vogt, and Wicks, PLB632 (2006)
12/5/2018
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14. What About Elastic Loss?
Appreciable!
Finite time effects small
Adil, Gyulassy, WAH, Wicks, PRC75 (2007)
Mustafa, PRC72 (2005)
12/5/2018
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15. Quantitative Disagreement Remains
v2 too small
NPE supp. too large
p0 v2
C. Vale, QM09 Plenary (analysis by R. Wei)
WHDG
Wicks, WAH, Gyulassy, Djordjevic, NPA784 (2007)
NPE v2
Pert. at LHC energies?
PHENIX, Phys. Rev. Lett. 98, (2007)
12/5/2018
Wayne State University Seminar

16. Strongly Coupled Qualitative Successes
T. Hirano and M. Gyulassy, Nucl. Phys. A69:71-94 (2006)
Blaizot et al., JHEP0706
AdS/CFT
PHENIX, PRL98, (2007)
Betz, Gyulassy, Noronha, Torrieri, PLB675 (2009)
12/5/2018
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17. Wayne State University Seminar
Jets in AdS/CFT
Model heavy quark jet energy loss by embedding string in AdS space
dpT/dt = - m pT
m = pl1/2 T2/2Mq
J Friess, S Gubser, G Michalogiorgakis, S Pufu, Phys Rev D75 (2007)
Similar to Bethe-Heitler
dpT/dt ~ -(T3/Mq2) pT
Very different from LPM
dpT/dt ~ -LT3 log(pT/Mq)
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18. Wayne State University Seminar
Compared to Data
String drag: qualitative agreement
WAH, PhD Thesis
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19. Wayne State University Seminar
pQCD vs. AdS/CFT at LHC
Plethora of Predictions:
WAH, M. Gyulassy, PLB666 (2008)
Taking the ratio cancels most normalization differences
pQCD ratio asymptotically approaches 1, and more slowly so for increased quenching (until quenching saturates)
AdS/CFT ratio is flat and many times smaller than pQCD at only moderate pT
WAH, M. Gyulassy, PLB666 (2008)
12/5/2018
Wayne State University Seminar

20. Not So Fast! Speed limit estimate for applicability of AdS drag
g < gcrit = (1 + 2Mq/l1/2 T)2
~ 4Mq2/(l T2)
Limited by Mcharm ~ 1.2 GeV
Similar to BH LPM
gcrit ~ Mq/(lT)
No Single T for QGP
smallest gcrit for largest T
T = T(t0, x=y=0): “(”
largest gcrit for smallest T
T = Tc: “]”
D3 Black Brane
D7 Probe Brane Q
Worldsheet boundary Spacelike if g > gcrit
Trailing
String
“Brachistochrone”
“z” x5
12/5/2018
Wayne State University Seminar

21. LHC RcAA(pT)/RbAA(pT) Prediction (with speed limits)
WAH, M. Gyulassy, PLB666 (2008)
T(t0): “(”, corrections likely small for smaller momenta
Tc: “]”, corrections likely large for higher momenta
12/5/2018
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22. Wayne State University Seminar
RHIC Rcb Ratio
pQCD
pQCD
AdS/CFT
AdS/CFT
WAH, M. Gyulassy, JPhysG35 (2008)
Wider distribution of AdS/CFT curves due to large n: increased sensitivity to input parameters
Advantage of RHIC: lower T => higher AdS speed limits
12/5/2018
Wayne State University Seminar

23. Universality and Applicability
How universal are th. HQ drag results?
Examine different theories
Investigate alternate geometries
Other AdS geometries
Bjorken expanding hydro
Shock metric
Warm-up to Bj. hydro
Can represent both hot and cold nuclear matter
12/5/2018
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24. Wayne State University Seminar
New Geometries
Constant T Thermal Black Brane
Shock Geometries
P Chesler,
Quark Matter 2009
Nucleus as Shock
DIS
Embedded String in Shock
Albacete, Kovchegov, Taliotis,
JHEP 0807, 074 (2008)
Before
After Q
vshock x z
vshock x z Q
WAH and Kovchegov, PLB680 (2009)
12/5/2018
Wayne State University Seminar

25. Asymptotic Shock Results
Three t-ind. solutions (static gauge):
Xm = (t, x(z), 0,0, z)
x(z) = x0, x0 ± m ½ z3/3
Constant solution unstable
Time-reversed negative x solution unphysical
Sim. to x ~ z3/3, z << 1, for const. T BH geom.
x0 - m ½ z3/3
x0 + m ½ z3/3 x0
vshock Q
z = 0
z = ¥ x
12/5/2018
Wayne State University Seminar

26. Putting It All Together
For L typical momentum scale of the medium
We’ve generalized the BH solution to both cold and hot nuclear matter E-loss
Recall for BH:
Shock gives exactly the same drag as BH for L = p T
12/5/2018
Wayne State University Seminar

27. Shock Metric Speed Limit
Local speed of light (in HQ rest frame)
Demand reality of point-particle action
Solve for v = 0 for finite mass HQ
z = zM = l½/2pMq
Same speed limit as for BH metric when L = pT
12/5/2018
Wayne State University Seminar

28. Back to pQCD: Quant. and Falsifiable
Requires rigorous pQCD estimates, limits:
Different pQCD formalisms, different results
Bass et al., Phys.Rev.C79:
024901,2009
12/5/2018
Wayne State University Seminar

29. Need for Theoretical Uncertainty
Want to rigorously:
falsify theories
quantify medium
Therefore need:
Precise observables
Precise theory
Distinguish between systematic uncertainties:
between formalisms
Due to diff. physics assumptions
within formalisms
Due to simplifying approximations
Focus specifically on opacity expansion
GLV; ASW-SH
12/5/2018
Wayne State University Seminar

30. Mechanics of Energy Loss
RAA ~ ∫(1-ϵ)n P(ϵ) dϵ
pf = (1-ϵ)pi
Opacity expansions finds single inclusive gluon emission spectrum
dNg/dxdkTdqT
12/5/2018
Wayne State University Seminar

31. Wayne State University Seminar
Poisson Convolution
Find P(ϵ) by convolving dNg/dx
Approximates probabilistic multiple gluon emission, Sudakov
assume independent emissions
NB: ϵ is a momentum fraction
Gyulassy, Levai, and Vitev NPB594 (2001)
12/5/2018
Wayne State University Seminar

32. Opacity Expansion Calculation
Want to find dNg/dx
Make approximations to simplify derivation
Small angle emission: kT << xE
Note: ALL current formalisms use collinear approximation
Derived dNg/dxdkT violates collinear approx
Both IR and UV safe
Enforce small angle emission through UV cutoff in kT
12/5/2018
Wayne State University Seminar

33. Uncertainty from Collinear Approx
Derived dNg/dxdkT maximally violates collinear approximation
WAH and B Cole, arXiv:
dNg/dx depends sensitively on kT cutoff
Despite UV safety
For effect on extracted prop., must understand x
Discovered through TECHQM Brick Problem
12/5/2018
Wayne State University Seminar

34. Two Standard x Definitions
ASW-SH: xE
Energy fraction
GLV: x+
Plus momentum fraction P
NB: gluon always on-shell
12/5/2018
Wayne State University Seminar

35. Coordinate Transformations
Same in the limit kT/xE → 0!
UV cutoff given by restricting maximum angle of emission
Previous comparisons with data took qmax=p/2
Vary qmax to estimate systematic theoretical uncertainty P q
12/5/2018
Wayne State University Seminar

36. Wayne State University Seminar
Jacobians
ϵ is fraction of longitudinal momentum
Need dNg/dxE to find P(ϵ)
A Jacobian is required for x = x+ interpretation
12/5/2018
Wayne State University Seminar

37. Rad. Gluon Kin. SensitivitiesUV
What about IR?
WAH and B Cole, arXiv:
12/5/2018
Wayne State University Seminar

38. Collinearity and Gluon Mass
Massless gluons:
Large IR cutoff sensitivity
Gluons with thermal mass ~
BDMS, JHEP 0109 (2001)
Larger x better respects kT << xE
12/5/2018
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WAH and B Cole, arXiv:

39. Wayne State University Seminar
Results
Quantitatively compare to PHENIX data
Assumed infinite Elastic precision
WAH and B Cole, arXiv:
12/5/2018
Wayne State University Seminar

40. Parton Energy Dependence
Dependence on parton energy
Uncertainty on qhat
Assume all formalisms equally affected
WAH and B Cole, arXiv:
12/5/2018
Wayne State University Seminar

41. Wayne State University Seminar
Conclusions
pQCD and AdS/CFT enjoy qualitative successes, concerns in high-pT HIC
RHIC suppression of lights and heavies
Future LHC measurements
Quantitative comparisons with rigorous theoretical uncertainty estimates needed for falsification/verification
Theoretical work needed in both in pQCD and AdS
In AdS, control of jet IC, large pT required
In pQCD, wide angle radiation very important, not under theoretical control
12/5/2018
Wayne State University Seminar