HERMES experiment

HERMES experiment. The HERMES experiment was a pioneering particle physics investigation conducted at the DESY laboratory in Hamburg. It utilized the unique polarized internal gas target of the HERA accelerator to study the spin structure of the nucleon through deep inelastic scattering. The collaboration made seminal contributions to our understanding of quark and gluon contributions to proton spin, significantly advancing the field of nuclear physics.

Overview

Operational from 1995 to 2007, the experiment was installed in the East Hall of the HERA storage ring. Its primary scientific motivation was to resolve the so-called "spin crisis" or "proton spin puzzle" that emerged from earlier experiments at CERN and SLAC National Accelerator Laboratory. By scattering a longitudinally polarized electron beam off polarized internal targets of hydrogen, deuterium, and helium-3, HERMES provided unprecedented data on spin-dependent structure functions. The international effort involved physicists from numerous institutions across Europe, North America, and Asia.

Experimental setup

The core of the detector was a forward spectrometer with a dipole magnet providing a 1.3 T field. Key tracking components included drift chambers, proportional chambers, and a transition radiation detector for pion/kaon separation. Particle identification was achieved with a ring-imaging Cherenkov detector, a lead-glass calorimeter, and a preshower detector. The polarized internal gas target, a major technical achievement, fed polarized atomic hydrogen or deuterium into an open-ended storage cell within the HERA electron ring. The Siberian snake technique was used to maintain the polarization of the stored electron beam, while target polarization was measured via Breit-Rabi polarimeter.

Physics goals and results

The central goal was the precise measurement of the helicity distributions of valence and sea quarks within the nucleon. HERMES produced landmark results on the quark spin contribution, finding the up quark spin aligned with the proton spin and the down quark spin anti-aligned. The collaboration made the first direct evidence for a non-zero polarization of strange sea quarks. Furthermore, it provided pioneering measurements of Generalized Parton Distributions through exclusive processes like deeply virtual Compton scattering and meson production. Studies of semi-inclusive deep inelastic scattering allowed for flavor separation of quark distributions, and data on transverse momentum dependent parton distributions offered insights into nucleon structure beyond one-dimensional collinear pictures.

Collaboration and institutions

The HERMES collaboration comprised approximately 200 physicists from over 30 institutions worldwide. Major contributing groups hailed from DESY itself, the University of Hamburg, and the University of Bonn in Germany. Significant partners included INFN sections in Italy, Nikhef in the Netherlands, and the University of Glasgow in the United Kingdom. North American participation involved groups from the University of Illinois, the University of Virginia, and TRIUMF in Canada. Institutes from Armenia, Israel, and Japan also played vital roles in detector construction, data analysis, and theoretical interpretation.

Legacy and impact

The experiment conclusively showed that quark spins account for only about 30% of the proton's total spin, compelling the field to focus on contributions from gluon spin and orbital angular momentum. Its rich dataset on Generalized Parton Distributions helped establish a new three-dimensional framework for nucleon structure, influencing the scientific programs of subsequent facilities like Jefferson Lab and the future Electron-Ion Collider. The technical innovations in polarized internal targets and forward spectrometer design informed experiments at COZY and other storage rings. HERMES fundamentally shaped the modern research agenda in quantum chromodynamics and spin physics, leaving a lasting legacy for the global high-energy physics community.

Category:Particle physics experiments Category:DESY Category:Nuclear physics