Challenges in QCD matter physics - The Compressed Baryonic Matter experiment at FAIR

Abstract: Substantial experimental and theoretical efforts worldwide are devoted to explore the phase diagram of strongly interacting matter. At LHC and top RHIC energies, QCD matter is studied at very high temperatures and nearly vanishing net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was created at experiments at RHIC and LHC. The transition from the QGP back to the hadron gas is found to be a smooth cross over. For larger net-baryon densities and lower temperatures, it is expected that the QCD phase diagram exhibits a rich structure, such as a first-order phase transition between hadronic and partonic matter which terminates in a critical point, or exotic phases like quarkyonic matter. The discovery of these landmarks would be a breakthrough in our understanding of the strong interaction and is therefore in the focus of various high-energy heavy-ion research programs. The Compressed Baryonic Matter (CBM) experiment at FAIR will play a unique role in the exploration of the QCD phase diagram in the region of high net-baryon densities, because it is designed to run at unprecedented interaction rates. High-rate operation is the key prerequisite for high-precision measurements of multi-differential observables and of rare diagnostic probes which are sensitive to the dense phase of the nuclear fireball. The goal of the CBM experiment at SIS100 (sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD matter: the phase structure at large baryon-chemical potentials (mu_B > 500 MeV), effects of chiral symmetry, and the equation-of-state at high density as it is expected to occur in the core of neutron stars. In this article, we review the motivation for and the physics programme of CBM, including activities before the start of data taking in 2022, in the context of the worldwide efforts to explore high-density QCD matter.

• The paper's main contribution is outlining the CBM experiment's design to map the QCD phase diagram at high baryon densities.
• It employs advanced detector systems and real-time data processing to capture rare observables like strangeness production and di-lepton spectra.
• Its findings are expected to refine our understanding of neutron star interiors and enhance theoretical models of dense QCD matter.

Overview of the CBM Experiment at FAIR

The paper "Challenges in QCD Matter Physics -- The Scientific Programme of the Compressed Baryonic Matter Experiment at FAIR" outlines the ambitious objectives and strategic scientific framework of the Compressed Baryonic Matter (CBM) experiment at the Facility for Antiproton and Ion Research (FAIR). The CBM experiment is poised to significantly deepen our understanding of the phase structure of quantum chromodynamics (QCD) matter at high net-baryon densities. It seeks to explore territories in the QCD phase diagram that were hitherto inaccessible, thereby enriching our comprehension of the strong force under high-density conditions akin to those in neutron star cores.

Scientific Objectives and Significance

The primary goal of the CBM experiment is to delineate the phase structure of QCD matter at large baryon-chemical potentials—conditions expected in the interiors of neutron stars. Key scientific objectives include:

1. Understanding QCD Phase Transitions: The CBM aims to investigate the existence of a first-order phase transition between hadronic and partonic matter, potentially terminating in a critical point. Such discoveries could resolve long-standing conjectures about QCD matter under extreme conditions.
2. Chiral Symmetry and Dense Matter: By probing chiral symmetry restoration, the CBM experiment seeks insights into the fundamental interactions and behavior of quarks in dense baryonic environments.
3. Neutron Star Physics: Exploration of the equation-of-state of dense QCD matter will inform theoretical models of neutron stars, adding empirical data to support or refute current theoretical formulations regarding their interior densities and properties.

Experimental Design and Capabilities

The CBM experiment is distinct in its capability to operate at unprecedented interaction rates, enabling high-precision measurements of rare and complex observables. This high-rate data acquisition demands rapid and efficient detector technology paired with extensive computing resources for real-time data processing:

• Detector Systems: The CBM features advanced detector technology including silicon tracking devices, time-of-flight systems, and calorimetry for particle identification. These components are tailored to handle heavy-ion collision environments characterized by high luminosity and rapid data throughput.
• Computing Infrastructure: The CBM employs sophisticated data acquisition systems capable of handling self-triggered data from multiple detector components, necessitating the development of efficient online event reconstruction algorithms.

Probes of Dense QCD Matter

CBM targets several observables that are pivotal for understanding high-density QCD matter:

• Multi-Variate Observables: By analyzing collective flow, fluctuations, and correlations, the CBM aims to map out the equation-of-state and potentially identify signs of a critical end-point in the QCD phase diagram.
• Strangeness Production: Studying strange and multi-strange baryons provides insights into particle production mechanisms in dense matter, with implications for the thermal models of heavy-ion collisions.
• Di-Lepton Analysis: Di-lepton spectra from decays of vector mesons offer a window into in-medium modifications, potentially revealing the onset of chiral symmetry restoration.
• Charmonium and Hypernuclei: The production of charm and strange quark states in high baryon density regimes represents unexplored terrain, potentially unveiling novel states and lending insights into non-perturbative QCD processes.

Anticipated Impact and Future Directions

The CBM experiment's findings will not only refine our theoretical models of QCD matter but also impact broader fields such as astrophysics and nuclear physics by relating laboratory results to astrophysical phenomena like neutron stars. Furthermore, the expected data from CBM could stimulate advancements in computational physics, detector technology, and the development of theoretical models. The project's forward-looking scope, bolstered by collaborative efforts within the FAIR research framework, sets the stage for a transformative understanding of the strong force and the unique characteristics of dense nuclear matter. The initiative to leverage portions of CBM components in existing international experiments bears testament to the collaborative spirit and foresight in advancing QCD matter research during the FAIR Phase 0 timeframe.