Development of the GEM tracker for the J-PARC E16 experiment
Introduction
Hadrons are considered to acquire their masses through the spontaneous breaking of the chiral symmetry. In hot and/or dense matter, it is predicted that the broken symmetry is partially restored and thus, the mass of hadrons changes. To have an experimental information on this phenomenon, the KEK-PS E325 experiment measured the masses and the widths of light vector mesons in nuclear matter [1]. They measured e+e− decays of mesons and K+K− decays of mesons in 12 GeV p+A reactions and reported significant mass modifications of those mesons.
The J-PARC E16 experiment [2] has been proposed to investigate such mass modifications more systematically. The experiment aims to collect a 100 times as large statistics of compared to the experiment KEK-PS E325 and to investigate the origin of the mass modification.
The J-PARC E16 spectrometer mainly consists of tracking detectors (called “GEM trackers”), Čherenkov detectors and lead-glass calorimeters. The whole spectrometer is composed of 26 modules each of which covers 0.22 sr as referred in Ref. [2]. In this paper, the development effort for the GEM tracker is reported. The spectrometer has three layers of position sensitive trackers. Three points of a particle track in the magnetic field are measured with those trackers and the momentum of the particle is reconstructed from them. The Gas Electron Multiplier (GEM) [3] is employed in each tracker to cope with the high rate environment of this experiment, which is at maximum 5 kHz/mm2. The GEM tracker is designed to measure the hit position of a track, and the finite angle of the incident track is caused by the magnetic field. The angle of incidence is defined as the angle of the track with respect to the axis normal to the GEM tracker. For the momentum region of interest, the required position resolution is for the tracks with an incidence angle of up to 30°. In the following sections, the structure of the GEM tracker and the performances obtained in test experiments are reported.
Section snippets
GEM tracker
The structure of the GEM tracker is shown in Fig. 1. The dimensions of the GEM foils used in our tests were 100 mm×100 mm. The amplification gas was a mixture of Ar (70%) and CO2 (30%) for the whole measurements.
The tracker consists of a triple-GEM stack, a two-dimensional readout board and a cathode plane made of aluminized Mylar. The length of the drift gap is 6 mm. The transfer and the induction gaps are 2.2 mm each. The insulating layer of the GEM foil is made of polyimide with a thickness of
Test experiment
Test experiments were performed at the J-PARC K1.1BR beam line in December 2012 and January 2013. The purposes of these tests were the following: the first was to establish analysis methods to minimize the position resolutions for all the incidence angles. The second was to evaluate the position resolution and the efficiency as a function of the gain of the triple-GEM and to optimize the operational voltage. The position resolution and the efficiency of the X side are discussed in the present
Summary
A particle detector (called “GEM tracker”) using a triple-GEM geometry and a two-dimensional readout board is developed for the J-PARC E16 experiment. The GEM tracker is tested with a beam with a momentum of 1.0 GeV/c at the J-PARC K1.1BR beam line. The position resolution of the tracker is evaluated with two methods, one is a “Center of Gravity (COG)” method and the other is a timing method. A position resolution of is achieved with the COG method for the 0° tracks, however it degrades
Acknowledgments
We would like to acknowledge the great efforts of the staff of the J-PARC accelerator and the Hadron experimental facility. The beam test would have been impossible without their assistance.
This study was partly supported by the KEK Detector Technology Project (KEKDTP), Japan Society for the Promotion of Science (JSPS), and by Grant-in-Aid for Scientific Research (Nos. 19654036, 19340075 and 21105004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
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