Report from the Early Time Dynamics in Heavy Ion Collisions
McGill University, Montréal, Canada, July 16-19, 2007
By Agnes Mocsy
For the past few decades, a goal of the nuclear physics community has been to produce and characterize a new state of matter, the Quark-Gluon Plasma (QGP). In 1999 the Relativistic Heavy Ion Collider (RHIC) at BNL turned on and began producing a state of matter more remarkable than predicted. In April 2005 the discovery of strongly coupled QGP, which behaves as a perfect liquid, was announced and covered extensively by mainstream media. The current experimental results, however, still cannot be understood in a comprehensive and quantitative way in lack of understanding the early time dynamics in heavy ion collisions. By early times, we mean the time between the physical contact of two heavy nuclei and the formation of the locally thermalized QGP. So, on Sunday July 15th physicists from different countries gathered beside wine and cheese in the Physics Department of McGill University in Montréal, Canada, to kickoff the conference on Early Times Dynamics in Heavy Ion Collisions. Through the following four intensive days, experts from 35 research institutions and universities discussed the latest experimental results and theory developments with the aim to forge a coherent picture of the important early time era. Thanks to the generosity of our sponsors, BNL and McGill University, the graduate and some early post-graduate level conference participants (25 out of 86) received financial support.
RHIC accelerates counter-rotating beams of nuclei to nearly the speed of light. The speed flattens the nuclei into Lorentz-contracted “pancakes” which approach each other and collide at intersection points near the center of the detectors. The collision creates a zone of complicated interactions that evolves into a QGP - a thermalized soup of the nuclear constituents, quarks and gluons. The collision zone typically starts with the shape of an ellipse. As pressure develops it causes the ellipse to expand and become round so that matter moves faster along the shorter axis. This is known as elliptic flow (“v2”) and is experimentally observed. The QGP continues to expand and cool until it regroups into different particles, which subsequently stop interacting, and fly to the detectors where they are observed. Hydrodynamic simulations of heavy ion collisions have enjoyed success in describing some of the phenomena observed by the experiments. The discovery of the perfect liquid is largely based on the remarkable agreement of ideal (zero viscosity) hydro with data on elliptic flow. Our community's collective conclusion that we have created the QGP depends in great part on hydro models. These, however, have several theoretical uncertainties in their input assumptions.
One of the unresolved issues is the thermalization time. A successful description of the data is possible only if the hydro evolution of a QGP starts at the very early time (about 0.6 fm/c ≈ 2x10-24 seconds) after the collision. But how is this achieved and what processes are responsible for such a fast thermalization? During this time, incoming partons (quarks and gluons) must not only collide and produce secondary partons, but also the produced partons must scatter enough times to produce a locally thermalized matter. Are scatterings enough? Existing theoretical calculations of the thermalization time yield larger numbers than this requirement, and these larger times spoil the agreement with data. In this case it’s reasonable to scrutinize the other parameters hydro takes as input (such as the equation-of-state). One can attempt to obtain the thermalization time from the underlying theory of QCD. Progress in this direction has been made, but the thermalization time has not yet been calculated. Also Weibel instabilities, unstable modes in plasmas that may occur if the momentum distribution is anisotropic, could provide a mechanism to isotropize and thermalize the system. The growth of plasma instabilities is one plausible way to possibly speed up thermalization. Nowadays it is hard to find a conference that deals with QCD in extreme conditions, without a talk based on ADS/CFT. This meeting was no exception. The time of isotropization was calculated in this framework to be about 0.3 fm/c in agreement with the initial thermalization time of 0.6 fm/c of hydro. This is an intriguing result, but the relevance of calculations based on ADS/CFT to QCD phenomena is still not established.
Another important issue is to determine the initial conditions of the heavy ion collisions that will eventually evolve into the configuration at which to initialize the hydrodynamic models. It is customary to model the initial nuclear overlap density that initializes hydro with a naïve geometric model of the colliding nuclei called the Glauber model. Recent calculations of the initial overlap density based on first principle QCD calculations deviate from Glauber calculations. This non-perturbative program, under the name of Color Glass Condensate (CGC), is the study of nuclear wavefuctions at extremely high energies by starting from classical QCD and additionally applying quantum corrections. Significant recent progress has been made in understanding heavy ion collisions using CGC initial conditions, bringing us closer to a coherent picture of the initial times coming from first principles (see also the sketched figure): The two accelerated nuclei are more like two-dimensional CGC sheets, extended in the transverse direction, perpendicular to the direction of motion. In these sheets the density of gluons is high, and therefore can be treated as a classical electric and magnetic fields. The collision of the sheets and the interplay of the charge densities are such that in a very short time the fields change from purely transverse to entirely longitudinal. This state of matter is called glasma. The glasma fields are unstable, meaning that some of the quantum-fluctuations may grow exponentially. Relevant aspects of these glasma instabilities is that the growth of quantum fluctuations could isotropize and thermalize the system (much like instabilities do this in the inflationary early universe) possibly giving us a handle on the thermalization as well. Let me point out that the direct connection between Weibel instabilities and glasma instabilities is not clear at this point and needs further investigation. A still outstanding issue is the transition between classical field degrees of freedom of the glasma to quantum mechanical particle degrees of freedom.
CGC based calculations of the initial energy density used to initialize hydro lead to elliptic flow larger than observed in data. This is because the CGC leads to a larger initial eccentricity than in the Glauber model. With these large initial eccentricities, the amount of viscosity allowed by the data would be larger since including viscosity in the model will reduce the prediction for v2 and bring it back in line with data. The ability of the models to adjust several parameters to match data is unsettling and first principle calculations and precise measurements are needed to remove the various ambiguities in interpretation. Another difference between the CGC and Glauber initial conditions is the degree to which eccentricity fluctuates. The Glauber model predicts large eccentricity fluctuations, which will manifest themselves as large in the fluctuations of v2. Initial estimates of the maximum contribution of eccentricity fluctuations to v2 fluctuations seems to indicate that the eccentricity fluctuations are much smaller than what is expected from a Glauber model, favoring a CGC initial condition instead. Further data analysis, however, is required to settle this hotly debated topic.
Early time dynamics is further important to some of the probes of QGP. The products of hard processes (processes involving large momentum transfers), such as jets, high momentum photons, and heavy quarks, are formed during the early times then traverse the QGP probing this. The modification of their properties due to the medium can in principle provide a sharp picture of the QGP. For instance, from the attenuation of jet strength coming out of a heavy ion collision (“jet quenching”), one can infer the energy density of the underlying matter through the jet-quenching parameter, qhat. One of the first experimental discoveries coming from RHIC was the suppression of the number of high momentum particles produced in the collision of heavy nuclei compared to collisions of protons only, this being one of the first indication of the dense and opaque nature of the produced medium. As a conclusion: due to the large energy loss qhat is thought to be large, but no agreement on it’s precise value has been reached between the varieties of QCD-based models. A promising new development considers high transverse momentum photons produced in jet-medium interaction to directly observe the energy distribution of the quarks in QGP. Excitement in the community was also triggered by the possibility of connecting initial state instabilities to what is called an anomalous viscosity – which in turn is related to the jet quenching parameter. The idea is based on the theory of transport of particles in turbulent plasmas, characterized by regimes of instability, the effect of which is to reduce the viscosity of the plasma and greatly increase the energy loss of the particles.
The RHIC data showed us some highly unexpected results in the measured energy loss of heavy quarks, which turned out to be of the same order as the energy loss of light quarks. This was a priori not expected since the masses of heavy quarks are orders of magnitude greater than that of the light ones, which affects their velocity and how they radiate gluons in the medium, and therefore affects the expected loss of their energy. At this conference the mystery of heavy quark energy loss was still debated. Furthermore, for twenty years suppression of bound states formed from heavy quark and its antiquark (“quarkonium”) were thought to give the golden signal for QGP formation. The state of the art presented at the meeting revealed that the wealth of the experimental data available for quarkonia suppression is not explainable with any of the currently existing theoretical models. Studying the initial time probes, as a matter of course, requires good understanding of their production at earliest times in heavy ion collisions.
Great interest is triggered by the experimental measurement of correlations narrow in the azimuth and wide in the longitude (“rapidity”) direction (the “ridge”). A possible theoretical explanation of these lies in the glasma instabilities mentioned above, which, in a timescale proportional with the inverse of the gluon density per unit area in the nuclear wavefunction (1/Qs), can grow to be of the order of the initial longitudinal fields, giving rise to a transverse component for the magnetic field. The strong transverse magnetic field in turn can bend the trajectory of a jet in the longitudinal direction. These jet-deflections could show up in the data as correlations in rapidity. At this point however, the direct link between the theoretical instabilities to the experimentally observed ridge is not yet established. If such direct relationship is proven then we would have an experimental handle on the glasma, and thus on the initial stage of heavy ion collisions described as an unstable color field configuration.
It is clear that understanding the initial times remains a barrier to understanding the properties of the matter that are created in heavy ion collisions. The conference in Montréal gathered some of the brightest and most creative minds in our fields to discuss the different aspects related to the initial stages. Younger as well as more established scientists, theorists as well as experimentalists, have been intensely discussing the progress made in this direction in the past decade. In a comfortable venue and relatively small and friendly setting 47 talks and 14 posters have been presented between July 16 -19. Also a special lunch discussion-session has been held. From this meeting the initial seeds of understanding seem to be growing into what is a coherent and self-consistent picture based soundly in QCD of the important early time era in the evolution of the heavy ion collisions.
Reference: The agenda of the meeting and other related details can be found here.