What we have (not)learned from the ultrarelativistic heavy ion collisions

TL;DR

This paper reviews the progress and ongoing challenges in understanding the processes and properties of matter created in ultrarelativistic heavy ion collisions, highlighting gaps despite decades of research.

Contribution

It provides a personal overview of what has been learned and the unresolved issues in the field, emphasizing the complexity and evolving nature of the research.

Findings

Insights gained from increased collision energies

Persistent gaps in understanding collision processes

New challenges arising from experimental and theoretical advancements

Abstract

The field of ultrarelativistic heavy ion collisions is today a flourishing activity both on the experimental and on the theoretical side. Although the theoretical justifications to study these collisions was given already more than three decades ago and the experimental studies have a history of more than 25 years we are still very much in the dark as to the details of the processes and of the characteristics of the matter created in collisions. Increasing the energy of collisions has brought new insights but has also resulted with new challenges. In the present paper I will try from a personal perspective to report on the answers we have collected and on the problems we are faced with. The account is partial, taking into account that it is impossible to render justice to every aspect of the field.