Falsyfing AdS/CFT Drag or pQCD Heavy Quark Energy Loss with A+A at RHIC and LHC
Nuclear Physics Seminar
William Horowitz (Columbia University)
We propose the novel observable of the double ratio of charm to bottom
nuclear modification factors as the only robust experimental measurement
that can falsify AdS/CFT, pQCD, or both formalisms as applied to
relativistic heavy ion reactions. Previous efforts to connect AdS/CFT
calculations with experiment focused on the entropy to viscosity ratio
which, due to its sensitivity to the uncontrolled initial conditions,
is a fragile probe of the ``perfection'' of the QGP fluid. The
nonphotonic electron puzzle poses a challenge to prevailing pQCD
prejudices but does not falsify pQCD methods. Imminent detector
upgrades in current experiments and the capabilities of near-future
colliders will for the first time allow identified heavy quark suppression
measurements. We show that current theoretical errors lead to weak
predictions for the individual heavy quark nuclear modification factors
but that these cancel to an astonishing degree in the ratio of the two,
leading to the possibility of experimental falsification. We will discuss
the ``speed limits'' of the classical supergravity approach, beyond which
the AdS/CFT techniques may not be applicable, from both field and string
theoretic perspectives. The double ratio signal will be most easily
observed at LHC, with its large momentum reach and relatively soft heavy
quark production spectra, but will also be measurable at RHIC, where the
lower medium temperature leads to higher speed limits.
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