Missing Navigational Beacons

In designing the PHENIX experiment circa 1990, much care was giving to maintaining good acceptance and mass resolution for the “onium” states. At that time, following the famous Matsui/Satz paper, the thinking was the Debye screening in the plasma would be signaled by suppression of the J/Psi, while the much smaller radius for the upsilon would argue against dissociation at the energy densities expected for RHIC. In the PHENIX Conceptual Design Report we wrote

“The upsilon serves as a calibration point in the measurement of the screening length since its radius is much smaller than any estimated plasma screening radius at expected temperatures, meaning it may well never be suppressed at plasma temperatures attainable at RHIC.”

That is, the upsilon was expected to serve as a navigational beacon, assuring us that our normalization of hard processes in comparing Au+Au to p+p collisions was correct.

Since then, RHIC phenomena have proven to be far more striking then we ever could have imagined. Not the least of these surprises has been the strong coupling of heavy flavor to the “bulk”. Conservative physicists argue that this is true only for charm, with bottom being so heavy that it is unlikely to flow and lose energy. Very considerable efforts are underway to upgrade PHENIX and STAR to provide the detector technology necessary to perform the b/c separation.

In  a recent preprint Thermal Width of the Upsilon at Large ‘t Hooft  Coupling Noronha and Dumitru argue that the thermal width of the upsilon is large enough that it too could be suppressed at RHIC. Their calculation is made in an AdS/CFT model and is instructive in several aspects. The setup is closely related to the one of a hanging string between two massive (static) ‘quarks’ first calculated by Maldacena at T=0.  The result is Coulombic, that is, for separations L the potential ~ 1/L. This is as it must be on the CFT side- if there is no other scale in the theory, energies must go as 1/L. The interesting part is understanding how this arises in the string setup on the AdS side. Since the string length increases as L increases, why doesn’t the potential also grow with L? This is where AdS does its magic- the string hanging down into the 5th dimension is in a highly warped rather than flat spacetime. The warp factor in the AdS metric is precisely what is needed to produce the 1/L required by the CFT side. (See Section 5.1 of Maldacena’s TASI lectures for a clear discussion from the master on this.)

Noronha and Dumitru extend this formalism to a QGP via the ‘usual’  trick of thermalizing it with a black brane located at some coordinate Uh in the 5th dimension. The value of Uh is proportional to the temperature T in the conformal theory. For small T (small Uh), the brane is very far ‘down’ in the bulk, and the long wavelength thermal vibrations of the ‘bottom’ of the string don’t know much about the presence of the horizon (keep in mind that the strings in this geometry are much more ‘U’ shaped than ‘V’ shaped). However, as the temperature is increased, two things happen: The horizon moves closer to the boundary at infinite U where the string endpoints (quarks) are located, and, because the temperature is higher, the amplitude of thermal vibrations of the string bottom become larger. At some temperature the lowest-mode oscillation of the string touches the horizon, which means the potential acquires an imaginary part, that is, the state has a non-zero width. Given the above arguments, it is not surprising that this width is very sensitive to the temperature, going at (LT)4 times the conformally-mandated energy  scale 1/L.

 Somewhat surprisingly, the numerical value of the width ~50 MeV for upsilons is not hugely different from that calcualted via pQCD. However, the parameteric dependence is both the coupling constand the temperature is very different, with the AdS/CFT results having a much higher dependence on T. The authors estimate that RAA for upsilons may be as small as 0.3, leading to the loss of the navigational beacon- but this loss may be another’s gain.

Does AdS/QCD really help us understand physics at RHIC ?

As the number of theoretical manuscripts applying AdS/CFT duality techniques to the physics of relativistic heavy ion collisions (and other QCD phenomena) proliferates, it is still unclear which aspects of the dual models in the strong ‘t Hooft coupling limit shed light on real physics governed by QCD. (more…)

Quark Matter 2009 Highlights

The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, colloquially referred to as Quark Matter 2009, took place in Knoxville, Tennessee from March 30th to April 4th, 2009. (more…)

What do we know about the shear-viscosity of QCD matter?

The success of ideal Relativistic Fluid Dynamics (RFD) in describing hadron spectra and elliptic flow at RHIC has led to a strong interest in the transport coefficients of QCD, in particular the shear- and bulk-viscosity as well as the shear-viscosity over entropy-density ratio η/s. Of course ideal RFD assumes η to be zero, which is unphysical, but its success has hinted at a very small value for η/s. The purpose of this post is to review what we currently know about η/s of QCD matter and put the different approaches used to study this quantity into context.


AdS/CFT duality and hard processes

Up to now, the AdS/CFT duality has been used to model thermal transport processes in a strongly coupled gauge plasma (see the Review in this VJ). It was unclear what theories with an AdS/CFT dual predict for hard processes, for which QCD permits rigorous calculations. In several recent articles,

Hatta et al. have shown how such processes can be explored in strongly coupled super-Yang Mills theories with a gravity dual. (more…)

What can super-YM theory teach us about thermal QCD ?

The AdS/CFT duality has allowed theorists to calculate previously incalculable quantities in a strongly coupled gauge theory (see the Review in this VJ), albeit one with infinitely many colors. However, the theoretical magic comes at a price: The (N=4) super-Yang Mills (SYM) theory is conformally invariant while QCD has an intrinsic scale, which allows it to be simultaneous confining at long distances and weakly coupled at short distances. The strongly coupled SYM theory, in its purest form, has neither of these properties. What are the phenomenological consequences of this difference between the two theories with regard to quantities of interest to relativistic heavy ion physics? (more…)

What a Difference a Term Makes

The recent post on the AMO competition for perfect fluidity offers a wonderful opportunity to compare and contrast the techniques of condensed matter physics versus those of relativistic heavy ion physics. (more…)

The Quest for the Ideal Fluid: Competition from Li-Atoms

In April of 2005 Brookhaven National Laboratory announced that scientists at RHIC had created the most ideal liquid ever observed in nature. Now, 3 years later, RHIC is facing competition to this claim from strongly interacting ultra-cold Fermi gases. (more…)

The String Theory Connection of RHIC

The idea that superstring theory may help inform the physics of relativistic heavy ion collisions begins with Maldacena’s remarkable discovery in 1997 that the (N = 4) supersymmetric Yang-Mills theory in (3+1)-dimensional space-time and 10-dimensional superstring theory in a geometrical background composed of the product of 5-dimensional Anti-deSitter space (AdS) and a 5-dimensional sphere describe the same physics. (more…)