GlueX Experiment Research Articles

Study of the Nature of the Confinement in the GlueX Experiment

Confinement is a fundamental property of quantum chromodynamics (QCD) associated with the unique role of the gluonic field responsible for binding quarks in hadrons. Understanding the role of gluons in the confinement of quarks is one of the most tantalizing topics in modern particle physics to be explored. The new experiment GlueX has been recently constructed at Jefferson Lab [1] . The experiment was designed to search for hybrid mesons with exotic quantum numbers using a beam of linearly polarized photons incident on a liquid hydrogen target. The spectrum of these states and their mass splitting from normal mesons may yield information on confinement. In addition, these observations in combination with detailed chromodynamics calculations such as on the Lattice can provide important tests for our understanding of the role of gluons. The production of exotic mesons is expected to be enhanced in γp interactions, where the experimental data is very limited. We present the description of the GlueX detector, beam line, and first results of the commissioning with photon beam.

The Drift Chamber for the Experiment to Study the Nature of the Confinement

The GlueX experiment was designed to search for hybrid mesons with exotic quantum numbers using a beam of linearly polarized photons incident on a liquid hydrogen target. The spectrum of these states and their mass splitting from normal mesons may yield information on confinement. The description of the GlueX spectrometer and Forward Drift Chambers (FDC) as a part of track reconstruction system is presented in the text. FDC‘s are multiwire chambers with cathode and anode read-out. The system allows reconstructing tracks of charged particles with ∼200mkm accuracy with angles from 20 ̊up to 1 ̊. One of the detector features is 1.64% X0 material amount in the active area. The cathode gain calibration procedure is presented. The results of such calibration using cosmic data and beam data are presented as well.

A drift detector system with anode and cathode readout in the GlueX experiment

A drift detector system designed to detect charged particle tracks in the GlueX experiment dedicated to study the nature of confinement is described. The key design features of the drift chambers associated with the requirement of a minimum material budget in the path of secondary particles are presented. The spatial resolution and the detection efficiency have been measured with cosmic rays using the automatic data acquisition system.

Application of the Silicon Photomultipliers for Detectors in the GlueX Experiment

The GlueX detector in Hall D at Jefferson Lab [1]is instrumented with about 5000 Silicon Photomultipliers (SiPM) manufactured by Hamamatsu Corporation [2]. These photo sensors have properties similar to conventional photomultipliers but can be operated at high magnetic fields. Silicon photomultipliers with a sensitive area of 3x3 mm2 are used to detect light from the following GlueX scintillator detectors: the tagger microscope, pair spectrometer, and start counter. Arrays of 4x4 SiPMs sensors were chosen for the instrumentation of the barrel electromagnetic calorimeter. The tagger microscope must operate at high rates (up to 2.5MHz) and provide time measurements with a resolution better than 0.3ns. The paper will describe some results of the characterization of SiPMs for various GlueX sub-detectors.

Recording of relativistic particles in thin scintillators

Results of investigating an assembly of thin scintillators and silicon photomultipliers for registering relativistic particles with the minimum ionization are presented. A high efficiency of registering relativistic particles using an Ej-212 plastic scintillator, BSF-91A wavelength-shifting fiber (Saint-Gobain), and a silicon photomultiplier (Hamamtsu) is shown. The measurement results are used for creating a scintillation hodoscope of the magnetic spectrometer for registering γ quanta in the GlueX experiment.

Exotic Hybrid Meson Spectroscopy with the GlueX detector at JLab

The GlueX experiment[1] is scheduled to begin taking data in 2015. The goal is to discover evidence for the existence of exotic hybrid mesons and to map out their spectrum in the light quark sector. Recent theoretical developments using Lattice QCD [2] predict exotic hybrid states in a mass range accessible using the newly upgraded 12GeV electron accelerator at Jefferson Lab. Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. The experiment will use 9 GeV linearly polarized photons produced via coherent Bremsstrahlung to produce the exotic hybrids. The decay products will be detected in the solenoid-based GlueX detector currently under construction at Jefferson Lab. The status of the GlueX experiment including detector parameters will be presented along with theoretical motivation for the experiment.

Novel Hamamatsu Multi-Pixel Photon Counter (MPPC) array studies for the GlueX experiment: New results

The novel Hamamatsu Multi-Pixel Photon Counter (MPPC) S12045(X) is an array of 16 individual 3×3mm2 MPPC devices each with 3600 Geiger-mode Avalanche Photodiode pixels of 50×50μm2. Each MPPC in the array operates at a reverse bias of approximately 70V. This paper summarizes our characterization of MPPC arrays used in the GlueX experiment in Hall D at Thomas Jefferson National Accelerator Facility (Jefferson Lab). We studied the main features of each of the 16 MPPC array channels for 2800 MPPC arrays at several different temperatures. Two measurement stations were built to extract gain, breakdown voltage, photon detection efficiency, optical crosstalk and dark rate for each of the 44800 MPPC array channels at each temperature setting. The hardware and the data analysis are described, and new analytical expressions for the mean number of photoelectrons and optical crosstalk are presented, as well as systematic trends of the performance parameters.

Characterization of novel Hamamatsu Multi Pixel Photon Counter (MPPC) arrays for the GlueX experiment

The novel Hamamatsu Multi Pixel Photon Counter Array S12045(X) is an array of 16 individual MPPCs (3×3mm2) (further in the paper MPPC array channel) each with 3600 G-APD (Geiger-mode Avalanche Photodiodes) pixels (50×50 [μm2]). Each MPPC in the array works with its individual reverse bias voltage mode (around 70V). The paper summarizes the characterization process of MPPC arrays used in GlueX experiment (Hall D, Jefferson Lab). We studied the main features of each MPPC array channel for 2800 MPPC arrays at different temperatures. Two measurement stations were built to extract gain, breakdown voltage, photo detection efficiency (PDE), optical crosstalk and dark rate for each MPPC array channel. The hardware and the data analysis are described, which includes new analytical expressions to obtain the mean number of photo-electrons and optical crosstalk. The dynamical behavior of characterization parameters is presented as well.

GlueX: Photoproduction of Hybrid Mesons

The goal of the GlueX experiment is to provide critical data to help understand the soft gluonic field responsible for binding quarks in hadrons. Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. Photoproduction is expected to be particularly effective in producing exotic hybrids but there is little data on the photoproduction of light mesons. GlueX will use the new 12-GeV electron beam to produce a 9-GeV beam of linearly polarized photons using the technique of coherent bremsstrahlung. A solenoid-based hermetic detector is under construction, which will be used to collect data on meson production and decays. These data will also be used to study the spectrum of conventional mesons, including the poorly understood excited vector mesons. This talk will describe the latest theoretical developments to help understand how the data of hybrid mesons can provide insights into the fundamental theory of strong interactions.

QCD confinement and the GlueX experiment at Jefferson Lab

An understanding of the confinement mechanism in QCD requires a detailed mapping of the spectrum of hybrid mesons. Understanding confinement means understanding the role of gluons and it is in hybrid mesons that the gluonic degrees of freedom are manifest. High statistics searches for such states with π and p beams have resulted in some tantalizing signals. There is good reason to expect beams of photons to yield hybrid mesons with JPC quantum numbers not possible within the conventional picture of mesons as qq̄ bound states. Meager data currently exist on the photoproduction of light quark mesons. This talk represents an overview of the available data and what has been learned. In looking toward the future, the GlueX experiment at Jefferson Laboratory represents a new initiative that will perform detailed spectroscopy of the light-quark meson spectrum. This experiment and its capabilities will be reviewed.

Experimental Status of Exotic Mesons and the GlueX Experiment

One of the unanswered and most fundamental questions in physics regards the nature of the confinement mechanism of quarks and gluons in QCD. Exotic hybrid mesons manifest gluonic degrees of freedom and their spectroscopy will provide the data necessary to test assumptions in lattice QCD and the specific phenomenology leading to confinement. Within the past two decades a number of experiments have put forth tantalizing evidence for the existence of exotic hybrid mesons in the mass range below 2 GeV. This talk represents an overview of the available data and what has been learned. In looking toward the future, the GlueX experiment at Jefferson Laboratory represents a new initiative that will perform detailed spectroscopy of the light-quark meson spectrum. This experiment and its capabilities will be reviewed.

Radiative transitions in charmonium from lattice QCD

Radiative transitions between charmonium states offer an insight into the internal structure of heavy-quark bound states within QCD. We compute, for the first time within lattice QCD, the transition form-factors of various multipolarities between the lightest few charmonium states. In addition, we compute the experimentally unobservable, but physically interesting vector form-factors of the $\eta_c, J/\psi$ and $\chi_{c0}$. To this end we apply an ambitious combination of lattice techniques, computing three-point functions with heavy domain wall fermions on an anisotropic lattice within the quenched approximation. With an anisotropy $\xi=3$ at $a_s \sim 0.1 \mathrm{fm}$ we find a reasonable gross spectrum and a hyperfine splitting $\sim 90 \mathrm{MeV}$, which compares favourably with other improved actions. In general, after extrapolation of lattice data at non-zero $Q^2$ to the photopoint, our results agree within errors with all well measured experimental values. Furthermore, results are compared with the expectations of simple quark models where we find that many features are in agreement; beyond this we propose the possibility of constraining such models using our extracted values of physically unobservable quantities such as the $J/\psi$ quadrupole moment. We conclude that our methods are successful and propose to apply them to the problem of radiative transitions involving hybrid mesons, with the eventual goal of predicting hybrid meson photoproduction rates at the GlueX experiment.

Gluonic excitations and the GlueX experiment

The physics goals of the GlueX experiment and theoretical expectations for exotic mesons are presented.

Studies of magnetic shielding for phototubes☆

Phototubes associated with a Cherenkov counter, with a wall of scintillation counters for time-of-flight measurements and with a wall of lead glass blocks of an electro-magnetic calorimeter will operate in the fringe field of a superconducting solenoid in the GlueX experiment. The solenoid will be operated with a central magnetic field of ≈2.5T. The maximum fringe field in the vicinity of the phototubes will be approximately 150G. Various techniques for magnetic shielding of phototubes were studied using a 1-m diameter Helmholtz coil arrangement operated with a maximum central field of 200G. Results are presented.

Studies of magnetic shielding for phototubes

Phototubes associated with a Cherenkov counter, with a wall of scintillation counters for time-of-flight measurements and with a wall of lead glass blocks of an electro-magnetic calorimeter will operate in the fringe field of a superconducting solenoid in the GlueX experiment. The solenoid will be operated with a central magnetic field of ≈2.5T. The maximum fringe field in the vicinity of the phototubes will be approximately 150G. Various techniques for magnetic shielding of phototubes were studied using a 1-m diameter Helmholtz coil arrangement operated with a maximum central field of 200G. Results are presented.

GlueX: The search for gluonic excitations

The phenomenon of confinement in QCD remains a fundamental, unanswered question, and so does the significance of hybrid mesons in understanding QCD in the confinement region. The GlueX experiment will search for all hybrid exotic and non-exotic mesons in the mass range up to 3.0 GeV/c2. This contribution deals exclusively with the search for exotic hybrid mesons.

Characteristics of the TOF counters for GlueX experiment

Timing characteristics of two time-of-flight scintillation counters were studied using particle beams at the IHEP (Protvino) accelerator. The bars were 2m long and had a cross-section of 2.5×6.0cm2. Each bar was viewed from both ends by XP2020 photomultipliers. A time resolution better than 40ps was achieved.