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Register a new userHigh-pT hadrons from nuclear collisions: Unifying pQCD with hydrodynamics
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J. Nemchik
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Hadrons inclusively produced with large pT in high-energy collisions originate from the jets, whose initial virtuality and energy are of the same order, what leads to an extremely intensive gluon radiation and dissipation of energy at the early stage of hadronization. Besides, these jets have a peculiar structure: the main fraction of the jet energy is carried by a single leading hadron, so such jets are very rare. The constraints imposed by energy conservation enforce an early color neutralization and a cease of gluon radiation. The produced colorless dipole does not dissipate energy anymore and is evolving to form the hadron wave function. The small and medium pT region is dominated by the hydrodynamic mechanisms of hadron production from the created hot medium. The abrupt transition between the hydrodynamic and perturbative QCD mechanisms causes distinct minima in the pT dependence of the suppression factor R_{AA} and of the azimuthal asymmetry v2. Combination of these mechanisms allows to describe the data through the full range of pT at different collision energies and centralities.
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A parton produced with a high transverse momentum in a hard collision is regenerating its color field, intensively radiating gluons and losing energy. This process cannot last long, if it ends up with production of a leading hadron carrying the main fraction z_h of the initial parton momentum. So energy conservation imposes severe constraints on the length scale of production of a single hadron with high pT. As a result, the main reason for hadron quenching observed in heavy ion collisions, is not energy loss, but attenuation of the produced colorless dipole in the created dense medium. The latter mechanism, calculated with the path-integral method, explains well the observed suppression of light hadrons and the elliptic flow in a wide range of energies, from the lowest energy of RHIC up to LHC, and in a wide range of transverse momenta. The values of the transport coefficient extracted from data range within 1-2 GeV^2/fm, dependent on energy, and agree well with the theoretical expectations.
By analyzing the available data on strange hadrons in central Pb+Pb collisions from the NA49 Collaboration at the Super Proton Synchrotron (SPS) and in central Au+Au collisions from the STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) in a wide collision energy range from $sqrt{s_{rm NN}}$ = 6.3 GeV to 200 GeV, we find a possible non-monotonic behavior in the ratio $mathcal{O}_text{K-$Xi$-$phi$-$Lambda$}$= $frac{N(K^+)N(Xi^-)}{N(phi)N(Lambda)}$ of $K^+$, $Xi^-$, $phi$, and $Lambda$ yields as a function of $sqrt{s_{rm NN}}$. Based on the quark coalescence model, which can take into account the effect of quark density fluctuations on hadron production, a possible non-monotonic behavior in the dependence of the strange quark density fluctuation on $sqrt{s_{NN}}$ is obtained. This is in contrast to the coalescence model that does not include quark density fluctuations and also to the statistical hadronization model as both fail to describe even qualitatively the collision energy dependence of the ratio $mathcal{O}_text{K-$Xi$-$phi$-$Lambda$}$. Our findings thus suggest that the signal and location of a possible critical endpoint in the QCD phase diagram, which is expected to result in large quark density fluctuations, can be found in the on-going Bean Energy Scan program at RHIC.
The production of vector boson tagged heavy quark jets provides potentially new tools to study jet quenching, especially the mass hierarchy of parton energy loss. In this work, we present the first theoretical study on $Z^0,+,$b-jet in heavy-ion collisions. Firstly utilizing a Monte Carlo transport model, our simulations give nice descriptions of the azimuthal angle correlation $Deltaphi_{jZ}$, transverse momentum imbalance $x_{jZ}$ for $Z^0,+,$jet as well as the nuclear modification factor $R_{AA}$ of inclusive b-jet in Pb+Pb collisions. Then we calculate the azimuthal angular correlation $Deltaphi_{bZ}$ of $Z^0,+,$b-jet and $Deltaphi_{bb}$ of $Z^0,+,2,$b-jets in central Pb+Pb collisions at $sqrt{s_{NN}}=$~5.02 TeV. We find that the medium modification of the azimuthal angular correlation for $Z^0,+,$b-jet has a weaker dependence on $Deltaphi_{bZ}$, as compared to that for $Z^0,+,$jet. With the high purity of quark jet in $Z^0,+,$(b-)jet production, we calculate the momentum imbalance distribution of $x_{bZ}$ of $Z^0,+,$b-jet in Pb+Pb collisions. We observe a smaller shifting of the mean value of momentum imbalance for $Z^0,+,$b-jet in Pb+Pb collisions $Deltaleftlangle x_{bZ} rightrangle$, as compared to that for $Z^0,+,$jet. In addition, we investigate the nuclear modification factors of tagged jet cross sections $I_{AA}$, and show a much stronger suppression of $I_{AA}$ in $Z^0,+,$jet than that of $Z^0,+,$b-jet in central Pb+Pb collisions.
Colliding high energy hadrons either produce new particles or scatter elastically with their quantum numbers conserved and no other particles produced. We consider the latter case here. Although inelastic processes dominate at high energies, elastic scattering contributes considerably (18-25%) to the total cross section. Its share first decreases and then increases at higher energies. Small-angle scattering prevails at all energies. Some characteristic features are seen that provide informationon the geometrical structure of the colliding particles and the relevant dynamical mechanisms. The steep Gaussian peak at small angles is followed by the exponential (Orear) regime with some shoulders and dips, and then by a power-law drop. Results from various theoretical approaches are compared with experimental data. Phenomenological models claiming to describe this process are reviewed. The unitarity condition predicts an exponential fall for the differential cross section with an additional substructure to occur exactly between the low momentum transfer diffraction cone and a power-law, hard parton scattering regime under high momentum transfer. Data on the interference of the Coulomb and nuclear parts of amplitudes at extremely small angles provide the value of the real part of the forward scattering nuclear amplitude. The real part of the elastic scattering amplitude and the contribution of inelastic processes to the imaginary part of this amplitude (the so-called overlap function) at nonforward transferred momenta are also discussed. Problems related to the scaling behavior of the differential cross section are considered. The power-law regime at highest momentum transfer is briefly described.
Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~mathrm{TeV}$. We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a threshold effect of the required double charm quarks for $T_{cc}^+$. Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.
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