About the CLASH project

The CLASH project is a collaboration between experimental and theoretical particle physicists at Lund University, which focuses on improving the understanding of some of the most debated experimental results from the Large Hadron Collider (LHC) at CERN. In particle collisions at the LHC, the kinetic energy can be converted into new particles according to Einstein's famous equation E=mc². In a lead-lead (Pb-Pb) collision at LHC, this results in a Small Bang: a recreation of the very dense medium of quarks and gluons, denoted the Quark Gluon Plasma (QGP), that dominated the very early Universe microseconds after the Big Bang. The QGP produced at the LHC is the hottest man made matter. It is more than 100,000 times hotter then the core of the sun; so hot that it melts the protons and neutrons that makes up atomic nuclei. The QGP expands and cools like a nearly perfect liquid and quenches energetic quarks and gluons traversing it, which indicates that it has extremely strong collective interactions.

What has surprised many is that QGP-like effects has also been observed in proton-proton (pp) and proton-lead (p-Pb) collisions. These are systems where one did not expect a QGP to be able to form since the systems are considered too dilute and too short lived. The goal of the CLASH project is to pin down the origin of collective effects in small collisional systems. It is funded by the Knut and Alice Wallenberg Foundation from summer 2018 to summer 2023.

• Information about CLASH at the KAW webpage (English and Swedish)

The three pillars of the CLASH project

The CLASH project utilizes that Lund University has a unique 30 year tradition in the studies of both large and small collisional systems. Peter Christiansen (PI) is a LHC experimentalist specialized in large collision systems, and Leif Lönnblad (co-PI) is the author of several microscopic models. CLASH is centered around proposed new experimental measurements and development of a new theoretical model. CLASH has three main branches:

• Angantyr. A theoretical goal is to develop new physics for the world famous PYTHIA generator. PYTHIA can describe most of the well known small system physics and Leif and his team want firstly to extend it so that it can also describe the new QGP-like effects in small systems. But also, secondly, to entend PYTHIA to be able to model larger collision systems such as p-Pb and Pb-Pb collisions. This extension is called Angantyr.
• Jet quenching in small systems? On the experimental side a goal is to search for other QGP-like effects in small collisional systems that has so far not been observed there. The most prominent unobserved QGP-like effect is jet quenching. Peter and his team wants to develop new more sensitive measurements to search for jet quenching in small systems.
• The CLASH. The main goal of the project is to directly confront the traditional paradigm for small collision systems (microscopic models based on quarks and gluons) with the QGP-paradigm originating in large collisional systems (macroscopic models based on relativistic viscous hydrodynamics). The goal is to identify the most sensitive observables for differentiating between the model descriptions using Angantyr and other models and to then carry out the most promising measurements.