arXiv subjects
Publications and source records attributed to Giorgio Torrieri.
We give a pedagogical introduction (suitable to upper level physics undergraduates) to the field of ultrarelativistic heavy ion collisions. We pay particular attention to our understanding of the thermodynamic and hydrodynamic properties of the matter created in heavy ion collisions at RHIC energies.
We explore the effect of hadronization by partonic coalescence on a "conical" signal at the partonic level. We show that, by transferring partons from a lower to a higher $p_T$, coalescence makes the conical signal stronger and hence less susceptible to thermal smearing, provided the signal is integrated over a large momentum bin and effects such as non-collinearity and a finite Wigner function width are taken into account. We explore the role of this effect in baryon/meson scaling and calculate the effect of resonances decays on such a conical signal.
We use (3+1)-dimensional ideal hydrodynamics to describe a variety of different jet energy loss scenarios for a jet propagating through an opaque medium. The conical correlations obtained for fully stopped jets, revealing a Bragg peak, are discussed as well as results from pQCD and AdS/CFT. Moreover, we investigate transverse flow deflection. It is demonstrated that a double-peaked away-side structure can be formed due to the different contributions of several possible jet trajectories through an expanding medium.
We propose to use the Kolmogorov-Smirnov test to uncover non-statistical differences between events created in heavy ion collisions within the same centrality class. The advantage of the method over other approaches which are currently in use, is that it is sensitive to any difference between the events and is not restricted to simple moments of the distribution of hadrons. The particular application examined here is the identification of the fireball decay due to spinodal fragmentation and/or sudden rise of the bulk viscosity.
We show that the five-fold constraints due to (1) the observed nuclear modification of heavy quark jets measured via non-photonic electrons $R_{AA}^e(p_T \sim 6 {\rm GeV})$ in central Au+Au collisions at 200 AGeV, (2) the "perfect fluid' elliptic transverse flow of low transverse momenta pions, $v_2(p_T\sim 1 {\rm GeV})$ reported for noncentral collisions, (3) the global pion rapidity density $dN_π/dy$, (4) the lattice QCD entropy density deficiency, $S/S_{SB}$, of strongly coupled Quark-Gluon Plasmas (sQGP), and (5) a causal requirement are analytically correlated in a class of gauge/string gravity dual models of sQGP dynamics. Current RHIC/BNL and lattice QCD data are found to be remarkably compatible with these models if the t'Hooft and Gauss-Bonnet coupling parameters lie in the range $λ\approx 10-25$ and $0< λ_{GB}< 0.09$. In addition, the observed five-fold correlation appears to favor color glass condensate over Glauber initial conditions within current systematic errors.
Multiplicity fluctuations and correlations are calculated within thermalized relativistic ideal quantum gases. These are shown to be sensitive to the choice of statistical ensemble as well as to the choice of acceptance window in momentum space. It is furthermore shown that global conservation laws introduce non-trivial correlations between disconnected regions in momentum space, even in the absence of any dynamics.
We propose an identified heavy quark jet observable to discriminate between weakly coupled pQCD and strongly coupled AdS/CFT models of quark gluon plasma dynamics in ultra-relativistic nuclear collisions at RHIC and LHC energies. These models are shown to predict qualitatively different associated hadron correlations with respect to tagged heavy quark jets. While both models feature similar far zone Mach and diffusion wakes, the far zone stress features are shown to be too weak to survive thermal broadening at hadron freeze-out. However, these models differ significantly in a near zone "Neck" region where strong chromo-fields sourced by the heavy quark jet couple to the polarizable plasma. Conical associated correlations, if any, are shown to be dominated by the jet induced transverse flow in the Neck zone and unrelated to the weak far zone wakes. Unlike in AdS/CFT, we show that the induced transverse flow in the Neck zone is too weak in pQCD to produce conical correlations after Cooper-Frye freeze-out. The observation of conical correlations violating Mach's law would favor the strongly-coupled AdS/CFT string drag dynamics, while their absence would favor weakly-coupled pQCD-based chromo-hydrodynamics.
We show that far zone Mach and diffusion wake ``holograms'' produced by supersonic strings in anti--de Sitter space/conformal field theory (AdS/CFT) correspondence do not lead to observable conical angular correlations in the strict $N_c\to\infty$ supergravity limit if Cooper-Frye hadronization is assumed. However, a special {\em nonequilibrium} ``neck'' zone near the jet is shown to produce an apparent sonic boom azimuthal angle distribution that is roughly independent of the heavy quark's velocity. Our results indicate that a measurement of the dependence of the away-side correlations on the velocity of associated identified heavy quark jets at the BNL Relativistic Heavy Ion Collider and CERN LHC will provide a direct test of the nonperturbative dynamics involved in the coupling between jets and the strongly-coupled Quark-Gluon Plasma (sQGP) implied by AdS/CFT correspondence.
We propose an application of the Kolmogorov-Smirnov test for rapidity distributions of individual events in ultrarelativistic heavy ion collisions. The test is particularly suitable to recognise non-statistical differences between the events. Thus when applied to a narrow centrality class it could indicate differences between events which would not be expected if all events evolve according to the same scenario. In particular, as an example we assume here a possible fragmentation of the fireball into smaller pieces at the quark/hadron phase transition. Quantitative studies are performed with a Monte Carlo model capable of simulating such a distribution of hadrons. We conclude that the Kolmogorov-Smirnov test is a very powerful tool for the identification of the fragmentation process.
We give an overview of the phenomenology of strangeness enhancement in heavy ion collisions, within the paradigm of the statistical model of particle production. We argue that, while strangeness enhancement data is suggestive of a phase transition, the mechanism triggering enhancement is still elusive. We study the feasibility to constrain this mechanism by determining the scaling variable of strangeness enhancement. We further argue that to test the applicability of the statistical model generally, and to confirm our interpretation of the statistical physics responsible for strangeness enhancement, the scaling of strange particle fluctuations ($K/π$ and other particles) w.r.t. yields has to be explored.
We solve (3+1)-dimensional ideal hydrodynamical equations with source terms that describe punch-through and fully stopped jets in order to compare their final away-side angular correlations in a static medium. For fully stopped jets, the backreaction of the medium is described by a simple Bethe-Bloch-like model which leads to an explosive burst of energy and momentum (Bragg peak) close to the end of the jet's evolution through the medium. Surprisingly enough, we find that the medium's response and the corresponding away-side angular correlations are largely insensitive to whether the jet punches through or stops inside the medium. This result is also independent of whether momentum deposition is longitudinal (as generally occurs in pQCD energy loss models) or transverse (as the Bethe-Bloch formula implies). The existence of the diffusion wake is therefore shown to be universal to all scenarios where momentum as well as energy is deposited into the medium, which can readily be understood in ideal hydrodynamics through vorticity conservation. The particle yield coming from the strong forward moving diffusion wake that is formed in the wake of both punch-through and stopped jets largely overwhelms their weak Mach cone signal after freeze-out.
We study the fate of the energy deposited by a jet in a heavy ion collision assuming that the medium created is opaque (jets quickly lose energy) and its viscosity is so low that the energy lost by the jet is quickly thermalized. The expectation is that under these conditions the energy deposited gives rise to a Mach cone. We argue that, in general, the behavior of the system is different from the naive expectation and it depends strongly on the assumptions made about the energy and momentum deposited by the jet into the medium. We compare our phenomenological hydrodynamic calculations performed in a static medium for a variety of energy-momentum sources (including a pQCD-based calculation) with the exact strong coupling limit obtained within the AdS/CFT correspondence. We also discuss the observability of hydrodynamical features triggered by jets in experimentally measured two-particle correlations at RHIC.
We give an review the HBT puzzle, and argue that its resolution requires the introduction of new physics close to the phase transition scale. We argue that a candidate for this new physics is bulk viscosity, recently postulated to peak, and even diverge, close to the phase transition temperature. We show that such a viscosity peak can force the system created in heavy ion collisions to become unstable, and filament into fragments whose size is weakly dependent on the global size of the system, thereby triggering freeze-out.
We compare away-side hadron correlations with respect to tagged heavy quark jets computed within a weakly coupled pQCD and a strongly coupled AdS/CFT model. While both models feature similar far zone Mach and diffusion wakes, the far zone stress features are shown to be too weak to survive thermal broadening at hadron freeze-out. Observable away-side conical correlations are dominated by the jet-induced transverse flow in near zone "Neck" region, which differs significantly for both models. Unlike in AdS/CFT, the induced transverse flow in the Neck zone is too weak in pQCD to produce conical correlations after Cooper-Frye freeze-out. The observation of conical correlations violating Mach's law would favor the strongly-coupled AdS/CFT string drag dynamics, while their absence would favor weakly-coupled pQCD-based hydrodynamics.
The near zone energy-momentum tensor of a supersonic heavy quark jet moving through a strongly-coupled $\mathcal{N}=4$ SYM plasma is analyzed in terms of first-order Navier-Stokes hydrodynamics. It is shown that the hydrodynamical description of the near quark region worsens with increasing quark velocities. For realistic quark velocities, $v=0.99$, the non-hydrodynamical region is located at a narrow band surrounding the quark with a width of approximately $3/πT$ in the direction parallel to the quark's motion and with a length of roughly $10/πT$ in the perpendicular direction. Our results can be interpreted as an indication of the presence of coherent Yang-Mills fields where deviation from hydrodynamics is at its maximum. In the region where hydrodynamics does provide a good description of the system's dynamics, the flow velocity is so small that all the nonlinear terms can be dropped. Our results, which are compatible with the thermalization timescales extracted from elliptic flow measurements, suggest that if AdS/CFT provides a good description of the RHIC system, the bulk of the quenched jet energy has more than enough time to locally thermalize and become encoded in the collective flow. The resulting flow pattern close to the quark, however, is shown to be considerably different than the superposition of Mach cones and diffusion wakes observed at large distances.
We solve the relativistic Navier-Stokes equations with homogeneous boost-invariant boundary conditions, and perform a stability analysis of the solution. We show that, if the bulk viscosity has a peak around $T_c$ as inferred from QCD-based arguments, the background solution "freezes" at $T_c$ to a nearly constant temperature state. This state is however highly unstable with respect to certain inhomogeneous modes. Calculations show that these modes have enough time to blow up and tear the system into droplets. We conjecture that this is how freeze-out occurs in the QGP created in heavy ion collisions, and perhaps similar transitions in the early universe.
We show that Mach-like correlations from heavy quark jets in the Gubser et al AdS/CFT string drag model arise not from the expected (weak) hydrodynamical Mach sound wakes zone but from a near ``Neck'' zone where strong chromo-viscous field effects dominated and lead to a distinctive stress in the plasma of the form $T^{μν}_{Neck} \propto \surd λT_0^2 Y^{μν}(x)/x^2$. We propose that measurement of the jet velocity dependence of moderate $p_T$ hadrons associated with future {\em identified heavy quark jets} at RHIC and LHC could be used to look for this novel source of conical correlations which are unrelated to Mach's law.
The energy-momentum tensor of a supersonic heavy quark jet moving through a strongly-coupled $\mathcal{N}=4$ SYM plasma is analyzed in terms of first-order Navier-Stokes hydrodynamics. We focus on the near zone ``head'' region close to the heavy quark where deviations from hydrodynamic behavior are expected to be large. For realistic quark velocities, $v=0.99$, we find that the non-hydrodynamical head region is confined at a narrow pancake surrounding the jet with a width $\sim 1/T$ and transverse radius $\sim 3/T$. Outside this jet head region, the jet induced stress is well approximated by Navier-Stokes hydrodynamics.