Successes, Failures, and
Uncertainties in the Jet Physics
of Heavy Ion Collisions
W. A. Horowitz
The Ohio State University
February 23, 2010
With many thanks to Brian Cole, Miklos Gyulassy, Ulrich Heinz, and Yuri Kovchegov
30-Jun-16
UCT Seminar
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Introduction
• What is a heavy ion collision?
• Why do people care?
– Phase diagram of QCD
– Big Bang
– Strongly coupled systems
• AdS/CFT
• What is jet physics and what can we learn
from it?
30-Jun-16
UCT Seminar
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Heavy Ion Collisions
• Collider machines: RHIC, LHC
Relativistic Heavy Ion Collider
Large Hadron Collider
• Cartoon of a collision
30-Jun-16
UCT Seminar
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Four Fundamental Forces
Electromagnetism
Gravity
starchild.gsfc.nasa.gov
John Maarschalk, travelblog.portfoliocollection.com
Weak
Strong
lhs.lps.org/staff/sputnam/chem_notes/tritium_decay.gif
30-Jun-16
UCT Seminar
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Strong compared to E&M
• Electromagnetism
• Strong
Hydrogen
+
Proton
nobelprize.org
– Electric charge (+)
– Color charge (r, g, b)
• electrons
• quarks
– Carriers: photons
– Field theory:
– Carriers: gluons
– Field theory:
• Quantum electrodynamics (QED)
30-Jun-16
• Quantum chromodynamics (QCD)
UCT Seminar
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Crucial QCD Facts
• Yang-Mills Theory: non-Abelian
– Gluons carry color charge
– “Strong” over long distances, “weak” over
short distances
• Rigorous analytic calculations only for large
momentum processes
– Large compared to LQCD ~ 200 MeV
• Quarks and gluons are confined
• no bare quark or gluon has ever been detected
– 1,000,000 USD Clay Millenium prize for proof of
confinement in Yang-Mills theories
30-Jun-16
UCT Seminar
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Standard Model and Phase Diagrams
• E&M, Strong, and Weak particle physics
well understood (few particles)
• Many body physics less well understood
Water
Hydrogen
www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/triplpt.html
30-Jun-16
UCT Seminar
Calculated, Burkhard Militzer,
Diploma Thesis, Berlin, 2000
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Phase Diagram in QCD
Long Range Plan, 2008
30-Jun-16
UCT Seminar
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Methods of QCD Calculation I: Lattice
Long Range Plan, 2008
• All momenta
• Euclidean correlators
Kaczmarek and Zantow, PRD71 (2005)
30-Jun-16
Davies et al. (HPQCD), PRL92 (2004)
UCT Seminar
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Methods of QCD Calculation II: pQCD
(perturbative QCD)
d’Enterria, 0902.2011
Jäger et al., PRD67 (2003)
30-Jun-16
• Any quantity
• Small coupling (large momenta only)
UCT Seminar
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Methods III: AdS/CFT
Maldacena conjecture: SYM in d  IIB in d+1
Gubser, QM09
• All quantities
• Nc → ∞ SYM, not QCD
• Probably not good approx. for p+p; maybe A+A?
• Applicable to condensed matter systems?
30-Jun-16
UCT Seminar
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Why High-pT Jets?
• Tomography in medicine
One can learn a lot from a single probe…
and even more with multiple
probes
PET Scan
30-Jun-16
http://www.fas.org/irp/imint/docs/rst/Intro/P
art2_26d.html
UCT Seminar
SPECT-CT Scan uses
internal g photons and
external X-rays
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Tomography in QGP
• Requires wellcontrolled theory of:
– production of rare, highpT probes
pT
f
, g, e-
• g, u, d, s, c, b
– in-medium E-loss
– hadronization
• Requires precision
measurements of
decay fragments
30-Jun-16
UCT Seminar
Invert attenuation
pattern => measure
medium properties
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QGP Energy Loss
• Learn about E-loss mechanism
– Most direct probe of DOF
pQCD Picture
AdS/CFT
Picture
30-Jun-16
UCT Seminar
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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
Gyulassy, Levai, and Vitev, NPB571 (200)
• mult. coh. em.
– Bethe-Heitler
– LPM
dpT/dt ~ -LT3 log(pT/Mq)
30-Jun-16
UCT Seminar
dpT/dt ~ -(T3/Mq2) pT
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Jets in Heavy Ion Collisions
• p+p
Y-S Lai, RHIC & AGS Users’ Meeting, 2009
30-Jun-16
• Au+Au
PHENIX
UCT Seminar
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High-pT Observable
Naively: if medium has no effect, then RAA = 1
30-Jun-16
UCT Seminar
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pQCD Success at RHIC:
(circa 2005)
Y. Akiba for the PHENIX collaboration,
hep-ex/0510008
– 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
30-Jun-16
UCT Seminar
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Qualitative Disagreement
e-
Djordjevic, Gyulassy, Vogt, and Wicks, PLB632 (2006)
30-Jun-16
UCT Seminar
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What About Elastic Loss?
• Appreciable!
• Finite time effects small
Adil, Gyulassy, WAH, Wicks, PRC75 (2007)
Mustafa, PRC72 (2005)
30-Jun-16
UCT Seminar
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Quantitative Disagreement Remains
Wicks, WAH, Gyulassy, Djordjevic, NPA784 (2007)
Pert. at LHC energies?
30-Jun-16
UCT Seminar
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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
– Similar to Bethe-Heitler
dpT/dt ~ -(T3/Mq2) pT
J Friess, S Gubser, G Michalogiorgakis, S Pufu, Phys Rev D75 (2007)
– Very different from LPM
dpT/dt ~ -LT3 log(pT/Mq)
30-Jun-16
UCT Seminar
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Compared to Data
• String drag: qualitative agreement
WAH, PhD Thesis
30-Jun-16
UCT Seminar
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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 WAH,
saturates)
M. Gyulassy, PLB666 (2008)
– AdS/CFT ratio is flat and many times smaller than pQCD at only moderate pT
30-Jun-16
UCT Seminar
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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
Q
Worldsheet boundary
Spacelike if g > gcrit
x5
Trailing
String
“Brachistochrone”
– 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: “]”
30-Jun-16
D7 Probe Brane
UCT Seminar
“z”
D3 Black Brane
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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
30-Jun-16
UCT Seminar
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Conclusions
• Heavy Ion Physics is fascinating
– Jets provide a unique tool for study
• Usual theory techniques in quantitative
disagreement
– New, exciting tool with AdS/CFT, successes
• Experimental signature: RcAA/RbAA
• Recent and future work:
– Double ratio at RHIC WAH, J.Phys.G35:044025,2008
– Ultimately want to be able to rigorously falsify
•
•
•
•
30-Jun-16
Universality of AdS result WAH and Y Kovchegov, PLB680:56-61,2009
Uncertainties in pQCD WAH and B Cole, arXiv:0910.1823 [hep-ph]
Comparison to additional exp. observables
Corrections to pQCD and AdS/CFT
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