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Register a new userThe thermal proton yield anomaly in Pb-Pb collisions at the LHC and its resolution
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We propose a resolution of the discrepancy between the proton yield predicted by the statistical hadronization approach and data on hadron production in ultra-relativistic nuclear collisions at the LHC. Applying the S-matrix formulation of statistical mechanics to include pion-nucleon interactions, we reexamine their contribution to the proton yield, taking resonance widths and the presence of nonresonant correlations into account. The effect of multi-pion-nucleon interactions is estimated using lattice QCD results on the baryon-charge susceptibility. We show that a consistent implementation of these features in the statistical hadronization model, leads to a reduction of the predicted proton yield, which then quantitatively matches data of the ALICE collaboration for Pb-Pb collisions at the LHC.
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We provide, within the hydrokinetic model, a detailed investigation of kaon interferometry in $Pb+Pb$ collisions at LHC energy ($sqrt{s_{NN}} = 2.76$ TeV). Predictions are presented for 1D interferometry radii of $K^0_SK^0_S$ and $K^{pm}K^{pm}$ pairs as well as for 3D femtoscopy scales in out, side and long directions. The results are compared with existing pion interferometry radii. We also make predictions for full LHC energy.
We briefly review the predictions of the thermal model for hadron production in comparison to latest data from RHIC and extrapolate the calculations to LHC energy. Our main emphasis is to confront the model predictions with the recently released data from ALICE at the LHC. This comparison reveals an apparent anomaly for protons and anti-protons which we discuss briefly. We also demonstrate that our statistical hadronization predictions for J/$psi$ production agree very well with the most recent LHC data, lending support to the picture in which there is complete charmonium melting in the quark-gluon plasma (QGP) followed by statistical generation of J/$psi$ mesons at the phase boundary.
Results are presented from a phenomenological analysis of recent measurements of jet suppression and modifications of jet fragmentation functions in Pb+Pb collisions at the LHC. Particular emphasis is placed on the impact of the differences between quark and gluon jet quenching on the transverse momentum ($p_{T}^{jet}$) dependence of the jet $R_{AA}$ and on the fragmentation functions, $D(z)$. Primordial quark and gluon parton distributions were obtained from PYTHIA8 and were parameterized using simple power-law functions and extensions to the power-law function which were found to better describe the PYTHIA8 parton spectra. A simple model for the quark energy loss based on the shift formalism is used to model $R_{AA}$ and $D(z)$ using both analytic results and using direct Monte-Carlo sampling of the PYTHIA parton spectra. The model is capable of describing the full $p_{T}^{jet}$ , rapidity, and centrality dependence of the measured jet $R_{AA}$ using three effective parameters. A key result from the analysis is that the $D(z)$ modifications observed in the data, excluding the enhancement at low-$z$, may result primarily from the different quenching of the quarks and gluons. The model is also capable of reproducing the charged hadron $R_{AA}$ at high transverse momentum. Predictions are made for the jet $R_{AA}$ at large rapidities where it has not yet been measured and for the rapidity dependence of $D(z)$.
We compute predictions for various low-transverse-momentum bulk observables in $sqrt{s_{NN}} = 5.023$ TeV Pb+Pb collisions at the LHC from the event-by-event next-to-leading-order perturbative-QCD + saturation + viscous hydrodynamics (EKRT) model. In particular, we consider the centrality dependence of charged hadron multiplicity, flow coefficients of the azimuth-angle asymmetries and correlations of event-plane angles. The centrality dependencies of the studied observables are predicted to be very similar to those at 2.76 TeV, and the magnitudes of the flow coefficients and event-plane angle correlations are predicted to be close to those at 2.76 TeV. The flow coefficients may, however, offer slightly more discriminating power on the temperature dependence of QCD matter viscosity than the 2.76 TeV measurements. Our prediction for the multiplicity in the 0-5% centrality class, obtained using the two temperature-dependent shear-viscosity-to-entropy ratios that give the best overall fit to RHIC and LHC data is $dN_{rm ch}/detabig|_{|eta|le 0.5} =1876dots2046$. We also predict a power-law increase from 200 GeV Au+Au collisions at RHIC to 2.76 and 5.023 TeV Pb+Pb collisions at the LHC, $dN_{rm ch}/detabig|_{|eta|le 0.5} propto s^{0.164dots0.174}$.
The systematics of Statistical Model parameters extracted from heavy-ion collisions at lower energies are exploited to extrapolate in the LHC regime. Predictions of various particle ratios are presented and particle production in central Pb-Pb collisions at LHC is discussed in the context of the Statistical Model. The sensitivity of several ratios on the temperature and the baryon chemical potential is studied in detail, and some of them, which are particularly appropriate to determine the chemical freeze-out point experimentally, are indicated. The impact of feed-down contributions from resonances, especially to light hadrons, is illustrated.
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