This repository contains predictions for Xenon-Xenon collisions at √s = 5.44 TeV.
The prediction figures are stored in the git repo plots folder for convenience, but they can easily be regenerated.
First make sure the prerequisite Python libraries are installed,
pip install -r requirements.txt
Then run python3 -m src.plots from the git repo directory to regenerate the
prediction figures.
The trento initial condition model takes as input the inelastic nucleon-nucleon cross section and an overall entropy normalization factor which should be tuned at each collision beam energy. To make predictions for Xe+Xe collisions at 5.44 TeV, we need to extrapolate their values from previous measurements.
The inelastic nucleon-nucleon cross section is measured experimentally. We use measurements made at 0.2, 2.76, 5.02 and 7 TeV listed with references on the trento documentation page. The cross section values are then fit with a second order polynomial in log(√s).
We find an inelastic nucleon-nucleon cross section at 5.44 TeV of 7.07 fm2.
In addition to the increasing cross section, we must extrapolate the increase in the normalization factor from 5.02 to 5.44 TeV.
We start by running three sets of trento initial condition events at 2.76, 5.02 and 5.44 TeV with the correct inelastic nucleon-nucleon cross section at each beam energy. We then then use the empirically verified scaling law dNch/dη ≈ norm * dS/dη to find the requisite entropy normalization at each beam energy to reproduce observed particle production at 2.76 and 5.02 TeV.
The two normalization factors, norm(2.76 TeV) and norm(5.02 TeV), are then fit with a power law to predict norm(5.44 TeV). We then convert this scaling normalization into a physical normalization by measuring the ratio norm(5.44 TeV) / norm(5.02 TeV) and using it to scale up our best fit Pb+Pb 5.02 TeV normalization from the combined Bayesian analysis.
This yields a modest ~2% increase in the entropy normalization from 5.02 to 5.44 TeV.
We show self consistent calculations for Pb+Pb collisions at 5.02 TeV and Xe+Xe collisions at 5.44 TeV using model parameters which have been calibrated to fit available data for Pb+Pb collisions at 2.76 and 5.02 TeV. Note that most of the 5.02 Pb+Pb data is missing, and hence our model calculations represent predictions for those observables as well. For observables where the 5.02 and 5.44 TeV measurements are missing, we use the same centrality bins as 2.76 TeV measurements.
The observables are calculated from ~1 million minimum bias Pb+Pb events and ~1.5 million minimum bias Xe+Xe events. The minimum bias events are sorted into centrality bins exactly as done by experiment.
Solid lines show model calculations for 5.02 TeV Pb+Pb collisions and dashed lines model calculations for 5.44 TeV Xe+Xe collisions. The inset at the bottom of each panel is the ratio of 5.44 TeV Xe+Xe to 5.02 TeV Pb+Pb.
Minimum bias events are generated using the trento initial condition model with a pre-equilibrium free streaming stage. The subsequent transport dynamics are then simulated using the iEBE-VISHNU hydro + micro model with shear and bulk viscous corrections.
It's worth noting that this version of the physics model is updated compared to the versions shown at Quark Matter 2017 and in previously published work. Namely, significant changes have been made to the particle sampler. These changes have not yet been documented, so please contact Jonah Bernhard for additional information.
The initial condition and hydro model parameters are chosen using maximum aposteriori (MAP) parameter values from a combined Bayesian analysis of particle yields, mean pT, mean pT fluctuations and multiparticle correlations in Pb+Pb collisions at 2.76 and 5.02 TeV. Specifically, the parameter values are determined by maximizing the posterior probability density of the obtained Bayesian posterior distribution.
We use the same parameter values for both 5.44 TeV Xe+Xe collisions and 5.02 TeV Pb+Pb collisions, except for the initial entropy normalization and inelastic nucleon-nucleon cross section which depend strongly on beam energy as discussed previously.
The shared MAP parameter values are:
| Parameter | Description | Value |
|---|---|---|
| p | Entropy deposition parameter | 0.0 |
| k fluct | Multiplicity fluct. shape | 1.186 |
| d min | Minimum nucleon distance | 1.267 fm |
| w | Gaussian nucleon width | 0.956 fm |
| τ-fs | Free streaming time | 1.162 fm/c |
| η/s min | Shear viscosity at Tc | 0.08 |
| η/s slope | Slope above Tc | 1.113 GeV-1 |
| η/s curv | Shear viscosity curvature param | -0.479 |
| ζ/s max | Cauchy dist. peak height | 0.052 |
| ζ/s width | Cauchy dist. scale | 0.022 GeV |
| ζ/s T0 | Cauchy dist. location | 0.183 GeV |
| T switch | Particlization temperature | 0.151 GeV |