Parton Rescattering Research Articles

Hard diffraction from parton rescattering in QCD

We analyze the QCD dynamics of diffractive deep inelastic scattering. The presence of a rapidity gap between the target and diffractive system requires that the target remnant emerges in a color singlet state, which we show is made possible by the soft rescattering of the struck quark. This rescattering is described by the path-ordered exponential (Wilson line) in the expression for the parton distribution function of the target. The multiple scattering of the struck parton via instantaneous interactions in the target generates dominantly imaginary diffractive amplitudes, giving rise to an "effective pomeron" exchange. The pomeron is not an intrinsic part of the proton but a dynamical effect of the interaction. This picture also applies to diffraction in hadron-initiated processes. Due to the different color environment the rescattering is different in virtual photon- and hadron-induced processes, explaining the observed non-universality of diffractive parton distributions. This framework provides a theoretical basis for the phenomenologically successful Soft Color Interaction model which includes rescattering effects and thus generates a variety of final states with rapidity gaps. We discuss developments of the SCI model to account for the color coherence features of the underlying subprocesses.

Determination of azimuthal features of jets without measuring the angle of the reaction plane

A new method is proposed for determining the coefficient of elliptic anisotropy of jets in ultrarelativistic heavy-ion collisions. This method, which does not involve directly reconstructing the reaction plane, is a generalization of the method used in current experiments to measure the coefficient of elliptic anisotropy of particle fluxes. It is shown that, for the spectrum of hadronic jets, the method makes it possible to explore the azimuthal asymmetry caused by the rescattering of hard partons and by their energy losses in dense quark-gluon matter formed in the region of the initial overlap of nuclei in collisions at a nonzero value of the impact parameter—in particular, under the conditions of the CMS experiment at LHC.

Net baryon density in Au+Au collisions at the relativistic heavy ion collider.

We calculate the net-baryon rapidity distribution in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) in the framework of the parton cascade model (PCM). Parton rescattering and fragmentation leads to a substantial increase in the net-baryon density at midrapidity over the density produced by initial primary parton-parton scatterings. The PCM is able to describe the measured net-baryon density at RHIC.

Photon Radiation Induced by Multiple Parton Rescattering inDeeply Inelastic Scattering

Photon radiation induced by multiple parton rescattering andcorresponding parton energy loss in eA deeply inelastic scattering areinvestigated by using the generalized factorization of higher twistparton distributions beyond the helicity amplitude approximation. Itturns out that the behaviour of the nuclear size dependence of theparton energy loss is different in the photon and gluon radiation cases.The parton energy loss due to photon radiation depends linearly,instead of quadratically, on nuclear size due to gluon radiation.

Parton rescattering and colour screening in Au+Au collisions at RHIC

We study the microscopic dynamics of quarks and gluons in relativistic heavy ion collisions in the framework of the Parton Cascade Model. We use lowest order perturbative QCD cross sections with fixed lower momentum cutoff p T min ⁡ . We calculate the time-evolution of the Debye-screening mass μ D for Au+Au collisions at √s = 200 GeV per nucleon pair. We also determine the energy density reached through hard and semi-hard processes at RHIC and analyze the extent of perturbative rescattering among partons.

Parton rescattering and screening in Au + Au collisions at RHIC

We study the microscopic dynamics of quarks and gluons in relativistic heavy ion collisions in the framework of the Parton Cascade Model. We use lowest order perturbative QCD cross sections with fixed lower momentum cutoff pminT. We calculate the time-evolution of the Debye-screening mass μD for Au+Au collisions at s=200 GeV per nucleon pair. The screening mass is used to determine a lower limit for the allowed range of pminT. We also determine the energy density reached through hard and semi-hard processes at RHIC, obtain a lower bound for the rapidity density of charged hadrons produced by semi-hard interactions, and analyze the extent of perturbative rescattering among partons.

Azimuthal asymmetry of jet production as a signal of parton energy losses in semicentral heavy-ion collisions

The question is investigated of whether an azimuthal asymmetry in the hadron-jet spectra can arise because of rescattering and energy losses of partons produced via hard processes in a dense quark-gluon matter formed in the region of the initial nuclear overlap in collisions characterized by a nonzero value of the impact parameter. Methods are discussed for determining the reaction-plane angle in ultrarelativistic heavy-ion collisions with the aid of the flux of semihard particles.

Initial conditions and charged multiplicities in ultrarelativistic heavy-ion collisions

At ultra-relativistic energies the minijet production in heavy-ion collisions becomes sensitive to semi-hard parton rescatterings in the initial stages of the process. As a result global characteristics of the event, like the initial minijet density, become rather insensitive on the infrared cutoff that separates hard and soft interactions. This allows to define a nearly parameter-free {\it saturation cutoff} at which the initial conditions may be computed. As an application we study the centrality dependence of the charged particle multiplicity, which is compared with present RHIC data and predicted at higher energies.

Minijet transverse spectrum in high-energy hadron-nucleus collisions

Hadron-nucleus collisions at CERN LHC energies are studied by including explicitly semihard parton rescatterings in the collision dynamics. Under rather general conditions, we obtain explicit formulas for the semihard cross section and the inclusive minijet transverse spectrum. As an effect of the rescatterings the spectrum is lowered at small ${p}_{t}$ and is enhanced at relatively large transverse momenta, the deformation being more pronounced at increasing rapidity. Its study allows to test the proposed interaction mechanisms and represents an important baseline to examine nucleus-nucleus collisions.

Hard parton rescatterings and minijets in nuclear collisions at the CERN LHC

The average number of minijets and the corresponding transverse energy produced in heavy ion collisions are evaluated by including explicitly semi-hard parton rescatterings in the dynamics of the interaction. At the CERN LHC semi-hard rescatterings have a sizable effect on the global characteristics of the typical inelastic event. An interesting feature is that the dependence on the cutoff which separates soft and hard parton interactions becomes less critical after taking rescatterings into account.

Semi-hard parton rescatterings in nuclear collisions at very high energies

Representing the semi-hard partonic interactions by the exchange of Lipatov's perturbative Pomeron, we express the semi-hard nuclear cross section as a self shadowing cross section. With the help of a generating functional technique, we obtain average numbers of multiple semi-hard partonic collisions without any need of using explicit expressions for the multi-parton distributions. The average number of semi-hard interactions of a given projectile parton against a target nucleus is estimated quantitatively and it is shown to grow very rapidly above one with increasing the c.m. energy. (Figures available on request)

D-meson as a probe of early parton rescattering

Initial parton scatterings produce a hot and dilute gluonic gas in high energy nuclear collisions. Pre-equilibrium gluon rescatterings are shown to have significant contributions to the charm production. In contrast to the gluon shadowing effect in the initial parton scattering, the enhancement of the final total open charm in the central region due to pre-equilibrium production could be used as a probe of the early dynamics of the partonic system.

Photon emission from very high energy nuclear collisions.

The number of high transverse momentum photons emitted by a free-streaming quark-gluon plasma created in a collision between heavy nuclei at energies reached at the CERN SPS, BNL RHIC, or CERN LHC is computed assuming a production-time momentum distribution that is exponential. Comparison to a hydrodynamically expanding plasma with the same initial conditions indicates that the photon yield is relatively insensitive to parton rescattering. The expected background coming from the decay of neutral pions is also computed and compared to the direct yield. Direct photons generally dominate decay photons for transverse momenta greater than 5 GeV.

Transverse momentum distributions of hadrons

The study of hadron production in heavy ion collisions is essential to the search for effects beyond independent nucleon-nucleon collisions, for example the predicted phase transition to quark matter. Hadron distributions are known over a large range of transverse momenta for p-p collisions, so a careful study of the differences can be made. The transverse momentum distributions of hadrons may provide global information about p-nucleus and nucleus-nucleus collisions, such as the degree of thermalization achieved, and perhaps provide evidence for collective expansion of the highly excited central region. Comparison of the p{sub t} and transverse mass, m{sub t}, distributions of different hadronic species are crucial to extract this kind of information. Hadronic p{sub t} spectra show effects of the collision dynamics, such as hard scattering processes, and possibly rescattering of partons as well as of the formed hadrons. Such modifications have been observed in p-nucleus collisions, and can be expected to be important in nucleus-nucleus reactions. The spectral shape changes arising in this manner cause a background in efforts to extract global information from hadronic p{sub t} spectra. Lastly, there is an excess of pions observed at low p{sub t} in p-A and A-A collisions. the origin of these softmore » pions is not yet well understood. The phenomenon represents a major difference between p-p and nuclear collisions. 31 refs., 8 figs.« less

Rescattering production of dileptons in nucleus-nucleus collisions

We present a space-time formulation for the rescattering of hard partons produced in primary QCD subcollisions within a nucleus-nucleus collision. As an application we calculate the rescattering rate for dilepton production in 100 GeV/A+100 GeV/A uranium-uranium collisions.