# QM2017: Status and Key open Questions in Ultra-Relativistic Heavy-Ion Physics

## TL;DR

This paper reviews the 30-year history of ultra-relativistic heavy-ion physics, highlighting major achievements, ongoing challenges, and open questions across different experimental eras from fixed target to LHC.

## Contribution

It provides a comprehensive overview of the evolution, key results, and unresolved issues in ultra-relativistic heavy-ion physics over three decades.

## Key findings

- Major results from fixed target, RHIC, and LHC eras
- Identification of key open questions in the field
- Historical perspective on experimental progress

## Abstract

Almost exactly 3 decades ago, in the fall of 1986, the era of experimental ultra-relativistic (\emph{E/m $\gg 1$}) heavy ion physics started simultaneously at the SPS at CERN and the AGS at Brookhaven with first beams of light Oxygen ions at fixed target energies of 200 GeV/A and 14.6 GeV/A, respectively. The event was announced by CERN \cite{cernpress1,cernpress2} with the usual superlatives "Break new ground.., World Record Energy ..", but also with the information that "up to 400 particles were created per collision" and that "over 300 physicists .. analyzing the data .. [try] to find out whether the famous quark-gluon plasma really has been achieved". One would have thought that with almost one physicist per particle, this would have been figured out rather quickly. However, as we know today, 30 years and 21 Quark Matter conferences later, the study of dense and hot matter, of the strong interaction in the non-perturbative regime, has been a long and winding road. The journey was much more difficult and time consuming, but also much more interesting and rewarding, than anyone could have anticipated, with many twists, some dead ends, and a never-ending string of surprises. This $30^{th}$ anniversary of heavy ion physics, and the start of the 26$^{th}$ Quark Matter in Chicago, is a good opportunity to look back and mention a few of the major results from each of the three eras (fixed target/RHIC/LHC), along with some of the answers they have provided us and some of the key questions which remain to be solved.