Symmetries and Spectroscopy with WASA
In the Standard Model of particle physics, we consider quarks and leptons as the fundamental constituents of matter. Only quarks feel the strong interaction, which is mediated by gluons as exchange particles. However, unlike the electromagnetic and weak interactions, we observe neither the fundamental constituents that are subjected to the interaction (quarks) nor the exchange particles (gluons) as free particles. They are confined to composite systems, which we call hadrons, and the fact that they are confined we call confinement.
Another difference compared to electromagnetic and weak interactions is that the coupling constant, which determines the strength of the interaction – just like the Sommerfeld constant describes the strength of the electromagnetic interaction – is energy dependent. At low and intermediate energies, that is the energy regime where we are doing physics with WASA, the coupling gets stronger, too strong that perturbative methods would work. The latter have shown at high energies (which correspond to small distances) that QCD is the correct theory to describe the strong interaction. However, very little is known about the solution of the theory at small energies, that means large distances which are comparable to the size of hadrons.
Therefore, studies of hadronic systems will help to understand confinement and the origin of mass as two of the grand challenges in contemporary physics. Moreover, identifying suitable hadronic processes, the symmetries of the underlying theory, QCD, can still be observed at the hadron level. A study of such fundamental symmetries, both in meson decays and dedicated production reactions, is the core physics programme for the WASA facility at COSY Jülich.
The data taking at WASA finished in 2014, but data analysis is still going on. The collaboration consists of more than 160 physicists from 32 institutes in eight countries.
WASA-at-COSY combines a unique target concept and a close to 4π setup with the high quality proton and deuteron beams available at COSY. WASA (Wide Angle Shower Apparatus) is a detector system for charged and neutral particles which has been designed to study production and decay of light mesons.
After successful operation at CELSIUS data taking as fixed target internal experiment at the COoler SYnchrotron and storage ring COSY at Forschungszentrum Jülich started in 2007. COSY delivered beams of polarised and unpolarised protons and deuterons in the momentum range between 0.3 and 3.7 GeV/c. In collisions with a hydrogen or deuterium target, mesons with masses up to the φ meson could be produced.
[WASA experimental setup] The WASA detector setup comprised a forward part to measure charged target-recoil particles and scattered projectiles, a central part to measure meson decay products, and a pellet target. With the pellet target it was possible both to reach the design luminosity of 1032 cm-2 s-1 and to detect particles close to the interaction point with almost full angular coverage.
Running an internal target experiment with a gas target one faces typically high gas loads arise close to the interaction region. If one still wants to achieve a reasonable (momentum) precision and life-time of the circulating beam this requires high pumping capacity. The consequence is, of course, that it is not possible to place detectors very close to the interaction point. Therefore, good vacuum conditions are typically achieved at the expense of acceptance or resolution of the vertex reconstruction.
[Droplet formation] The unique pellet target concept avoids to a large extent this trade-off: With a vibrating nozzle at typical frequencies around 70 kHz, a jet of liquid hydrogen or deuterium near triple point conditions is broken up into droplets with typical diameters of about 35 μm in the case of hydrogen. The droplets freeze by partial evaporation in a droplet chamber and form a stream of small frozen microspheres, which are called pellets. After collimation, the pellets move at velocities in the order of 100 m/s and with an angular spread of about 1 mrad. They are directed through a thin 2 m long pipe into the scattering chamber (to the interaction point) and are collected further down in a beam dump. In an optimum scenario, there should be always exactly one pellet crossing the accelerator beam. So far, average distances between consecutive pellets as short as 9 – 20 mm have been reached.
In general, the pellet target provides a point-like well-defined interaction region and the space for an almost 4π angular coverage. Effective target thicknesses of > 1015 cm-2 allow – with available beam intensities at COSY – to reach an average luminosity of 1032 cm-2 s-1. This is one prerequisite to obtain high-statistics data samples and to study rare hadron physics processes.
The WASA pellet target has been the only pellet target in operation at internal storage ring experiments. Another pellet target is presently being developed for the PANDA experiment at the future FAIR facility.
[Forward Detector PID] The forward part of the WASA detector – four proportional chamber layers consisting of drift tubes (straws) and 13 planes of plastic scintillators – was used to measure scattered projectiles and charged recoil particles like protons, deuterons and He nuclei. Particle identification was based on the ΔE – E method using the energy deposit E in the so called Range Hodoscope layers. Particles were stopped in the scintillator material up to kinetic energies ranging from 350 MeV for protons to 1000 MeV for alpha particles, and the energy could then be reconstructed with a precision of about 3%. In addition, the particle time-of-flight could be used, especially at higher energies, where the accuracy of the ΔE – E method deteriorates.
The central part of the detector around the interaction point was designed to detect meson decay products: photons, electrons and charged pions. 1012 sodium-doped CsI scintillating crystals in the shape of truncated pyramids of about 16 radiation lengths form the electromagnetic calorimeter, covering 96% of the full solid angle. The energy of electromagnetic showers from photons, electrons and positrons could be measured with a resolution of 5%/√ E[GeV]. The scintillator material primarily has been chosen due to its comparatively short scintillation decay time, which is advantageous for high-rate applications, as well as its radiation hardness.
The calorimeter enclosed a thin (0.18 radiation lengths) superconducting solenoid which provides an axial magnetic field up to 1.3 T. [Vertex reconstruction] With few per cent accuracy, charged particle momenta could be determined from the track curvature measured in the cylindrical straw chamber placed inside the solenoid. If more than one charged particle track in the central detector part is reconstructed, this allows to determine the reaction vertex with high accuracy. A precise vertex definition is important to distinguish meson decays with lepton-antilepton pairs from radiative decays, where the photon may undergo external conversion, for example on the beam pipe. To minimize such effects, the beam pipe inside the straw chamber was made of 1.2 mm thin beryllium.
[Central Detector PID] The chamber was surrounded by a barrel arrangement of plastic scintillators, consisting of a cylindrical part and two end-caps, placed inside of the solenoid. Charged particle identification in the central detector is achieved from combining reconstructed momenta with either the barrel energy loss information or the energy deposit in the calorimeter. The scintillator barrel could also be used as a veto signal on charged tracks for photon identification.
The data acquisition system used the state-of-the-art FPGA technologies for event and buffer management combined with fast communication paths to achieve low system latencies and highest possible event rates. For the digitizing layer TDCs and dedicated slow and fast QDC boards were developed. The QDCs sampled the analog signals by Flash ADCs with 80 MHz for the electromagnetic calorimeter and 160 MHz for the plastic scintillator channels, respectively, prior to charge integration. A fast feature extraction capability allowed to include particle energy information at the trigger level. The TDCs used, are based on the F1 (straw chambers) and GPX (plastic scintillators) ASICs from acam messelectronic GmbH, while time-stamping for the calorimeter signals was done by the FPGAs on the QDC boards. As a typical performance number, the electronic dead-time of the system amounts to 20 μs per event and data rates of 5-10 kHz and 80 Mbytes/s could be written to disk.
The WASA detector allowed to measure both charged and neutral final states with high acceptance at the same time. This reduced systematical uncertainties significantly. In typical decay studies, mesons were produced in the tagging reactions pp → ppX and pd → 3He X, where X denotes a neutral meson or meson system.
In proton-proton reactions the production cross sections are generally higher, and allow for rare decay searches. The 3He X final state was easier to tag by the energy loss of 3He ions in the forward detector and has lower background from multi-pion production. [η Meson Tagging] The meson X is identified by combining the missing four-momentum with respect to the pp system or the 3He detected in the forward detector with the invariant mass of the mesonic decay products reconstructed from the central detector information.
The flexible trigger system allowed data taking with a minimum bias on the decay system. Several or even all decay modes, charged and neutral, of a given meson could be tagged using the forward detector trigger conditions on the pp or 3He system.