CERN Accelerating science

Measurements of $\alpha_s$ as a function of the energy scale $Q$, as reported in the Quantum Chromodynamics review of the Particle Data Group ~\cite{PDG2018}. From each set of experiments, there is a value of $\alpha_s$ that has been extracted using different levels of QCD perturbation theory, which are indicated in brackets.

On the left we can see the screening effect that quark loops provide for the electric charge, which corresponds to the notion of an effective charge $e(r)$ that becomes smaller with larger distances. This is encoded in the QED beta function $\beta(r) = -\frac{de(r)}{d \log (r)}$, which is positive. On the right we see that the color charge gets screening from the quark loops but antiscreening from the gluon loops, which corresponds to the two contributions with opposite signs to $b_0$ in the QCD beta function. The QCD beta function is negative and so the effective strong coupling becomes small at short distances~\cite{alan}.

OPAL Collaboration measurement of the $\rho$ (vector meson, $I^G/J^P = 1^+/1^-$) and $a_1$ (axial-vector meson, $I^G/J^P) = 1^-/1^+$) mass spectral distributions. The $\rho$ spectrum peaks at $\sim 0.6$ GeV$^2$ while the $a_1$ spectrum peaks at $\sim 1.2$ GeV$^2$~\cite{parityfig}

On the left, the phase diagram for QCD matter shows the Hadron Gas phase and the Quark-Gluon Plasma phase in the temperature ($T$) and baryon chemical potential ($\mu_B$) plane~\cite{phase-diag-conj}. On the right, the phase diagram for QCD matter shows the Hadron Gas phase, the Quark-Gluon Plasma phase and the Quarkyonic phase in the temperature ($T$) and baryon density ($n$) normalized to the cold nuclei baryon density ($n_o$) plane~\cite{phase-diag-NICA}.

Four stages of a heavy-ion collision and its evolution. (a) Shows the two Lorentz contracted heavy-ions moving towards each other in the CM frame, then (b) shows the overlaping nuclei at the collision stage. (c) Just after the collision the heavy-ions travel accross each other and generate a volume of high temperature and energy density and finally in (d), the system expands and cools down, eventually fragmenting into hadrons that travel to the detector.\cite{hiccol}

On the left, a schematic representation of the collision \textit{centrality}. The ions may collide head-on or may only partially overlap at the collision stage, so in the overlap region there are conditions that facilitate QGP formation. On the right, the collision centrality promotes a characteristic initial shape in the transverse plane to the formed medium, that tends to be more lenticular as the collisions are more head-on or central \cite{centralcern, flowcoeff}.

On the left, modelling the colliding nuclei as made of transparent spheres where the dashed circles represent spectator nucleons that travel down the beam pipe and the solid circles represent the wounded or participant nucleons which collide with at least one other nucleon. On the right, the number of binary collisions and the number of participants are plotted for 1000 events in Au+Au collisions at 200 GeV with respect to the impact parameter, using MCGlauber\cite{mcglauber}.

On the left, thermodynamic properties calculated with Lattice QCD \cite{hotqcd}, preassure, energy and entropy densities. On the right, $v_2$ for most hadrons at RHIC which uses a hydrodynamics description of the QGP \cite{Zajc, KolbHeinz}.

On the left, thermodynamic properties calculated with Lattice QCD \cite{hotqcd}, preassure, energy and entropy densities. On the right, $v_2$ for most hadrons at RHIC which uses a hydrodynamics description of the QGP \cite{Zajc, KolbHeinz}.

In the upper panel, comparison of the ratio $4\pi ~k~\eta/\hbar s$ in natural units~\cite{longrangeplan}. On the lower panels, a viscous hydrodynamics calculation for a collision between heavy ions, as a one-time snapshot at $t= 5$ fm/c ~\cite{smallsystem,bigquestions}, where the colour scheme indicates the temperature and the arrows indicate the fluid velocity.

In the upper panel, comparison of the ratio $4\pi ~k~\eta/\hbar s$ in natural units~\cite{longrangeplan}. On the lower panels, a viscous hydrodynamics calculation for a collision between heavy ions, as a one-time snapshot at $t= 5$ fm/c ~\cite{smallsystem,bigquestions}, where the colour scheme indicates the temperature and the arrows indicate the fluid velocity.

On the left, the phase diagram of water which has continous lines representing the phase transitions and a critical end-point. On the right, the phase diagram of nuclear matter also shows the phase transitions with continous lines and a critical end-point, where the cross-over region begins.~\cite{rajawater}

Skewness and kurtosis of net-proton number is reported by the STAR Collaboration~\cite{sknetproton}.

Hadron species show \textit{quenching} at RHIC\cite{phenixquench} and single-inclusive jet quenching at LHC\cite{atlasquench}.