Fast Partons Research Articles

The wake potential in the viscous quark–gluon plasma

Abstract Based on the dielectric function derived from the viscous chromohydrodynamic approach, we investigate the wake potential induced by a fast parton traveling through the viscous quark–gluon plasma. An oscillatory wake potential and a Lennard-Jones potential are found in the backward direction for the fast parton speeds v = 0.99 c and v = 0.55 c , respectively. In the forward direction, there is a modified Coulomb screening potential for both two speeds. When v = 0.99 c , shear viscosity makes the oscillation of potential more remarkable with the increase of η / s . While v = 0.55 c , the viscous effects on the wake potential are very trivial in both the forward and backward directions. Finally, we give some explanations for the speed-dependent viscous effects on the wake properties.

Asymptotic behavior of double parton distribution functions

The double parton distribution functions are investigated in the region of small longitudinal momentum fractions in the leading logarithm approximation of perturbative QCD. It is shown that these functions have the factorization property in the case of one slow and one fast parton.

Finite-size effects on the radiative energy loss of a fast parton in hot and dense strongly interacting matter

We consider finite-size effects on the radiative energy loss of a fast parton moving in a finite temperature strongly interacting medium, using the light cone path integral formalism put forward by Zakharov. We present a convenient reformulation of the problem which makes possible its exact numerical analysis. This is done by introducing the concept of a radiation rate in the presence of finite-size effects. This effectively extends the finite-temperature approach of AMY (Arnold, Moore, and Yaffe) to include interference between vacuum and medium radiation. We compare results with those obtained in the regime considered by AMY, with those obtained at leading order in an opacity expansion, and with those obtained deep in the LPM (Landau-Pomeranchuk-Migdal) regime.

Effects of expansion and equation of state on the conical propagation in the QGP

We study the dynamics of the QGP fluid in terms of the momentum-weighted angular distribution generated by a fast parton propagating in the medium. We analyze the effects of the change of the speed of sound during the expansion of the system. We also discuss the effect of transverse expansion and energy–momentum deposition rates on the appearance of peaks in the distribution.

Transition in Yield and Azimuthal Shape Modification in Dihadron Correlations in Relativistic Heavy Ion Collisions

Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements of π^{0} trigger particles at transverse momenta p{T}{t}=4-12 GeV/c and associated charged hadrons (p{T}{a}=0.5-7 GeV/c) vs relative azimuthal angle Δϕ in Au+Au and p+p collisions at sqrt[s{NN}]=200 GeV. The Au+Au distribution at low p{T}{a}, whose shape has been interpreted as a medium effect, is modified for p{T}{t}<7 GeV/c. At higher p{T}{t}, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p{T}{a}, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in Au+Au relative to p+p (I{AA}) is suppressed at high p{T} (I{AA}≈0.35-0.5), but less than for inclusive suppression (R{AA}≈0.2).

Momentum broadening of a fast parton in a perturbative quark-gluon plasma

The average transverse momentum transfer per unit path length to a fast parton scattering elastically in a perturbative quark-gluon plasma is related to the radiative energy loss of the parton. We first calculate the momentum transfer coefficient $\stackrel{^}{q}$ in terms of a classical Langevin problem and then define it quantum mechanically through a scattering matrix element. After treating the well-known case of a quark-gluon plasma in equilibrium, we consider an off-equilibrium unstable plasma. As a specific example, we treat the two-stream plasma with unstable modes of longitudinal chromoelectric field. In the presence of the instabilities, $\stackrel{^}{q}$ is shown to exponentially grow in time.

Non-abelian effects on wake potential in quark–gluon plasma

In this paper, based on the dielectric function calculated with the hard thermal loop (HTL) resummation approach, we study the wake potential induced by a fast parton traveling through quark–gluon plasma (QGP). In contrast to the wake potential calculated in the HTL approximation, our results show that non-abelian effects from the resummation calculation enhance the anisotropy of the wake potential. By investigating the relation between the anisotropic properties of the wake potential and the imaginary part of the dielectric function, we give some explanations for the enhancement of the anisotropy.

The sound generated by a fast parton in the quark-gluon plasma is a crescendo

The total energy deposited into the medium per unit length by a fast parton traversing a quarkgluon plasma is calculated. We take the medium excitation due to collisions to be given by the well known expression for the collisional drag force. The parton's radiative energy loss contributes to the energy deposition because each radiated gluon acts as an additional source of collisional energy loss in the medium. In our model, this leads to a length dependence on the differential energy loss due to the interactions of radiated gluons with the medium. The final result, which is a sum of the primary and the secondary contributions, is then treated as the coefficient of a local hydrodynamic source term. Results are presented for energy density wave induced by two fast, back-to-back partons created in an initial hard interaction.

Partonic effects on dijet correlations in relativistic heavy ion collisions

Within a parton transport model, we show that diffusion wakes and Mach cone wakes produced from energy loss by a fast parton propagating in the quark-gluon plasma lead to the observed azimuthal angular correlations of hadrons in central $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC. The magnitude and shape of the wakes are shown to be sensitive measures of the initial parton density and the parton scattering cross section. The shear viscosity to entropy density ratio, $\ensuremath{\eta}/s$, of the plasma is found to be remarkably similar to the lower bound $\ensuremath{\eta}/s=1/4\ensuremath{\pi}$ obtained in the AdS/CFT conjecture.

Medium-modified evolution of multiparticle production in jets in heavy-ion collisions

The energy evolution of medium-modified average multiplicities and multiplicity fluctuations in quark and gluon jets produced in heavy-ion collisions is investigated from a toy QCD-inspired model. In this model, we use modified splitting functions accounting for medium-enhanced radiation of gluons by a fast parton which propagates through the quark–gluon plasma. The leading contribution of the standard production of soft hadrons is found to be enhanced by the factor while next-to-leading order (NLO) corrections are suppressed by , where the nuclear parameter Ns > 1 accounts for the induced soft gluons in the hot medium. The role of next-to-next-to-leading order corrections (NNLO) is studied, and the large amount of medium-induced soft gluons are found to drastically affect the convergence of the perturbative series. Our results for such global observables are cross-checked and compared with their limits in the vacuum, and a new method for solving the second multiplicity correlator evolution equations is proposed.

Sound Produced by a Fast Parton in the Quark-Gluon Plasma is a “Crescendo”

We calculate the total energy deposited into the medium per unit length by fast partons traversing a quark-gluon plasma. The medium excitation due to collisions is taken to be given by the well-known expression for the collisional drag force. The radiative energy loss of the parton contributes to the energy deposition because each radiated gluon acts as an additional source of collisional energy loss in the medium. We derive a differential equation which governs how the spectrum of radiated gluons is modified when this energy loss is taken into account. This modified spectrum is then used to calculate the additional energy deposition due to the interactions of radiated gluons with the medium. Numerical results are presented for the medium response for the case of two energetic back-to-back partons created in a hard interaction.

Medium-modified average multiplicity and multiplicity fluctuations in jets

The energy evolution of average multiplicities and multiplicity fluctuations in jets produced in heavy-ion collisions is investigated from a toy QCD-inspired model. In this model, we use modified splitting functions accounting for medium-enhanced radiation of gluons by a fast parton which propagates through the quark–gluon plasma. The leading contribution of the standard production of soft hadrons is enhanced by a factor \(\sqrt{N_{s}}\) while next-to-leading order (NLO) corrections are suppressed by \(1/\sqrt {N_{s}}\) , where the parameter Ns>1 accounts for the induced soft gluons in the medium. Our results for such global observables are cross-checked and compared with their limits in the vacuum.

Mach cones in the quark-gluon plasma: Viscosity, speed of sound, and effects of finite source structure

I use the space-time distribution of energy and momentum deposited by a fast parton traversing a perturbative quark-gluon plasma as a source term for the linearized hydrodynamical equations of the medium. A method of solution for the medium response is presented in detail. Numerical results are given for different values of the shear viscosity to entropy density ratio, $\ensuremath{\eta}/s$, and speed of sound, ${c}_{s}$. Furthermore, I investigate the relevance of finite source structure by expanding the source term up to first order in gradients of a \ensuremath{\delta} function centered at the fast parton and comparing the resulting dynamics to that obtained with the full source. It is found that, for the source term used here, the medium response is sensitive to the finite source structure up to distances of several femtometers from the source parton.

Study of photon-tagged jet events in high-energy heavy ion collisions with CMS

The energy loss of fast partons traversing the strongly interacting matter produced in high energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The collisional and radiative energy loss of the partons will modify the fragmentation functions depending on the path length in the medium. Pb + Pb collisions at \(\sqrt{s_{NN}}=5500\) GeV at the LHC will allow detailed measurements of the in-medium modifications of fragmentation functions of parton initiated jets, using the γ-jet channel. Since the photon does not strongly interact with the medium, the initial transverse energy of the fragmenting parton can be related to the photon transverse energy. This in turn allows for precision studies of the fragmentation function underlying these jets.

Comparing different freeze-out scenarios in azimuthal hadron correlations induced by fast partons

I review the linearized hydrodynamical treatment of a fast parton traversing a perturbative quark–gluon plasma. Using numerical solutions for the medium’s response to the fast parton, I obtain the medium’s distribution function, which is then used in a Cooper–Frye freeze-out prescription to obtain an azimuthal particle spectrum. Two different freeze-out scenarios are considered which yield significantly different results. I conclude that any meaningful comparison of azimuthal hadron correlation functions to RHIC data requires implementing a realistic freeze-out scenario in an expanding medium.

Generation of complete events containing very high-p T jets

The study of very high transverse-momentum jets will be an important issue at the LHC, in particular since the corresponding cross sections will be considerably larger than at RHIC energies. Jets are expected to provide information on QGP formation, due to the energy loss of fast partons in the medium. Jet cross sections can in principle be compared to simple pQCD calculations, based on the hypothesis of factorization. But often it is useful or even necessary to not only compute the production rate of the very high-p T jets, but in addition the “rest of the event”. The proposed talk is based on recent work, where we try to construct an event generator—fully compatible with pQCD—which allows one to compute complete events, consisting of high-p T jets plus all the other low p T particles produced at the same time. Whereas in “generators of inclusive spectra” like Pythia one may easily trigger on high-p T phenomena, this is not so obvious for “generators of physical events”, where in principle one has to generate a very large number of events in order to finally obtain rare events (like those with a very high-p T jet). We shall discuss how we overcome these difficulties in the framework of the EPOS model.

Fast partons as a source of energy and momentum in a perturbative quark-gluon plasma

I derive the space-time distribution of energy and momentum deposited by a fast parton traversing a perturbative quark-gluon plasma by considering the fast parton as the source of an external color field interacting with the medium. I include the medium's response to screen the fast parton's color field by incorporating dielectric functions and compare to the unscreened result.

Sonic Mach cones induced by fast partons in a perturbative quark-gluon plasma

We derive the space-time distribution of energy and momentum deposited by a fast parton traversing a weakly coupled quark-gluon plasma by treating the fast parton as the source of an external color field perturbing the medium. We then use our result as a source term for the linearized hydrodynamical equations of the medium. We show that the solution contains a sonic Mach cone and a dissipative wake if the parton moves at a supersonic speed.

The sound of a fast parton in QCD

The spacetime distribution of energy and momentum deposited by a fast parton traversing a weakly coupled quark-gluon plasma is calculated by treating the fast parton as the source of an external color field perturbing the medium. When the result is used as a source term in the linearized hydrodynamical equations of the medium, the solution is found to contain a sonic Mach cone and a dissipative wake if the parton moves at supersonic speed.

Limiting fragmentation from scale-invariant merging of fast partons

Exploiting the idea that the fast partons of an energetic projectile can be treated as sources of colour radiation interpreted as wee partons, it is shown that the recently observed property of extended limiting fragmentation implies a scaling law for the rapidity distribution of fast partons. This leads to a picture of a self-similar process where, for fixed total rapidity Y, the sources merge with probability varying as 1/y.