Region Of Heavy-ion Collisions Research Articles

Distribution of nuclear matter and radiation in the fragmentation region

We study the fragmentation (far forward/backward) region of heavy ion collisions by considering an at-rest nucleus which is struck by a relativistic sheet of colored glass. By means of a simple classical model, we calculate the subsequent evolution of baryons and the associated radiation. We confirm that the struck nucleus undergoes a compression and that the dynamics of the early times of the collision are best described by two separate fluids as the produced radiation's velocity distribution is very different to the velocity distribution of the matter in the struck nucleus.

Nucleon-number scalings of anisotropic flows and nuclear modification factor for light nuclei in the squeeze-out region

The number of nucleon (NN) scaling of the directed flow $v_1$ and elliptic flow ($v_{2}$) as well as the nuclear modification factor ($R_{cp}$) are tested for light nuclei which are produced in 0.4$A$ GeV $^{197}Au + ^{197}Au$ collisions at different impact parameters with two different in-medium nucleon-nucleon cross sections in the framework of an isospin-dependent quantum molecular dynamics (IQMD) model. In that energy domain, the emission of light nuclei can be well described by the squeeze-out phenonomenon. The results show a nice NN scaling behavior for flow parameters $v_{1}$, $v_{2}$ and ${R_{cp}}$. These results demonstrate that the nucleon coalescence mechanism is responsible for nucleon-number scaling of above physical observables in squeeze-out region in heavy-ion collisions at intermediate energy.

Dimuon transverse momentum spectra as a tool to characterize the emission region in heavy-ion collisions

Previous dilepton measurements in heavy-ion collisions have mainly focused on invariant mass spectra to clarify in-medium changes of vector meson properties. However, a dimuon is characterized by two quantities - invariant mass M and transverse momentum p{sub T}. Like transverse momentum spectra of hadrons, p{sub T} spectra of dileptons arise from an interplay between emission temperature and collective transverse flow, whereas the invariant mass is insensitive to flow. Having two control parameters of which only one is sensitive to flow allows one at a given M to characterize the emission region in terms of average temperature and flow. Thus, one can study what phases of the fireball evolution radiate into a given mass window. We demonstrate this technique using the dimuon transverse momentum spectra measured by the NA60 Collaboration and present strong arguments that a thermalized evolution phase with T>170MeV leaves an imprint in the spectra.

Jet tomography in the forward direction at RHIC

Hadron production at high-$p_T$ displays a strong suppression pattern in a wide rapidity region in heavy ion collisions at RHIC energies. This finding indicates the presence of strong final state effects for both transversally and longitudinally traveling partons, namely induced energy loss. We have developed a perturbative QCD based model to describe hadron production in $pp$ collision, which can be combined with the Glauber -- Gribov model to describe hadron production in heavy ion collisions. Investigating $AuAu$ and $CuCu$ collisions at energy $\sqrt{s}=200$ $A$GeV at mid-rapidity, we find the opacity of the strongly interacting hot matter to be proportional to the participant nucleon number. Considering forward rapidities, the suppression pattern indicates the formation of a longitudinally contracted dense deconfined zone in central heavy ion collisions. We determine parameters for the initial geometry from the existing data.

Hadron production in the transfragmentation region in heavy-ion collisions

We study the production of hadrons in Au+Au collisions in the region $0.6<x_F<1.2$, which we refer to as the trans-fragmentation region (TFR), since it corresponds roughly to $\eta'>0$, where $\eta'=\eta-y_{\rm beam}$, depending on $p_T$. We show how hadrons can be produced in that region when the hadronization process is parton recombination. The inclusive $x$-distributions for proton and pion production are calculated with momentum degradation taken into account. The results show that the proton yield is significantly higher than that of the pions in the TFR. Without particle identification the existing data cannot be used for comparison with our result on the $p/\pi$ ratio. Without $p_T$ determination it is not feasible to relate the $x$ distribution to the experimental $\eta'$ distribution. Nevertheless, on theoretical grounds we have shown why the production of hadrons in the TFR is not forbidden by momentum conservation.

Limiting fragmentation from the color glass condensate

We show how the limiting fragmentation phenomenon can arise from the Color Glass Condensate model of high energy QCD. We consider the very forward rapidity region in relativistic heavy ion collisions and argue that in this region, nucleus-nucleus collisions are similar to proton-nucleus collisions (up to shadowing corrections). We then use the known results for proton-nucleus cross sections to show that it leads to the phenomenon of limiting fragmentation in the very forward region of heavy ion collisions as observed at RHIC.

Creation of strange matter at low initial microT.

We demonstrate that the creation of strange matter is conceivable in the midrapidity region of heavy ion collisions at Brookhaven RHIC and CERN LHC. A finite net-baryon density, abundant (anti)strangeness production, as well as strong net-baryon and net-strangeness fluctuations, provide suitable initial conditions for the formation of strangelets or metastable exotic multistrange (baryonic) objects. Even at very high initial entropy per baryon $S/{A}^{\mathrm{init}}\ensuremath{\approx}500$ and low initial baryon numbers of ${A}_{B}^{\mathrm{init}}\ensuremath{\approx}30$ a quark-gluon-plasma droplet can immediately charge up with strangeness and accumulate net-baryon number.

A parton description of the nucleus fragmentation region in heavy-ion collisions

In nucleus-nucleus collisions, the rapidity distribution of partons in the nucleus fragmentation region is highly asymmetrical. Thermalization that randomizes the momenta of partons far apart in rapidity cannot be expected. A local thermalization model is introduced which relates temperature to the range of parton interaction in rapidity. The parton number density and energy density are then calculated.