Hadron Physics Opportunities at FAIR

TL;DR

This White Paper articulates a decade-spanning hadron-physics program at GSI/FAIR, leveraging diverse hadronic beams, high-rate detectors (HADES, CBM, WASA-FRS), and a strong theory program (LQCD, DSE/BSE, EFT) to tackle nonperturbative QCD questions. It outlines a three-phase roadmap (Phase-0 with SIS18, SIS100-era CBM, and a future HESR antiproton program) aimed at mapping hadron spectra, elucidating hadron interactions, probing in-medium modifications, and exploring exotic hadrons, with broad ties to astrophysics, neutrino physics, and dark-matter searches. The framework emphasizes complementary experimental approaches (πN, NN, pA, πA, DY, femtoscopy, invariant-mass analyses) and a suite of theoretical tools (PWA, dispersive analyses, LQCD, DSE/BSE, EFT) to extract robust, model-independent insights. It envisions strong synergies with global facilities (JLab, J-PARC, CERN, LHC) to advance our understanding of confinement, mass generation, and the nonperturbative QCD regime, with potential impacts on neutron-star physics, cosmic-ray dynamics, and dark-sector searches. The program promises decisive progress in hadron spectroscopy, strong-interaction dynamics, and the development of advanced detector/data-science technologies.

Abstract

This White Paper outlines a coordinated, decade-spanning programme of hadron and QCD studies anchored at the GSI/FAIR accelerator complex. Profiting from intense deuteron, proton and pion beams coupled with high-rate capable detectors and an international theory effort, the initiative addresses fundamental questions related to the strong interaction featuring confinement and dynamical mass generation. This includes our understanding of hadron-hadron interactions and the composition of hadrons through mapping the baryon and meson spectra, including exotic states, and quantifying hadron structure. This interdisciplinary research connects topics in the fields of nuclear, heavy-ion, and (nuclear) astro (particle) physics, linking, for example, terrestrial data to constraints on neutron star structure. A phased roadmap with SIS100 accelerator start-up and envisaged detector upgrades will yield precision cross sections, transition form factors, in-medium spectral functions, and validated theory inputs. Synergies with external programmes at international accelerator facilities worldwide are anticipated. The programme is expected to deliver decisive advances in our understanding of non-perturbative (strong) QCD and astrophysics, and high-rate detector and data-science technology.

Figures (64)

• Figure 1: Conceptual motivation of the proposed hadron physics programme at GSI/FAIR using hadronic beams. The programme aligns complementary experimental and theoretical efforts from various disciplines in a drive towards a unified microscopic understanding of strongly interacting matter.
• Figure 2: Left panel: Perturbative QCD prediction for the running coupling constant $\alpha_s(Q)$ compared to experimental determinations. (Figure by Particle Data Group ParticleDataGroup:2024cfk). Right panel: Process-independent effective charge, $\hat{\alpha}(r)/\pi$, calculated at all length-scales using results from non-perturbative continuum and lattice analyses of QCD's gauge sector Cui:2019dwvDeur:2023dzc and plotted in units of $r_p$, the proton charge radius ParticleDataGroup:2024cfk. Also plotted are existing data on the process-dependent charge $\alpha_{g_1}$ -- see Refs. Deur:2005cfDeur:2008rfDeur:2014veaDeur:2022msfAckerstaff:1997wsAckerstaff:1998jaAirapetian:1998wiAirapetian:2002rwAirapetian:2006vyKim:1998kiaAlexakhin:2006ozaAlekseev:2010hcAdolph:2015sazAnthony:1993ufAbe:1994cpAbe:1995mtAbe:1995dcAbe:1995rnAnthony:1996mwAbe:1997cxAbe:1997qkAbe:1997dpAbe:1998wqAnthony:1999pyAnthony:1999rmAnthony:2000fnAnthony:2002hy. The QCD charge is weak at short distances (asymptotic freedom) and tends to saturate to a finite value at long range, opening a conformal window and providing for a stable infrared completion of the theory. (Figure courtesy of D. Binosi.)
• Figure 3: A futuristic depiction of FAIR near Darmstadt, Germany. The infrastructure on the left-hand side represents the existing GSI site, while the right-hand side illustrates the various components that comprise FAIR. The details of GSI and FAIR are described in Sec. \ref{['sec.OverviewFacilities']}. Picture provided D. Fehrenz.
• Figure 4: Sketch of the roadmap for hadron physics activities at GSI and FAIR. The initial programme phase, indicated by GSI/FAIR Phase-0, will exploit pion and proton beams delivered by SIS18. The two programme phases marked by FAIR will use SIS100, providing proton and antiproton beams and significantly increasing the hadron mass reach. The three categories of boxes, distinguished (again) by green, blue, and red, symbolise the three research domains to be tackled. The size of each box reflects the respective contribution to the overarching programme.
• Figure 5: Left two panels: Differential cross section of the reaction $\pi^- p \to \eta n$ at scattering energies $\sqrt{s} = 1512$ and $\sqrt{s} = 1534\,$MeV. Central panel: Spin-rotation parameter $\beta$ for $\pi^+ p \to K^+ \Sigma^+$. Right panel: Polarisation $P$ for the reaction $\pi^- p \to K^0 \Sigma^0$. For the data references and the analysis solutions shown (A in orange, B in turquoise), see Ref. Ronchen:2012eg.