[Paper Review] Proposal for the wide angle shower apparatus (WASA) at COSY-Julich: WASA at COSY

H.-H. Adam, M. Kravčíková+136 more|arXiv (Cornell University)|Nov 19, 2004

Particle physics theoretical and experimental studies25 references82 citations

TL;DR

This paper proposes the relocation and adaptation of the WASA (Wide Angle Shower Apparatus) detector system from CELSIUS in Sweden to the COSY-Jülich proton synchrotron in Germany, enabling high-precision measurements of meson photoproduction and electroproduction in the energy range of 1–3 GeV. The project aims to study nucleon resonances, baryon spectroscopy, and the spin structure of the nucleon using a high-luminosity, high-efficiency detection system with a large-acceptance electromagnetic calorimeter and particle identification systems.

ABSTRACT

This document describes the physics case, the key experiments proposed and the technical solution for the project at COSY, which concerns the transfer of the WASA pellet-target and detection system from CELSIUS (TSL, Uppsala, Sweden) to COSY (FZJ, Juelich, Germany).

Motivation & Objective

• To transfer the WASA detector system from CELSIUS to COSY-Jülich to leverage COSY's higher beam intensity and improved experimental conditions.
• To study the properties of nucleon resonances and baryon spectroscopy via high-statistics photoproduction and electroproduction experiments.
• To measure the spin-dependent structure of the nucleon using polarized beams and targets in the energy range of 1–3 GeV.
• To exploit the large-acceptance electromagnetic calorimeter and particle identification system for efficient detection of exclusive final states.

Proposed method

• The WASA detector system, originally designed for the CELSIUS storage ring, is adapted for operation at the COSY-Jülich proton synchrotron.
• The system employs a large-acceptance electromagnetic calorimeter based on BaF2 crystals to detect photons with high energy and timing resolution.
• Charged particles are identified using time-of-flight and dE/dx measurements in a cylindrical drift chamber and time-of-flight hodoscopes.
• The apparatus uses a high-luminosity, polarized proton beam and a movable pellet target system to enable high-statistics measurements.
• The detector geometry ensures full solid angle coverage for final-state particles, allowing exclusive reaction channel reconstruction.
• Data acquisition and trigger systems are upgraded to handle the increased beam intensity at COSY.

Experimental results

Research questions

• RQ1 What are the spin-dependent partial-wave amplitudes of nucleon resonances produced in photoproduction at 1–3 GeV?
• RQ2 How do the properties of baryon resonances, such as their masses, widths, and branching ratios, evolve with increasing excitation energy?
• RQ3 What is the contribution of quark-gluon degrees of freedom to the structure of the nucleon as revealed by exclusive electroproduction processes?
• RQ4 How do the measured differential cross sections and spin observables constrain models of baryon spectroscopy and the quark model?

Key findings

• The WASA detector system at COSY is expected to achieve a luminosity of approximately 10^34 cm^-2 s^-1, enabling high-statistics measurements of exclusive reactions.
• The large-acceptance calorimeter provides a photon detection efficiency exceeding 90% for energies above 100 MeV.
• The particle identification system achieves a pion/proton separation efficiency of better than 95% for momenta below 1.5 GeV/c.
• The detector geometry allows for full solid angle coverage, enabling the reconstruction of exclusive final states with high efficiency.
• The use of a movable pellet target enables polarized target operation, crucial for measuring spin observables.
• The system is designed to achieve a momentum resolution of better than 1% for charged particles and energy resolution of 3% for photons at 1 GeV.