Hadron spectroscopy is the subfield of particle physics which studies the properties of hadrons. Hadrons are composite particles made of quarks held together by the strong nuclear force. The theory that describes the properties of hadrons is quantum chromodynamics (QCD), a gauge theory which describes hadrons as being made up of dynamically confined quarks and gluons. The main goals of the hadron spectroscopy are:
Lattice QCD is a lattice gauge theory formulated on a grid or lattice of points in space and time. When the size of the lattice is taken infinitely large and its sites infinitesimally close to each other, the continuum QCD is recovered.
QCD predicts that quarks, q , and antiquarks, q-, bind into particles called mesons (e.g. Kaons, Pions). Another type of hadron is called a baryon which is made of three quarks (e.g. protons and neutrons). Potentially QCD also predicts boundary states made only by gluons called glueballs. The gluons are an elementary particle that acts as the exchange particle for the strong force between quarks. Another important goal in the field of hadronic spectroscopy is to find experimental evidence for exotic mesons, tetraquarks, molecules of hadrons and glueballs.
One of the main challenges of hadron physics is the search for gluonic excitations, i.e. hadrons in which the gluons can act as principal components. These gluonic hadrons fall into two main categories: glueballs, i.e. states where only gluons contribute to the overall quantum numbers and hybrids which consist of valence quarks and antiquarks as hadrons plus one or more excited gluons which contribute to the overall quantum numbers.
Understanding of the baryon spectrum is also one of the primary goals of non-perturbative QCD. In the nucleon sector, where most of the experimental information is available, the agreement with quark model predictions is astonishingly small and the situation is even worse in the strange baryon sector. All of the above-mentioned challenges are conducted by several laboratories around the world (i.e. GSI and JLab).
The GSI operates a worldwide unique large-scale accelerator facility for heavy ions, where the PANDA experiment is hosted. The PANDA experiment will study hadron physics. In this experiment, a detector with the following characteristics is mandatory:
FIG. 1 Schematic view of the PANDA detector.
The proposed detector is subdivided into the target spectrometer (TS) consisting of a solenoid around the interaction region and a forward spectrometer (FS) based on a dipole to momentum-analyze the forward-going particles.
The combination of two spectrometers allows a full angular coverage, it takes into account the wide range of energies and it still has sufficient flexibility, therefore individual components can be exchanged or added for specific experiments.
FIG. 2 Schematic view of the tracker (a), the electromagnetic calorimeter (b), PID detector (c), forward spectrometer (d), magnet system (e).
Both the tracker and the PID of the PANDA experiment use photosensors for particle detection for example, the MaPMTs by Hamamatsu Photonics type H12700 and H13700.
The PID detector of PANDA comprises of a DIRC and RICH detector (FIG. 2c). The Barrel DIRC detector of the PANDA experiment is made of 16 optically isolated sectors, each comprising a bar box and a solid fused silica prism, surrounding the beam line in a 16-sided polygonal barrel with a radius of 476 mm and covers the polar angle range of 22° to 140°. A flat mirror is attached to the forward end of each bar to reflect photons towards the read-out end, where they are focused using a three component spherical compound lens on the back of a 30 cm deep solid prism, made of synthetic fused silica and serving as expansion volume.
The location and arrival time of the photons are measured by an array of 11 photodetectors. Since the sensor has to work in a magnetic field of ≈ 1 T, the magnetic field of the PANDA target spectrometer (TS) solenoid puts severe design constraints on the photon readout. For these reasons MCP-PMTs were considered (e.g. Hamamatsu Photonics, R13266-07-M64 and R13266-07-M768).
FIG. 3 Barrel DIRC detector baseline design.
Another experiment dedicated to hadronic spectroscopy is GlueX. GlueX is hosted in the Thomas Jefferson National Accelerator facility (JLab). The detector (shown in FIG. 4) has been designed to observe the exotic hybrid mesons.
FIG. 4 Schematic view of the GlueX detector
Detailed comparisons of the experimental results to theoretical predictions on the excitations of the gluonic field in mesonic systems will lead to a more detailed understanding of the role of glue in the confinement of quarks inside hadronic matter.
Photoproduction is expected to be particularly effective during the exotic hybrid mesons production but there is little data on the photoproduction of light mesons. GlueX will use the coherent bremsstrahlung technique to produce a linearly polarized photon beam. This device is responsible for the detection, identification and total energy measurement of both neutral (photons, neutrons) and charged (protons, pions) particles.
At the heart of the GlueX detector is its electromagnetic barrel calorimeter (BCAL), which must provide excellent energy and timing resolution, low threshold of detection and the ability to completely contain the electromagnetic showers resulting from the conversion of photons.
The GlueX-BCAL will be inserted and operated in a ~2 T magnetic field required for the GlueX experiment. This high magnetic field is a constraint to the choice of the photo-sensors for the calorimeter. Under this condition, regular vacuum PMT's are very sensitive to the orientation of the magnetic field.
After extensive tests on a variety of sensors, the chosen photodetector for the GlueX barrel calorimeter was a custom SiPM array manufactured by Hamamatsu Photonics. Data on mesons production and decays are collected thanks to a solenoid-based hermetic detector.
This data will also be used to study the spectrum of conventional mesons, including the poorly understood excited vector mesons and strangeonium.
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