1. RHIC Challenges and LHC Outlook
W. A. Horowitz
The Ohio State University
June 21, 2010
With many thanks to Brian Cole, Miklos Gyulassy,
Ulrich Heinz, Jiangyong Jia, and Yuri Kovchegov
11/13/2018
INT Workshop 2010

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
11/13/2018
Wayne State University Seminar

3. Success of pQCD in p+p Calibrated probe in p+p
Note need for NLO for production
d’Enterria,
11/13/2018
INT Workshop 2010

4. Why High-pT Particles in HI?
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
11/13/2018
INT Workshop 2010

5. Tomography in QGP Indep. check of hydro
geom. meas. vital for quant. h/s
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
11/13/2018
INT Workshop 2010

6. QGP Energy Loss Learn about E-loss mechanism Most direct probe of DOF
AdS/CFT Picture
pQCD Picture p h g p e-
11/13/2018
INT Workshop 2010

7. pQCD Success at RHIC: (circa 2005) Null Control: RAA(g)~1
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
11/13/2018
INT Workshop 2010

8. Irony of Multiple Observables (I)
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
PHENIX, Phys. Rev. Lett. 98, (2007)
11/13/2018
INT Workshop 2010

9. Irony of Multiple Observables (II)
IAA
WHDG soon
Baryon/Meson Ratio
STAR pT
pQCD w/ KKP
pQCD w/ DSS
WAH, in preparation
Jamie Nagle, QM09
Pert. at LHC energies?
11/13/2018
INT Workshop 2010

10. Experimental Convergence on NPE
c/b production ~ FONLL predictions
Large b contribution
STAR & PHENIX now consistent in p+p
RAA?
Bertrand Biritz, QM09
Wei Xie, DIS10
11/13/2018
INT Workshop 2010

11. Need for pQCD Theory Improvement
NB: pQCD calculations so far assume M << E
b corrections likely large
c and b separation
Success of AdS/CFT?
dpT/dt = - m pT
m = pl1/2 T2/2Mq
WAH, PhD Thesis
11/13/2018
INT Workshop 2010

12. v2 Marches On Data improves, pQCD falsified to higher pT
Does v2 becomes constant, >> pQCD?
Rui Wei, for PHENIX, QM09
11/13/2018
INT Workshop 2010

13. v2 Comparison to WHDG Failure at > 9 GeV!
Future: improve density scaling pred.
WS Glauber
HN Mixed
KLN CGC
2008 PHOBOS
NB: KLN CGC and Hirano/Nara tuned to old (2002) PHOBOS data
Horowitz and Jia, in preparation
11/13/2018
J Jia, JET Collab Meeting
INT Workshop 2010

14. Decreasing the Disagreement
How much can we sharpen the medium?
Use CGC IC
Include initial state fluctuations
Correct mismatch between reaction plane and participant plane
Horowitz and Jia, in preparation
11/13/2018
INT Workshop 2010

15. pQCD L1-2 Seemingly Not Enough
CGC, fluctuations not enough for pQCD
AdS/CFT: DE ~ L3
Better job?
20-30% PHENIX p0
Glauber
CGC
Solid: analytic
Dashed: fluctuating
WAH and J Jia, in preparation
Marquet and Renk, PLB685 (2010)
Largest medium density w/i unc. from RAA
11/13/2018
INT Workshop 2010

16. Theoretical Uncertainties: Different Formalisms, Different Results
Very diff. ideas about relevant physics
pQCD
AMY, BDMPS, GLV, HT
Is this a systematic th. unc.? Can we falsify?
Bass et al., Phys.Rev.C79:
024901,2009
What about AdS/CFT?
11/13/2018
INT Workshop 2010

17. 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
11/13/2018
INT Workshop 2010

18. 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)
11/13/2018
INT Workshop 2010

19. 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
11/13/2018
INT Workshop 2010

20. Uncertainty from Collinear Approx
Derived dNg/dxdkT maximally violates collinear approximation
WAH and B Cole, PRC81, 2010
dNg/dx depends sensitively on kT cutoff
Despite UV safety
For effect on extracted prop., must understand x
11/13/2018
INT Workshop 2010

21. Two Standard x Definitions
ASW-SH: xE
Energy fraction
GLV: x+
Plus momentum fraction P
NB: gluon always on-shell
11/13/2018
INT Workshop 2010

22. 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
11/13/2018
INT Workshop 2010

23. Jacobians ϵ is fraction of longitudinal momentum
Need dNg/dxE to find P(ϵ)
A Jacobian is required for x = x+ interpretation
11/13/2018
INT Workshop 2010

24. Rad. Gluon Kin. SensitivitiesUV
What about IR?
WAH and B Cole, PRC81, 2010
11/13/2018
INT Workshop 2010

25. Collinearity and Gluon Mass
Massless gluons:
Large IR cutoff sensitivity
Gluons with thermal mass ~
BDMS, JHEP 0109 (2001)
Larger x better respects kT << xE
11/13/2018
INT Workshop 2010
WAH and B Cole, PRC81, 2010

26. Results Quantitatively compare to PHENIX data
Assumed infinite Elastic precision
WAH and B Cole, PRC81, 2010
11/13/2018
INT Workshop 2010

27. Parton Energy Dependence
Dependence on parton energy
Uncertainty on qhat
Assume all formalisms equally affected
WAH and B Cole, PRC81, 2010
11/13/2018
INT Workshop 2010

28. JET Collaboration Meeting at LBL
v2: “Collinearly Safe”
Fix dNg/dy from RAA, calculate v2
Expect larger v2 for smaller opening angle
( tcoh ~ 1/kT2 )
IAA: coming soon
Rad Only
Rad + El v2 pT
20-30% p0
20-30% p0 v2 pT
Data
Data
WAH, in preparation
11/13/2018
JET Collaboration Meeting at LBL

29. LHC Predictions Elastic still significant at LHC energies
LHC: Power of power law of production more constant
RHIC
LHC
Power
v2 still too small at LHC?
11/13/2018
INT Workshop 2010

30. Need for Null Control Jets at LHC? T Isobe, JPhysG34 (2007) 11/13/2018
INT Workshop 2010

31. WHDG LHC Predictions As shown in QM’06 Large rise for WHDG
DE/E ~ Log(E)/E, dies faster than power increases
Note that ASW & PQM used EKRT extrapolation with dNg/dy ~ 7000
11/13/2018
INT Workshop 2010

32. “PHENIX Extrapolation”
Flat out to infinity
PHENIX PRC77
11/13/2018
INT Workshop 2010

33. Heavy Quark Probes in AdS/CFT
Model heavy quark 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)
11/13/2018
INT Workshop 2010

34. 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)
11/13/2018
INT Workshop 2010

35. Worldsheet boundary Spacelike if g > gcrit
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
11/13/2018
INT Workshop 2010

36. 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
11/13/2018
INT Workshop 2010

37. Additional Discerning Power
Adil-Vitev in-medium fragmentation rapidly approaches, and then broaches, 1
Does not include partonic energy loss, which will be nonnegligable as ratio goes to unity
CERN Heavy Ion Forum

38. 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
11/13/2018
INT Workshop 2010

39. Conclusions pQCD is in trouble in A+A at RHIC
v2 is just too large, out to > 9 Gev!
Large sys. th. unc. from large angle radiation
Problem remains for higher orders in opacity, dyn. sc. cent.
Despite unc. in RAA, global analysis yields small unc. for v2
More calculated obs. on the way
Saved by NLO, better geom, better elastic, fewer approx.?
NPE: need c and b sep. & need MQ ~ E theory
AdS applied to jets tantalizing
Exciting days to come at LHC
pQCD vs. AdS Drag vs. In-medium diss.
Need detailed estimates of unc. for falsification (global analysis)
Will need null control
11/13/2018
INT Workshop 2010